30th TONMEISTERTAGUNG – VDT INTERNATIONAL CONVENTION, November 2018

Is single microphone position enough for immersive system
equalization and level calibration in production monitoring?
A. Mäkivirta1, T. Lund1
1

Genelec Oy, Iisalmi, Finland. Email: aki.makivirta@genelec.com

Abstract
Immersive monitoring systems contain a large number of loudspeakers, each strongly and differently influenced
acoustically by the room boundaries. This creates uncertainty regarding the actual in-room frequency response, resultsin large acoustical differences between individual loudspeakers, and can reduce the transparency and reliability ofmonitoring on such system. The aim of this work is to demonstrate this influence and then study the importance ofusing multiple measurement microphone positions instead of one measurement microphone position when preforming
in-room frequency response calibration and system alignment. An NHK 22.2 immersive monitoring setup
demonstrates the variability typical of immersive layouts, and the influence individualized in-room monitoring
loudspeaker equalization can bring. After this, various spatial distributions in the vicinity of the main listening locationin 12 rooms are compared to the typical situation of using single microphone location at the listening location. Whenusing equalization to remove response differences between loudspeakers, no significant differences were foundbetween single point measurements and spatial averages for small spatial averaging displacements in rooms having
low reverberation times.

1. Introduction
Loudness-based production in broadcast relies on accuratemonitoring for judging balance, formats, speech
intelligibility, and audio quality. Reproduction systems formonitoring immersive audio presentations [1],[2] sharecertain assumptions about the monitoring system setup. These
include flat frequency response at the listening location, thesame level and time of flight from all monitors at the listening
location, and normalized monitoring level [3],[4]. Roomrelated acoustic effects modify frequency responses ofloudspeakers. For immersive monitoring layouts, theseloadings are typically unique to loudspeaker locations.
Because of this, loudspeakers for critical monitoring are
equalised to reduce the acoustical room influence to improveaccuracy in high quality professional listening [5].
The goal in professional monitoring is to hear the audio
recording content without change. The monitoring system, theloudspeaker in the room, ideally approximates an allpass
system within the audible frequency range.
|𝐻(𝑒 𝑗𝜔 )| = 1{−

d arg{ 𝐻(𝑒 𝑗𝜔 ) }dω= const.

(1)

Then, the monitoring system does not change the spectral
content and merely causes a constant delay at all frequencies
while delivering the audio for the engineer’s ears.
Recommendations consequently call for flat in-room responseat the listening location, constant reverberation time in theroom for audible frequencies, and sufficiently short room
reverberation to not interfere excessively with the direct
sound radiated by the loudspeaker. Minimum-phase
implementations of loudspeakers lead to group-delay
variation which, however, can be compensated to fulfil
Equation 1 [6]. As loudspeakers are three-dimensionalradiators, recommendations for directivity characteristics arealso given, e.g. [5].

The obvious method for improving the sound colour at thelistening location is electronic equalization. This is not a newidea. H. Tremaine’s Audio Cyclopedia claims that RCA’s
John Volkman was the first to use electronic filters for room
response equalization already in the 1920s [7]. Adjustablefilters for room equalization have been available in thevacuum tube era (e.g. [8]). Parametric equalizer filters wereintroduced in the 1970s [9],[10] and digital signal processing
has made both measurements and room equalization easy andprecise.
While there is a consensus that a neutral loudspeaker has a flat
anechoic frequency response, some researchers havequestioned the suitability of the allpass target for the in-room
frequency response, suggesting a down sloping character
across the audio band. However, this largely seems to be
motivated by listener preference [11],[12],[13],[14],[15],[16],[17],[18] while possibly too little attention has been paid
to solving the problem with the ‘circle of confusion’. Thisrefers a self-referenced system where existing recordings areused to evaluate the room sound [18],[19].

1.1. One or more measurement locations
The present work concentrates on enabling reliablemeasurement of the in-room frequency response at thelistening location, but not on the preferred in-situ response,
which we regarded a separate issue.
The sound pressure measurable in a room at a single
microphone location is local. At high frequencies the typicalnotching due to multi-way propagation and reverberation inrooms particularly at high frequencies varies when the microphone location changes. This effect is usually reduced byapplying in-frequency smoothing with a sliding variablewidth averaging window to the frequency response. Part ofthe late reverberation may also be rejected using a time
domain window on the impulse response but this leads to loss

30th TONMEISTERTAGUNG – VDT INTERNATIONAL CONVENTION, November 2018of low frequency information. Frequency smoothing reduces
acoustic comb filtering and locality of measurement while
extracting subjectively important low Q colorations in the
midrange frequencies.
Spatial averaging of responses at multiple microphone
positions can extract the common frequency response features
visible in the measurement points. Spatial sampling of theroom response in vicinity of the listening location has beensuggested to improve the reliability of measurement and isassumed to produce a better representation of room acoustics
over single point measurement, and thereby be more reliable
starting point for system equalization.
Spatial sampling is done with several motivations. In thecontext of large seating area applications, such as cinema
theatre equalization, even spatial sampling across the floorarea aims to accomplish the best average equalization [5].
Similarly, spatial sampling can create information about the
average coloration within an area, such as the large mixing
console.

both parts is to demonstrate the magnitude response at thenominal listening location. The study has been conducted in
the context of professional monitoring spaces, typicallyhaving relatively low reverberation time and low level of theearly reflections at the listening position.

Figure 1a. Method of suspending the upper layer monitors.

1.2. Immersive audio
Immersive reproduction systems are intended to create aspatially stable presentation of at least the frontal virtual
sound images for a wide seating area [20] by having a large
number of real loudspeaker sources correctly localizable at all
seating position. Virtual sound images in immersive
reproduction systems are created using two, three or more
loudspeakers together, and localize the virtual images within
the area span by these speakers, even if location error occurs
for off-centre listening positions.

Figure 1b. Method of installing the lower layer monitors.

The combination of equalizing individual loudspeaker
responses and careful alignment of playback levels and delays
can reduce acoustic differences and can improve virtual soundimage precision. Controlled in-room spectral response is thefoundation of meaningful level calibration [4].
Immersive reproduction systems place loudspeakers on
multiple height layers around the listener. The acousticalinfluence of the room differs more than for stereophonic orsingle-layer multichannel (surround) layouts, where it iseasier to design room acoustics affecting the monitors to be
similar. Increasing the number of height layers andloudspeaker channels complicates the acoustical playback
system and increase acoustical differences between monitors.

Figure 1c. Side view to the 22.2 immersive speaker layout.

1.3. Paper structure

2. Case study

The purpose of this work is to study the sufficiency of thesingle microphone position acquisition of the acousticalresponse at the listening position. Frequency domainsmoothing (averaging in frequency) may in itself be able toincrease spatial stability in a measured frequency response forfeatures relevant to subjective experience of sound colour andfor compensating the sound colour to reduce such colorations.
First, a case study of the effects of electronic room calibrationof an immersive NHK 22.2 system is presented. Then,
research reported partially in [21] and [22] is revisited andadditional analysis is provided. This study uses stereo
loudspeaker setup in several monitoring rooms to study thebenefit of spatial averaging for room calibration. The aim inA three-layer immersive monitoring system was set up
according to the NHL 22.2 format. The installation room
reverberation time in mid frequencies (f > 250 Hz) is 0.25 s
on average and at low frequencies (f < 200 Hz) 0.5 – 0.8 s,
increasing towards low frequencies. The room has an isolated
prominent modal resonance at 44 Hz.
The loudspeaker system width is 3.22 m (from FL to FR, as
well as from BL to BR). The channel name abbreviations areaccording to the SMPTE 2036-2 [2],[20]. The room height is
2.5 m, width 5.56 m and length 7.2 m. The listening position(measurement position) is 2.8 m from the left and right walls
and 2.70 m from the front wall. The microphone height is 1.2
m. The upper layer monitors are suspended on rails attached30th TONMEISTERTAGUNG – VDT INTERNATIONAL CONVENTION, November 2018
to the ceiling (Figure 1a-1c). This places the upper layer
loudspeaker front at 2.16 m height. The median distance toloudspeakers is 2.28 m, ranging from 0.96 m (TpC, roomheight limitation) to 3.01 m (TpFL). The time-of-flight (ToF)
variation range was 4,97 ms (5,96 ms when TpC is included),
see Table 1.

Table 1. Loudspeaker arrangement for the case example.
Loudspeaker distance (DIST) and distance-related time-offlight difference (DTOF) are given.

lower

DIST.
(cm)
211

DTOF
(ms)
2.62

ROLE

TYPE

LAYER

BtFC

8330A

BtFL

8331A

lower

257

1.28

BtFR

8331A

lower

261

1.16

BC

8250A

middle

130

4.97

BL

8240A

middle

208

2.70

BR

8340A

middle

209

2.67

FC

8330A

middle

213

2.56

FL

8330A

middle

272

0.84

FLc

8351A

middle

230

2.06

FR

8330A

middle

265

1.05

FRc

8351A

middle

230

2.06

SiL

8430A

middle

248

1.54

SiR

8430A

middle

226

2.18

TpBC

8320A

upper

139

4.71

TpBL

8330A

upper

223

2.27

TpBR

8330A

upper

214

2.53

TpFC

8330A

upper

247

1.57

TpFL

8330A

upper

301

0.00

TpFR

8330A

upper

292

0.26

TpSiL

8320A

upper

233

1.98

TpSiR

8320A

upper

214

2.53
5.96

TpC

8320A

upper

96

Sub L

7380A

sub

280

Sub R

7370A

sub

280

Figure 2. All response before (grey) and after equalization(black), aligned at 0.5 – 2 kHz and staggered by 25 dB to aid
visibility.

Loudspeakers were measured at the listening position. In each
loudspeaker, 6…10 second-order parametric notch filters andtwo low-frequency shelving filters were optimized for
frequencies below 2 kHz, to reduce peaking above thereference level (mean level on log(f) scale in the range 0.5 –2 kHz).
The outputs were adjusted to give the same mean output at all
speakers. The times-of-flight were set the same using
electronic delays in speakers.
Figure 3 presents shows the bi-smoothed loudspeaker
responses (1/24 smoothing f < 200 Hz, then 1/3 octave). The
1/24 octave smoothing gives more detailed presentation of the
SPL variation due to room modes at low frequencies. There,
large level variation is typical due to sparse modal resonances
compared to higher frequencies.
The maximum and mean levels recorded (Fig. 5, N=22) havebeen reduced. The audibility of mid and high Q peaks ishigher than similar notches [23],[24],[25]. Equalizing thepeaks reduces audible colorations and thereby renders
loudspeaker responses more similar.

30th TONMEISTERTAGUNG – VDT INTERNATIONAL CONVENTION, November 2018
The equalization does not significantly change the amount oflevel shifting needed to bring the loudspeaker levels inalignment to the reference levels (Fig. 4).
The magnitude equalization reduces values exceeding thereference level, seen in reduction of the maximum and median
response level (Fig. 5).
Looking at the pooled values exceeding the reference for all
monitors (N=22), before equalization the level distribution
shows a higher incidence of large deviations toward highpressure from the reference level (Fig. 6) than after theequalization (Fig. 7).
The case example demonstrates typical acoustical effects andlevel shifting seen in realistic, slightly acoustically suboptimal
monitoring rooms. Equalization, level alignment and time-offlight alignment can reduce acoustical differences and help
create a predictable and reliable monitoring system.

Figure 3. Minimum, maximum and mean responses afteraligning the mean value calculated in log(f) in the range 0.5 –2 kHz (N=22). The level scale is relative. The responses havebeen smoothed with a 1/3 octave window above 200 Hz andwith a 1/24 octave window below this frequency.

Figure 5. Histogram of the values exceeding the reference
level before equalization (N=22).

Figure 6. Histogram of the values exceeding the reference
level after equalization (N=22).

3. Sufficiency of single measurement
3.1. Method
This section studies the ability of single position magnitude
response measurement to render essentially the same
information as spatial averaging. Methodology, choice ofrooms and measurement data set used in this study has beenreported in [21].
The measurement microphone has an electret capsule withomnidirectional characteristics. The main microphone
position and 17 offset positions located at increasing distances
are measured. The offset distances are 0.1 meters to the front,
0.1, 0.25, and 0.5 meters to the side and back, and 1.5 meters
to the back, see Figure 1. Two-way loudspeakers were used
(5 in woofer, 2/3 in tweeter, type Genelec 8330 or similar).

Figure 4. The median and maximum of the values exceeding
the reference level, before and after calibration.

The impulse response is measured using a log sine sweep withlength 256 kilo-samples (2.97 s, fs = 44.1 kHz). Two sets of
18 measurements are taken for each room. In total, 432
measurements in 24 series are taken in the study.

30th TONMEISTERTAGUNG – VDT INTERNATIONAL CONVENTION, November 2018

Figure 7. The principle of the loudspeaker layout follows thestandard stereo pair placement. The main microphone
position is marked.

Each spatial average response is calculated using the 1/3
octave smoothed data. All measurements in the spatial
average receive the same weight. The weight of an individual
measurement depends on the number of measurements
included in the spatial average. After smoothing, thedifferences of the spatial averages to the single microphone
position set at the listening location are calculated. This
results in three sets of difference pressure graphs for eachroom, one for each spatial averaging case.
The mean pressure within an octave is calculated, resulting in
11 octaves from 16 Hz to 16 kHz. This results in 24 data
points for each octave.
The spatial averages are compared to the single microphone
position measurement. Measurements in all rooms and for all
speakers are pooled. Box-and-whiskers plots are generated foreach octave and for each spatial averaging distance. These
describe the variability of deviation in the pressure from thesingle microphone position, in the octaves across rooms andloudspeaker position.

Case A

Case B

Case C
Figure 8. Definitions of the spatial average cases A to C
showing the spatial averaging areas (shaded) and the numbersof measurement positions used (dark colour).

The magnitude response measurements are smoothed tostandard 1/3 octave and 1/12 octave. Differences in distances
are removed by level-normalizing responses. Measurements
are therefore normalized in level using the mean response
level across the 500 Hz to 5 kHz. The impulse responses areprocessed to generate (1) the main microphone position alone,(2) main position response averaged with the microphone
responses at 0.1 m distance offset, (3) main response averaged
with the responses at 0.25 m offset, and (4) main response
averaged with the responses at 0.5 m offset.

room

vol.
(m3)

base
width
(m)

RT60 (s)

TER
(ms)

LER
(dB)

L

R

L

R

A

29.6

1.30

0.49

8.9

2.7

-11.9

-11.8

B

115.1

1.30

0.66

1.5

1.7

-10.5

-10.3

C

39.5

1.17

0.43

8.6

8.7

-10.8

-14.0

D

39.5

1.32

0.46

2.4

2.5

-13.5

-13.2

E

39.5

1.20

0.44

3.3

3.3

-15.9

-16.7

F

16.6

1.15

0.31

0.85

0.85

-10.5

-11.8

G

97.6

2.27

0.19

5.6

5.7

-10.8

-11.7

H

22.6

0.86

0.50

4.0

3.7

-16.7

-17.7

I

18.2

1.01

0.56

5.9

7.5

-17.2

-12.2

J

69.4

1.30

0.55

4.6

11.0

-14.8

-20.9

K

80

1.31

0.48

4.1

4.2

-17.2

-16.7

L

90.5

1.31

0.48

4.6

6.2

-20.9

-20.0

Table 2. Listening rooms used in the study. Reverberation
time (RT60), highest early reflection level (LER) and theassociated early reflection delay (TER) at the listening
location for left (L) and right (R) loudspeakers.

3.2. Octave decay time in rooms
The twelve rooms share low background noise, acceptablereverberation time, and reasonably low level and acceptabledirection of early reflections at the listening position (seeTable 1).
The 12 rooms included in the study were evaluated for octave
decay time. Three measurements were taken, at the reference
location, 0.5 meters to the right and 1.5 meters to the back of
the reference location.
The octave decay is compared to the recommendations [5],Fig. 9 and 10. The rooms fit recommendations for high30th TONMEISTERTAGUNG – VDT INTERNATIONAL CONVENTION, November 2018resolution listening location in terms of the median octave
decay time, but the quartile limits exceed slightly therecommended flatness for octave decay time.
Notably, the lowest octave evaluated (63 Hz) shows large
variability and two rooms show large reduction in decay time
at high frequencies. However, the rooms are not untypical of
the acoustic in real monitoring environments [26], Fig. 11,
making the data collected in these environments useful forpractical evaluations of in-room acoustics and the effect of
spatial sampling of responses close to the listening location.

Figure 11. Distribution of decay time in professional
monitoring rooms according to [26].

Figure 9. Distribution of the octave decay time in rooms.
Three measurements were taken in each room.

Figure 12. 0.1 m average vs. single measurement

Figure 10. Deviations in octave decays in each room. Roomaverage decay time in range [200Hz…4kHz] has been scaled
to zero for each room, to enable comparison.

Figure 13. 0.25 m average vs. single point measurement

30th TONMEISTERTAGUNG – VDT INTERNATIONAL CONVENTION, November 2018

spatial average offset
0.1 m

0.25 m

0.5 m

mean

0,074

0,190

0,327

std

0,67

0,89

1,02

min

-3,50

-2,65

-3,00

max

4,30

3,30

4,80

Table 3. Pooled octave band statistics across all rooms for the fullaudio bandwidth [21].
spatial average offset

Figure 14. 0.5 m average vs. single point measurement

3.3. Frequencies f > 500 Hz
Differences to the single microphone position increase withincreasing distance of the offset points.

0.1 m

0.25 m

0.5 m

mean

0,0

0,016

0,011

std

0,16

0,27

0,24

min

-0,45

-0,80

-0,35

max

0,40

0,60

0,70

Table 4. Pooled octave band statistics across all rooms for the octave
bands 500 Hz and higher [21].

The 0.1 m offset case (Fig. 10) shows small differences to thesingle measurement. The median difference remains less than
0.5 dB for all offset cases in frequencies f > 500 Hz. Quartiles,
describing 50% of the deviation, are within ± 0.5 dB.
Extremes (minimum, maximum in octave) remain less than ±1 dB for f > 500 Hz. These are close to the just noticeable limit(JND) [24].
As a conclusion, using single measurement appears to resultin similar equalization as using a spatial average within thespatial offset range not exceeding 0.5 m.

3.4. Frequencies f < 500 Hz

Figure 15. Mean level difference in octave bands, averaged acrossthe audio band for the spatial offset cases 0.1, 0.25 and 0.5 m. The
decay time is the maximum within the frequencies 0.4-4 kHz [21].

For low frequencies f < 500 Hz, acoustic interaction of theroom with the loudspeaker radiation leads to larger roomdependent variability.
Spatial averages taken using smaller offset distance show
smaller variability, as expected. The octave median levels
across the rooms and speakers show good agreement with thesingle measurement position for small displacement spatial
averaging (0.1 m).
For frequencies f > 63 Hz the medians in all spatial averagecases remain in agreement with the single microphonemeasurement to ± 1 dB. The lowest frequency medians (32and 16 Hz) show larger deviation, but even then, the deviation
to single microphone measurement does not exceed 2 dB,
close to the just noticeable limit (JND) [23].
Quartiles range covering 50% of data in each octave remains
within ± 2 dB while extremes (minimum, maximum) are seen
deviating by as much as 6 dB from the single point
measurement at frequencies f < 125 Hz and this happens
mainly toward smaller pressure, indicating that this may berelated to notches in the frequency response at low
frequencies. Notches at low frequencies are known to havepoor audibility [23].

Figure 16. Mean level difference in octave bands 500 Hz and above,
for the spatial offset cases 0.1, 0.25 and 0.5 m. The decay time is the
maximum within the frequencies 0.4-4 kHz [21].

3.5. Significance of spatial averaging
Spatially averaged responses do not deviate significantly fromthe single point measurement at the listening position forsmall spatial averaging displacements (± 0.5 m) except forvery low frequencies. The largest differences between the
single point octave mean levels and the spatial average octave
mean levels occur at low frequencies, in octave bands with

30th TONMEISTERTAGUNG – VDT INTERNATIONAL CONVENTION, November 2018
frequency lower than 500 Hz. The differences are small at 500
Hz and higher frequency octave bands (tables 2 and 3).
When full bandwidth data is considered, a slight trend exists
where increasing distance from the reference position andincreasing reverberation measured inside the octave bands
0.4-4 kHz is linked to larger difference between the single
point measurement and spatial average (Fig. 7) but this trend
is mainly created by low frequency room effects and if octave
bands at and above 500 Hz are considered, the trend
disappears (Figure 8). The reverberation time appears to havea stronger effect on this than the spatial average distance.

4. Discussion
For immersive reproduction systems, controlled in-room
spectral response is the foundation for meaningful loudness
calibration and accurate monitoring [2],[3],[4]. For example,
the ATSC recommended practice A/85 [1] devotes an entire
chapter to monitoring system calibration to dismantling what
they call the ‘circle of confusion’ for audio monitoring.
Reproduction system calibration is the key to achieving this.
We have studied the ability of single position magnitude
response measurement to render essentially the same
information as spatial averaging. The design of the present
research is suitable for modelling typical audio engineering
workflow, where decisions are taken primarily in seatedposition. The spatial average collection microphone position
layout in our work consequently reflects the single engineer
situation. Fixed microphone height at the listener’s ear height
was used, therefore the present work cannot reveal soundcolour stability in vertical orientation although similarfindings can be expected also for vertical direction.

When in-frequency smoothing is applied mostly nonsignificant differences were found in our study between the
single-point measurement and spatial averages taken with
distances less than 0.5 meters for listening rooms similar toprofessional rooms in decay characteristics. Large differences
between a single point measurement and a spatial average areconnected to strong modal resonances in the room, typically
happening at low frequencies where the natural room
absorption is not sufficient to reduce the decay time.
However, at low frequencies the audibility of such mediumto-high Q resonances is much reduced comparing to mid
frequencies. Studio monitoring rooms typically haverelatively short reverberation times, reducing local variationin pressure and further decreasing significance of spatial
averaging.

5. Conclusion
The use of single point equalization appears to be a safe
choice for measurement of studio monitoring rooms typicallyhaving relatively low reverberation times and well controlled
room modes. The use of spatial average is not likely tosignificantly change or improve the outcome of equalizationin the case of equalizing an audio reproduction system for alimited listening area essentially intended for one person.

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In-frequency smoothing is the typical tool used for in-room
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