A Survey Study Of In-Situ Stereo And Multi-Channel
Monitoring Conditions
Aki V. Mäkivirta and Christophe Anet
Genelec OY
Olvitie 5, Iisalmi, Finland
aki.makivirta@genelec.com
chris.anet@genelec.com
ABSTRACTThe in-situ responses of a total of 372 loudspeakers in 164 professional monitoring rooms around the world havebeen measured after acoustical calibration. All measured rooms have been equipped with factory calibrated three
way monitors and acoustically calibrated with standardized apparatus. The results provide a thorough understanding of typical monitoring conditions for stereo and multi-channel rooms, distribution in room parametersand quality of reproduced audio. Results are compared to current standards and recommendations.
INTRODUCTION
The audio quality of monitoring determines the resulting quality of
the recording process. The monitor loudspeakers strongly effect theaesthetic decisions of artists and engineers producing audio material. The installation of the speakers and the monitoring room itself
has typically an even stronger influence, and an increasing numberof standards and recommendations have been issued to ensure consistency and high quality of the monitoring process both with andwithout an accompanying picture [1-5]. These give recommendations of aspects of in-situ reproduction quality such as
•
•
•
•
•
•
•

magnitude response at the listening location
dynamic range and noise leveltime-of-flight difference
idealized layout configuration
nominal listening level
reverberation and reflection in the room
quality of the reproduction system

METHODSAll rooms included in the present study are equipped with a factory
calibrated 3-way main monitors, produced by one manufacturer1.
Measurement Apparatus and Method
The measurement method used an MLS sequence2 of period 16383
samples (217ms) at a sampling rate of 75.47kHz. An impulse response of length 16383 samples was stored for each loudspeaker.
The measurement apparatus was laboratory calibrated to a reference
measurement system before and after the measurements. The microphone3 was calibrated by producing a separate calibration filethat was used during analysis to equalize the magnitude response
calculated from an impulse response. To ensure high consistency ofthe measurement process one person used the measurement appa-

1

Genelec OY, Iisalmi, Finland
MLSSA measurement system, DRA Laboratories.
3
Neutrik Type 3382.
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IN-SITU MONITORING CONDITIONS

ratus throughout the study to perform all on-site calibrations andmeasurements.

To assess the level of late energy, several early-late energy ratios
were calculated (C35, C50, C80, C95) for each impulse response.

Detection of impulse response start
The onset of an impulse response is determined as 10 samples before the impulse response power estimate exceeds 3% (–30dB) of its
peak value. This corresponds closely to an impulse response startdefinition given in standards of room reverberation measurement[6].

Room Operational Response Curve
The target for the magnitude response at the listening location, or
the room operational response curve, is defined as the third-octavesmoothed magnitude response. The target for this is given as an
acceptance window function centered at the mean value in this
response calculated over the frequency range 50Hz-16kHz.

Signal-to-Noise Ratio and Listening Distance
The stored impulse response contains the natural time-of-flight
before the impulse begins. This section in the recorded signal represents the total system plus room noise level, and it is used as anindication of the quality of measurement (signal-to-noise ratio) andmeasurement distance.

Sliding third octave smoothed magnitude responses are calculated
for each impulse response. Each magnitude response is level alignedby setting the mean value calculated over the frequency range
50Hz-16kHz to one (or zero dB).

The signal-to-noise ratio D of a measurement is calculated as a ratioof the recorded impulse response data a(t) peak value to the noiselevel rms value within the time-of-flight period [t0, t1] before theonset of the actual impulse response

The median, 50% and 90% percentiles of this distribution are extracted.

D=

max a (t )1 t1 2a (t ) dt
T ∫ t0(1)

The listening distance is calculated from the time-of-flight assuming
the speed of sound in air as 344m/s.
Reverberation Time
Nonlinear fitting technique due to Karjalainen et al. [8] is used toestimate the reverberation time in both the full-band and octaveband cases. This method is accurate in the presence of noise andworks well also in sparse reverberation fields typical of high quality
control rooms.
Karjalainen's method fits three parameters into time envelope data.
These are the initial amplitude A, decay time constant τ and final
constant level An modeling the noise level of a measurement within
the time span [t1, t2] or interest. The modeling is formulated as aleast-squares minimization task4t2minA,τ , An∫ [a (t ) − y (t )] dt
2

(2)

t1

The 60-dB decay time T60 is related to the decay time constant τ by

1
6.908
T60 = − ln(10 −3 ) ≈
τ
τ

(3)

The initial and final levels A and An are not recorded in our study.
The octave bands included in the study are from 63Hz to 16kHz.
Before performing the fitting, the start of fitting time span t1 is updated to point to the maximum value in an impulse response if the
maximum value of an octave-band impulse response does not occur
at the detected impulse response start.
Early-Late Energy Ratio
The early-late ratio Cx describes the energy level ratio before andafter a certain point in time, and is linked to the reverberation timein a space. The early-late ratio Cx is defined as

C x = 10 log10

4

∫

x 2
p (t )dt0∞ 2∫xp (t )dt

Implementation in Matlab uses the lsqcurvefit function.

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At each frequency, level aligned magnitude values from all responses are collected to form the magnitude response distribution.

Notches at f < 1kHz
Boundary reflections and room modes can produce comb filtering
effects and standing wave cancellation in the monitoring space
displayed as notches in the magnitude response. These effects arequantified for frequencies 40Hz < f < 1kHz (the search frequencyband). Ten deepest notches are sought in the raw magnitude response within the specified frequency band.
Notches are identified with the following process. A detection levelis moved up starting from the global minimum in the data within the
search frequency band. A monotonously increasing neighborhood of
an identified minimum is recorded. The detection level is increased
until 10 largest minimums are found.
All minimums found in this manner are notches, but the monotonously increasing neighborhood for some of them is quite narrow,and therefore Q-value calculation for the notches is not attempted.
The center frequency and maximum depth of the notches is recorded.
Speaker Pair Comparisons
To assess the similarity of magnitude responses of monitor speakers
in a room, the magnitude responses of monitor speakers are compared pair-wise in three groups•••

left-right stereo pairs in a room
left-center-right front speaker triplets in a room
five-channel systems in a room

The difference between Left-Right stereo pair is calculated.
The Left-Center-Right triplets are compared to the Center channel.
The five-channel system analysis compares separately third octavesmoothed responses of front monitors (left-center-right triplet) andrear left-right triplet because the front-back balancing is not performed as a part of system alignment.
LOUDSPEAKER MATERIAL
A total of 372 loudspeaker in 164 professional monitoring rooms
around the world have been measured after acoustical calibration.
All rooms included in the present study are equipped with a factory
calibrated 3-way main monitor system, produced by one manufacturer5.
Of the 372 recorded impulses, 277 (75%) have been measured at theengineer's position (Tables 1-3). Of the measurements at the engineer's position, 250 (90%) have also been measured on the acoustical axis of the loudspeaker, implying that a speaker's acoustical axis
is aimed toward the engineer's position.

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Genelec Oy, Iisalmi, Finland2MÄKIVIRTA AND ANET

Additional data has been recorded for the impulse response files,
tabulated for several factors known to affect measured responses.
The factors included are –installation type (free-standing, flush-mounted in a soffit)•
measurement location (at or outside the engineer's position)•measurement taken on a speakers acoustical axis (yes/no)•a speaker vertically tilted toward the listening point (yes/no)•monitor height (below/at/higher than the listening point)•room size (small/medium/large)•
ceiling (soft/hard)•side walls (soft/hard)•back wall (soft/hard)•room geometry (square, rectangular but not square, other)•
Loudspeaker type is given in Table 2, indicating also the type of
speaker installation. The 1037A, 1037B and 1038A are mostly usedfree-standing, but even loudspeakers intended for in-wall installation (1034, 1035, 1036, 1039) were sometimes used free-standing.
The detailed specifications of all monitor speakers included in the
study can be found on the web site of the manufacturer.6

IN-SITU MONITORING CONDITIONS

Table 1. Type of installation and measurement axis.
Type of installation

Yes

No

Yes

In-wall

129

23

152

Free standing

121

4

125

In-wall66

066Free standing

29

0

29

345

27

372

No
Total

Total

Table 2. Speaker height and vertical tilt in monitoring rooms ofvarious sizes.
Room Size

Ear Level

Higher

Vertical Tilt

Vertical Tilt

yes

yes

no

n.a.

Signal-to-Noise Ratio
The dynamic range in a measurement is typically 60dB (Fig. 1) andvaries from less than 40dB to over 70dB.

Total

Speaker Height

ROOM-RELATED PARAMETERS

The dynamic range measured in this way contains the noise contributions of both the measurement apparatus and the room in which
the measurement was taken. Typically the contribution of the measurement apparatus is not significant.

Measured on
acoustical axis

Measured atengineer's position

Small
Medium

10
2

Large
Total

2

no

2

4

6

8

12

30

11

56

33

102

34

130

70

234

55

196

119

372

Table 3. Acoustical axis orientation at the engineer's position.
At Engineer's position

Room Size

On-AxisTotal

Off-Axis

n.a.

6

6

Small

24

24

Medium

66

10

76

Large

154

17

171

250

27

277

Total
Fig. 1. Signal-to-noise ratio of impulse responses (N = 372).

Table 4. Loudspeaker type and type of installation.
Listening distanceThe distribution of listening distance has been analyzed for thosemonitor speakers having their acoustical axis directed toward andthe impulse responses recorded at the listening position. Of all recorded impulse responses, 250 were measured at the listening position, and were analyzed for distance.
The distances for studied three-way monitor systems range from 1.2
meters to 4.2 meters. The average listening distance is 2.49 meters(Fig. 2). The distances estimated in this manner correspond to actual
listening distances and can be taken to indicate the listening distance
distribution for main monitors including both front and rear monitors in multichannel audio configurations. The mean distance ofindividual channel location is given in Table 5.
The measurement signal-to-noise ratio correlates only weakly to thelistening (measurement) distance (Fig. 2).

6

Monitor TypeType of Installation
In-Wall

Total

Free Standing

1033A

10

10

1034A, 1034B

21

101035A, 1035B

34

2

36

1036A

10

4

14

31

1037A, 1037B

29

78

107

1038A, 1038AC

81

52

133

1039A

33

8

41

Total

218

154

372

www.genelec.com

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Table 5. Mean listening distances of individual channels for leftright stereo rooms, left-center-right triplet rooms and five-channel
full surround rooms. Distances are in meters.
Channel
L
C
R
SL
SR

StereoN = 79
2.36
2.39

L/C/R
N = 15
2.66
2.66
2.66

5-chan.
N=8
2.68
2.71
2.72
2.70
2.69

The distribution in listening distances (Figs. 3-13) is the widest, as
can be expected, in stereo pairs.

Fig. 4. Listening distance to left speaker in left-right stereo pairs,(mean = 2.36m, N = 79).

The minimum listening distance for a three-way system is about 1
meter. However, it is clear that listening distances this short may
significantly modify the frequency response as the listening is donein the near field, and 3-way loudspeaker products included in the
study are not intended for near-field listening.
A typical listening distance of a three-way main monitor in a stereo
configuration is 1.5–2.5 meters. This is shorter than that of L/C/Rtriplets (Figs. 6-8). The mean listening distance of L/C/R-tripletsand five-channel surround systems (Figs. 9-13) is about 0.3 meters
longer.
An electronic delay device may be use to equalize the time-of-flightdelay differences. Since the measurement system used for alignment
is connected straight to the speaker input, any electronic delay devices are excluded from this analysis.

Fig. 5. Listening distance to right speaker in left-right stereo pairs,(mean = 2.39m, N = 79).

Fig. 6. Listening distance to center speaker in left-center-right triplets, (mean = 2.66m, N = 15).
Fig. 2. The regression of the listening distance and measurement
signal-to-noise ratio estimated in impulse responses (N = 250).

Fig. 7. Listening distance to left speaker in left-center-right triplets,(mean = 2.66m, N = 15).
Fig. 3. The distribution of listening distance, all loudspeakers including surround. Mean distance to a loudspeaker is 2.49 meters(N = 250).

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IN-SITU MONITORING CONDITIONS

Fig. 8. Listening distance to right speaker in left-center-right triplets, (mean = 2.66m, N = 15).

Fig. 12. Listening distance to rear left speaker in five-channel surround systems, (mean = 2.72m, N = 8).

Fig. 9. Listening distance to front left speaker in five-channel surround systems, (mean = 2.68m, N = 8).

Fig. 13. Listening distance to rear right speaker in five-channel
surround systems, (mean = 2.72m, N = 8).
Reverberation Time
Distributions of reverberation time are given (Figs. 15-24) for thefull bandwidth and octave-band measurements.
At each octave band, the median, 50% and 90% bounds are given in
Fig. 14. The mean reverberation time RT60 is 380ms from 200Hz to
4kHz.
Most rooms show reverberation times that conforms to present
standards and recommendations for high quality monitoring rooms,
but there are large differences, related to the use of either absorption
or diffusion to control the reverberant decay field.

Fig. 10. Listening distance to front center speaker in five-channel
surround systems, (mean = 2.71m, N = 8).

Fig. 11. Listening distance to front right speaker in five-channel
surround systems, (mean = 2.72m, N = 8).

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Fig 14. The minimum, maximum, 90% and 50% limits and the
median for reverberation time RT60 in octave bands in measured
rooms (N = 372). German Surround Sound Forum limit showncentered at the median of mean RT60 levels.

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IN-SITU MONITORING CONDITIONS

Fig. 15. Distribution of full bandwidth reverberation time RT60
(N = 372).

Fig. 19. Distribution of reverberation time RT60 of 500Hz octave
band (N = 372).

Fig. 16. Distribution of reverberation time RT60 of 63.5Hz octave
band (N = 372).

Fig. 20. Distribution of reverberation time RT60 of 1kHz octave band
(N = 372).

Fig. 17. Distribution of reverberation time RT60 of 125Hz octave
band (N = 372).

Fig. 21. Distribution of reverberation time RT60 of 2kHz octave band
(N = 372).

Fig. 18. Distribution of reverberation time RT60 of 250Hz octave
band (N = 372).

Fig. 22. Distribution of reverberation time RT60 of 4000Hz octave
band (N = 372).

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Fig. 23. Distribution of reverberation time RT60 of 8kHz octave band
(N = 372).

IN-SITU MONITORING CONDITIONS

Fig. 26. The distribution of early-late energy ratio of energy earlier
and later than 95ms after onset of the impulse (C95) in the measured
rooms (N = 372).

Magnitude Response
The overall frequency balance at the listening position for loudspeakers aimed at the listening position (N = 250) is represented bythird octave smoothed frequency response deviations (Fig. 27) at thelistening position for loudspeakers aimed at the listening position(N = 250). To obtain the distributions, each frequency response isnormalizing to the mean level between 50Hz and 16kHz as proposed by the German Surround Forum [5]. Also shown in are theroom operational curve limits as proposed in [5] set relative to the
50Hz – 16kHz mean of the median of distributions.
Fig. 24. Distribution of reverberation time RT60 of 16kHz octave
band (N = 372).

The 50% variation limit is within the proposed limits [5] for frequencies f > 130Hz, and 90% of rooms for frequencies f > 400Hz.
We can see the responses generally suffering loss of level above
16kHz. Only 5% of rooms show straight response up to 20kHz.

Early-Late Energy Ratio
C35, C50, C80, C95 early-late energy ratios have been estimated but
only the extremes are presented for clarity in Fig. 25 and 26. The
C35 represents the level of early energy relative to the direct soundin the monitoring room. The C95 represents the level of late reverberation relative to the combined direct sound and early energy.

As expected, the frequency response at the listening position forloudspeakers aimed at the listening position shows a decreasing
variation toward high frequencies. Notches in the magnitude responses are displayed as a larger spread toward negative values.

The reverberation characteristics of rooms can also be viewed by
C35 and C95. These show the ratio of power integrated in the interval
0…35ms and after 35ms, or 0…95ms and after 95ms, respectively.
Some rooms have strong resonant behavior leading to low C35 andC95 figures.

Fig. 25. The distribution of early-late energy ratio of energy earlier
and later than 35ms after onset of the impulse (C35) in the measured
rooms (N = 372).

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Fig. 27. Third octave smoothed sound pressure level measured at
the listening position (N = 250) for speakers aimed at the listening
position. 50Hz-16kHz mean level normalized to 0dB. Also shown
are German Surround Forum proposed limits.

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IN-SITU MONITORING CONDITIONS

Response Notches for f < 1kHz
To demonstrate notches in the frequency response recorded at the
primary listening position for speakers that have been aimed optimally toward the listener, we studied measurements taken at thelistening positions for those speakers having their acoustical axis
directed toward the listening position
The median notch depth is 14.2dB, but 30dB notches are not uncommon (Fig. 28).
In our material the most typical notch frequency (Fig. 29) is 100Hz,
but deeper notches appear at higher frequencies (Fig. 30).
It is important to note that our data contains information of 10 deepest notches in each impulse response measurement, not all notches.
As can be seen, also notches up to 6dB level are included, but thisshould not be taken to mean that all notches up to this level are
invariably included. Some rooms show deeper notches, and the lessdeep notches come simply from better rooms.

Fig. 29. Notch frequency distribution, bin size is 20Hz (N = 250).

MAGNITUDE RESPONSE MATCH
In order to look at the magnitude response matching after calibration, we have studied those 105 rooms of the total of 164 rooms that
had speakers with acoustical axis aimed at the engineer's position.
The total number of speakers measured at and aimed toward theengineer's position was 250.
The pair-wise comparison included 243 speakers with eight fivechannel surround setups (40 speakers), 15 L/C/R triplets (45 speakers) and 79 L/R stereo pairs (158 speakers).
Three of the remaining seven speakers were single speakers, implying that other speaker(s) in the room have not been measured ator aimed toward the engineer's position. These speakers have been
excluded because there was no pair to match.
Four of the remaining speakers have been measured in a fivechannel surround room but the surround speakers have not beenmeasured at the engineer's position, and therefore this set of speakers was excluded from the pair-wise comparison.
Left-Right Pairs
The left-right stereo pairs in two-channel rooms (Fig. 31, number ofrooms N=79) show a very good agreement.
The 50% bounds are about ±2dB above 1kHz and ±4dB below thatfrequency. The 90% distribution limits are ±4dB for f > 150Hz,±8dB for 50Hz < f < 150Hz.

Fig. 30. The scatter of notch frequency versus gain (N = 250).
The agreement is better than that of the L/C/R triplets when the
responses are compared to the center channel, probably because
typical installations are symmetrical for the Left-Right pair whereas
the center speaker usually has differing radiating condition fromthose of left and right front speakers
The typical increase in distribution toward low frequencies is seen.
The speakers used in this study have carefully controlled radiation
characteristics at mid and high frequencies, minimizing room effects, and resulting in improved pair-match except for low frequencies. Despite that 50% of systems show a mismatch of more than
2dB below 1kHz, and a mismatch of this magnitude is likely toaffect auditory imaging, and probably result in reduced sharpness ofstereo imaging.

Fig. 28. Notch depth for 10 deepest notches within a frequency band
50Hz to 1kHz. Size of a bin is 1dB, median notch depth is 14.2dB(N = 250).

Fig. 31. Third octave smoothed sound pressure level difference of
Left-Right pair in L/R stereo systems (N = 79).

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Left-Center-Right Triplets
Left/center/right-triplets (L/C/R) are found as front speakers in
multichannel reproduction rooms using typically two-way systems
as rear speakers.
The 90% distribution for the L/C/R triplets (Fig. 32 and 33, number
or rooms N = 15) increases below 400Hz and above 10kHz.
The 50% distribution is within a ±3dB window for frequency f >1kHz (except for f > 15kHz) and in a 6dB window below that frequency.

IN-SITU MONITORING CONDITIONS

balance is not adjusted as a part of the acoustical calibration due tothe differing operational standards in monitoring rooms included in
the study. Therefore the SL/SR level is not compared with the
L/C/R level. The front channel match is studied separately from therear channels.
In five-channel setups third octave-smoothed responses (Figs. 34,35) of left and right speakers with respect to the center speakershow smaller response deviations than for the left/center/right triplets (Figs. 32, 33). Also the rear left-right difference is small (Fig.
36). The number of rooms in this category is rather small.

Comparing to stereo left-right pair match, the larger distribution is
produced because the comparison is made to the center speaker.
This is valid in the sense that in multichannel systems the center
speaker is receiving an increasingly important role in forming thesound stage.

For the studied five-channel setups the mean distance from themeasurement position to the monitor speakers is the same for all
speakers within few centimeters.

Fig. 32. Third octave smoothed sound pressure level difference ofleft-center pair in L/C/R systems (N = 15).

Fig. 34. Third octave smoothed sound pressure level difference ofleft-centre pair in five-channel surround systems.

Fig. 33. Third octave smoothed sound pressure level difference ofright-center pair in L/C/R systems (N = 15).

Fig. 35. Third octave smoothed sound pressure level difference ofright-centre pair in five-channel surround systems.

Five-Channel Surround Set-Ups
Five-channel surround systems use three-way speakers for all fiveaudio channels. Rooms with such setups are fairly large.
A left-center-right (L/C/R) triplet in the five-channel reproduction
system is calibrated to have a proper subjective balance during
acoustical calibration. Also the rear left-right (SL/SR) speaker pairis calibrated to correct subjective balance. However, the front-rear

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IN-SITU MONITORING CONDITIONS

Horizontally, the speaker acoustical axis is typically oriented toward
the listening position. This is also mostly recommended in the literature [1,2,5,8] although there is even advice to direct the acoustical axis past the listening position [9], forcing monitoring with offaxis response. Even with modern constant directivity design approaches monitor loudspeakers are typically optimized for on-axis
response, and off-axis response can be significantly degraded.
Speaker size or front baffle size is not discussed as a parameter
affecting speaker placement in recommendations although it isrecognized that large speakers may have to be placed high. Current
recommendation of 1.2m height for the acoustical axis can lead tonotches in the 80...120Hz frequency region, causing a deteriorationin the bass frequency response.

Fig. 36. Third octave smoothed sound pressure level difference ofrear left-right pair in five-channel surround systems.

DISCUSSION
Loudspeaker Geometry
There is a significant discrepancy between the recommendations forspeaker placement for surround monitoring and what actually happens.
The widely accepted reference configuration for a 3/2 format surround sound reproduction places the Left and Right speakers at ±30o
and the surround speakers at ±100…120o relative to the Center
speaker. All speakers are on the same horizontal plane at the engineer's ear height (nominally 1.2 meters) [1,2,5] or at maximum 10
degrees inclined above this level [3,4] at an equal distance of 2…4m
[1,2,5,8]. Some recommendations allow the rear speakers to be
inclined more, up to 15 degrees [1,5,8].
The room size is discussed [3,8] but exact location of the speaker set
in the room is not discussed, although a minimum distance toneighboring walls is given and the aim of a symmetric placement isstressed.
The recommendation for speakers to be placed at least 1m away
[4,8,9] from walls appears too optimistic. In that case, if the wallbehind a speaker has insufficient absorption, a quarter wavelength
notch at 86Hz can be produced as a result of a reflection off the wallbehind the speaker, causing severe irregularity of a free-standing
system bass response.
Increasing the distance brings down the notch frequency. A minimum distance in concert with the recommendation for the operational curve to extend down to 50Hz would be 2m, demonstrating
the value of flush mounting even for small two-way monitor systems. Providing sufficient absorption to remove a wall reflection at
low frequencies can be difficult and expensive.
Listening distances for three-way systems range from 1.2 meters to
4.2 meters. An average listening distance is 2.5 meters, agreeing
well with recommendations. However, in measured rooms theacoustical axis of three-way speakers is rarely placed at the recommended 1.2m height.
The acoustical axis is typically elevated (pointing downwards to theengineer's position) up to 15 degrees. Most large rooms have monitors placed above the listening height with the bottom of the speakerat the height of 1.2m, bringing the acoustical axis 0.5…1m above
the listening level. This reduces problems with low order floor
reflections and is particularly relevant to flush-mount speakers witha large front baffle.

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There is also some discrepancy concerning the surround speakerheight. EBU recommends [4] the same height and vertical tilt forsurround and front speakers while other recommendations [1,5,8]
allow only surround speakers to be placed higher. In our material
surround speakers are frequently placed higher than front speakers.
Very few rooms have five identical speakers in a surround setup.
Rear speakers are typically not of the same type as front speakers.
Reverberation Characteristics
Most rooms show reverberation time that conforms to present recommendations for high quality monitoring rooms.
The decay properties of monitoring rooms are typically not similar
to those of larger rooms, but show a reflection-free time zone andafter that an evoking dense reflection field, requiring a careful consideration of the measurement of reverberation time for thesespaces.
An increase in the reverberation time can be seen when strong diffusion is used. Insufficiency of bass trapping in some rooms is indicated by an increase in the reverberation time distribution limits
toward low frequencies. On the other hand, some rooms exhibitedvery exactly constant reverberation time over the whole frequency
band, including low bass frequencies. This indicates that there is
still work to be done to increase the quality of monitoring roomdesign.
Notches below f = 1kHz
Most of the notches found in our study are produced by low orderreflections. As we look at the notch distribution and spread, we see
notches around 100Hz more frequently than on other frequencies.
The reflections can be very strong, essentially the same level as theaudio signal, producing notch depths up to –40dB.
Room modal resonances begin to dominate listening location notch
characteristics below 200Hz where the wavelength typically becomes large compared to size of objects in a room. At higher frequencies notches are created because of low-order reflections anddiffractions in the room.
The reasons for notches in the frequency response are typically –
•
•
•
•

•

First order floor reflections.
Interaction of the console size and shape with the first orderfloor reflection.
Incorrect front wall design, e.g. discontinuities in the front
wall, such as non-aligned windows, large TV screens or cavities and recesses.
In case of a soft front wall, if front wall bass trapping is not
sufficient, LF notches can be created due to reflections fromthe hard wall behind a speaker. This creates a quarterwavelength notch at the frequency corresponding to the distance from the speaker front baffle to the hard (front) wall behind the speaker. This distance can be larger than the depth of
the speaker cabinet.
With free-standing monitors, notches are typically createdbecause of a reflection from the (front) wall behind a monitor.

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Sustained standing waves or modes in a room can cause severenotching and irregularity in the response at the mix position.

Magnitude Response
The magnitude response of an individual monitor loudspeakershould be flat to within ±2dB in anechoic conditions using thirdoctave smoothing. All loudspeakers included in this study fulfill thisrequirement.
In-situ frequency response measured at the engineer's position using
third-octave smoothing should be flat within ±3dB from 50Hz to
16kHz with some level reduction allowed at high frequencies [3,8].
In multichannel rooms, the sound pressure level 50% distribution
limits indicate a good frequency response control above 1kHz but
an increasing magnitude response distribution below this frequency.
Opposite to this, the 50% distribution limits in stereo rooms remain
consistently within ±3dB bounds also at low frequencies.
The 90% distribution limits of magnitude response indicate thatsome rooms have problems in their frequency response below 1kHz.
This is primarily a product of room design and loudspeaker installation. There are still frequent failures in low frequency design ofmonitoring spaces and the management of low order (early) reflections.
Most large modern control rooms can achieve adequate low frequency damping and have a properly designed acoustic treatment
enabling a very high quality monitoring. Small control rooms with
free-standing monitor systems and compromised acoustical treatment can exhibit large variations in low frequencies.
Magnitude Response Match
According to recommendations the magnitude response difference
between front loudspeakers in anechoic conditions should be lessthan 0.5dB within 250Hz…2kHz [8]. Monitor speakers included in
the present study fulfill this requirement. Random loudspeaker pairmatch after factory calibration is small enough to render production
tolerance insignificant to the pair match measured in this study.
The measured median difference is larger than 2dB for all cases
studied within this frequency band. The L/C/R triplets show a median difference of 4dB. This demonstrates that room effects play animportant role in determining the in-situ frequency response.
The pair match of the stereo pairs and surround left-right pairs istypically better than that of L/C/R triplets. One reason for this is thefact that the Center channel is exposed to different radiating conditions than the Left and Right speakers, and the match was calculated
by comparing the Left and Right speakers to the Center speaker.
The Left and Right speakers have very similar and symmetricalradiation conditions in modern monitoring room designs while it
may be very difficult, if not impossible, to design similar radiating
conditions for the Center speaker being typically in the middle of
the room. Many installations have large objects such as computer
screens, furniture, racks etc. placed centrally near the engineer's
position. These objects create strong reflections resulting in comb
filtering in mid and high frequencies.
In the case of L/C/R triplets we typically have two-way speakers assurround speakers. This implies smaller rooms and smaller listening
distances. Many small 5.1 rooms have non-ideal layout and equipment positioning. This is displayed in the L/C and R/C pair matchfor the L/C/R triplets where mid and high frequency distribution ishigher than in the five-channel systems.

AES 111TH CONVENTION

IN-SITU MONITORING CONDITIONS

The five-channel systems included in the study use three-way
speakers also as surround speakers, implying large monitoring
spaces. The directivity control of the waveguide structures incorporated in the three-way speakers is apparent in measurements of the
five-channel setups. The pair match of the L/C and R/C pairs is very
good above 500Hz demonstrating minimal low order reflections at
the engineer's position.
The present study can not answer questions regarding the psychoacoustic significance of mismatch in sound level between the
speakers, but it is clear that this mismatch becomes an increasingly
acute problem in the future with fast spreading of multichannel
audio. New approaches are necessary in monitoring room design toproduce rooms capable of accurate reproduction. Better control of
directivity in the loudspeaker may also decrease problems in poorly
designed environments, but will never be a substitute for a carefully
designed room. Flush mounting proves once again a valuable
method of decreasing low frequency problems due to reflections off
the nearest walls.
References
1.
ITU Recommendation ITU-R BS.775-1, "Multichannel
Stereophonic Sound System with and without an accompanying Picture", Geneva 1992-1994.
2.

ITU Recommendation ITU-R BS.1116-1, "Methods for theAssessment of Small Impairments in Audio Systems Including Multichannel Sound Systems", Geneva 1994.

3.

EBU Document Tech. 3276-1998 (2nd ed.), "Listening Condition for the Assessment of Sound Programme Material:
Monophonic and Two-Channel Stereophonic", 1982.

4.

EBU Document Tech. 3276-1999, "Supplement 1: Multichannel Sound", 1999.

5.

Surround Sound Forum Recommended Practice SSF01/98,"Listening Conditions and Reproduction Arrangements forMultichannel Stereophony", http://www.tonmeister.de/
foren/surround/surround.htm, 1998.

6.

Surround Sound Forum Recommended Practice SSF-02/1-E2, "Multichannel Recording in 3/2 Format", http://www.
tonmeister.de/foren/surround/surround.htm, 1999.

7.

ISO 3382, "Acoustics – Measurement of the reverberation
time of rooms with reference to other acoustical parameters",
International Standards Organization, Geneva, 1997.

8.

AES Technical Committee on Multichannel and Binaural
Audio Technology (TC-MBAT), "Multichannel SurroundSound Systems and Operations", Technical Document (version 1.5), 2001.

9.

AES Recommended Practice for Professional Audio, "Subjective Evaluation of Loudspeakers", AES20-1996, publishedin J. Audio Engineering Soc., Vol. 44, No. 5, May 1996.

10.

M. Karjalainen, P. Antsalo, A. Mäkivirta, T. Peltonen, and V.
Välimäki, “Estimation of Modal Decay Parameters from
Noisy Response Measurements”, Proc. AES 110th Convention, Amsterdam, The Netherlands, 2001 May 12–15, preprint 5290.

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