29th TONMEISTERTAGUNG – VDT INTERNATIONAL CONVENTION, November 2016

Measurement and Visualization of Room Impulse Responses with Spherical
Microphone Arrays(Messung und Visualisierung von Raumimpulsantworten mit kugelförmigen
Mikrofonarrays)
Michael Kerscher1, Benjamin Vonrhein1, Gunnar Heilmann1, Sébastien Barré2, Peter Weigel3
1

gfai tech GmbH, Berlin, Germany, Email: kerscher@gfaitech.de, vonrhein@gfaitech.de, heilmann@gfaitech.de2
GFaI e.V., Berlin, Germany, Email: barre@gfai.de
3
ADAM Audio GmbH, Berlin, Germany, Email: peter.weigel@adam-audio.com

Abstract
Room impulse responses (RIR) contain a vast amount of information about the acoustic properties of a room in a very
compact form. Conventional RIR acquisition methods mostly use a single measurement microphone placed to various
positions in the room. The use of a spherical microphone array with a high channel number allows for a way morecomprehensive analysis as it includes the direction of the incoming soundwaves. Thus a realistic description of the real
3D sound field is possible. The application of a so called delay-and-sum beamforming algorithm permits a precise
localization and depiction of the direct sound and the reflections on a 3D model of the room. This paper presents thebasic principle and some measurement examples of omnidirectional room impulse responses.

1. Introduction
Multi-array technologies are well-known in the audio industry
nowadays. 3D-audio playback systems become more andmore popular and give engineers, artists and Tonmeisters the
chance to realize a virtual acoustic scenario. On the recordingside multi-channel microphone arrays prevail more and more.
They have the big advantage of adding a spatial information
to the recording by evaluating the run-time delays between the
microphones on the grid. With this information sound can bevisualized. This technique is already well-established in manyindustry branches but can also immensely contribute to room
acoustics by the visual analysis of room impulse responses
and therefore understanding the sound propagation in a room.
Multi-channel microphone arrays can be constructed in manydifferent ways. For room acoustic measurements 3D-arrays
are most suitable as they can record the sound field in a threedimensional way. To minimize the interaction of the arraywith the sound field the microphones are placed on an
acoustically transparent sphere and to ensure a constantsampling of the sound field a uniform microphone distribution
is used.
Spherical microphone arrays can replace single channel
measurement microphones for room acoustic measurements.
Applying deconvolution techniques, which are based on
spectral division, directional room impulse responses for all
microphone channels can be calculated.

2. Room Impulse Responses and how tomeasure them
Room impulse responses are a well-known andcomprehensive format to evaluate a room´s acoustic
properties. The method is derived from system theory, where
impulses are used to identify systems. In our case the system
under test is the room, which is excited with an impulsive
signal and the way the room “responses” to this excitation isrecorded with a microphone.
This response contains all relevant information about theroom acoustic properties in a very compact way, e.g. thereverberation time, early decay time, initial time delay gap
and pre-delay. Also further room acoustic parameters can bederived from the RIR like definition, clarity, centre time etc.
But there is way more information that can be extracted fromthe RIR if acquired with directional information: the temporaland spatial development of sound in the room. Therefore ameasurement method is needed which realizes a visualisationof sound in general and the locations of the reflections in ourcase. Such a measurement procedure will be introduced in thispaper.

2.1. Structure of Room Sound
But first let´s take a look at the general structure of RIRs. They
can be divided into various temporal stages or phases: direct
sound, first and early reflections and diffuse sound.

Common methods in array technology are beamforming
algorithms. One of the oldest and best-known is the so-called
delay-and-sum beamformer which evaluates the run-timedelays of the microphone pressure signals. By feeding thisbeamformer with the directional room impulse responses thelocations of the reflections can be determined and be depicted
on a 3D-model of the room: room impulse responses become
visible!

Fig. 1: direct sound path

29th TONMEISTERTAGUNG – VDT INTERNATIONAL CONVENTION, November 2016
When sound propagates in a room then of course the first
relevant propagation path is the direct sound from the source
to the receiver like in figure 1.

As shown in figure 4 the obtained microphone signal alreadyis the required RIR. This is also the biggest advantage of thedirect method, no additional calculations are needed.

The next relevant paths are the first reflections. They arecaused by single sound reflections on the floor, ceiling andside walls, the sound does not take any further detours.

An ideal impulse is a theoretically infinitely short time
domain signal with a broad and flat frequency response. Any
deviation from this ideal leads to a not perfectly flat frequency
response, which is important for exciting the room with all
frequencies in the audible range. Shots and bursting balloons
are very close to this, but cannot be seen as perfect impulse.
Thus their frequency response is not perfectly flat and differsfrom shot to shot. This lack of reproducibility is another big
drawback of the direct method.

2.3. RIR Measurement
Signalswithdeterministic

That´s why indirect methods with deterministic measurement
signals are mostly more preferable. They use other broadband
signals for the excitement of the room, e.g.
Fig. 2: first reflections

Then multiple reflections start and the reflection pattern gets
more and more complicated. Finally single reflections cannot
be separated and the sound field becomes diffuse.

-sine sweeps (linear or logarithmic)

-

noise

-

other measurement signals like MLS etc.

These signals have a flat frequency response and aredeterministic. Therefore they can be repeated and reproduced
very precisely. To guarantee a uniform excitement of theroom omnidirectional loudspeakers are used. Such a situation
is depicted in figure 5.

Fig. 3: diffuse sound2.2. RIR Measurement with Impulse Signals
But how can we get an RIR? The easiest way is to directly
excite the room with an impulsive signal, to which the room
directly “responses”. Common methods are bursting balloons,pistol or gun shots and claps.

Fig. 5: indirect RIR measurement with deterministic measurement
signals

This method does not lead to the RIR directly. Further
calculations have to be performed with the microphonesignals, which is described in chapter 4.
Since the sound takes different propagation paths through theroom depending on the source and receiver position eachcombination of excitation and microphone position has its
own reflection pattern. This must be taken into account when
performing the measurements. Hence, it is not sufficient tojust use a single source and microphone position, as it
represents only one situation. In concert halls acousticians
mostly place the sound source on the stage and themicrophone at various seats in the audience.

Fig. 4: direct RIR measurement with an impulsive signal

Depending on the main focus of the measurement an
averaging over various measurement positions can be madeor a single position can be assessed individually.

29th TONMEISTERTAGUNG – VDT INTERNATIONAL CONVENTION, November 2016

3. Delay-and-Sum Beamforming
The main focus of this work is the visualization of room
impulse responses. Beamforming methods can be used to
produce and reproduce directional sound. One of the oldest
and best-known algorithm is the so-called delay-and-sum
beamforming and it can be used for our visualization task. It
is based on the evaluation of run-time delays from a soundsource to various receivers, e.g. microphones. A similar
concept is partially used for sound localization with thehuman hearing. The run-time delay between the two ears isinterpreted by the human brain to locate sound sources in theenvironment and tells us, where a sound is actually coming
from.

this is the room and the algorithm works on the points of apoint cloud 3D-model or the edges of a mesh 3D-model of theroom. For each point a corresponding set of distances ri andrun-time delays ti can be calculated. This is shown in figure 7.

Multi-channel microphone arrays use a much higher channelcount. The microphone data is used to calculate the soundpressure level for a certain position using the following
formula:Fig. 7: Photo plane and microphone plane

1 M
p ( x, t )  pi (t  i ). (1)M i 1
In (1) p denotes the corresponding sound pressure level for agiven time t at a position x on a reference plane. M is thenumber of microphones on the array, pi the sound pressure of
the i-th microphone on the gird and Δi its related delay.

Hence, the delay-and-sum beamformer allows for thedetermination of the sound pressure level for all the points of
the 3D-model. The points are dyed according to a colour
scale, which defines certain sound pressure levels. This socalled acoustic map is overlaid with the 3D-model of the room
as in figure 8. This end product is called acoustic image. Heresound pressure levels are represented by colours, similar toheat cameras, where colours represent temperatures: soundbecomes visible!

Fig. 8: Overlay of acoustic map and 3D-room modelFig. 6: The principle of time-domain beamforming

Figure 6 shows the situation more in detail. A sound event
reaches a number of microphones (in this case four) placed at
different positions x at different times ti, because of the
distances between the sound event and the individual
microphone positions. The sound reaches the second
microphone first, then the first, then number three and finally
number four. Using the speed of sound the run-time delays
from the source to each microphone can be calculated. These
are temporarily equalized by a set of delays Di. In the next
step the delayed microphone signals are summed up, which
results in a level that is actually too high. Therefore this levelmust be divided by the numbers of microphones, in this casefour. The end result is then the sound pressure level of thesound event f at the position x and for the time t.
This process is now applied to a number of picture points in aphoto plane. There the device under test is located. In our caseFor our measurements the spherical microphone array infigure 9 with 120 measurement microphones and a diameterof 60 cm was used. This array can localize sound sources three
dimensionally and is therefore capable of evaluating the entire
sound field in the room.

Fig. 9: The spherical microphone array29th TONMEISTERTAGUNG – VDT INTERNATIONAL CONVENTION, November 2016

4. Directional Room Impulse Responses
The analysis principle which is used for sound visualization
is based on the delay-and-sum beamformer and is now welldescribed. But what can be used as input signal for the
beamformer? First we replace the single measurement
microphone in figures 4 and 5 by our spherical microphonearray. Due to the 3D-spatial arrangement of the microphones
on the sphere this allows for the recording of directional soundand therefore directional RIRs. There are again two
possibilities of obtaining these RIRs:-

Direct measurements with impulsive sound sources-

Indirect measurements with deterministic signals

This was already described in Chapter 2. The same procedures
apply for the measurements with the array.
The direct methods lead to the RIRs without any further
calculation. The recorded directional RIRs are directly fed
into the beamformer which can display them using thealgorithm described in the previous chapter. Measurement
examples will be shown in the next chapter.

measurements with bursting balloons and different kinds ofgun shots were performed.

Fig. 11: RIR measurement at ADAM Audio

The array was placed in the centre of the room and for theexcitation 3 different positions were chosen. In figure 12 thefirst 26 ms of all 120 recorded RIRs are depicted.

The indirect method needs further calculation to get thedirectional RIRs. Barré, Jaeckel and Bauer-Diefenbach
describe a linear deconvolution method in [2] which is
depicted in figure 10:

Fig. 12: The first 26 ms of the 120 directional RIRs

Fig. 10: Linear deconvolution algorithm [2]The measurement signal s(t) is reproduced in the room withan omnidirectional loudspeaker. A reference microphone
placed close to the speaker records a reference signal, which
also includes its frequency response. The N microphonechannels of the array acquire the spatial sound and are thentransformed into the frequency domain using an FFTalgorithm. The same is done for the reference signal. All N
array signals are now spectrally divided by the reference
signal. After an inverse Fourier Transform we obtain the N
directional RIRs g1(t) to gN(t). They can be used as input
signal for the delay-and-sum beamformer which realizes thevisualisation task.

5. Measurement Examples
5.1. RIR Measurement with Impulse Signals
The following measurement was performed in a quite large
room at ADAM Audio speaker factory (figure 11). This roomshould be optimized for loudspeaker measurements. To
evaluate the current acoustic properties of the room

For the following analysis the different steps of the RIRs weremarked and the beamforming algorithm was applied to them.
To obtain the 3D-room model the room was first scanned witha 3D-laser scanner.

Fig. 13: Laser scan of the room

Figure 14 shows the direct sound. The actual shot was madeabout 2 meters in front of the wall, but as the shooter is notpart of the 3D-model the direct sound source is mapped on thenearest points of the 3D-model, which is the back wall. Thisis another disadvantage of the direct method.

29th TONMEISTERTAGUNG – VDT INTERNATIONAL CONVENTION, November 2016

Fig. 14: Direct sound source at measurement position 1 projectedon the back wallFig. 17: Begin of multiple reflections

Figure 15 shows the first reflection on the floor:

Fig. 18: The diffuse sound field
Fig. 15: First reflection

By looking at the RIR step by step the way the sound travels
through the room can be visualized and retraced.

In figure 16 the second reflection on the ceiling can be seen:Fig. 19: Retracing the sound in the room

Fig. 16: Second reflection

5.2. RIR Measurement with deterministic Signal

The third reflection still is located at a discrete position on theside wall. After the fourth reflection multiple reflections
begin. This is displayed in figure 17.

The next example explains a measurement that was done in a
meeting room at the GFaI e.V. in Berlin. Here a constant
power sound source was used as deterministic signal. A single
channel measurement microphone was placed close to it toacquire the reference channel. Again here the 120 channel
spherical array was used. Figure 20 shows the setup. The
objective of this experiment was to determine the influence ofabsorbing material on room acoustics and its visualization.
Therefore Basotect material was put in front of the windowsof the room.

The last and longest stage of the RIR is the diffuse field. Thiscan also be visualized with the beamforming algorithm. The
sound power is more equally distributed and reflections areless discrete. Such a situation is shown in figure 18.

29th TONMEISTERTAGUNG – VDT INTERNATIONAL CONVENTION, November 2016

Fig. 20: Measurements setup in meeting room with constant
power source and spherical microphone array

Fig. 22: Diffuse sound field without (above) and with (below)
absorbing material
Fig. 21: Positions of array, source and absorbers

Speech intelligibility is of utmost importance in meetingrooms. Poor values lead to fast tiring of the meeting
participants. Acoustic parameters like the reverberation timeand definition can be derived from the RIRs and evaluate thisin an effective way. As table 1 summarizes in this experiment
the absorbers could improve all these values in an effective
way. Measurement 1 was performed in the plain room without
any absorbers. For number 2 the upper windows in figure 21
were covered with 10 cm material and for number 3 both sides
were covered with absorbers. RT20 denominates the soundenergy decay in the room by 20 dB (similar to RT60), D50 thedefinition and C80 the clarity.
Measurement
12
3

RT20 [sec]
0,50
0,34
0,29

D50 [%]
81,5
88,5
91,3

C80 [dB]
10,3
14,8
16,4

Tab. 1: Room acoustic values without and with treatment
As demonstrated in chapter 5.1 the beamforming algorithm
can be used to visualize the phases of the RIR. Looking at the
diffuse sound field here reveals the effect of the absorbers. In
figure 22 the influence can be clearly seen. The above picture
visualizes the diffuse sound field without absorber, the below
picture with absorber. In figure 22 the dynamic is the same inboth picture, this means, the colours represent the same soundpressure levels. Thus the influence of the absorber becomes
obvious. By the colour scale readings on the right a soundattenuation of about 5 dB caused by the absorbers can bedetected.

6. Conclusion
Spherical microphone arrays can be used to record directional
RIRs. Their evaluation provides a vast amount of information
about the real 3D sound field. With the energy decay curve,
which is derived from the RIR, parameters like reverberation
time, early decay time, definition and clarity etc. can becalculated. But with the application of imaging procedures
based on delay-and-sum beamforming a comprehensive
understanding of the way the sound takes to travel through aroom can be achieved. The visualization helps to understandthe complex context in a much easier way.
The performance of this technique was shown in two
experiments: locations of reflections can be clearly detected.
This can be used for the evaluation of rooms and form thebasis for room acoustic improvements e.g. the equal
distribution of first reflections over the audience by placing
reflectors at the correct positions, the detection of flutter
echoes or the efficient mounting of absorbers.

7. References[1] Gunnar Heilmann, Magdalena Böck, Dirk Döbler:
Exploring the limitations and expectations of soundsource localization and visualization techniques;InterNoise 2014[2] Sebastien Barré, Olaf Jaeckel, Ralf Bauer-Diefenbach:Analysis of sound field variations in concert halls via
visualization and objective parameter comparison;
Berlin Beamforming Conference BeBeC 2016