Subwoofer system using a passive radiator

Subwoofer system using a passive radiator
United States Patent [191 p
Sep. 21, 1982 '
S} G
Matthew 8. Polk’ Baltimore, M(1.
55-73197 6/1980 Japan ............................. .. 179/1 GA
[73] Assignee: Polk Audio, Inc., Baltimore, Md.
Prim’? Examiner-R1- Hickey
Attorney, Agent, or Firm-Cushman, Darby & Cushman
A' l N
pp . _
[22] Filed:
224 451
Jan. 12, 1981 .
Cg‘: """"""""""""" "
' '
A loudspeaker subwoofer system is disclosed which
' """"""""""""""" "
combines signals from more than one audio channel. A
181/1 5’5_ 181/165
single enclosure mounts an active driver for each of a
[58] Field of Search‘ ................. .. 119/1 G, 1 ,GA, 1 E;
Plural“? °f audi° channels’ which active d‘iYe“ are
respectively. connected
to each respective
audio chan.
181/145, 151, 154, 155, 163
nel. A passive radiator loudspeaker 15 also mounted 1n
References Clted
the single enclosure. The passive radiator is acoustically
coupled to the active drivers and functions as a sub
Klasco ........... ., ............. .. 181/155 x
4,276,446 6/1981 Taddeo
181/145 X
4,326,099 4/1982 Maille ............................... .. 179/1 G
5 Claims, 3 Drawing Figures
I ______ _ _ _/_ _ _ _.
I 12%
single passive radiator acoustically coupled to the
driver loudspeakers.
Brie?y, in accordance with one embodiment of the
invention, there is provided a loudspeaker housing. At
5 least two actively driven loudspeakers are mounted in
the housing, and means are provided for respectively
This invention pertains to a loudspeaker apparatus,
and more particularly pertains to a subwoofer loud
electrically coupling the actively driven loudspeakers
speaker system which utilizes a passive radiator.
to at least two signal sources. A passive radiator loud
A common limitation of loudspeaker systems, partic
speaker is also mounted in the housing. The passive
ularly those intended for high quality stereo sound re
radiator has no direct electrical connections, but rather
production, is relatively inadequate low frequency per
is acoustically coupled to the actively driven loudspeak
formance. Of course, one possible solution for those
who are not satis?ed with the low frequency perfor
mance of their stereo loudspeakers is to replace them
ers by the air mass contained within the loudspeaker
housing. In this manner the signals from the at least two
with larger (and more expensive) loudspeaker systems.
Another possibility is the addition of what is known as
trical coupling means.
subwoofer systems to existing loudspeaker systems.
Other objects and advantages of the present invention
However, there are several problems that have ad
versely affected the performance and marketability of
subwoofer systems.
will appear from the detailed description of one embodi
One of the most serious problems affecting the sale
ment thereof, taken in conjunction with the drawings.
FIG. 1 is an oblique view of a typical loudspeaker
housing (with the grille removed) illustrating an exem
ability of a subwoofer is that two subwoofers must be
used, one for each‘of the channels in a stereo high
?delity system. This effectively doubles the price. Since
very low frequency sound (i.e. below 150 Hz) tends to
signal sources are acoustically coupled to the passive
radiator loudspeaker without the necessity of any elec
have no directional effects in a stereo system, the im
plary mounting of individual loudspeakers.
FIG. 2 is a circuit diagram of one embodiment of the
provement in low frequency performance afforded by a
invention illustrating the electrical connections to the
single subwoofer would be adequate in most cases.
However, the use of a single subwoofer to which the
stereo channels were coupled would unbalance the
FIG. 3 is a circuit diagram illustrating aspects of an
alternate embodiment of the invention.
active driver loudspeakers.
stereo channels.
Several previous designs have attempted to combine
Referring now to FIGS. 1 and 2, there is illustrated a
the electrical signals from the two stereo channels so
that a single center channel subwoofer can be used. This
typical loudspeaker housing 11. Although loudspeaker
is somewhat dif?cult in view of the substantial electrical 35 enclosures typically have a grille of some kind the hous
ing 11 isshown without any grille, for clarity of illustra
power and low frequencies involved. Usually, it is ac
tion. In accordance with one embodiment of the inven
complished through a “matrix” type crossover which
tion two active driver loudspeakers 12 and 13 are
includes both the large inductors required for the low
mounted in the front 11a of the housing 11. The active
crossover frequency and the transformers required to
mix the two channels. One example of this general kind 40 driver speakers 12 and 13 have voice coil and magnet
structures 12a and 13a, which are respectively electri
of arrangement in which the signals on the channels are
cally connected to terminals 14 and 16. As shown in
electrically summed to feed a single low frequency
FIG. 2, the coupling of the voice coil and magnet struc
speaker, is shown in U.S. Pat. No. 3,637,938.
tures 12a and 13a to the terminals 14 and 16 may include
Such an approach entails several problems. It is ex
pensive and requires common ground outputs from an 45 inductors 17 and 18, respectively, forming ?rst order
ampli?er. Further, it is electrically inefficient and has
low pass ?lters. These ?lters can serve as crossover
networks in a manner akin to crossover networks for
poor performance due to inevitable saturation of trans
former cores. Moreover, it also offers no ?exibility and
is too complicated for most consumers.
loudspeaker systems as known to those skilled in this
art. For a woofer or subwoofer part of a loudspeaker
Another prior approach has been simply to build two 50 system, it is desirable to eliminate as much as possible all
subwoofers into a single divided cabinet, with or with
mid and high frequency components, as known to those
out separate built-in crossovers. This obviously reduces
the cabinet cost but is otherwise equal to the cost of two
separate subwoofers, and would result in a very bulky
skilled in this art.
As shown in FIG. 2, the terminals 14 and 16 are
adapted for connection to two respective signal inputs,
55 labeled channels A and B in the drawing. For a stereo
Accordingly, it is an object of this invention to pro
vide a loudspeaker system which overcomes the de?
ciencies of these prior approaches.
More speci?cally, it is an object of this invention to
provide a novel apparatus and method for summing at
one loudspeaker the signals on a plurality of sound
It is a still more speci?c object of this invention to 65
provide a subwoofer system including a single enclo
sure containing two active driver loudspeakers for di
rect electrical coupling to two signal channels, and a
system, these can correspond to the left and right chan
FIGS. 1 and 2 also illustrate a passive driver 19
mounted to the enclosure 11. As can be seen in FIG. 2,
the passive driver 19 does not have any electrical con
nections thereto, nor does it have any magnet and voice
coil structure. The passive radiator 19 simply comprises
a diaphragm of suitable design for operating in its in
tended range, i.e. low frequency. In operation, acoustic
energy generated inside housing 11 by the active driver
loudspeakers 12 and 13 is transmitted or coupled, via
the air mass within the enclosure 11, to the low fre
quency passive radiator. In accordance with one em
bodiment of the invention, values of the ?lters are se
lected in accordance with the loudspeaker sizes so that
ing Society. (A. N. Thiele, “Loudspeakers in Vented
the system impedance is generally high below 80 Hz
and drops rapidly at higher frequencies. This contrib
“Vented Box Loudspeaker System-Parts l, 2, 3, 4",
June through October 1973, JAES).
Boxes: Part I”, May 1971 JAES. and R. H. Small,
utes to a faster roll-off of the illustrated ?rst order low
Turning now to a consideration of FIG. 3, there are
pass ?lter shown and hence improves performance at a
lower cost. With this arrangement, the necessary elec
trical separation between the channels is maintained,
shows aspects of possible alternate embodiments of the
invention. In FIG. 3, components that correspond to
the components in FIG. 2 are given identical reference
and the outputs at the two active driver loudspeakers is
numerals. Thus two active driver loudspeakers 12 and
acoustically summed at frequencies below the acoustic
turnover to the passive radiator.
13 and a passive radiator loudspeaker 19 are mounted in
e.g., the front panel of enclosure or housing 11. The
active driver loudspeakers 12 and 13 are respectively
coupled through low pass ?lters 17 and 18 to channel A
and B signal inputs at terminals 14 and 16. As shown in
The prior art does include examples of loudspeaker
systems in which there might be one component which
is not electrically connected to the signal source, i.e. a
passive radiator. These include U.S. Pat. Nos.
FIG. 3, additional ?lter elements 21 and 22 can be re
1,988,250; 3,772,466; and 3,780,824.
spectively provided in series with the ?lter channels 17
and 18. A double pole, single throw switch 23 having
switch contacts 230 and 23b is provided. By manual
operation of the switch 23, the ?lter elements 21 and 22
can be either left in the system (when switch 23 is open)
or electrically removed by short-circuiting when the
Some of these prior art systems use the air mass in the
enclosure to acoustically couple a signal on a smaller
active driver loudspeaker to a larger passive radiator.
However, the prior art appears not to have realized that
such a technique, when applied according to the princi-,
ples of this invention, can be used to advantage to con
switch 23 is closed. In this manner, the crossover point
struct a single subwoofer for a stereo system without
and/or efficiency of the system can be adjusted.
Also, referring to FIG. 3, an additional double pole,
single throw switch 24 can be provided. With such a
switch in the stereo position as shown in FIG. 3, both
the necessity for complex matrix electrical coupling
between the channels.
Optimized design of a loudspeaker system in accor
dance with the present invention requires that attention
terminals 14 and 16 would be connected to the same
be paid to several parameters. The in-cabinet reso
nances of the low frequency passive radiator 19 and the
active driver loudspeakers 12 and 13 will determine the
input signal, i.e., both channels A and B would be the
same, and two loudspeaker systems such as shown in
FIG. 3 would be provided, one for each stereo channel.
Alternatively, with the switch 24 in the center channel
acoustic crossover point and the mechanical Q of each
system will determine the slope of the crossover. The
enclosure volume, as related to the piston area of the
active drive speakers, will sharpen the high-end rolloff
of the low frequency passive radiator and will deter
mine the degree of acoustic coupling between the active
position, each of the active driver loudspeakers 12 and
13 would be fed a different signal, so that the loud
system of FIG. 3 would function as a center
channel loudspeaker subwoofer system. Provision of
the switch 24 would provide the maximum system ?exi
driver loudspeakers and the passive radiator. An addi
tional advantage may accrue from the use of relatively
small active driver speakers to drive the larger low
frequency passive radiator in that the mechanical ad
vantage obtained results in improved transient response
and a high mechanical damping factor.
In accordance with a speci?c presently preferred best
following parameters:
Diaphragm diameter—5"
Mechanical Q-—1.0
radiator 19 has a diaphragm 9%" in diameter weighing
140 grams (approx). The system resonance of the active
drivers is approximately 50 Hz and the system reso
nance of the passive-radiator is approximately 16 Hz.
Each active driver is connected to one channel of a
stereo ampli?er through a series inductor 17 or 18
What is claimed is:
1. A loudspeaker apparatus for use in a'stereo sound
reproducing system having a left channel signal and a
right channel signal comprising
a housing containing an air mass,
a left channel loudspeaker and a right channel loud
B1 product-5.8 Weber/meter
The volume of enclosure 11 is 2.1 cu. ft. The passive
should be understood that various modi?cations within
the skill of those in this art may be made, without de- '
‘ parting from the true spirit and scope of the invention.
mode embodiment of the invention as illustrated in
FIGS. 1 and 2, the two active drivers 12 and 13 have the 45
‘DC resistance-6.40
Free air resonance-22 Hz
While the present invention has been described by
reference to presently preferred embodiment thereof, it
speaker mounted in said housing, each of said left
and right channel loudspeakers having voice coils,
respective means for connecting the left channel sig
nal to said left channel loudspeaker voice coil and
the right channel signal to said right channel loud
speaker voice coil,
a passive radiator loudspeaker mounted in said enclo
sure and acoustically coupled by the air mass to
said left and right channel loudspeakers for acousti
cally summing their acoustic outputs.
whose value is approximately 11.2 mH. This value is
chosen to provide a 3 db down point at 60 Hz. In addi 60 2. A loudspeaker apparatus in accordance with claim
1 wherein said respective means for connecting the left
tion, each inductor has a center tap allowing a choice of
and right channel signals to’ said left and right channel
5.6 mH giving a 3 db down point of 100 Hz.
speakers each includes an inductor forming low pass
In general, the system can be considered to be two
vented systems each with a single driver and passive
3. A loudspeaker apparatus in accordance with claim
radiator mounted in a cabinet whose volume is equal to
2 wherein said respective means each includes an addi- ,
the entire cabinet volume. The system parameters can
tional inductor and switching means for selectively
then be determined by the methods outlined by Thiele
short-circuiting said additional inductor, so that the low
& Small in papers in the Journal of the Audio Engineer
pass ?lter characteristics of the loudspeaker apparatus
can be selectively altered.
4. A loudspeaker apparatus in accordance with claim
1 wherein said passive radiator is of a substantially
‘larger size than said active driver loudspeakers, said
passive radiator intended for sound reproduction of low
frequency signal components.
5. A loudspeaker system in accordance with claim 1
including switch means for connecting the at least two
active driver loudspeakers in parallel.
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