1GA61_0e
Measurement of Acoustic
Properties of Rooms Using
Audio Analyzer R&S® UPV
Application Note
Products:
|
R&S UPV
|
R&S UPV66
|
R&S UPV-K1
|
Thomas Lechner
29th June 2012-1GA61_0e
Application Note
Acoustic properties like reverberation time
and early reflections are important for test
rooms as well as for speech intelligibility
and music performance in assembly
rooms and concert halls. This application
note explains the basics behind these
properties and offers an application
program for their measurement.
Table of Contents
Table of Contents
1GA61_0e
1
Sound Propagation and Room Acoustics............................ 4
1.1
Sound Theory ...............................................................................................4
1.1.1
Sound Pressure and Sound Velocity .........................................................4
1.1.2
Acoustic Wave ..............................................................................................4
1.1.3
Sound Intensity.............................................................................................4
1.1.4
Radiation of a Point Source ........................................................................5
1.2
Reflection and Absorption ..........................................................................5
1.2.1
Reflection on a Plane Surface.....................................................................5
1.2.2
Absorption on Surfaces...............................................................................5
1.2.3
Sound Attenuation During Propagation ....................................................6
1.3
Early Reflections ..........................................................................................6
1.4
Diffuse Sound Field......................................................................................7
1.4.1
Stationary condition.....................................................................................7
1.4.2
Reverberation Time......................................................................................7
1.4.3
Critical Distance ...........................................................................................8
2
Measurement Principle.......................................................... 9
2.1
Reverberation Time......................................................................................9
2.2
Reflectogram.................................................................................................9
2.3
Critical Distance ...........................................................................................9
3
The Application Program..................................................... 10
3.1
Installing the Software ...............................................................................10
3.1.1
Prerequisites...............................................................................................10
3.1.2
Required File...............................................................................................10
3.1.3
Installation...................................................................................................10
3.2
Measurement Setup ...................................................................................10
3.2.1
Loudspeaker ...............................................................................................10
3.2.2
Microphone .................................................................................................10
3.2.3
Connections................................................................................................11
3.3
Starting the Software .................................................................................11
3.4
User Interface..............................................................................................11
3.4.1
Test Signal Type .........................................................................................12
3.4.2
RMS Generator Output Voltage ................................................................12
Rohde & Schwarz Measurement of Acoustic Properties of Rooms 2
Table of Contents
1GA61_0e
3.4.3
Measurement Frequency Range ...............................................................12
3.4.4
Autoscale Result Display ..........................................................................12
3.4.5
Status Display.............................................................................................12
3.4.6
Peak Output Voltage ..................................................................................12
3.4.7
High Pass Filter ..........................................................................................12
3.4.8
Number of Averages ..................................................................................12
3.4.9
Autoscale Result Display ..........................................................................13
3.4.10
Live Update .................................................................................................13
3.4.11
Minimize during Measurement and / or when Completed......................13
3.5
Sample Results...........................................................................................13
3.6
Working with the Source Code .................................................................14
3.6.1
Loading the Source Code to the IDE ........................................................14
3.6.2
Running the Program in the Debugger ....................................................14
3.6.3
Modifying the Program ..............................................................................15
4
Literature............................................................................... 16
5
Ordering Information ........................................................... 16
Rohde & Schwarz Measurement of Acoustic Properties of Rooms 3
Sound Propagation and Room Acoustics
Sound Theory
1 Sound Propagation and Room Acoustics
1.1 Sound Theory
1.1.1 Sound Pressure and Sound Velocity
Sound can be viewed as the AC component of air pressure. For example for a sine
tone, the air pressure can be described as
p(t ) = p0 + pˆ * sin(2 * * f * t )
Where p0 is the static atmospheric pressure and
pressure.
p̂ is the amplitude of the sound
According to the physical laws for gases, changes in the air pressure are associated
with changes in the mass density. A change in mass density is associated with a
movement of air. The speed of air particles caused by the changing air pressure is
called particle velocity v(t ).
1.1.2 Acoustic Wave
Since the compliance and mass of the gas introduce a phase shift on the sound
pressure and velocity with increasing distance from a sound source, the sound
propagates as a wave. At a fixed point in time, the phase of the sound depends on the
distance from the sound source. The distance which corresponds to a phase shift of
2 * is called wavelength . The speed c with which the wave propagates is called
sound velocity.
c= *f
The sound velocity should not be confused with the particle velocity. With usual sound
levels, the particle velocity is much smaller than the sound velocity.
The ratio between sound pressure and sound velocity is called specific acoustic
impedance. It depends on the physical properties of the gas and on the shape of the
propagating wave which in turn depends on the shape and type of the sound source.
For example, a large moving diaphragm causes a plane wave at least for higher
frequencies and in the vicinity of the diaphragm. A small opening can cause a spherical
or semi-spherical wave, depending on whether it is in more or less free space or on a
hard wall.
1.1.3 Sound Intensity
The power which is transported by an acoustic wave per unit area is called sound
intensity I. The sound intensity can be calculated from the RMS values of sound
pressure and particle velocity as
1GA61_0e
Rohde & Schwarz Measurement of Acoustic Properties of Rooms 4
Sound Propagation and Room Acoustics
Reflection and Absorption
I=~
p * v~
1.1.4 Radiation of a Point Source
If the dimensions of a sound source are small compared to the wavelength and there
are no obstacles around, the sound propagates as a spherical wave in all directions.
Under the condition that the losses in the sound propagation can be neglected, all
spherical shells around the sound source are penetrated by the same amount of
power: the radiated power of the source. As the surface of the spherical shells is
proportional to the square of their radius, the sound intensity decays with the square of
the radius:
I~
1
r2
The sound pressure is approximately inversely proportional to the distance form the
sound source:
1
~
p~
r
1.2 Reflection and Absorption
1.2.1 Reflection on a Plane Surface
If an acoustic wave hits a hard flat surface, it is reflected back into the room due to the
large difference in the acoustic impedance between the air and the material of the hard
surface. Just like in optics, the trajectory of the reflected wave lies in the same plane as
the trajectory of the incident wave and the surface normal. The angle between the
trajectory of the reflected wave and the surface is the same as the angle between the
trajectory of the incident wave and the surface.
1.2.2 Absorption on Surfaces
Real surfaces absorb a portion of the energy of the incident sound wave. Absorption is
usually depending on the frequency. The absorption coefficient is the ratio between the
intensity Ii of the incident wave and the intensity difference between incident and
reflected wave:
=
(I i
Ir )
Ii
An open window in a room has more or less 100% absorption ( = 1) at least for
frequencies where the wavelength is small compared to the dimensions of the window.
1GA61_0e
Rohde & Schwarz Measurement of Acoustic Properties of Rooms 5
Sound Propagation and Room Acoustics
Early Reflections
Fabric absorbs acoustic energy due to the sound velocity which moves filaments in the
fabric. As the particle velocity is zero on the surface of a hard wall, fabric mounted on
hard walls absorb only high frequencies where a quarter wavelength is within the fabric
layer, because sound velocity reaches a maximum at a quarter wavelength distance
from the hard surface. Absorption of fabric layers can be extended to lower frequencies
if the fabric is mounted in a distance from the hard reflecting wall. For example a
curtain hanging in plaits at a certain distance from a wall or window glass can act as
absorber for medium and high frequencies.
Low frequencies can best be absorbed by resonance absorbers. One form of
resonance absorbers are wooden panels mounted in a distance from the hard surface
of the wall. They resonate at a frequency determined by there mass and by the
stiffness of the suspension and the air layer between panel and wall. Another form of
resonant absorbers are Helmholtz resonators which consist of the stiffness of air in a
cavity and the mass of air in one or more opening(s) from this cavity to open air. Such
a construction can be found in perforated panel mounted in a certain distance in front
of a wall.
1.2.3 Sound Attenuation During Propagation
Sound propagation in gases is not entirely lossless. Depending on the size of a room,
dissipation of acoustic energy during propagation may have an influence on the diffuse
sound field and therefore on the reverberation time. The dissipation is linked to sound
velocity. Therefore it is higher for high frequencies than for low frequencies.
L denotes the attenuation of acoustic waves per traveled distance in dB/m. It depends
on the frequency and on properties of the medium like humidity.
1.3 Early Reflections
A sound impulse starts from a sound source traveling at equal speed in all directions.
In different distance from the sound source at different directions, the impulse hits a
hard surface of a wall or other obstacle in the room and is reflected. At a certain point
of observation, the direct sound traveling the shortest distance from the source to the
point of observation arrives first. One after the other, reflections arrive from different
directions, delayed according to the additional distance the impulse had to travel due to
its detour. If the sound intensity at the point of observation is plotted over time, a socalled reflectogram is obtained.
The pattern of reflections arriving at a certain point of observation in the room depends
on the position of the sound source as well as on the point of observation. The
reflectogram should be investigated for a certain sound path, e.g. from one musician’s
place to another musician’s place to find our how well they can hear each other’s
performance, or from a musician’s place to the place of the conductor or to a place in
the audience.
1GA61_0e
Rohde & Schwarz Measurement of Acoustic Properties of Rooms 6
Sound Propagation and Room Acoustics
Diffuse Sound Field
For a good common music performance it is important that the reflectogram from one
musician’s place to another’s should have as much energy as possible as early as
possible. For a good impression of a music performance in the audience it is more
important to have energy evenly distributed over time to a large number of reflections,
as opposed to a few prominent reflections.
1.4 Diffuse Sound Field
1.4.1 Stationary condition
When the source starts radiating sound, the acoustic wave will propagate across the
room, get reflected at boundary areas with a certain absorption, and the reflected
waves will be reflected again, with the amplitude decreasing with each reflection. After
a certain time the room will be flooded with sound incident from random directions at
any point in the room after multiple reflections. In the stationary state a level of the
diffuse field will be reached where the total power absorbed at the boundary areas and
dissipated during propagation equals the sound power radiated by the sound source.
1.4.2 Reverberation Time
When the diffuse sound field is in a stationary state, and the sound source is switched
off, the total acoustic energy in the room decays by absorption at boundary layers and
dissipation during propagation. The absolute value of lost power, which represents the
change of energy in the sound field per time unit, is proportional to the energy present
in the sound field. This leads to a simple differential equation. The solution of the
differential equation is an exponential decay of the energy which is equivalent to an
exponential decay of sound energy and sound pressure and a linear decay of sound
pressure level.
The reverberation time is defined as the time in which the sound pressure level of the
diffuse sound field decays by 60 dB. Due to the linear decay of the sound pressure
level, the reverberation time is independent of the absolute level.
With V being the volume of a room, S its surface area, the average absorption
coefficient of the walls and L the coefficient for sound dissipation in the air, the
reverberation can be calculated as
TR s
0.16 * V m 3
* S m 2 + 0.46 *
L
dB
*V m 3
m
This equation is called Sabine’s law. If the dissipation in air can be neglected, Eyring’s
law can be applied:
TR s
1GA61_0e
0.16 * V m 3
ln(1
) * S m2
Rohde & Schwarz Measurement of Acoustic Properties of Rooms 7
Sound Propagation and Room Acoustics
Diffuse Sound Field
1.4.3 Critical Distance
As set forth in section 1.1.4, the sound pressure generated by a point source in
distance r decays approximately with 1/r. The sound pressure of the diffuse field is
approximately constant in the whole room. The total level generated by a point source
in a reverberating room is shown by the red curve in the following diagram. The
distance at which the sound pressure of the direct sound has the same magnitude as
the sound pressure of the diffuse field, is called critical distance rc. Closer to the
source than rc, the direct sound dominates. Further away than rc the diffuse field
dominates.
At the critical distance, the sound pressure level is 3 dB above the general level of the
diffuse field.
Critical distance
Sound pressure level [dBSPL]
100
90
80
70
60
50
40
0.1
rc
1
10
Distance from source [m ]
Total level
Direct sound
Diffuse field
Figure 1: Sound pressure depending on the distance form the source, and critical distance
If the sound dissipation in the air can be neglected, the critical distance can be
calculated from the volume of the room and the reverberation time as
rC m
1GA61_0e
V m3
TR s
Rohde & Schwarz Measurement of Acoustic Properties of Rooms 8
Measurement Principle
Reverberation Time
2 Measurement Principle
2.1 Reverberation Time
Reverberation time can be measured by generating a stationary diffuse sound field and
measuring the slope of its decay after the sound source has been switched off.
As the decay of energy follows an exponential law, the sound pressure level as the
logarithm of the sound pressure, plotted over time, decays approximately linearly. The
slope of a linear approximation of the level decaying over time can be used to calculate
the reverberation time.
Reverberation time is an integral property of a room which is more or less independent
of the place of the observer in the room. Therefore the placement of the sound source
and the measurement microphone has little influence on the result. The distance of the
measurement microphone from the sound source must be much larger than the critical
distance, in order to pick up only diffuse sound, and the sound source should radiate
more or less as a point source without directivity. Both the sound source and the
microphone should be placed far away from acoustic obstacles.
As periodic signals produce standing waves in reverberating rooms, random noise
should be used as test signal. Absorption and therefore also the reverberation time
depends strongly on the signal frequency. For assessment of room acoustics the
dependence of the reverberation time on the frequency is of interest. Therefore
bandpass-filtered noise like 1/3 octave noise or octave noise should be used. The level
of the diffuse sound field is limited by the power capabilities of the sound source. To
achieve a good dynamic range even in presence of noise in the room, a bandpass filter
with the same frequency range as the test signal should be used on the measurement
side.
2.2 Reflectogram
The easiest way to measure a reflectogram is to send an impulse from the sound
source and record the sound pressure at the microphone over time. For a good signalto-noise ratio a train of impulses can be used for which the microphone signal is
averaged synchronously.
2.3 Critical Distance
For determination of the critical distance, a selective RMS measurement with long
integration time is required to measure the octave or 1/3-octave level depending on the
distance form the sound source. For rooms with a small critical distance, the sound
source must be small enough to allow the measurement at close distance.
1GA61_0e
Rohde & Schwarz Measurement of Acoustic Properties of Rooms 9
The Application Program
Installing the Software
3 The Application Program
3.1 Installing the Software
3.1.1 Prerequisites
Running the application program requires option key UPV-K1. The VB.NET IDE
(development environment) is not required for running the installed program. Firmware
version 3.0 or later should be installed on the UPV.
3.1.2 Required File
Copy the installer “1GA61.msi” to a location on the UPV hard disk, e.g. “D:\R&S
Software\Application Notes”.
3.1.3 Installation
Start the installer “1GA61.msi” and follow the instructions on the screen.
3.2 Measurement Setup
3.2.1 Loudspeaker
There are special omnidirectional sound sources offered which are equipped with
speaker chassis all around a regular body. In many cases sources made up by four or
five speakers radiating in different directions or just one small speaker can be used as
well.
3.2.2 Microphone
The microphone should have an omnidirectional directivity. Frequency response is
uncritical because absolute level has no influence on the results.
A battery powered electret microphone can be directly connected to UPV Analyzer
input 1. If necessary use appropriate adapters for the microphone connector. A
measurement microphone usually needs a power supply inserted between the
preamplifier on which the microphone cartridge is mounted, and the analyzer input.
1GA61_0e
Rohde & Schwarz Measurement of Acoustic Properties of Rooms 10
The Application Program
Starting the Software
3.2.3 Connections
Connect a suitable active speaker or speaker with power amplifier to generator
output 1.
Connect an omnidirectional microphone to analyzer input 1.
3.3 Starting the Software
The installer creates shortcuts “1GA61 Room Acoustics” on the desktop and in folder
“R&S UPV Applications” in the programs menu.
It is possible to assign one of the quick launch buttons in the UPV toolbar for starting
the application program.
3.4 User Interface
Measurement of reverberation time
Type of test signal
Generator output voltage
Measurement frequency range
Autoscale result display
Start and cancel measurement
Status display for both measurements
Measurement of reflectogram
Generator output voltage
High pass filter cutoff frequency
Number of waveforms to be averaged
Autoscale result display
Start and cancel measurement
Minimize window of application
program while measurement is running
Update UPV window
during measurement
End program
Figure 2: User interface of the application program
1GA61_0e
Minimize window of application
program after measurement has
completed
Rohde & Schwarz Measurement of Acoustic Properties of Rooms 11
The Application Program
User Interface
3.4.1 Test Signal Type
Random signal is recommended. The other signal types are for evaluation purpose.
3.4.2 RMS Generator Output Voltage
rd
The voltage entered here is the actual RMS value of the 3 octave signal. The peak
output of the generator is limited and depends on the crest factor of the signal and the
bandwidth ratio of the filtered to the unfiltered signal.
If the peak generator output voltage resulting from the settings exceeds the capability
of the generator output, a message is displayed and the measurement is aborted.
3.4.3 Measurement Frequency Range
The loudspeaker frequency range and the microphone frequency range must include
the selected measurement frequency range.
3.4.4 Autoscale Result Display
The sweep graph which shows reverberation time over frequency after the
measurement is complete is automatically scaled according to the maximum value
when this checkbox is checked.
3.4.5 Status Display
Shows the progress during the measurement. With reverberation time measurement,
the current center frequency is displayed. With reflectogram measurement, the
average count is displayed.
3.4.6 Peak Output Voltage
Amplitude of the impulses at the generator output.
3.4.7 High Pass Filter
For improvement of the signal-to-noise ration, the analyzer input signal is high pass
filtered. This parameter sets the cutoff frequency of the high pass filter.
3.4.8 Number of Averages
The algorithm acquires the specified number of waveforms and averages them sample
by sample. This also serves to increase the signal-to-noise ratio.
1GA61_0e
Rohde & Schwarz Measurement of Acoustic Properties of Rooms 12
The Application Program
Sample Results
3.4.9 Autoscale Result Display
The waveform graph which shows the averaged reflectogram after the measurement is
complete is automatically scaled according to the maximum value when this checkbox
is checked.
3.4.10 Live Update
Allows to watch the level over time during reverberation time measurement and the
acquired waveforms before averaging during reflectogram measurement.
3.4.11 Minimize during Measurement and / or when Completed
The window of the application program is minimized at the described condition in order
to put the UPV window on top of the screen content.
3.5 Sample Results
The following graphs have been generated using the hardcopy function of the UPV
“Source” was set in the config panel to “Active Graphics”, and a printer-friendly
graphics profile has been loaded for “File”.
Figure 3: Example result of reverberation time over frequency
Figure 3 shows the reverberation time over frequency in an empty staircase almost
without any absorbing surfaces.
1GA61_0e
Rohde & Schwarz Measurement of Acoustic Properties of Rooms 13
The Application Program
Working with the Source Code
An example of a reflectogram taken in a typical medium-sized office room is shown in
figure 4. The first and highest peak is associated with the direct sound from the
speaker which is placed in about 4 m distance from the microphone. The subsequent
peaks are caused by the walls which are closest to the speaker and to the microphone,
and by side walls half way between speaker and microphone. Smaller peaks occurring
after 70 ms delay are caused by multiple reflections or by surfaces which are farther
away from microphone and speaker.
Figure 4: Example reflectogram
3.6 Working with the Source Code
3.6.1 Loading the Source Code to the IDE
I
I
I
Download “1GA61_VB2003.zip”, “1GA61_VB2005.zip” or “1GA61_VB2008.zip”
according to the version of the VB.Net IDE installed on the UPV
Unzip the file to the VB.Net projects folder on the UPV hard disk
Open file “RoomAcoustics.sln” from the VB.Net IDE
3.6.2 Running the Program in the Debugger
When running measurements, the program loads setup files “Reverb.set”,
“Reverb_Result.set”, “Reflectogram.set” and “Dirac_2s.ARB” from the program folder
“C:\Program Files\Rohde&Schwarz\1GA61” of the application to the UPV firmware. If
the program folder has not been created by installing the application program, it has to
be created manually, and the files have to be copied there.
1GA61_0e
Rohde & Schwarz Measurement of Acoustic Properties of Rooms 14
The Application Program
Working with the Source Code
The program can be started in the debugger from the VB.NET IDE with menu item
“Debug
Start” of the IDE menu or with the “run” button in the IDE tool bar (with
green triangle).
3.6.3 Modifying the Program
The user interface and program code can be modified and enhanced in the editor
according to special requirements.
Open the solution explorer in the IDE, right-click on “Form1.vb” and Click “View
Designer” or “View Code”, respectively.
To add a new control to the user interface in the designer, select a control from the
toolbox on the left side of the IDE window and drag it onto the window of the
application program in the designer. Edit the appearance of the control in the
“Properties” window. Double-click the new control in the designer and enter code in the
source code editor which is to be executed upon actuation of this control.
1GA61_0e
Rohde & Schwarz Measurement of Acoustic Properties of Rooms 15
Literature
Working with the Source Code
4 Literature
[1]
Jürgen Meyer: “Acoustics and the Performance of Music: Manual for
Acousticians, Audio Engineers, Musicians, Architects and Musical Instrument
th
Makers”, Springer, 5 Edition 2009
[2]
H. Kuttruff: “Raumakustik“, in “Taschenbuch der Technischen Akustik”, edited
nd
by M. Heckl and H.A. Müller, Springer, 2 Edition 1994
[3]
M.Zollner, E. Zwicker: “Elektroakustik”, Springer, 3 Edition 1993
rd
5 Ordering Information
Ordering Information
Type of instrument
Description
Audio Analyzer
Universal Sequence Controller
1GA61_0e
Instrument type
Ordering number
®
1146.2003.02
®
R&S UPV66
1146.2003.66
R&S®UPV-K1
1401.7009.02
R&S UPV or
Rohde & Schwarz Measurement of Acoustic Properties of Rooms 16
About Rohde & Schwarz
Rohde & Schwarz is an independent group
of companies specializing in electronics. It is
a leading supplier of solutions in the fields of
test and measurement, broadcasting,
radiomonitoring and radiolocation, as well as
secure communications. Established more
than 75 years ago, Rohde & Schwarz has a
global presence and a dedicated service
network in over 70 countries. Company
headquarters are in Munich, Germany.
Environmental commitment
I Energy-efficient products
I Continuous improvement in
environmental sustainability
I ISO 14001-certified environmental
management system
Regional contact
Europe, Africa, Middle East
+49 89 4129 12345
[email protected]
North America
1-888-TEST-RSA (1-888-837-8772)
[email protected]
Latin America
+1-410-910-7988
[email protected]
Asia/Pacific
+65 65 13 04 88
[email protected]
China
+86-800-810-8228 /+86-400-650-5896
[email protected]
This application note and the supplied
programs may only be used subject to the
conditions of use set forth in the download
area of the Rohde & Schwarz website.
R&S® is a registered trademark of Rohde & Schwarz
GmbH & Co. KG; Trade names are trademarks of the
owners.
Rohde & Schwarz GmbH & Co. KG
Mühldorfstraße 15 | D - 81671 München
Phone + 49 89 4129 - 0 | Fax + 49 89 4129 – 13777
www.rohde-schwarz.com
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