Title: Loudspeaker and listener positions for optimal low-frequency spatial reproduction in listening rooms.
1 Loudspeaker and listener positions for optimal
low-frequency spatial reproduction in listening
rooms.
- David Griesinger Location Lexicon, 3 Oak Park
Dr., Bedford, MA 01730-1441
2Main Message(s)
- 1. Low frequency spatiality (envelopment) is
beautiful. - 2. We perceive it through fluctuations in the
apparent azimuth of low frequency signals. - 3. Fluctuations are produced by the acoustics of
large spaces, and not by small spaces. We must
reproduce them from information in the recording. - 4. A suitable recording must contain
decorrelated reverberation in at least two
channels. - 5. Reproducing the perception of envelopment
from a recording requires least two full-range
speakers. - In anechoic space these speakers should be at the
sides of the listener - 6. Perceiving envelopment in a small room
requires that the pressure gradient at the
listening position should be independent of the
pressure. - And that each should be driven by an independent
component of the recorded reverberation - 7. The above conditions are most commonly met
when pressure gradients produced by assymetric
lateral room modes overlap with pressure from
medial modes. - The overlap must be both in frequency and at the
location of the listener. - 8. Room dimensions and loudspeaker placements
are critical. - Their effects can be predicted by diagrams of the
first few room modes.
3Envelopment is beautiful
- Pictured the Deutches Staatsoper, Berlin
- A Lares system has been in constant use for many
years. - The principle benefit of this system is to
increase low frequency envelopment. - The Lares system is not perceived by the audience
as adding to the reverberation. - The system is considered essential for the
production of most operas and all ballets. - With the system on the low frequencies come
alive, giving the audience a feeling of direct
involvement with both the music and the emotion
of the scene.
4Low frequency envelopment is perceived through
fluctuations in azimuth
- There is not time in this paper to discuss the
theory of interaural fluctuations. See the
references. In briefest outline - Reverberation in large spaces is chaotic in its
direction of incidence, fluctuating randomly even
during long notes. - Large spaces are very seldom in steady-state, so
the ITD at the listener is in constant state of
change - We perceive this fluctuation as envelopment the
sense of being inside the sound. - The perception is VERY DIFFERENT from having the
sound inside the head! - Reverberation in small spaces does not fluctuate
- because for most musical notes the reverberation
quickly reaches a steady-state condition. - To reproduce these fluctuations we must first
record them - This is easy.
- Then we must find a way of reproducing them at
the listening position - This is hard
5Reverberation in recordings
- Reverberation in most two-channel recordings is
independent in the two channels - Reverberation is frequently recorded by two
microphones spaced by more than the reverberation
radius - Artificial reverberation (which is frequently
used) is deliberately made such that the two
output channels are completely independent.
6Reproducing Fluctuating ITD
- Fluctuating ITDs are easy to reproduce in
anechoic spaces if the loudspeakers are placed
at the sides of the listeners - Since the reverberant sound pressure is
independent in the two speakers, both the
pressure and the pressure gradient fluctuate. - The pressure gradient is produced by the
difference signal of the two loudspeakers - The pressure is produced by the sum signal
- The pressure gradient and the pressure are
independent of each other, so a random ITD is
produced.
7Anechoic space - standard stereo
- Standard stereo gives little envelopment in an
anechoic space because the speakers are not
lateral. - The pressure gradient is proportional to the sine
of the speaker angle
8Envelopment in small rooms
- Envelopment can be reproduced directly in small
rooms using the inhomogenious (non-modal)
component of the room sound - IF the low frequency loudspeakers are at the
sides of the listeners!
9Envelopment with room modes
- To create envelopment with room modes we must use
the information in the recording to create a
fluctuating ITD at the listening position. - This can only be done by separately driving the
pressure gradient and the pressure at the
listening position. - Lets look at examples
10Large room at HSG at 32Hz
- The large demo room at HSG is 17x23x9.
- The ceiling is highly absorbent.
- The first asymmetric lateral mode is at 32Hz.
- The pressure gradient from this mode (the sound
velocity) is maximum along the front/back center
line of the room. This area is shown in green. - The sound pressure from this mode is a minimum
along this line. So we do not directly hear this
mode
11Large room at HSG at 32Hz and 48Hz
- The front/back mode at 48Hz has a pressure
maximum at the center of the room. (shown in
red) - Note that the 48Hz pressure mode overlaps the
pressure gradient from the 32Hz mode in the
center of the room. - The 32Hz mode is driven by the difference signal
from the loudspeakers, and the 48Hz mode is
driven by the sum signal. - So envelopment will be heard at frequencies
between 32Hz and 48Hz if the Q of these two modes
is low.
12Large room at 96Hz
- There is an asymmetric lateral mode at 96Hz,
which produces areas of high pressure gradient
(green) - There is a front/back mode at 96Hz which produces
high pressure zones (red) - Where the two overlap we will have high
envelopment. (areas shown in blue.) - Note that in this room envelopment is widespread
at frequencies near 96Hz.
13Mixing room at HSG (poor spatiality)
- The mixing room has dimensions of 19x21x9.
The ceiling is absorbing. - The room is nearly square (bad idea)
- At 30Hz the assymetric lateral mode produces a
pressure gradient shown in green. - The 30Hz front/back mode produces the pressure
zones shown in red. - Notice there is NO overlap in the center of the
room which was the listening position. - There is thus no envelopment at these frequencies.
14Mixing room at 84Hz
- There is an asymmetric lateral mode at 87hz, and
a front back mode at 84Hz. - There is no envelopment at the center but there
is envelopment at other positions - But ONLY if the speakers are in the corners!
15Mixing room at 84Hz as used
- Alas, the marketing department does not put the
speakers in the corners. - The speakers look better when they are closer
together. - But this puts them at pressure nulls for the
asymmetric lateral mode. - So NO envelopment is produced at these
frequencies.
16Mixing room at 55Hz
- There are two modes that overlap at 55Hz A
symmetric lateral mode, and a symmetric
front/back mode. - No envelopment is produced because both modes are
driven by the sum component of the stereo signal.
17Mixing room at 55Hz asymmetric listening
position
- If we move the listening position to the left, we
can produce envelopment at 55Hz. - Although we are using the same modes as the
previous slide, now the lateral mode is excited
by the difference signal in the recording. - This speaker arrangement also produces some
envelopment at 30Hz, because the lateral mode is
driven by the left speaker only. - This arrangement is one of the best for this room
but envelopment is still poor compared to the
Large room.
18Mixing room best solution
- The best solution for this unfortunate room is to
put two powerful subwoofers at the sides of the
listening area, and damp the room modes as
completely as possible (either with bass traps or
by electronic equalization.) - Envelopment will be produced through the
non-modal component of the room transfer function.
19Small room the authors studio 32Hz to 48Hz.
- The authors studio is 12x12x9 with a 3x6 bay
window. - The 32Hz asymmetric lateral mode produces a
pressure gradient shown in green. - The 48Hz front/back mode produces the pressure
zones shown in red - There is considerable overlap at the listening
position, and envelopment is high.
20Small room at 45-73Hz
- The symmetric lateral mode at 73Hz overlaps with
a front/back mode at 45Hz. - Normally this would not produce envelopment, but
here the left speaker is near the pressure null
of the lateral mode. So this mode is driven
primarily by the right speaker only, while the
pressure mode is driven by the sum of left and
right. - Some envelopment is audible at the listening
position. - There is also envelopment from the asymmetric
lateral mode at 105Hz.
Overall this is a very pleasant room for music
mixing or listening. Envelopment seems uniform
and high near the listening position.
21Small room conventional speaker positions
- All speaker configurations with the speakers
pointing along the long axis of the room sound
poor in this space. - The first asymmetric lateral mode is at 46Hz, and
the overlap with pressure is poor. - The next asymmetric lateral mode is at 138Hz,
which is too high to give a spacious bass. - Conclusion the speaker positions along the long
wall have advantages for envelopment because they
maximize the overlap between the lateral pressure
gradient and the total pressure.
22Envelopment in the front/back direction.
- All the discussion so far has assumed the
listener is facing forward. The assumption is
seledom true. - The same arguments can be applied to the
front/back direction. - If we want envelopment to be high in both
directions (which is highly desirable) we need at
least three LF drivers. (and four is better.) - We also need at least one additional decorrelated
source channel. (we can derive this using Logic
7) - The large room at HSG is a good example of a room
which is excellent in both directions. It sounds
uniformly spacious as you rotate your head. - Achieving good front/back as well as lateral
envelopment is one of our goals in automobile
sound system design - Automobiles rely on inhomogenious (non-modal)
sound propigation, so this task is not as
difficult as it would seem.
23Reducing the Q of room modes electronically
- The large room at HSG has several modes with high
Q. - Of particular interest is the one at 48kHz, which
adds an obnoxious buzz to bass transients.
24After electronic equalization
- We can find the exact frequency and Q of these
modes by examining the waterfall plot. - We can insert a dip filter with the identical
frequency and Q. - The result is electronic damping of the mode.
Unlike conventional room equalization, this
damping works everywhere in the room. - Electronic damping improves the transient
response, the frequency uniformity of the bass,
and the envelopment.
25Conclusions
- The perception of envelopment with sound
reproduction in small rooms is possible and
desirable. - But in many common cases the room dimensions and
speaker layout conspire to make the effect nearly
inaudible - Rooms of approximately square dimension set up
symmetrically are particularly poor at
reproducing envelopment. - In such a room envelopment and the spatial
properties of bass in general is simply not
audible. - These rooms can often be greatly helped by moving
the subwoofers to the sides of the listening
position, and by damping the room modes as much
as possible. - Lack of envelopment a result of unfavorable room
dimensions and speaker layout. - It does not imply that envelopment in small rooms
cannot exist. - In many cases a lack of envelopmetnt can be
corrected by making a diagram of the first few
room modes, and seeing where they overlap. - In many cases the lack of envelopment is a direct
result of putting the main loudspeakers at nulls
for an important lateral asymmetric mode. Once
again, putting subwoofers at the sides of the
listeners, and raising the crossover frequency,
is a solution. - Drawing a few pictures of how the asymmetric
lateral modes and the front/back modes overlap in
a proposed listening room is a highly worth-while
exercise. - If you are stuck with a square room seriously
consider altering it. You will be glad you did.
26References
- Griesinger, D. The Psychoacoustics of Apparent
Source Width, Spaciousness and Envelopment in
Performance Spaces Acta Acustica Vol. 83 (1997)
721-731 (preprint available on the authors
web-page.) - Griesinger, D. Objective measures of
spaciousness and envelopment Presented at the
16th international conference of the AES.
(preprint available on the authors web-page) - Griesinger, D. Speaker Placement,
Externalization, and Envelopment in Home
Listening Rooms AES preprint 4860 (preprint
available on the authors web-page.) - Griesinger, D. General overview of spatial
impression, envelopment, localization, and
externalization presented at the 15th
international conference of the AES, Denmark
1998. (web page) - Griesinger, D. Spatial impression and
Envelopment in Small Rooms AES preprint 4638.
(web page)