Varla et al.

Design of rooms for multichannel audio monitoring

DESIGN OF ROOMS FOR MULTICHANNEL AUDIO MONITORING
A.VARLA, A. MÄKIVIRTA, I. MARTIKAINEN, M. PILCHNER1, R. SCHOUSTAL1, C. ANET
Genelec OY, Finland
genelec@genelec.com
1

Pilchner Schoustal Inc, Canada

acoustic@pilchner-schoustal.com

This paper presents an overview of the practical aims, methods and problems of designing monitoring rooms for
multichannel audio. The main problem areas encountered in practical multichannel audio monitoring room aredescribed with a case study of a practical high quality monitoring room and with scale model measurements. The scalemodel is also used to investigate the effect of structural modifications to the room suggested in literature as methods toachieve better performance in multichannel audio reproduction. The design principles and methodology applied in theindustry is reviewed, and some suggestions are developed to guide the monitoring room designer.

INTRODUCTION
A modern audio production facility has to be able toserve productions in a large number of different
formats.
The change from mono and stereo to multichannel
reproduction has produced a lot of problems, both inconverting existing production facilities to multichannel
format and in new installations.
The audio formats that must be handled by a modern
production facility include currently
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construction to a medium-to-large scale monitoring
facility.
The scale model was necessary to allow investigation of
the effects of structural changes to the room and theloudspeaker installations.
The aim of the work is to investigate the feasibility ofthe methods suggested in the contemporary literature to
implement multichannel audio monitoring. The
particular areas of interest are

mono, stereo
matrixed four channel format
five channels (later 5.0 reproduction)
five channels with a separate Low
Frequency Enhancement channel (later 5.1
reproduction)

MATERIAL AND METHODS

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surround loudspeaker installation methods toachieve similarity with the front loudspeakersystemmethods to achieve specified performance at
low frequencies were the summation of theacoustic signals from multiple sources
becomes coherent
methods to obtain large high quality
monitoring area in the room
the frequencies of interest f < 1000Hz

1.1
Aims of the experimental work
The properties of a multichannel monitoring systemwere studied in room comparable in size and

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Design of rooms for multichannel audio monitoring

Figure 1. Monitoring room construction dimensions, and the placement of the measurement grid points.

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Design of rooms for multichannel audio monitoring

1.2
Monitoring room and scale model
The floor area of the room is 63m2. Free volume is
163m3. The room was furnished as an auditorium(Figure 1). A 10:1 scale model was produced of theroom, including the absorbents and the loudspeakerinstallations.
The loudspeaker system in the room includes three flush
mounted front loudspeakers, to implement the Left,
Center and Right outputs. The surround loudspeakers
are installed, as they typically are, next to the rear bass
trap and the side wall. Optically they are in the corner ofthe room, but acoustically there are reflecting
boundaries close to these radiators such that theirradiation to the room happens in conditions significantly
differing from those of the frontal main speakers.
All loudspeakers are full bandwidth units, withacceptably low lower cut-off frequencies (front 3 pcs
Genelec 1036A, surround 2 pcs Genelec 1038A).
The scale model was used to model mid-to-low
frequency behaviour of the room. The radiators were 1”
dome tweeters, modelling the low frequency drivers.
1.3
Measurement gridFrequency responses were measured in the actual roomin a grid of measurement points with an interval of one
meter. Front-to-back there were four measurement
point, from the center of the room to the right side there
were three points, totalling 12 measurement points. The
room is symmetrical, so only one side of the room wasmeasured. The total floor area covered by themeasurements was 12m2. The grid interval of one meter
was adequate to cover the frequencies of interest.
1.4
Data processing
The measurements were taken using a 7mm diameter
microphone and the MLSSA measurement system. The
impulse responses in all grid points were stored. Similar
measurements were taken in the scale model.

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Figure 2. Cancellation of direct sound by rear wall
reflection (top) in room at 50Hz, and (bottom) in modelat 500Hz.

Table 1. Similarity of frequency responses, deviation dB
in band 20Hz…1kHz after 0.33 octave smoothing. “C”
center loudspeaker, “L” left, “R” right, “SL” surround
left, “SR” surround right.

RESULTS

2.1
Accuracy of the model
Measurements in the real room were compared to thescale model. A good agreement was observed.
As an example, the reflection of the front speaker soundfrom the back wall produces a cancellation notch whichmoves in frequency as the microphone moves towards
the back of the room (Figure 2).

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position
00
10
20
30
01
11
21
31
02
12
22
32

C
14
11
10
12
10
11
8
15
15
14
13
14

L
10
8
12
15
15
13
13
24
23
17
20
22

R
SL SR
14
21
23
10
20
21
13
21
19
19
19
18
9
20
14
8
16
9
17
18
11
17
14
13
18
18
13
23
18
14
18
17
15
16
13

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2.2
Similarity of frequency responses
The similarity of the frequency responses is investigatedby calculating the place dependent deviation from linear
frequency response.
The frequency responses were measured at all grid
points in the room. A 1/3-octave smoothing wasapplied. The difference between the minimum andmaximum values between frequencies 20Hz…1kHz ispresented in Table 1.
The variation is typical of a high quality monitoringroom. It is larger for the rear channels, and caused byboundary reflections in the room.

Design of rooms for multichannel audio monitoring

2.3
Frequency response in the listening area
Signals from all loudspeakers combine to form soundexperience at the listening point.
The signal level generated when all front loudspeakers
are operating was calculated at two points, “10” on thecenter symmetry axis, and point “11” one meter to theside from that, for in-phase and non-coherent material.
The summation was calculated by complex averaging(non-coherent summation) and by power averaging(coherent summation).
The on-axis summation for both coherent and noncoherent signals shows good agreement, while a fairly
small displacement off-axis shows a significant
difference in the sound level produced for in-phase
input signals (coherent summation) and other signals.
2.4
Bass response change in front-back direction
When bass frequencies are produced from radiator(s) in
the front of the room, the solid rear wall behind apossible bass trap produces a reflection of the sound thatinterferes with the primary sound if bass trapping in theis not adequate. This is often the case.
The shape of the rear wall modifies the level of theinterference. A concave rear wall is sometimes used,
and this increases the amplitude of the interference at
the center of the room, in the best listening area.
The solid back wall in the studied room was straight,
but still the effect of the back wall reflection is clearlyvisible as loss of level in the low bass frequencies.
The interference notch moves up in frequency as the
microphone approaches the back wall (Figure 4). A
distinct cancellation notch becomes visible atmeasurement point “30” at 40Hz. This bass cancellation
was visible for all front loudspeakers, and resulted inloss of bass output and then a change in tonality when
the listener moves towards the back of the room.

Figure 3. On-axis (top, point “10”) and off-axis(bottom, point “11”) signal summation for coherent(dashed) and non-coherent signals (solid).

Any subwoofer placed in the front would suffer thesame loss in its radiating energy. Experience with many
monitoring room designs shows that the rear wall
reflection is a common cause of a lack of bass
performance in rooms designed according to highstandard established design methods.

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Design of rooms for multichannel audio monitoring

2.5
Surround loudspeaker installation
The wall installation of the surround loudspeakers
typically significantly differs from that of the front
loudspeakers. It is typical to see surround loudspeakers
installed close to the side walls of the room. Because ofradiating angles defined in current standards [1,2], they
become placed in a position where a significant amountof energy is radiated to the side wall.
Figure 1 shows the placement of the surroundloudspeakers in the room used in this study. The scalemodel was used to investigate the effect various
possibilities to install the surround loudspeakers. Bothfully absorbing (free field) and soffit installations havebeen proposed in the literature.
A baffle around the surround loudspeaker (Figure 5)
appeared to reduce the notch produced by a secondary
image radiator produced by the side wall on which thesurround loudspeaker was installed. The absorbing
material around the surround loudspeaker seemed tohave minimal effect and failed to produce free fieldradiating conditions for the surround loudspeaker.

Figure 4. First order reflection modifies the bass levelof the Right loudspeaker as the microphone moves backon the center symmetry axis of the room. From top tobottom: measurement point “00”,” 10”, “20”,”30”.

Figure 5. Effect of fully absorbing (top, solid) and baffle(bottom, solid) installation relative to the actual(dotted).

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Design of rooms for multichannel audio monitoring

Loudspeaker radiation can also be analysed by using
energy-time curves that more specifically provide
information about the individual reflections that
contribute to the measured frequency response.

For front radiation, additional absorbance on the samewall with the loudspeaker produced more reduction inearly reflection level than if the absorbing material wasplaced on the opposite wall (Figure 7).

The scale model was used to investigate the effect ofmodifications to the absorbance of the surfaces in theroom.

The surround loudspeaker radiation can also reflect off
the wall opposite to it, and from the faces of the front
loudspeakers (Figure 8).

If the baffles around surround loudspeakers are
implemented, they will reflect some sound energy fromthe front loudspeakers, increasing the level of earlyreflections associated with the front sound (Figure 6).
This radiation is coming from the back of the listener,
so it is likely to be discerned as a separate sound event[3].

If the front wall is made absorbing, the loudspeakers in
the front can still contribute significantly to the surroundaudio reflections from the front.
2.7
Coherence of front bass radiators
Griesinger [8] recommends a constant phase difference
for reproduction of bass frequencies from multiple front
speakers to increase the envelopment. Other authors [9]have advocated the use of a single subwoofer instead ofmultiple sources to minimise loudspeaker-room
interaction and the problems of coherent summation of
the energy at low frequencies from multiple sources.

Figure 6. Reflection from the Left (top, solid) and Right(bottom, solid) surround loudspeakers in baffles,
standard installation without baffle (dashed).

2.6

Control of reflections by additional absorbing
material on walls
Because the loudspeakers are calibrated for on-axis
listening, they are normally aimed towards the listeners.
In our case, the loudspeakers were to optimise themonitoring quality at point “10”.

Figure 7. Effect of absorbing material on the same wall
(top, solid) or opposite wall (bottom, solid) of the Rightloudspeaker; no added absorption (dashed).

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Design of rooms for multichannel audio monitoring

practical applications as electrical filtering as well asmechanical and acoustical filtering.
The phase difference on the transition band caused by
the band limiting filter relative to other sound radiators
may actually decrease the sound level and the systemefficiency.

Figure 9. Effect of an additional off-axis subwoofer,
Right loudspeaker with additional subwoofer (solid),
Right speaker alone (dashed).

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Figure 8. Right surround loudspeaker radiation with
(top, solid) absorbing material on the opposite left wall,
over the left front speaker (middle, solid) or around theleft front speaker (bottom, solid), relative to standard
surround loudspeaker installation (dashed).

To model an additional subwoofer in our configuration,a low pass filtered version of the Right loudspeaker
response to point “11” was calculated and summed to
the response of the Right loudspeaker, modelling an
non-symmetrically placed front subwoofer radiatingwith excellent coherence in the passband (Figure 9).
The subwoofer signal is band limited, and the filtering
associated with this band limitation is implemented in

DISCUSSION

3.1
Design aims
Volker [6] summarises the development of themonitoring room performance specification over some
60 years. The design aims of a modern day reproduction
system can be structured into [1,2,7]• wide listening area
• equality of sound level at all frequencies in
the listening area (flatness of frequency
response)• similarity of frequency responses of all fullbandwidth loudspeakers (similarity of
frequency responses)• equal times-of-flight of the audio signal
from all full bandwidth sources• faithful reproduction of the standardised
loudspeaker angles of the full bandwidthsources• sound sources at the listening height• good integration of low frequency soundsources, including the Low Frequency
Enhancement Channel, to produce an evenfrequency response, in-line with thefrequency response at higher frequencies

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There are several reasons why the design aims for
multichannel monitoring room can frequently not be
fulfilled. These arise when old production facilities areconverted for new audio systems, or newly designed
facilities face design limitations that hinder these aims
to be respected. Majority of problems can be grouped to• properties of the loudspeaker(s)• room geometry and loudspeaker — room
interaction• use of subwoofer(s)3.2Properties of the Loudspeakers
The assumptions of the current standards for 5.0 and 5.1
reproduction [1,2] call for similar loudspeakers to beinstalled. Often, this recommendation is difficult tofulfill. The surround loudspeakers may differ in size andimplementation from the front loudspeakers. The center
loudspeaker may be different from the left and rightloudspeakers. The calibration of frequency responses isfrequently overlooked.

3.3

Room geometry and loudspeaker – room
interaction
The stereophonic reproduction has generated few fairly
stabilised and accepted principles of design such as areflection free zone in front of the room, the Live EndDead End principle, left-right symmetry of themonitoring room, symmetrical placement of the Leftand Right loudspeakers. The stereophonic designprinciples do not directly extend to multichannel
reproduction, and the current lack of clear design
approach is generating a lot of debate. Proponents oftraditional design approach attempt to extend the
traditional approach to multichannel rooms, claimingthat this would be possible although they at the sametime recognise many problems [4,5].

Design of rooms for multichannel audio monitoring

Various solutions have been suggested to ensure similarradiating conditions for all loudspeakers• rooms with sound absorbing material around
all walls, including the front• some additional baffling around surroundloudspeakers
We demonstrated potential problems in applying eitherof these approaches.
3.4
Use of subwoofer(s)One or more subwoofers are typically used because of(1) the reproduction of a separate Low Frequency
Enhancement Channel [1], and because (2) the output
capability of the main loudspeakers is compromised at
low frequencies even though the recommendations callfor full bandwidth loudspeakers. This is typical in small
room installations, where the physical size of the main
loudspeakers is limited, and thereby the low frequency
handling capability is not adequate.
The frequency band of the Low Frequency
Enhancement channel overlaps with the bandwidth ofmain loudspeakers. If a subwoofer is used to reproduce
the Low Frequency Enhancement channel without
proper bass management, a possible consequence is
non-coherent radiation of the same programme content
by the subwoofer and the main loudspeakers. This may
result in unpredictable summing of audio (Figure 10).
Although the standard [1] assumes that the Low
Frequency Enhancement is carrying additional soundinformation, such as sound effects, in practice it is likelythat similar material appears on the main channels.
When this happens, and the bass management does notaccount for this possibility, there may actually be a
reduction at some frequencies in the bass level becauseof an additional subwoofer, not an increase.

The traditional approach installs the front loudspeakers
flush with the hard front wall, generating an extended
baffle in the front. The back wall typically contains a
thick soft bass trap. The surround loudspeakers are
typically installed either free-standing or embedded
flush with the surface of the bass trap material, resulting
in conditions similar to a free-standing placement at low
frequencies and a variable, undefined boundary
condition at higher frequencies.
It may be difficult to maintain the angles defined in thestandards for the surround loudspeakers and to maintainequidistant placement of all full bandwidth radiators insmall rooms and retrofit installations.
The need to produce similar radiating conditions for all
full bandwidth loudspeakers has been recognised [4,5].

Figure 10. Anechoic acoustical sum of a full bandwidth
loudspeaker (solid) and a subwoofer with 33…120 Hz
passband. Severe signal cancellation occurs because ofphase mismatch in overlapping frequencies caused byacoustical design.

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The management of low frequency information under
the limitations above has not been specified in standards
and recommendations, making it an implementation
dependent property.
The contemporary implementations can be divided into
• single subwoofer• several subwoofers to increase output
capability• surround subwoofer to enhance the surroundloudspeakers’ low frequency capability• distributed bass reproduction from Left andRight main loudspeakers• distributed bass reproduction from frontal
loudspeakers• distributed bass reproduction from all
loudspeakers

Design of rooms for multichannel audio monitoring

REFERENCES[1]
ITU-R BS.775-1 Recommendation 1992/1993,
Multichannel stereophonic sound system withand without acomppanying picture. International
Telecommunication Union, Geneve.
[2]

ITU-R BS.1116 Recommendation 1996,
Methods for the subjective assessment of small
impairments in audio systems including
multichannel sound systems. International
Telecommunication Union, Geneve.

[3]

Brian Moore, An introduction to the psychologyof hearing. Academic Press, London, 1989,
ISBN 0-12-505624-9.

[4]

Tom Hidley, Full bandwidth surround soundmusic monitoring. Audio Media Jan/Feb 1999.

[5]

Phillip Newell, Surrounded or besieged? AudioMedia, Feb 1999.

[6]

E. J. Voelker, Acoustics in control room – thatrecurring, burdensome subject. 105th AES
Convention (1998), preprint 4832.

[7]

EBU Recommendation 22-1997, Listening
conditions for the assessment of soundprogramme material. European Broadcasting
Union, Geneva, 1997.

[8]

David
Griesinger,
Speaker
placement,
externalization, and envelopment in home
listening rooms. 105th AES Convention (1998),
preprint 4860.

[9]

Tomlinson Holman, The basics of bass
management. Surround Sound Professional,
October 1998, vol 1, number 1, pp. 48—54.

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The problems seen in installations, both big and smallrooms, are• low order reflection induced place
dependent interference (notably the backwall reflection)• dissimilar frequency response characteristics
from several low frequency radiators
because of differences in the method ofroom
installation
andradiation
characteristics• wrong management of low frequency signal
sources

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CONCLUSIONSA descriptive study of an implemented multichannel
monitoring facility demonstrated the problems of
multichannel monitoring environment acoustical design.
Surround loudspeaker installation, low frequency soundintegration and the control of low order reflections werediscussed.