What do we measure – and hear – in rooms?
by
Floyd E. Toole
and
Allan Devantier
Science and technology
in the service of art
is our business
1
A very quick review of the
science of
loudspeaker/room/listener
interactions.
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EXAMPLE:
A loudspeaker with directivity problems
10
woofer
midrange
tweeter
0
dB
-10
-20
20
ON AXIS
30 DEGREES OFF AXIS
60 DEGREES OFF AXIS
50 100
500 1K
5K
FREQUENCY (Hz)
10K 20K
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Step One: collect data using the
“spinorama”
70 amplituderesponse
measurements
on horizontal and
vertical axes
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Step two: process the data
= on-axis response
= energy sum of
the appropriate off-axis angles for
the type of room being used.
=
total sound power = energy sum of
all sound radiated in all directions.
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Predictions of room events from many
anechoic measurements
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PREDICTED“ROOM CURVE”
10
dB
0
-10
20
DIRECT SOUND
EARLY REFLECTIONS
LATE REFLECTIONS
50 100
500 1K
5K
FREQUENCY (Hz)
10K 20K
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Real vs. Predicted Room
Curves
30
20
dB
From: Toole, JAES 1985
This is not new!
10
0
-10
20
50 100
500 1K
5K
FREQUENCY (Hz)
10K 20K
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There is a region where the room dominates,
and one where the loudspeaker dominates.
30
20
Room
dB
Transition
Loudspeaker
10
0
-10
20
50 100
500 1K
5K
FREQUENCY (Hz)
10K 20K
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For different rooms: fc = Schroeder frequency
Small domestic room: 70 m3, RT= 0.4s,
Room
Transition
fc = 150 Hz
Loudspeaker
Smallest room ST 202: 125 m3, RT=0.2s
fc= 80 Hz
≈ 350 seat cinema: 1900 m3, RT=0.5s
fc = 32 Hz
≈ 800 seat cinema: 4200 m3, RT=0.65s
fc = 24 Hz
20
50 100
500 1K
5K
FREQUENCY (Hz)
10K 20K
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So, for the rooms in which we
are interested:
• The loudspeaker and the screen are the
dominant factors over most of the
frequency range
• In cinemas standing waves are not a likely
problem, except for LFE
• In smaller rooms some means of
controlling low-frequency standing waves
will be necessary – and there are options.
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What makes this so important?
• From comprehensive anechoic data on
loudspeakers it has been possible to:
– Predict room curves with good accuracy, and
– Predict listener sound quality ratings – when
listening in a room – with good accuracy.
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What was learned?
• A flat on-axis response is important.
• Both on-axis and off-axis performance
must be smooth – and similar in shape.
• Smoothness of the curves – i.e. absence
of resonances – yields higher scores.
• 1/20-octave data yield better correlations
than 1/3-octave data. Conclusion: fine
details are audible.
• Bass performance accounts for about 30%
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of the factor weighting.
So, a combination of on- and off-axis
anechoic data can be processed by an
algorithm to yield good predictions of sound
quality.
But, where do room/house curves fit into
this?
Let us examine what it is that we are
measuring.
13
Measurements in a 125 m3 room – the smallest room
included in SMPTE ST 202:2010
Anechoic
spinorama
6-seat
average
room curve
Sound power
alone is not
enough at HF.
We must add
some direct
sound to the
prediction.
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Measurements in a 125 m3 room – the smallest room
included in SMPTE ST 202:2010
Anechoic
spinorama
This loudspeaker,
although imperfect,
has been equaled,
but not surpassed,
in double-blind
listening tests.
6-seat
average
room curve
The targets are not appropriate,
certainly not for a room that cannot accommodate 30 persons
15
A Problem:
A loudspeaker/room combination that is a
good example of the “state-of-the-art” in
sound reproduction would be degraded by
forcing it to meet any of the ISO/SMPTE
target room curves.
16
Measurements in a 125 m3 room – the smallest room
included in SMPTE ST 202:2010
Anechoic
spinorama
This loudspeaker,
is flawed, and it
receives mediocre
scores in doubleblind listening
tests.
6-seat
average
room curve
17
For known environments it is possible to develop simple predictive algorithms.
For typically furnished domestic rooms and home theaters a sum of:
12% listening window (direct sound),
44% early reflections and
44% sound power works well. Different rooms may need different proportions.
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Directivity is a MAJOR factor in
what is measured in a room.
• At low and middle frequencies in-room
curves are dominated by far off-axis,
reflected, sound.
• However at the highest frequencies, the
direct sound is likely to be dominant.
• Let us see how this translates to a large
cinema.
19
This suggests that
all that is needed
is to turn the bass
down and go
20
home
But, we haven’t corrected for screen loss, and that affects the direct sound
from the loudspeaker. We might also wish to compensate for air
attenuation. Both of these are known impediments to good sound from
loudspeakers, and both can be compensated for.
The predicted
(dotted) curve
anticipates the
bass rise
(reduced DI),
but does not
include screen
and air
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attenuation.
When we modify the measured steady-state room curve to correct for
screen and air attenuations, as shown in the top curves, we get an
estimate of the room curve without those losses – that now approximates
the predicted curve, which assumed no such losses.
Where did Curve-X come from?
22
Interim summary:
• A state-of-the-art domestic forward-firing
loudspeaker in a 125 m3 room yields a relatively
flat in-room curve.
• A state-of-the-art (1985) cinema loudspeaker in
a 5660 m3 room, when compensated for the
screen and air attenuation yields a relatively flat
in-room curve.
• Both were predicted from direct sound and
directivity-index data on the loudspeakers.
• The physical, loudspeaker/screen/room, side of
the issue seems to be well understood.
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Why the X curve?
• Based on (undocumented) listening tests done in 1971,
with the limited, and limiting hardware and software of
the day, it was concluded that a high frequency rolloff in
a large room was subjectively preferred. Allen notes:
“The reason for the apparently desirable HF droop is not
very easy to explain”. And it wasn’t.
• Since then international committees have massaged the
“HF droop” to its present forms with no documented
subjective evaluations in support of the decisions.
• If a HF droop is necessary, it needs to be demonstrated
in 2012, with current hardware, software and scientific
methodology.
• At the small room end of the size scale there is no
justification, and there is abundant science to show it.
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What is the most basic information
we need about a loudspeaker?
• The on-axis, preferably the listening window,
performance describing the direct sound. This
should be smooth and flat.
• The sound power radiated by the loudspeaker,
or
• The directivity index (DI) as a function of
frequency. DI is the difference between the
sound power and the direct sound. Either of
these should be smoothly changing or constant,
or a combination of the two.
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In new installations:
• We should be able to obtain these data
from loudspeaker manufacturers and,
• If the loudspeakers are to be located
behind a screen, the screen attenuation
must be known, and
• In large rooms, compensation for air
attenuation should be included – this is in
standard tables.
26
In existing installations:
• We will need to measure:
– Direct sound over an appropriate listening window.
This will include the on-axis screen attenuation.
This curve should be flat and smooth but lowfrequency data cannot be trusted because of
compromised resolution due to windowing.
– Sound power, as modified by the screen, air
attenuation, and absorption at the room
boundaries. Only at lower frequencies can this
steady-state data be trusted; at higher frequencies
corrections will need to be made for at least the
screen and air attenuations. This can be done.
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In a few installations:
• We need to perform controlled, randomized, blind or
double-blind subjective evaluations.
• These evaluations should include program from
known, documented, facilities with traceable
pedigree, otherwise we are trapped in the “circle of
confusion”. Soundtracks from dubbing stages with
500-seat X-curve equalization need not apply.
• They should include music program and speech,
from non-film sources.
• The events should be recorded binaurally, so that
listeners elsewhere can share at least the timbral
essence of the experience. It is the timbre that is at
issue here, not direction and space.
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• Now, moving forward, we need some
thorough 2012 technical data on:
– loudspeakers,
– screens and their interactions with
loudspeakers and baffles,
– cinema acoustical treatments, and
– in-situ measurements of sufficient detail that
they are not fatally contaminated by
acoustical interference effects (i.e. we need
many mics in many locations).
• The following are some suggestions – a
basis for discussions.
29
In-situ measurements – looking
back to the Westwood Bruin.
• Three sets of five measurements
were made = 15 curves.
• But because of symmetry, only 8 of
them were distinctive.
30
In-situ measurements – looking
back to the Westwood Bruin.
• Three sets of five measurements
were made = 15 curves.
• But because of symmetry, only 8 of
them were distinctive.
• If we had decided that the
measurements were to learn about
the loudspeakers, rather than to
measure what we think people are
hearing, we could have changed one
measurement, and acquired 5 more
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distinctive measurements.
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The mic array
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A ≈ 350 seat assymetrical stadium seating venue
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This is the “scientific” phase
• After we have learned what we can, and
have decided on what measurements we
wish to move forward with, we can get
down to simplifying the process, and to
optimizing its application in different venue
sizes and geometries.
• But first things first.
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