Keyboard instrument
United States ‘Patent [19]
[11]
[45]
Koike et al.
5,031,500
Patent Number:
Date of Patent:
Jul. 16, 1991
[54] KEYBOARD INSTRUMENT
[75] Inventors: Tatsuhiro Koike; Yoichi Misawa;
Toshiyuki Suzuki, all of Hamamatsu,
3,410,947 12/1968
Japan
[73] Assignee: Yamaha Corporation, Hamamatsu,
Japan
[21] Appl. No.: 366,748
4,365,113 12/1982 Soma et al.
Attorney, Agent, or Firm—Spens1ey, Horn, Jubas &
Lubitz
[22] Filed:
Jun. 15, 1989
[30]
Foreign Application Priority Data
Keyboard instruments incorporating sound systems for
11111.21,
Jun. 21,
Jun. 21,
Jun. 22,
1988
1988
1988
1988
4/1970
George
3,643,000
2/1972
Andersen . . . . . . . .
3,718,747
2/1973
Martin et a1.
4,741,040
4/1988
. ... ... ... .
. . . . ..
. . . ..
84/718
84/ 1.01
........ .. 84/600
181/144 X
Kaizer ................................. .. 381/96
Primary Examiner-W. B. Perkey
[57]
ABSTRACT
' generating musical tones are disclosed. In a ?rst key
[JP]
[JP]
[JP]
[JP]
board instrument, speakers are mounted near a key
board, and are arranged upright to be directed to the
‘keyboard on the front side, thereby allowing a per
former to clearly grasp the sound quality of perfor
'................. .. 84/600; 84/718;
' mance tones. In a second keyboard instrument, which
comprises a Helmholtz resonator consisting of a reso
181/141; 381/118; 381/159; 381/96
nance port and a cabinet, an electro-acoustic transducer
Field of Search ............... .. 84/718, 600, 644, 670,
mounted on the outer surface of the cabinet and a driver
for driving the transducer so as to cancel an air counter
action from the resonator, the size of a sound system is
[51]
Int. Cl.5 ........................ .. GIOH 3/26; H04R 3/00 ‘
[s2]
u.s.c1. ............ ..
[58]
84/DIG. 21», 189-196, v27o, 294, 296; 181/141,
148, 156, 160; 381/93, 94, 96, 98, 118, 159'
[56]
Behnke et a1. ...................... .. 84/600
3,506,773
reduced, and the frequency characteristics, especially
the low-frequency reproduction characteristics of the
References Cited
U.S. PATENT DOCUMENTS
1,953,544
4/1934
3,064,515 11/1962
sound system are improved.
Stein .................................... .. 84/189
4 Claims, 11 Drawing Sheets
Markowitz ......... ..
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FIG. l6
5,031,500
5,031,500
1
board instruments comprises a closed or phase-inver
sion (bass-re?ex) type speaker system and a power am
KEYBOARD INSTRUMENT
pli?er,‘ having a substantially zero output impedance,
BACKGROUND OF THE INVENTION
1. Field of the Invention
2
A sound system incorporated in conventional key
' for constant-voltage driving the speaker system. In this
' case, the lowest reproduction frequency of the speaker
' system is mainly determined by the volume of a cabinet
’
‘The present invention relates to a keyboard instru
_ ment incorporating a sound system for generating musi
(e.g., a main bodycase) and the characteristics (f0, Q0,
cal tonesand, more particularly, to an electric or elec
tronic keyboard instrument designed to reduce a, sound
‘ and the like) of a speaker'unit used in the system. That
system in size and improve sound quality or frequency
tones having lower frequencies are to be produced, a
cabinet having a larger volume is required, resulting in
' ‘considerable limitation in design. In addition, perfor
is, in the conventional keyboard instruments, if musical
characteristics.
2. Description of the Prior Art
In keyboard instruments, design, frequency charac
mance may be interfered depending on an arrangement
teristics, operability, and the like are very important 15 'ofthe cabinet, and other problems are posed in terms of
factors.
operation. FIG. 16 shows an outer appearance of a
"
In conventional electric or electronic keyboard in
keyboard instrument designed by integrally forming a
struments incorporating speakers, regarding the design,
rear frame 21 and a cabinet 7. Referring to FIG. 16,
in' order to satisfy a demand for a low-pro?le keyboard >
instrument, a speaker unit is arranged at a rear portion 20
of a keyboard section. These speaker units are ?xed so
that, their sound radiating directions (axial direction of a
reference symbols 9a and 9b respectively denote bass
reflex ports (resonance ports).
SUMMARY OF THE INVENTION
diaphragm) are directed to various directions. In a con
The present invention has been made in consideration
of the problems posed in the above-described conven
upper surface of an instrument main body case, and the ‘ 25‘ tional keyboard instruments, and has at its ?rst object to
diaphragm of the speaker is'arranged to oppose the slit
provide a keyboard instrument which allows a per
ventional instrument,a slit (tone escape) is formed in the
in a substantially horizontal state.
In another electric or electronic keyboard instru
former to directly and clearly grasp performance
sounds and which has a low pro?le and allows periph-v
ment, a speaker is arranged to generate musical sounds
toward the rear portion of the main body case. That is,
eral units to be arranged on the upper surface of an
a tone escape is arranged to be open to a side opposite
to a performer.
I With such a conventional arrangement of a speaker,
It is a second object of the present invention to pro
vide a‘ keyboard instrument which can reduce the size of
however, it is dif?cult for a performer to directly grasp
the sound quality of generated tones. conventionally,
the performer grasps performance sounds from sounds
reflected by the wall of the rear portion of the instru
ment main body. In addition peripheral units such as an
MIDI unit cannot be mounted on, e.g., the main body
case.
Regarding the frequency characteristics of such a
keyboard instrument, for example, an 88-key piano has
a lowest bass tone (A0) of 27.5 Hz, and the frequency of
instrument case.
_y_a_sound system, especially the size of a cabinet constitut
ing a speaker system, without impairing frequency char
acteristics, especially low-frequency characteristics, to
improve ?exibility in design and operability, or which
can improve low-frequency characteristics without
increasing the cabinet size.
According to a ?rst aspect of the present invention,
there is provided a keyboard instrument comprising a
box-like main body case, a keyboard arranged on the
front side of the main body case, a tone escape formed
in the main body case so as to be open to the front side
a fundamental wave of a bass drum during automatic
thereof, and a speaker unit which is arranged upright in
rhythm performance is about 30 Hz. These ultra low
45 the case on the rear side of the keyboard while the axis
of the diaphragm of the speaker unit is directed to the
bass tones pose no problem to the performer in monitor
tone escape, wherein the speaker assembly includes a
ing (grasping) a normal performance, even though a
large-diameter speaker unit and a small-diameter
fundamental wave itself is not produced enough. This is
speaker unit, the inclination of the large-diameter
because if harmonic waves are reproduced, the bass
tones are compensated in audible levels. However, for 50 speaker unit being smaller than that of the small-diame
ter speaker unit.
example, in the bass drum, if a fundamental wave of
According to the keyboard instrument of the ?rst
about 30 Hz is slightly output at a level exceeding an
aspect, desired musical tones are generated by a musical
audible sound pressure limit, and a harmonic overtone
tone generating section through the speaker assembly
of 50 to 60 Hz is sufficiently output, the generated tone
is felt as a heavy bass tone by the performer. In contrast 55 upon keyboard performance or various switch opera
tions. More speci?cally, bass tones are generated from
to this, if a sound system having of lowest reproduction
the large-diameter speaker unit, whereas treble tones
frequency of about 70 Hz or more is used, generated
are generated from the small-diameter speaker unit.
tones become less richer in low frequency region, thus
These musical tones are generated by vibrating the
exhibiting a great difference in sound quality.
diaphragms of the respective speaker units. In this case,
Many recent keyboard instruments employ a PCM
the axes of the diaphragms of the speakers of the
sound source as a sound source. For this reason, if input
speaker units are directed to the tone escape. The tone
signals to the sound system are directly reproduced, the
escape is open toward the front side of the main body
sound quality of reproduced tones is very high. In order
case. As a result, performance sounds directly reach a
to reproduce musical tones with high ?delity, a strong
demand has arisen for a sound system with improved 65 performer, and hence the performer can clearly hear
them.
?delity. The reproduction characteristics of a sound
In addition, peripheral units can be mounted on the
system are mostly determined by the reproduction char
upper surface of the main body case. Further, the pro
acteristics of a speaker system.
3
5,031,500
4
?le ofI the main body case can be decreased since the
tone (resonance tone) can be generated from the Helm
large-diameter speaker unit is inclined more than the
holtz resonator.
small-diameter speaker unit.
According to the sound system obtained by combin
ing'the speaker system having a resonance port of the
' tion, there is provided a keyboard instrument character 5 present invention and the driving means for driving the
According to a second aspect of the present inven
ized by incorporating a sound system comprising a
speaker system having a resonance port, and a driving
means. The speaker system is similar in shape to a bass
transducer of the speaker system so as to cancel an air
counteraction from the resonator side during a drive
period of the Helmholtz resonator, the volume of the
_re?ex type speaker system, and has 'an electro-acoustic
cabinet can be reduced compared with a case wherein a
transducer arranged on the outer wall of a cabinet hav
conventional bass-re?ex type speaker system is con
ing a ‘resonance port, which constitutes a Helmholtz
resonator. The transducer drives the Helmholtz resona
starit-voltage driven. In addition, by elongating the
' tor on the inner surface side of its vibrating body and
directly radiates a sound on the outer surface side of the‘
resonance port to decrease the resonance frequency of
the resonator, lower bass tones can be reproduced.
As described above, according to the second aspect
' vibrating body. The driving means drives the trans 15 of the present invention, the cabinet can be reduced in
ducer so as to cancel an air counteraction from the
Helmholtz resonator to the vibrating body.
_ ,According to ‘the second aspect, the speaker system
comprises a Helmholtz resonator similar to a bass-re?ex
typev speaker system. Therefore, a_ sound is ‘directly
radiated from the vibrating body of the electro-acoustic
‘transducer,’ and at the same time, a sound is also radiated
from the Helmholtz resonator driven by the vibrating
body. The frequency characteristics of an output sound
volume and pro?le.
‘As the pro?le of the cabinet is decreased and the ratio
of ‘a maximum length, width or height to a minimum
length, width or height is increased, characteristics as a
duct are enhanced. As a result, duct resonance tones
having wavelengths corresponding to i, l, . . . of the
maximum size are generated, and their levels and fre
quencies become noise or distorted components which
cannot be neglected.
pressure of the speaker system are equivalent to those 25
According to a third aspect of the present invention,
obtained by mixing a direct radiation sound from the
at least part of the inner wall of the cabinet is consti
vibrating body of the electro-acoustic transducer with a
tuted by a damping material for preventing duct reso
resonance sound from the resonator. For this reason,
nance. With this arrangement, generation of noise or
the low-frequency characteristics of this speaker system
distorted components due to duct resonance can be
can be improved compared with those of a closed type 30
speaker system for radiating only a direct radiation
prevented.
If the resonance frequency of the Helmholtz resona
_
tor is decreased and the Q value is increased to repro
Inthe second aspect, a driving means for the electro
d'uce lower bass tones in the arrangement of the second
_acoustic transducer drives the transducer so as to cancel
an air counteraction from the resonator side during a 35 aspect, the reproduction frequency characteristics drift.
This frequency drift can be compensated by increasing
drive period of the Helmholtz resonator. As such a
/decreasing an input signal voltage, especially boosting
driving means, a known circuit may be employed, e.g.,
a signal component with a low sound pressure. How
a negative impedance generating circuit for equiva
ever, in consideration of a case wherein a given key may
lently generating a negative impedance component
be kept depressed, the maximum output of a sound
(—Z()) in an output impedance or a motional feedback
sound by the extent of the resonance sound.
(MFB) circuit for detecting a motional signal corre
sponding tomovement of the vibrating body by a cer
tain method and negatively feeding back the detected
signal to the input side.
system in a keyboard instrument, especially a driving
means must be considered in terms of continuous rating.
. In this case, the driving means requires a capacity sev
eral times larger than that of a normal audio ampli?er
If the electro-acoustic transducer is driven to cancel a 45 whose maximum output can be set in terms of instant or
intermittent rating. As described above, such a driving
counteraction to the vibrating body of the transducer in
means (ampli?er) is used to drive the transducer so as to
this manner, when, for example, an air counteraction is
cancel an air counteraction from the resonator side, and
completely canceled, the transducer is driven in a so
is basically required to have a relatively large capacity.
called dead state wherein the transducer is sufficiently
damped to be free from the influences of the air counter 50 Therefore, it is dif?cult to further increase the capacity
of such a driving means (ampli?er) to compensate
action from the resonator side, i.e., the cabinet side. For
(boost) the above-described frequency drift.
this reason, the frequency characteristics of a direct
In contrast to this, in a low frequency range below
radiation sound are not influenced by the volume of the
several tens of Hz, since a wavelength becomes several
space at the back of the transducer. Hence, the volume
of the cabinet can be minimized as long as no inconve 55 meters or more, a sound image is basically not clear, and
the position of a sound source is not much of a problem.
nience occurs as a cavity of the Helmholtz resonator
According to a fourth aspect of the present invention,
and a casing of the transducer. When viewed from the
a plurality sets of sound systems each of which is identi
Helmholtz resonator side, driving the transducer to
cal with the above sound system are arranged, and the
cancel an air counteraction from the resonator side
during a drive period of the resonator means that the 60 Helmholtz resonators of the respective sound systems
are set to have different resonance frequencies. With
vibrating body of the transducer serves as an equivalent
this arrangement, the drift of sound pressure character
wall, i.e., part of a resonator inner wall which cannot be
istics obtained by mixing sounds radiated from speaker
driven by the resonator side. Therefore, the Q value of
systems of the respective sound systems is decreased
the Helmholtz resonator is not influenced by the char
acteristics of the transducer. Even if the resonance fre 65 because resonance tones from the respective Helmholtz
resonator compensate for each other. As a result, the
quency based on the resonance port and the cabinet is
boost amount of the driving means is decreased, and the
decreased, a suf?cient Q value can be ensured. For this
maximum output of the driving means can be decreased.
reason, even if a cabinet is reduced in size, a heavy bass
5,031,500
5
BRIEF DESCRIPTION OF THE DRAWINGS:
arranged upright in the main body case 111 so as to
v I FIG. 1 is a side view showing an arrangement of a '
form predetermined angles with respect to the horizon
‘- tal plane, e.g., the lower surface of the main body case
keyboard instrument according to a ?rst embodiment of
5 111. Reference numeral 123 denotes a bracket for ?xing
the present invention;
thespeaker units 115A and 115B.
The middle-frequency or low-frequency speaker unit
FIG. 2 is a perspective view of a speaker unit accord
ing to the first embodiment;
’
6
These speaker units 115A and 115B are respectively
_
(squawker or woofer) 115A has a larger diameter than
FIG. 3 is a perspective view showing an outer ap-'
pearance of a keyboard instrument according to a sec; " the high-frequencyspeaker unit (tweeter) 115B. The
inclination of the speaker unit 115A is smaller than that
ond ‘embodiment of the present invention;
' '
of the speaker unit 115B (see FIG. 1). The tweeter 115B
FIG. 4 is a sectional view taken along a line A—A in
has a diameter of, for example, 12 cm, whereas the
FIG. 3;
'squawker 115A has diameter of, for example, 20 cm.
FIG. 5 is a perspective view showing a state wherein
Both the speaker units 115A and 115B are arranged
exterior components are detached from the keyboard .
upright
to generate musical tones forward. The axis of a
instrument in FIG. 3;
.
FIGS. 6(a) and 6(b) are top and right side views, ' diaphragm of the small-diameter speaker unit 115B is
directed to the tone’ escape 131. The axis of a diaphragm
of the large-diameter speaker unit 115A is directed to
the case located slightly above the tone escape 131.
respectively, showing a cabinet in FIG. 3;
, FIG. 7 is a circuit diagram showing a fundamental
arrangement of a sound system provided for the-key
board instrument in FIG. 3;
.
20 Note that the tone escape 131 is formed at the front
surface of the main body case 111 located at a position
above the rear side of the keyboard 113.
FIG. 8 is an equivalent circuit diagram of the sound
system in FIG. 7;
_
.
In the keyboard instrument having the above
FIG. 9 is a graph showing frequency characteristics
described arrangement, vibrations of the diaphragms of
of sound pressures of sounds radiated from the sound
25 the‘ speaker units 115A and 115B are transmitted to a
systems in FIGS. 3 and 7;
performer through the 'tone escape 131. As a result, the
FIG. 10 is an equivalent circuit diagram of the instrua
performer can directly and clearly discriminate sound
ment in FIG. 7 when Zy- Z0=O;
v‘quality of performance tones, degradation in sound
FIGS. v11(0), 11(b), and 11(c) are graphs respectively
showing frequency characteristics of the sound system
quality, and the like.
The upper surface of the main body case 111 can be
30
in FIG. 7;
formed to be ?at, and a slit such as the tone escape 131
FIGS. 12 and 13 are circuit diagrams respectively
‘need not be formed in the upper surface. Therefore, a
showing fundamental circuits for generating negative
peripheral unit such as an automatic performance unit
impedance;
and the like can be placed on the upper surface. In
FIG. 14 is a circuit diagram showing a detailed ar
35 addition, a music desk can be formed on the upper desk
rangement of a negative resistance driver;
as with the case of the conventional instruments.
FIGS. 15(a), 15(b), and 15(c) are views showing an
Note that the inclinations of the speaker units 115A
arrangement of a 3D system according to another em
' bodiment of the present invention; and
FIG. 16 is a perspective view showing an outer ap
pearance of a keyboard instrument incorporating a
sound system for constant-voltage driving a conven
and 115B can be arbitrarily set.
Second Embodiment
FIG. 3 showsv an outer appearance of a keyboard
instrument according to a second embodiment of the
tional bass-re?ex type speaker system.
present invention. This keyboard instrument employs a
DETAILED DESCRIPTION OF THE
PREFERRED EMBODIMENTS
speaker system with a resonance port as a speaker sys
45 tem constituting a sound system. This speaker system
Embodiments of the present invention will be de-’_
scribed below with reference to the accompanying
drawings.
First Embodiment
FIGS. 1 and 2 show a keyboard instrument according
to a ?rst embodiment of the present invention.
As shown in FIGS. 1 and 2, an electronic keyboard
instrument comprises a box type main body case 111. A
comprises Helmholtz resonators like a conventional
bass-reflex type speaker system, and is similar in shape
to the bass-reflex type speaker system. However, the
volume of the cavity of each Helmholtz resonator of
this speaker system is greatly decreased to several liters
which is very small in comparison with a volume of 20
to 30 liters of the conventional bass-re?ex type speaker
system. In addition, each resonance port is elongated to
set the resonance frequency of the resonator to be 50 to
keyboard 113 and a speaker assembly 115 are arranged 55 60 Hz which is equal to or lower than that of the con
ventional bass-re?ex type speaker system.
in the main body case 111. The keyboard 113 is ar
FIG. 4 is a sectional view taken along a line A-A in
FIG. 3. FIG. 5 is a perspective view showing a state
wherein some exterior components omitted. Referring
The keyboard 113 is constituted by a plurality of 60 to FIGS. 3 to 5, a shelf plate 1 is held by two vertical leg
ranged on the front side of the main body case 111 so as
to be vertically swingable. A tone escape 131 is open to
an upper portion on the rear side of the keyboard 113.
aligned keys 121. The rear end of each key 121 is swing
ably supported by a pin 122 as a fulcrum. Switches for
portions 2a and 2b at a predetermined height. A key
' board 3, speaker mounting bases 5a and 5b for left and
right channels, on which speaker units 4a and 4b are
detecting depression of these keys 121 and switches for
mounted, and electric circuits (not shown) including a
detecting vdepression strength are arranged around the
65 sound source and ampli?ers for driving the speakers of
keys 121.
A speaker unit 115A for bass tones and a speaker unit
1153 for treble tones are respectively arranged on the
rear side of the keyboard 113 of the main body case 111.
the respective channels are mounted on the shelf plate 1.
In addition, openings 6a and 6b are formed in the shelf
plate 1, and a cabinet 7 is formed under the shelf plate 1.
5,031,500
.
8
7
Cavities and resonance ports are formed between the
shelf plate 1 and the cabinet 7.
As shown in FIGS. 6(a) and 6(b), opening ports 9a
be formed in a top plate 16, and hence musical scores,
ornaments, and the like can be placed on the top plate
16 without worrying about sound quality.
and 9b are formed in a bottom plate 8 of the cabinet 7,
A frame 21 is formed between the leg portions 20 and
» and the interior of the cabinet v7 is partitioned by an
2b below the cabinet 7 so as to reinforce a structure
intermediate plate 10 and partition plates 11a to 11d.
Portions partitioned by the opening ports 9a and 9b and
constituted by the shelf plate 1, the leg portions 20 and
2b, and the like. -
the partition plates 11a to 11d, which respectively com
FIG. 7 is a circuit diagram for explaining a fundamen
municate with the ports 9a vand 9b, constitute resonance 1 tal arrangement of an acoustic unit (sound system) in
.ports of the Helmholtz resonators when the cabinet 7 is. 10 corporated in the keyboard instrument shown in FIG.
‘mounted on the shelf plate 1 and the upper portion of
the cabinet 7 is closed. Spaces other than the resonance
' _ ports constitute cavities when the cabinet 7 is mounted
on the shelf plate 1 and the upper portion of the cabinet
7 is closed, ‘and is divided into two cavities for the left
' and right speaker systems by the intermediate plate 10.
vWhen the keyboard instrument is completed, these
spaces respectively communicate with spaces formed
on the rear sides of the left and right speaker mounting
bases 50 and 5bthrough the openings 6a and 6b, and
cavities of the Helmholtz resonators are formed-by the
spaces of the cabinet 7 and’ the spaces of the speaker ,2
mounting bases 5a and 5b. In this case, the intermediate .
3. This acoustic unit includes the speaker system with
the resonance port and an ampli?er for driving the
speaker system. FIGS. 3' and 6 show an arrangement of
each speaker system mounted in the instrument.
In the speaker system 40 shown in FIG. 7, a hole is
formed in the front surface of ‘a cabinet 7, and a dynamic
type electro-acoustic transducer (speaker unit) 4 (4a, 4b)
is mounted in the hole. An resonance port 18 which has
a sound path 17 opening to outward of the cabinet 7
through a opening port portion 9 (9a, 9b) is arranged
below the transducer 4. The resonance port 18 and the
cabinet 7iform a Helmholtz resonator. In this Helmholtz
resonator, an air resonance phenomenon occurs due to
plate 10 is attached to be slightly shifted from the center
toward the right side, so that the volumes of the cavities 25 an air spring in the cabinet 7 as a closed cavity and an air
mass in the sound path 17 A resonance frequency f0p is
for the left and right channel speaker systems are re
given by the above mentioned formula (1):
spectively set to be about 5.5 and 4.5 liters. In addition,
the resonance frequencies of the left and right Helm
holtz resonators are set to be different from each other,
i'.e., 50 and 60 Hz, respectively. If the velocity of sound
is represented by c; the sectional area of a resonance
In FIG. 7, the driver circuit 50 comprises a frequency
port, S; the length of the resonance port, 1; and the
characteristics compensation circuit 51, negative impe
dance driver 52 and the like. The negative impedance
volume of a cavity,”V, a frequency fop of such a Helm
driver 52 comprises a ampli?er 53, resistor R5, and
holtz resonator can be obtained by the following equa
feedback
circuit 54.
tion:
35
In the negative impedance driver 52, an output from
the ampli?er 53 having a gain A is supplied to the
speaker unit 4 of the speaker system as a load ZL. A
Felts 12a and 12b are bonded to the bottom plate 8 of
current IL ?owing through the speaker unit 4 is de
the cabinet 7. In this embodiment, since the height of
tected by the resistor R5, and the detected current is
the cabinet 7 is as l/l0 small as its‘width, the above 40 positively fed back to the ampli?er 53 through the feed
space portion strongly exhibits characteristics as a duct.
back circuit 54 having a transmission gain B. With this
If the wall enclosing this space consists of a rigid mate
arrangement, an output impedance Z0 of the circuit is
?aws/Wm
(1)
'rial such as a wood, plastic, or metal material, duct
‘resonance tones having wavelengths corresponding to
i, l, . . . the width of the space portion are generated. In 45
this case, the'felts 12a and 12b are bonded to prevent the
duct resonance. In place of the felts 12a and 12b, other
materials having air-permeability and acoustic resis
tance, e.g., a sponge, an unwoven fabric, and a woven
fabric may be used as such a duct resonance preventing
means. In addition, the duct resonance preventing
means may be constituted by a material having ?exibil
ity and viscoelasticity, e.g., rubber. Such ?exible, visco
elastic material exhibits a pressure reducing effect sub
stantially equivalent to the air-permeability of the felt
and the like due to its flexibility, and serves as a resistor
for consuming energy upon ?exing due to its viscoelas
ticity.
calculated as:
Zo=Rs(1—Al3)
(2)
If AB>1 is established in this equation, Z0 becomes an
open stable type negative resistance.
FIG. 8 shows an arrangement of an electric equiva
lent circuit of the portion comprising the speaker sys
tem shown in FIG. 7. In FIG. 8, a parallel resonance
circuit Z1 is formed by the equivalent motional imped
ances which are caused by the motion of the unit vibra
tion system comprising the diaphragm 41 of the speaker
unit 4. In the circuit Z1, reference symbol to denotes an
equivalent resistance of the vibration system; So, an
equivalent stiffness of the vibration system; and mo, an
equivalent mass of the vibration system. A series reso
The cabinet 7 is reinforced by mounting triangular
prism-like reinforcing members 130 to 13f at several 60 nance circuit Z2 is formed by an equivalent motional
positions.
Referring to FIGS. 3 to 5, slit-like opening groups
15a and 15b are formed in a front panel 14 of the key
board instrument. Direct radiation tones from the
speaker units 40 and 4b obliquely arranged in the instru
ment are output to the outside through the opening
groups 15a and 15b, respectively. With this arrange
ment, openings for the direct radiation tones need not
impedance of a Helmholtz resonator constituted by the
resonance port and the cavity. In the circuit Z2, refer
ence symbol rc denotes an equivalent resistance of the
cavity of the resonator; SC, an equivalent stiffness of the
cavity; rp, an equivalent resistance of the resonance
port; and mp, an equivalent mass of the resonance port.
In the Figure, reference symbol A denotes a force coef
ficient. When the speaker unit 4 is a dynamic direct
9
5,031,500
10
are converted into a resistance series-connected to mo
radiation speaker, A=Bly where B is the magnetic flux
density in'a magnetic gap, and l, is the total length of a ' ' tional impedances Scand mp in FIG. 8, and coefficients
assigned to the respective elements are omitted.
_'voice coil conductor. In theFigure, reference symbol
ZVdenotes an internal impedance (non-motional impe~
The equivalent circuit diagram reveals the following
facts.
The two ends of the parallel resonance circuit Z1
mainly comprises a resistance R Vof the voice coil, and , , formed by the equivalent motional impedance of the
speaker unit 4 are short-circuited at a zero impedance in
includes a small inductance.
“an AC manner. Therefore, the parallel resonance circuit
The operation of the acoustic apparatus having the
Z1 has a Q value of 0, and can no longer serve as a
arrangement shown in FIGS.'7 and 8 will be described
dance) of the speaker unit 4. When the speaker unit 4 is
a dynamic direct radiation speaker, the impedance ZV
below.
resonance circuit. More speci?cally, this speaker unit 4
'
When a drive signal is supplied from the driver ciré
loses the concept of a lowest resonance frequency
cuit 50 having a negative impedance drive function to
_ which is present in a state wherein the speaker unit 4 is
merelymounted on the Helmholtz resonator. In the
. the speaker unit 4, the speaker unit 4 electro-mechani
cally converts this signal to reciprocate its diaphragm
41 forward and backward (to the left and right inFIG.
5
following description, the lowest resonance frequency
f0 or equivalent of the speaker unit 4 merely means the
essentially invalidated concept. In this manner, since the
unit vibration system (parallel resonance circuit) Z1
7). The diaphragm 41 mechanoacoustically converts the '
reciprocalmotion. Since the driver circuit 50 has the
does not essentially serve as a resonance circuit, the
negative impedance drive function, the internal impe
danceof the speaker unit 4 is equivalently reduced ,20 resonance system in this acoustic apparatus is only the
ideally invalidated). Therefore, the speaker unit 4 drives
the diaphragm 41 while faithfully responding to the
_ drive signal input to the driver circuit 50, and indepen
dently supplies drive energy to the Helmholtz resonator
constituted by the resonance port 18 and the cabinet 7.
In this case, the front surface side (the right surface side
in FIG. 7) of the diaphragm 41 serves as a direct radia
tor portion for directly radiating acoustic wave to the
outward, and the rear surface side (the left surface side
in FIG.‘7) of the diaphragm 41 serves as a resonator
driver portion for driving the Helmholtz resonator con
stituted by the resonance port 18 and the cabinet 7.
For this reason, as indicated by an arrow a in the
FIG. 7, an acoustic wave is directly radiated from the
diaphragm 41, and air in the cabinet 7 is resonated, so
‘that an acoustic wave having'a suf?cient sound pressure
is resonantly radiated from the resonance radiation por
tion (the opening portion 9 of the resonance port.18), as
indicated by an arrow b in the Figure. By adjusting an
Helmholtz resonance system (series resonance circuit)
2;.
Since the speaker unit 4 does not’ essentially serve as
the resonance circuit, it linearly responds to a drive
25
signal input in real time, and faithfully electro-mechani
cally converts an electrical input signal (drive signal
E0), thus displacing the diaphragm 41 without transient
response That is, a perfect damped state (so-called
“speaker dead” state) is achieved. The output sound
pressure-frequency characteristics around the lowest
resonance frequency f0 or equivalent of this speaker in
this state are 6 dB/oct. Contrary to this, characteristics
of a normal voltage drive state are 12 dB/oct.
The series resonance circuit 2; formed by the equiva
lent motional impedance of the Helmholtz resonator is
connected to the drive signal source En at a zero impe
dance. Thus, the circuit Z2 no longer has a mutual de
pendency with the parallel resonance circuit Z1. Thus,
the parallel resonance circuit Z1 and the series reso
air equivalent mass in the resonance port 18 of the 40 nance circuit 2; are present independently of each
other. Therefore, the volume (in inverse proportion to
8,) of the cabinet 7, and the shape and dimension (in
proportion to m,,) of the resonance port 18 do not ad
fa[_,(=fo¢/\/2) which is a standard setting value as a
versely influence the direct radiation characteristics of
conventional'bass-reflex speaker system (where foc is the
lowest resonance frequency of the speaker unit 4 sup 45 the speaker unit 4. The resonance frequency and the Q
value of the Helmholtz resonator are not in?uenced by
posed to be attached to a conventional bass-reflex type
the equivalent motional impedance of the speaker unit
cabinet), and by adjusting the equivalent resistance of
Helmholtz resonator, the resonance frequency fop is set
to be lower than the Helmholtz resonance frequency
the resonance port 18, the Q value is set to be an appro
4. More speci?cally, the characteristic values (fop, Q,,,,)
priate level, so that a sound pressure of an appropriate
level can beobtained from said opening portion of the
resonance port 18. By these adjustments and by in
creasing/decreasing the signal level input to the driver
of the Helmholtz resonator and the characteristic values
(f0, Q0) of the speaker unit 4 can be independently set.
Furthermore, the series resistance of the series reso
nance circuit 2; is only rc+rp, and these resistances are
sufficiently small values, as described above. Thus, the
Q value of the series resonance circuit Z2, i.e., the
circuit, sound pressure-frequency characteristics shown
by, for example, solid lines in FIG. 9 can be obtained.
Note that, in FIG. 9, alternate one long and two dashed 55 Helmholtz resonator can be set to be sufficiently high.
From another point of view, since the unit vibration ‘
lines represent a frequency characteristic and a impe
system does not essentially serve as a resonance system,
dance characteristic of conventional closed type
speaker system, and dotted lines represent a frequency . the diaphragm 41 constituting the unit vibration system
is displaced according to a drive signal input E0, and is
characteristic and a impedance characteristic of con
not influenced by an external force, in particular, an air
ventional bass-reflex type speaker system,
counteraction caused by the equivalent stiffness S, of
An operation when a speaker system utilizing the
the cabinet. For this reason, the diaphragm 41 equiva
Helmholtz resonator is driven by a negative impedance
lently serves as a wall when viewed from the cabinet
will be described below.
side, and the presence of the speaker unit 4 when
FIG. 10 shows an electric equivalent circuit when
ZV-— 20:0 in FIG. 8, i.e., when the internal impedance 65 viewed from the Helmholtz resonator is invalidated.
(non-motional impedance) of a speaker unit 4 is equiva
lently completely invalidated. In the Figure, equivalent
Therefore, the resonance frequency fop and the Q value
resistances rt and rp of a resonance port 18 and a cavity
non-motional impedance of the speaker unit 4. Even
Q0p of the Helmholtz resonator do not depend on the
5,031,500
11
when the resonance frequency is set to be a value so that _
the Q vvalue is considerably decreased in a conventional
12
can be made compact in size without impairing a fre
quency characteristics and a sound quality In addition,
acoustic radiation quite independently of the unit vibra-f
tion'system. Although the virtual speaker is realized by
since the resonance frequencies and the Q values of the
respective resonance systems may be set in a relatively
optional manner when the driver circuit of this embodi
ment is used, the sound quality can be improved or the
acoustic reproduction range, in particular, a bass sound
' a small diameter corresponding to the port diameter, it
corresponds to one having a considerably large diame
‘ter as an actual speaker in view of its bass sound repro
speaker system, as compared with the case wherein the
speaker system is driven by a conventional constant
drive method, the Q value can be maintained to be a
‘sufficiently large value. The Helmholtz resonance sys-'
tem is present as a virtual speaker which performs
duction power.
'
range, can be easily expanded by driving an existing
_voltage driving system.
[The system and apparatus of the present invention
In the above description, the case of ZV-Z0=0 has
described above will be. compared with a conventional
system wherein a bass-re?ex speaker system is driven by
an ordinary power ampli?er. In the conventional Sys
tem, as is well known, a plurality of resonance systems,
been exempli?ed. However, in this embodiment, ZV
-Z0>0v may be allowed if -Z0<0. In this case, the
i.e., the unit vibration system Z1 and the Helmholtz
characteristic values and the like of the unit vibration
system and the Helmholtz resonance system become
intermediate values between the case of ZV—Z0 =0 and
resonance system 2;, are present, and the resonance , thecase of the‘conventional constant voltage drive
system according to the value of above-mentioned im
frequencies and the Q values of the resonance systems
closely depend on each other. For example, if the reso 20 pedance Zy- Z0. Therefore, ‘by positively utilizing this
nance port is elongated or its diameter is reduced (mp is
nature, the-Q value of the Helmholtz resonance system
can ‘be adjusted by adjusting the negative impedance
increased). to lower the resonance frequency of the
—Z() instead of adjusting the port diameter or inserting
Helmholtz resonance system 2;, the Q value of the unit
vibration system 2] is increased and the Q value of the
a‘ mechanical Q damper such as glass wool or felt in the
Helmholtz resonance system 2; is decreased. If the 25 cabinet.
volume of the cabinet is decreased (Sc is increased), the
FIGS. 11(a), 11(b), and 11(c) are graphs simulating
p Q value and the resonance frequency of the unit vibra
the electric characteristics of the acoustic unit in FIG. 7
using the speaker system with the resonance port and a
tion system Z1 are increased,'and the Q value of the
driver 50. In this case, the nominal impedance of a
Helmholtz resonance system Z2 is further decreased
even if the resonance frequency of the Helmholtz resoé
speaker unit 4 is set to be 8 (I; an AC input voltage e of
nance system 2; is kept constant lay-elongating the port
a negative impedance generator 52 of the driver 50, l V; '
or decreasing its diameter. More specifically, since the
and an output impedance Z0,- 7 9..
output sound pressure-frequency characteristics of the
Referring to FIG. 11(0), a solid curve a represents the
speaker system are closely related to the volume of the
frequency characteristic of an impedance ZL of the
cabinet and the dimensions of the port, a high-grade 35 speaker system with the resonance port; a broken curve
design technique is required to match them. Thus, it is
b, the frequency characteristic of an impedance due to
generally not considered that a cabinet (or system) can
an equivalent inductance A280 of the speaker unit 4; a
easily be made compact in size without impairing the
broken curve c, the frequency characteristic of an impe
frequency characteristics of an output sound pressure,
dance due to an equivalent capacitance mo/A2 of the
in particular, a bass range characteristics. The relation 40 speaker unit 4; a broken curve d, the frequency charac
ship between the frequency lower than the resonance
teristic of an impedance due to an equivalent inductance
frequency and a resonance acoustic radiation power in
AZ/SC of the cabinet 7; a broken curve e, the frequency
the Helmholtz resonance system Z2 is decreased at a
characteristic of an impedance due to an equivalent
rate of 12 dB/oct with respect to a decrease in fre
capacitance m_,,/A2 of the cabinet 7; and an alternate
quency when viewed from the sound pressure level. 45 long and short dashed curve f, the frequency character
Thus, when the resonance frequency is set to be ex
istic of an impedance of a unit resonance system Z1. In
tremely lower than that of the basic concept of the
the Figure, the ‘resonance frequency of the unit reso
bass-re?ex speaker system, correction by increasing/de
nance system is set to be a value corresponding to the
creasing an input signal level is very difficult to achieve.
intersection point between the broken curves b and c,
Furthermore, adverse influences on sound quality 50 i.e., about 35 Hz, and the resonance frequency of a port
caused by the high Q value and the abrupt change in
phase of the unit vibration system around the lowest
resonance system is set to be a value corresponding to
an intersection point between the broken curves d and
resonance frequency cannot be eliminated.
e, i.e., about 40 Hz. Referring to FIG. 11(b), a solid
curve g represents an output terminal voltage V of the
above, since the speaker system utilizing Helmholtz 55 negative impedance generator 52; a broken curve h, the
resonance is driven by a negative impedance, the char
output sound pressure characteristic of a resonance
acteristics, dimensions, and the like of the unit vibration
radiation sound from the port resonance system; a bro
system and the Helmholtz resonance system can be‘
ken curve i, the output sound pressure characteristic of
independently set. In addition, even if the resonance
a direct radiation sound from the unit resonance system;
frequency of the Helmholtz resonance system is set to
and a solid curve j, the synthetic output sound pressure
In the driver circuit of this embodiment, as described
be low, the large Q value and the high bass sound repro
duction power can be maintained, and the resonator
drive power of the unit vibration system can be in
characteristic as the speaker system obtained by mixing
the broken curves h and i. The output terminal voltage
V is obtained by:
creased (6 dB/oct). Therefore, nonuniformity of the
frequency characteristics can be advantageously cor 65
V= ZLe/(ZL+ —Zo)
(3)
rected by increasing/decreasing an input signal level
like in normal sound quality control. For this reason, a
cabinet can be rendered compact and speaker system
Therefore, if ——Z0 and Z]_ are respectively replaced
with pure resistances -R0 (= —-7 Q) and RL, the volt
5,031,500
13
age V is changed as follows: V: —8 V for R1,: — 8 Q;
V=4.5 V for RL=9 Q, . . . .
FIG. 11(0) shows a case wherein a ?at output sound
pressure characteristic can be obtained at frequencies of
50 Hz or moreas indicated by a solid curve j’ by in
creasing/decreasing the input voltage e of the negative
14
toward the low-frequency side, piano tones and the like
at the bass tone side sound like con?ned tones and be
come different from actual tones. For this reason, fun
damental wave components are reduced or removed in
the‘sound source to adjust sound quality.
FIG. 12 shows the basic arrangement of a negative
impedance generator 52 by using a frequency c_haracter-__ ' impedance generator 52 for driving a vibrator (speaker
istic compensation circuit 51 of the driver 50 in accor- ' unit) by negative impedance.
dance with a frequency and compensating the output
voltage from the generator 52 as indicated by a solid
In the circuit shown in the Figure, an output from an
ampli?er 53 having a gain A is supplied to a load ZL
curve g’. Referring to the Figure, a broken curve k
constituted by a speaker system. A current I], flowing
_ represents the output power (Watt) characteristic of an
the load ZL is detected, and the detected cur
ampli?er 53 (i.e., the negative impedance generator 52) through
rent
is
positively
fed back to the ampli?er 53 through a
when the output sound pressure characteristic is to be
feedback
circuit
54
having a transmission gain B. Thus,
15
made flat.
the
output
impedance
Z0 of the circuit is given by:
In the keyboard instrument shown in FIG. 3, the port
resonance frequencies of the speaker systems in the
acoustic units of left and right channels are set to be
Zo=Zs(l—AB)
(4)
different, i.e., 50 and 60 Hz, respectively. With this
arrangement, a synthetic frequency characteristic of the 20 From equation (4), If A> 1, Z0 is an open stable type
negative impedance. In the equation, Z5 is the impe
left and right speaker systems is shaped like as a sound
pressure characteristic having a peak at 50 Hz, which is
obtained from the resonance port of the left speaker, is
dance of a sensor for detecting the current.
Therefore, in the circuit shown in FIG. 12, the type
' of impedance Z5 is appropriately selected, so that the
' added to an output sound pressure characteristic exhib
iting‘ a ?at characteristic at frequencies of 60 Hz or 25' output impedance can include a desired negative impe
dance component. For example, when the current IL is
' more, which is obtained from the right speaker system.
As a result, the uniform reproduction range can be
detected by a voltage across the two end of the impe
widened toward the low-frequency side If the charac
dance Z5, if the impedance Z5 is a resistance R5, the
teristics of the frequency characteristic compensation . negative impedance component is a negative resistance
circuit 51 are properly set, the low~frequency side of the 30 component; if the impedance Z5 is an inductance L5, the
uniform reproduction range can be widened to 50 Hz by
negative impedance component is a negative inductance
using only the right channel. In this case, however, the
component; and if the impedance Z5 is a capacitance C5,
output voltage of the ampli?er 53 must be increased
vthe negative impedance component is a negative capaci
near the port resonance frequency, as indicated by the
tance component. An integrator is used as the feedback
broken curve k in FIG. 11(c). Referring to FIG. 11(0), 35 circuit 54, and a voltage across the two end of the in
in order to widen the low-frequency side of the uniform
ductance L5 as the impedance Z5 is detected by integra
reproduction frequency by 10 Hz, the output power of
the ampli?er 53 must be increased by 6 dB (four times).
Especially in a keyboard instrument, the capacity of the
ampli?er 53 must be determined in terms of continuous
rating in consideration of a case wherein keys are kept
depressed. If the nominal output is assumed to be equal,
the above-described system requires a power ampli?er
tion, so that the negative impedance component can be
a negative resistance component. A differentiator is
used as the feedback circuit 54, and a voltage across the
two end of the capacitance C5 as the impedance Z5 is
detected by differentiation, so that the negative impe
dance component can be a negative resistance compo
nent. As the current detection sensor, a current probe
having an output several‘ times larger than that of an
audio ampli?er whose output can be determined in 45 such as a CT. (current transformer) or a Hall Element
can be used in place of, or in addition to these impe
terms of an intermittent or instant maximum output. If
the output of the power ampli?er must be further in
dance element R5, L5 and C5.
creased to flatten the frequency characteristic, a load in
An embodiment of the above-mentioned circuit is
circuit design is excessively increased. In this embodi
described in, e.g., Japanese Patent Publication No. Sho
ment, therefore, the resonance frequency of the left port
59-5l77l.
‘ is set to be 50 H: which is lower than that of the right
port by 10 Hz. Output sound pressures at frequencies
around 50 Hz are mainly radiated from the resonance
port of the left speaker system so as to reduce the load
of the right driver 50. Similarly, sounds at around 60 Hz
are mainly radiated from the resonance port of the right
speaker system so as to reduce the load of the left driver
50.
-
.
In a low-frequency range below several tens Hz,
since a wavelength becomes several meters or more, the
directivity of sound is weak. Therefore, whether a
given sound is radiated from the left or right channel
poses little problem. That is, even if sounds having
different sound pressures are radiated from the left and
right speakers as described above, problems such as 65
localization of sound images are scarcely posed.
In this embodiment, since the output sound pressure
characteristic of the acoustic unit is further widened
Current detection can be performed at a nonground
side of the speaker 3. An embodiment of such a circuit
‘ is described in, e.g., Japanese Patent Publication No.
Sho 54-33704 FIG. 13 shows a BTL connection. This
can be easily applied to the circuit shown in FIG. 12. In
FIG. 13, reference numeral 56 denotes an inverter
FIG. 14 shows a detailed circuit of ampli?ers which
include a negative resistance component in its output
impedance.
The output impedance 20 in the ampli?er shown in
FIG. 14 is given by:
15
5,031,500
Modi?cation of the Embodiment
The present invention is not limited to the above
of a cabinet.
According to the third aspect of the present inven
tion, since at least part of the inner wall of a cabinet is
described embodiments, but can be variously modi?ed
For example, since, the second embodiment vis'de
signed to improve the low-frequency characteristics of
a speaker system, only portions corresponding to the
16
acteristics can be improved without increasing the size
5
constituted by a damping material for preventing duct
resonance, even if the pro?le of the cabinet is decreased
in association with an arrangement, design, and the like
of the cabinet, noise due to duct resonance or an in
low-frequency speaker unit 15A and its driver of the
firstembodiment are described-However, the high-frei ‘ crease in distortion can be prevented.
quency speaker unit 15B can be arranged as needed.
7 According to the fourth aspect of the present inven
In the second embodiment, as the above-described
driver, any circuit capable of driving the vibrating body
tion, since a low-frequency range is shared by a plural
ity of sets of speaker systems in units of bands, each
' so as to cancel a counteraction from’ its surroundings
speaker system can be efficiently operated. Therefore,
during a drive period of the resonator can be used. ‘For
example, a so-called MFB circuit disclosed in Japanese
Patent Publication (Kokoku) No. Sho 58-31156 may be
the capacity of a driving means can be decreased, or an
‘ used in jaddition to the negative impedance generator.
By setting proper frequency characteristics for the
above output‘ impedance, the freedom of setting the ‘QM,
and Qop, and the like can be increased, characteristics,
especially output sound pressure characteristics near
the resonance frequencies foe and f,,,, can be adjusted, orv
' an increase in distortion due to nonlinearity of a voice
coil inductance component can be suppressed in a high
frequency range.
In addition, duct resonance tones may be removed by
outputting an output from the resonance port through a
mechanical acoustic ?lter In this case, as shown in FIG.
I5('a),' a so-called 3D (three-dimensional) system may be
constituted by commonly using a single ?lter for the
increase in capacity thereof can be suppressed.
What is claimed:
1. A keyboard instrument comprising a keyboard, a
'_ sound source for generating a musical tone signal on the
basis of a key operation of said keyboard, and a sound
system for vconverting the musical tone signal into a
sound and radiating the sound, wherein
said sound system comprises:
a cabinet and resonance port constituting a resonator;
an electro-acoustic transducer, arranged on an outer
wall of said cabinet, for driving said resonator at
one surface of a vibrating body of the transducer
_ and directly radiating a sound from another surface
of the vibrating body; and
a driving means for driving said transducer so as to
cancel an air counteraction from said resonator to
said vibrating body.
right and left channels. In this case, by setting the 30
2. An instrument according to claim 1, wherein said
lengths of the left and right resonance ports ‘18a and 18b
resonator is a Helmholtz resonator.
to be different from each other, different resonance
pass, band eliminate, and low-pass ?lters or a ?lter hav
ing any structure'may be used as long as it can ?lter port
3. A keyboard instrument comprising a keyboard, a
sound source for generating a musical tone signal on the
basis of a key operation of said keyboard, and a sound
vsystem for converting the musical tone signal into a
resonance tones to eliminate duct resonance tones. For
sound and radiating the sound, wherein
frequencies can be set for the left and right channels,
respectively As a mechanical ?lter, any one of band
example, ?lters shown in FIGS. 15(b) and 15(c) may be
said sound system comprises:
used. FIG. 1p5(b) shows a ?lter obtained by forming an
a cabinet and resonance port constituting a resonator,
at least part of an inner wall of said cabinet consist
‘opening 91 in a cabinet 70, which serves as a low-pass
?lter for passing only components of frequencies below
duct resonance tones. FIG. 15(c) shows a ?lter obtained
by providing a passive vibrating body 92 such as a
drawn cone for a cabinet 7c, which serves as a band-pass
?lter for passing only components of frequencies in a
band including port resonance tones. The resonance
ports 18a and 18b having volumes shown in FIG. 6 or 7
may be stored in the cabinet 7a, 7b, or 7c. In this case,
the overall system can be further reduced in size.
ing of a damping material for preventing duct reso
nance;
an electro-acoustic transducer, arranged on an outer
wall of said cabinet, for driving said resonator at
one surface of a vibrating body of the transducer
and’ directly radiating a sound from another surface
of the vibrating body; and
a driving means for driving said transducer so as to
cancel an air counteraction from said resonator to
said vibrating body.
50
Effects
4. A keyboard instrument comprising a keyboard, a
sound source for generating a musical tone signal on the
As has been described above, according to the ?rst
basis of a key operation of said keyboard, and at least
aspect, a performer can directly and clearly grasp per
two sets of sound systems for converting the musical
formance tones. In addition, peripheral units and the
tone signal into a sound and radiating the sound,
like can be placed on the upper surface of the case of an
55 wherein
instrument.
said sound systems each comprise
Since a large-diameter speaker unit for middle and
bass tones is arranged to be more inclined than a small
diameter unit for treble tones, a demand for a low
a cabinet and resonance port constituting a resonator;
an electro-acoustic transducer, arranged on an outer
pro?le main body case can be fully satis?ed. In this case,
wall of said cabinet, for driving said resonator at
one surface of a vibrating body of the transducer
and directly radiating a sound from another surface
of the vibrating body, the resonance frequencies of
said resonators of said sound systems being differ
ent from each other; and
since degradation in sound quality of middle and bass
tones is less in?uential than that of treble tones, perfor
mance tones remain substantially the same for a per
former.
According to the second aspect, a cabinet of a
speaker unit can be reduced in size, and operability and 65
freedom of design of a keyboard can be increased with
out impairing reproduction low-frequency characteris‘
tics. In addition, the reproduction low-frequency char
a driving means for driving said transducer so as to
cancel an air counteraction from said resonator to
said vibrating body.
it
i
i
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