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40 EBU Technical Review Winter 1997
Hoeg et al.
Subjective assessment of audio quality
– the means and methods within the EBU
W. Hoeg (Deutsch Telekom Berkom)
L. Christensen (Danmarks Radio)
R. Walker (BBC)
1. Introduction
The existing EBU Recommendation, R22 [1],
states that “the amount of sound programme ma-
terial which is exchanged between EBU Members,
and between EBU Members and other production
organizations, continues to increase” and that
“the only sufficient method of assessing the bal-
ance of features which contribute to the quality of
the sound in a programme is by listening to it.”
Therefore, “listening” is an integral part of all
sound and television programme-making opera-
tions. Despite the very significant advances of
modern sound monitoring and measurement
technology, these essentially objective solutions
remain unable to tell us what the programme will
really sound like to the listener at home. The
human ear alone is able to judge the aesthetic or
artistic quality of programme material and, in-
deed, certain aspects of the technical quality as
well.
This article presents a number of
useful means and methods for the
subjective quality assessment of
audio programme material in radio
and television, developed and verified
by EBU Project Group, P/LIST.
The methods defined in several new
EBU Recommendations and
Technical Documents are suitable for
both operational and training
purposes in broadcasting
organizations.
An essential prerequisite for ensuring a uniform
high quality of sound programmes is to standard-
ize the means and methods required for their as-
sessment. The subjective assessment of sound
quality has for a long time been carried out by
international organizations such as the (former)
OIRT [2][3][4], the (former) CCIR (now ITU-R)
[5] and the Member organizations of the EBU
itself. It became increasingly important that com-
mon rules for subjective sound assessment should
be specified and, consequently, the EBU set up
Project Group P/LIST to develop tools for the sub-
jective assessment of sound programme quality.
These tools are described in several EBU Recom-
Original language: English
Manuscript received 1/12/97.
41EBU Technical Review Winter 1997
Hoeg et al.
mendations, Technical documents and other
printed material.
2. Listening conditions
2.1. General
Typically, the monitoring of programme material
in sound production and broadcasting is done by
listening in a certain room using loudspeaker pre-
sentation. (Listening by headphones is also used
in certain cases, but is not covered in this article.)
It is self-evident that both the acoustics environ-
ment and the electro-acoustic properties of the
loudspeakers must be controlled, in order to allow
consistent subjective assessments to be made.
The main components of the reproduced sound
field are the direct sound, the early reflections and
the later reflections which form the reverberant
field. All these components are time- and fre-
quency-dependent.
The following is a brief summary of the parame-
ters and other requirements for loudspeaker pre-
sentation, as specified in EBU document Tech.
3276 [6]. They also largely meet the requirements
given in ITU-R Recommendation BS.1116 [7].
2.2. Requirements of the reference
sound field.
Direct sound
The quality of the direct sound is mainly deter-
mined by the relevant loudspeaker parameters, as
measured in anechoic conditions (see Section
2.3.).
Early reflections
The levels of reflections earlier than 15 ms rela-
tive to the direct sound should be at least 10 dB
below the level of the direct sound for all frequen-
cies in the range 1 kHz to 8 kHz.
Reverberation field
The reverberation field should be sufficiently dif-
fuse over the listening area to avoid perceptible
acoustical effects such as flutter echoes.
The nominal reverberation time (Tm ) for the
1/3-octave bands from 200 Hz to 4 kHz is found as
follows:
Tm � 0.25�(Room�volume���Ref.�volume�(100))
1
3
Tm should lie in the range: 0.2 < Tm < 0.4 s
As a function of frequency, the reverberation time
(T) should conform to the tolerances shown in
Fig. 1.
Operational room response curve
The tolerance limits for the operational response
curves, measured at any point in the listening
room, are given in Fig. 2. Lm is the mean value of
the 1/3-octave bands from 200 Hz to 4 kHz. The
tolerances should be met for each channel sepa-
rately. For stereophonic reproduction, the close
matching of the room response of each channel is
important.
Listening level
For pink noise at the “alignment signal level”, the
gain of each loudspeaker channel is adjusted so
that the sound pressure level at the reference lis-
tening point is:
LLISTref � 85� 10 log(n)�dB(A)
where: n = number of reproduction channels
in the total configuration.
–0.05
0.05
0
0.1
0.2
0.3
–0.1
Tm
Frequency (Hz)
63
100 1000
4000 8000
10000
200
D
iff
er
en
ce
 in
 r
ev
er
b.
 ti
m
e 
(T
 –
 T
m
)
3
0
–3
–6
R
el
at
iv
e 
le
ve
l (
dB
)
Lm
1 dB per octave
50
100 1000
2000
10000
16000
Frequency (Hz)
Figure 1 (upper)
Tolerance limits for
the reverberation
time.
Figure 2 (lower)
Tolerance limits of
the operational room
response curve.
42 EBU Technical Review Winter 1997
Hoeg et al.
The measurement signal is available from the
EBU tape of R-DAT Levels [8].
Background noise
The sound pressure level (SPL) of the continuous
background noise should not exceed the ISO
Noise Rating Curve, NR 15, and should prefer-
ably not exceed NR 10. It should not be percepti-
bly impulsive, cyclical or tonal in nature.
2.3. Geometrical conditions for the
listening arrangement
Positioning of sound sources and listeners
The height of the acoustical centre of the loud-
speaker monitor should be at least 1.2 m above
floor level and the inclination angle of its refer-
ence axis in relation to the horizontal plane should
not exceed 10°. The monitor’s reference axis
should intersect the reference listening point at the
height of the ears of a seated person.
If the loudspeaker monitor is not installed into the
wall, the distance of its acoustical centre from the
surrounding walls should be at least 1 m. All lis-
tening positions should be situated at least 1.5 m
from the side walls and the back wall of the room.
Stereo listening
Two loudspeaker monitors should be placed in the
listening room for two-channel stereophonic re-
production according to the layout given in Fig. 3.
The base width, b, should be within the range 2 to
4 m.
Separate bass loudspeakers
If separate bass loudspeakers are used, the opti-
mum cross-over frequency between the bass and
the main loudspeakers depends on many factors,
including the positions of the loudspeakers in the
room, the room acoustics and the desired overall
frequency response. To prevent the separate bass
source locations from becoming perceptible,
lower cross-over frequencies (between 80 and
160 Hz) will be required for bass loudspeaker
positions which are located further from the main
loudspeakers (for more details, see [6]).
Room dimensions
The minimum floor area should be:
– 40 m2 for a reference listening room;
– 30 m2 for a high-quality sound control room.
The volume should not exceed 300 m3.
The following limits for the length-to-height and
the width-to-height ratios should be observed:
1.1w / h � l / h � 4.5w / (h – 4)
l < 3h
w < 3h
where: l = larger dimension of floor plan,
irrespective of orientation;
w = shorter dimension of floor plan,
irrespective of orientation;
h = height.
Ratios of l, w and h which are within �5% of inte-
ger values should be avoided.
Careful design and good workmanship in the
construction of listening rooms can greatly en-
hance the acoustic environment. Additional de-
sign considerations are given in [6].
2.4. Requirements of the
monitoring loudspeakers
Frequency response curve
The frequency response curve is measured in
1/3-octave bands with a pink noise test signal. The
measurements are taken on the main axis

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