Room Acoustics
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Room Acoustics
----& Theory& ----& The best rooms& ----& The listening room& ----&
Introduction
Loudspeaker directivity and room
Room reverberation time T60
- Sound waves between two walls
- Sound waves in a rectangular, rigid
- Reverberation distance
- Rooms for multi-channel sound
- Amplifier power to obtain Reference
- Room response time
Loudspeaker and listener placement
as a merging of room acoustics and auditory perception
- Introduction
Much has been written in the popular and
professional audio press about the acoustic treatment of rooms. The purpose of
such treatment is to allow us to hear more of the loudspeaker and less of the
room. I am convinced that a properly designed sound system can perform well in a
great variety of rooms and requires only a minimum of room treatment if any at
To understand this claim let's look at the
typical acoustic behavior of domestic size listening rooms, which have linear dimensions
that are small compared to the 17 m wavelength of a 20 Hz bass tone, but are
acoustically large when compared to a 200 Hz or 1.7 m wavelength midrange tone (G1 on the
piano keyboard).&
Below 200 Hz the acoustics of different locations
in the room are dominated by discrete resonances. Above 200 Hz these resonances
become so tightly packed in frequency and space that the room behaves quite
uniformly and is best described by its reverberation time RT60 ().
Room treatment can be very effective above 200
Hz, but the same result may be obtained more aesthetically with ordinary
furnishings, wall decoration, rugs on the floor and the variety of stuff we like
to surround ourselves with. How much treatment is needed, or how short the
reverberation time should be, depends on the polar radiation characteristics of
the loudspeaker. For my open baffle speaker designs a room becomes too dead when
its RT60 falls below 500 ms.
We can think of sound as propagating like a light
ray. Thus, we can use a mirror to find the region on the side wall or ceiling
where sound from the speaker might be reflected towards the preferred listening
location. It depends on driver, crossover and baffle design, i.e. the polar
radiation pattern, whether the region so found is illuminated by sound to any
significant degree. If so, then a variety of commercial surface coverings are
available to scatter and/or absorb the offending reflection.
The acoustically most problematic frequency range
is below 200 Hz, because of the spatially and frequency wise irregular
distribution of room resonances. Many computer programs have been written that
calculate the resonant modes of a given room and recommend optimum loudspeaker
and listener placements. Usually, real rooms are much more complex than the
calculated models. Walls are not infinitely stiff, rooms have windows, doors,
openings, suspended floors or ceilings, etc. In addition, it is the polar
pattern and the acoustic source impedance of the given loudspeaker that
determines which of the potential room modes are actually excited and to which
degree. The usefulness of such programs is marginal at best. Likewise,
recommended proportions for room length, width and height should not be taken
more seriously than other proportions that may be based on visual aesthetics.
The conventional closed or vented box design,
that is used for the majority of loudspeakers on the market, contributes
significantly to the room problems below 200 Hz. These designs are
omni-directional radiators and they tend to excite a maximum number of room
resonances, particularly when located in room corners. While this adds to the
perceived bass output at certain frequencies, it can lead to a falsification of
the recorded material, namely when the room resonance decays more slowly than
the original sound. In general, the low frequency response of omni-directional
speakers in small rooms is quite non-uniform. Attempts to treat the room with
absorbers will make only marginal differences unless very many absorbers or
large absorbing surfaces are used. It is best to attenuate peaks in the bass
response with parametric equalization. Holes in the response cannot be filled in
By far the perceptually most uniform response in the range
below 200 Hz is obtained with an open-baffle, dipole or figure-of-eight
radiating source. Because of its directionality, the dipole excites far fewer
room resonances than an omni-directional source. The measured room response is
not necessarily any smoother than that for an omni-directional source. But the
perceived difference in bass
reproduction is startling at first, because we are so used to hearing the
irregular and booming bass of the typical box speaker in acoustically small
rooms. Quickly one learns to recognize the
of this combination and it becomes intolerable.
For evaluating a given room and loudspeaker
combination a
is available. It contains unique sound
tracks to identify room resonances and their effect upon the clarity of sound
reproduction. Many of the tests require no instrumentation other than your ears.
illustrates wave propagation and reflection in a single plane and
gives an indication of the complexity of sound propagation in acoustically small
spaces like living rooms.&
Also see and listen to a talk about &Accurate sound
reproduction from two loudspeakers in a living room& under
and read the &
stereophonic sound reproduction& page.)
and room response
When a loudspeaker is placed in a room we hear both its
direct sound, i.e. the sound which arrives via the shortest path, and the room
sound due to the resonances, reverberation and reflections caused by the
boundaries of the room and the objects in it. The two sounds superimpose and
influence our perception of timbre, timing and spatial location of the virtual
sound source. Thus, the off-axis radiation of the speaker has great influence on
the naturalness of sound reproduction even when you listen on-axis and the more
so, the further you sit away from the speaker.
Two basic and fundamentally different sources of sound are
the monopole and the dipole radiator. The ideal monopole is an acoustically
small pulsating sphere, and the ideal dipole is a back and forth oscillating
small sphere. The monopole radiates uniformly into all directions, whereas the
dipole is directional with distinct nulls in the plane vertical to its axis of
oscillation. The 3-dimensional radiation or polar pattern of the monopole is
like the surface of a basketball, the dipole's is like two ping-pong balls
stuck together. At +/-45 degrees off-axis the dipole response is L = cos(45) =
0.7 or 3 dB down, the monopole is unchanged with L = 1.&
The graph above shows characteristic radiation patterns of
different sound sources for very low, mid and high frequencies and with flat
on-axis response.&
are neither pure monopoles nor pure dipoles except at low frequencies where the
acoustic wavelengths are large compared to the cabinet dimensions.
The ideal monopole is omni-directional at all frequencies.
Very few speaker designs on the consumer market approach this behavior. This
type of speaker illuminates the listening room uniformly and the perceived sound
is strongly influenced by the room's acoustic signature. The result can be quite
pleasing, though, because a great deal of acoustic averaging of the sound
radiated into every direction takes place. The speakers tend to disappear
completely in the wide sound field. Unfortunately, the direct sound is
maximally masked by the room sound and precise imaging is lost, unless the
listening position is close to the speakers.
The typical box speaker, whether vented, band-passed or
closed, is omni-directional at low frequencies and becomes increasingly
forward-directional towards higher frequencies. Even when flat on-axis, the
total acoustic power radiated into the room drops typically 10 dB (10x) or more
between low and high frequencies. The uneven power response and the associated
strong excitation of low frequency room modes contributes to the familiar (and
often desired :-( ) generic box loudspeaker sound. This cannot be the avenue to
sound reproduction that is true to the original.
The directional response of the ideal dipole is obtained
with open baffle speakers at low frequencies. Note, that to obtain the same
on-axis sound pressure level as from a monopole, a dipole needs to radiate only
1/3rd of the monopole's power into the room. This means 4.8 dB less contribution
of the room's acoustic signature to the perceived sound. It might also mean 4.8
dB less sound for your neighbor, or that much more sound to you. Despite this
advantage dipole speakers are often not acceptable, because they tend to be
constructed as physically large panels that interfere with room aesthetics, and
they seem to suffer from insufficient bass output, critical room placement and a
narrow &sweet spot&.
These claims are true to varying degree depending on the
specific design of a given panel loudspeaker. Because of the progressive acoustic short
circuit between front and rear as the reproduced signal frequency decreases, the membrane of an open-baffle speaker has to
move more air locally than the driver cone of a box speaker for the same SPL at
the listening position. This demands a large radiating surface area, because
achievable excursions are usually small for electrostatic or magnetic panel
drive. The obtained volume displacement limits the maximum bass output.
Non-linear distortion, though, is often much lower than for dynamic drivers.
Large radiating area means that the panel becomes multi-directional with
increasing frequency which contributes to critical room placement and listening
speaker is built with conventional cone
type dynamic drivers of large excursion capability, then adequate bass output
and uniform off-axis radiation are readily obtainable in a package that is more
acceptable than a large panel, though not as small as a box speaker. Such
speakers were built by Audio Artistry Inc. and a DIY project is described on
this web site in the PHOENIX pages. This type of speaker has a much more
uniform power response than the typical box speaker. Not only is its bass output
in proportion to the music, because room resonance contribution is greatly
reduced, but also the character of the bass now sounds more like that from real
musical instruments.&My hypothesis is that three effects combine to produce
the greater bass clarity:
1 - An open baffle, dipole speaker has a figure-of-eight radiation pattern and
therefore excites fewer room modes.
2 - Its total radiated power is 4.8 dB less than that of a monopole for the same
on-axis SPL. Thus the strength of the excited modes is less.&
3 - A 4.8 dB difference in SPL at low frequencies is quite significant, due to
the bunching of the equal loudness contours at low frequencies, and corresponds
to a 10 dB difference in loudness at 1 kHz.
Thus, bass reproduced by a dipole would be less masked by the room, since a
dipole excites fewer modes, and to a lesser degree, and since the perceived
difference between direct sound and room contribution is magnified by a
psychoacoustic effect.,&
The off-axis radiation behavior of a speaker determines
the degree to which speaker placement and room acoustics degrade the accuracy of
the perceived sound. Worst in this respect is the typical box speaker, followed
by the large panel area dipole and the truly omni-directional designs. Least
affected is the sound of the open-baffle speaker with piston drivers. ()&
Often concern is expressed over the fact that the
from a dipole is out of phase with the front radiation, and that
thus any sound reflected from a wall behind the speaker would cancel sound
coming from the front of the speaker. Cancellation can only occur when direct
and reflected sounds are exactly of opposite phase (180 degrees) and of the same
strength. Since direct and reflected sounds travel paths of different length,
they undergo different amounts of phase shift. Thus, the phase and magnitude
conditions for cancellation are given only at certain frequencies, if at all. At
some other frequencies direct and reflected sounds will add. The same also
applies to a monopole speaker in front of a wall. The only difference is in the
frequencies for which addition and subtraction occur. The best remedy is to move
the speaker away from the wall, or to make the wall as sound absorptive or
diffusive as possible. ()
Reverberation time is the single most important parameter
describing a room's acoustic behavior. The following discussion might get a
little technical but will illustrate how sound builds up and decays in a room
and the effect it has upon clarity of reproduction.
- Sound waves between two walls
Take the example of a speaker in a wall and a second wall at distance L in
front of it. As the cone vibrates it will send out an acoustic wave which gets
reflected back by the second wall, returns to the first wall, gets reflected
again back to the second wall and so on. If the frequency of vibration is such
that the distance L corresponds to half of a wavelength, then the cone movement
is in phase with the reflected wave and the sound pressure keeps building up.
Eventually an equilibrium is reached between the energy supplied by the cone
movement and the energy absorbed by the two walls and the air in between.
This is a standing wave resonance or mode condition and if
we change the frequency of cone vibration, we trace out the resonance curve that
is typical for any simple system containing mass, compliance and energy
loss. As frequency is increased another resonance occurs when L equals to a full
wavelength, to 3/2 wavelengths, 4/2 and so on. The lowest possible frequency
fmin = c / (2 L)& Hz,& where c=343
m/s&&& (1)
If the excitation is applied as a step function, then the
sound pressure will rise from 10% to 90% of its steady-state level within a time
Trise = 0.7 / BW&&& (2)&
where BW is the width of the resonance curve in Hz at the
half power (-3 dB) level. The SPL will decay to one thousand's (-60 dB) of its
full level after a time&
T60 = 2.2 / BW&&& (3)
The quality factor or Q of the resonance is&
Q = n fmin / BW&&
(4)&&&& with n = 1, 2, 3, etc.
Example 1&
L=25 ft (7.63 m), then& fmin =
343/(2*7.63) = 22.5 Hz and no resonance below this frequency. The next higher
resonance will be at 45 Hz, then 67.5 Hz, 90 Hz, 112.5 Hz and so on.
If we had measured Trise = 202 ms at 45 Hz,
then from (2)& BW = 0.7/0.202 = 3.5 Hz and T60 = 2.2/3.5 = 630
ms from (3).
Q = 45/3.5 = 12.9 and if T60 stays constant with increasing
frequency, then Q increases, for example Q = 112.5/3.5 = 32.1
in a rectangular, rigid room
In a rectangular room we have six surfaces and the number of possible
standing waves is much larger than for the two wall example. The frequencies at
which they can occur are calculated from
f = ( c / 2 ) [ ( l / L )2 + ( w / W )2
+ ( h / H )2 ]1/2&&&&
[Hz]&&&& (5)
l, w, h = 0, 1, 2, 3 etc.
See , a spreadsheet for
calculating and plotting room modes and other room parameters discussed here.&
At frequencies
pressure will increase at a rate of 12 dB/oct for a closed box speaker that is
flat under anechoic conditions, assuming that the room is completely closed and
its surfaces are rigid. This case has some significance for the interior of
automobiles. Under the same circumstances the sound from a dipole speaker will
stay flat.&
Domestic listening spaces are seldom completely closed, nor are sheet rock walls
rigid, making a prediction of very low frequency in-room response extremely
difficult.
Note: Calculations of room modes, though popular, are not
practical for
predicting optimum speaker placement or listener position. For this one would
need to calculate the transfer function between speaker and listener. The
transfer function is related to the room modes, but much more difficult to
determine. Never-the-less, room mode calculations are often invoked to predict
&optimum& room dimensions. They fail to take into account any
specifics about speaker placement, source directivity and source type (monopole
vs. dipole) that determine which modes are excited, and in combination with the
absorption properties of different room surfaces, to which degree these
resonances build up. Some people think that by making the room other than rectangular or
using curved surfaces, that they can eliminate standing waves. They merely
change frequencies, shift their distribution and make their calculation a lot
more difficult.
Room modes can be identified by peaks and dips in the
frequency response of the acoustic transfer function between speaker and
listening position, though only at low frequencies (&150 Hz) where their
density is not too high. Such measurements are location dependent and are
difficult to interpret as to their audible effect. Listening to a
at different frequencies gives audible indication of which room
locations and frequency regions suffer the greatest degradation in the
articulation of bass sounds (). With this
information in hand it is then possible to identify and electronically equalize
the worst offenders in the acoustic transfer function response.
Several room parameters can be calculated that give
insight into the general behavior of a closed space.&
The number of modes N between zero and a given upper
frequency limit fm can be estimated (H. Kuttruff, Room Acoustics,
1991) from
N = (4 p / 3)
V (fm/c)3 + (p /
4) S (fm /c)2 + (1 / 8) Le (fm
/c)&&& (6)
V = L W H&&&& [m3]
S = 2 ( L W + L H + W H )&&&& [m2]
Le = 4 ( L + W + H )&&&& [m]
p = pi = 3.14...
The number of modes increases very rapidly with frequency
and they move ever more closely together. Their average separation at fm
df = c3 / ( 4
)&&&& [Hz]&&&& (7)
Take a room with L = 25', W = 16' and H = 9' (7.62m x 4.88m x 2.74m), then
V = 3600 ft3 = 102 m3
S = 1537 ft2 = 143 m2
Le = 200 ft = 61 m
Below fm = 100 Hz, 200, 300 and 400 Hz the number of modes N and
their average separation df
at fm are respectively&
If we assume that the modes in this room decay at T60
= 630 ms, then each resonance occupies a 3 dB bandwidth BW = 3.5 Hz from (3)
above. Somewhere between 100 Hz and 200 Hz the average separation df
between modes is 1.2 Hz and thus 3 modes fall within the 3.5 Hz bandwidth
resulting from T60. This occurs at& fs = 157 Hz as calculated
from the simple formula for 3 overlapping modes per BW:
fs = 2000 ( T60 / V )1/2&&&&
[Hz]&&&& (8)
The frequency fs is also called the
frequency and denotes approximately the boundary between reverberant room
behavior above and discrete room modes below.
The sound decay time or reverberation time T60
is related to the average wall absorption coefficient a
by Sabine's formula
T60 = 0.163 V / ( S a
)&&&& [s]&&&& (9)
a = 18%& for the Example 2
room with T60 = 630 ms.&
containing unique test
signals is available. It allows to evaluate the effect of room modes upon the
clarity of sound reproduction.
When we consider radiation in the reverberant frequency range above 149 Hz,
the sound at the listening position is composed of the direct sound from the
source and the reverberant sound that is more or less uniformly distributed in
the room. The direct sound pressure level decreases inversely to distance from
the source and will equal the reverberant sound pressure at distance xr.
The ‘reverberation distance’ xr (also called 'critical distance')
is calculated from
xr = 0.1 ( G V / (p
T60) )1/2&&&&
[m]&&&& (10)
where the directionality gain G is unity for a monopole
and G = 3 for a dipole radiator. A dipole, thus, has a 31/2 = 1.73
times larger reverberation distance.
A typical reverberation distance is actually quite small,
0.72 m (2.4 ft) for the monopole and 1.24 m (4.1 ft) for the dipole in the
example room. Never-the-less,& the ratio of direct sound Ld to
reverberant sound pressure level Lr is 4.8 dB greater for the dipole
than for a monopole with the same direct sound output.& Thus, at 3 m
distance from the source, the direct sound would be 20*log(3/0.72) = 12.4 dB
below the reverberant sound field for the monopole and only 20*log(3/1.24) = 7.7
dB below it for the dipole.&
The 4.8 dB lower level of the reverberant field in the
case of the dipole significantly reduces the masking influence of the room upon
sonic detail. It eliminates the sensation of overload of the room during loud
passages of program material and makes your listening sessions much less noisy
to your neighbors.&
You have often experienced the poor intelligibility of spoken words from PA
systems in enclosed public spaces. Usually a central cluster of loudspeakers
aims at the audience. In reality the speakers are not very directional and too
much sound is radiated towards useless spaces, only to bounce around and raise
the reverberant sound level. It does not help to increase the volume to obtain
more direct sound, because it also raises the reverberant sound level. Speech
modulation gets lost in this, somewhat like the loss of articulation in my .
- Rooms for multi-channel sound
It has been suggested (R. Walker, BBC, 1998) that the reverberation time T60
over the 200 Hz to 4 kHz frequency range be adjusted to
T60 = 0.3 (V/V0)1/3&&
[s]&&&& where V0 = 100 m3&&&
with a tolerance of +/-50 ms which is allowed to increase
linearly to +300 ms between 200 Hz and 63 Hz.
The room of Example 2 should thus have T60 = 300 ms +/-50 ms. This
makes for a subjectively quite dead room, which is fine if the room is dedicated
solely to Home Theater and surround sound, but is in my opinion a very
overstuffed environment for normal living. It has the effect of making the
reverberation distance xr = 1.04 m for the monopole and xr
= 1.8 m for the dipole. At a viewing/listening distance of 2 m the direct sound
is only about 6 dB below the reverberant level of the monopole which is good for
sound clarity.&
Instead, you could use a dipole, increase T60 to a much more livable
600 ms and have the same direct-to-reverberant ratio as for the monopole for
which the specification was developed.
- Amplifier power to obtain Reference Level
When you know the equivalent sensitivity Ls of your speaker in dB SPL at 1 W
(2.83 V across 8 ohm) and 1 m distance and the reverberation time T60 of your room, then
you can estimate the amount of power Pref& required to obtain a
specified reference level Lref& at the listening distance xl.
First calculate the reverberation distance xr from (10). Then the
level of the reverberant field for 1 W into the speaker is
Lr(1W) = Ls - 20 log(xr)&&&
[dB SPL]&&& (12)
If the listening distance xl is greater than xr,
then the amplifier power in dBW is&
Pref = Lref - {Ls - 20
log(xr)}&& [dBW]&&&&&&&
for xl & xr&&& (13)
Ls = 89 dB SPL at 1 W, 1 m
Lref = 85 dB SPL
xr = 1.04 m for T60 = 300 ms
Lr(1W) = 89 - 20 log(1.04) = 88.7 dB SPL
Pref = 85 - 88.7 = -3.7 dBW, equivalent to 10(-3.7/10) =
xr = 1.8 m for T60 = 300 ms
Lr(1W) = 89 - 20 log(1.8) = 83.9 dB SPL
Pref = 85 - 83.9 = 1.1 dBW, equivalent to 10(1.1/10) = 1.3
With a suggested 20 dB of SPL (= 100 x power) headroom over reference
level the monopole requires 40 W and the dipole 130 W to set up a 105 dB SPL
reverberant sound field. The dipole's direct sound, though, is 4.8 dB higher
than the monopole's and will be 105 - 20 log(3/1.8) = 100.6 dB SPL at 3 m
distance. The increased clarity could be traded off for a more lively room with
larger T60 and the same 40 W amplifier power and
direct-to-reverberant SPL ratio as for the monopole.
It takes time to build up the reverberant sound field in a room. Combining
the expressions for rise time (2) and T60 (3) we obtain
Trise = 0.32 T60&&&
[s]&&& (14)&
You can think of Trise as the time constant of
the room. If music or speech varies faster than the time constant, then the room
will not respond fully and you hear predominantly the direct sound from the
speaker. For 630 ms reverberation time and 200 ms rise time this covers
modulation envelopes of a sound down to 1/200ms = 5 Hz which, in my opinion, is
preferable over the 10 Hz envelope rate of a T60 = 300 ms room.&
In all practical cases the room response time is large
compared to the time it takes a reflected sound to reach the listener and
therefore reflections will not be masked by the reverberant field. Depending
upon the directivity of the source and the proximity of reflecting surfaces and
objects specific absorptive or diffusive treatment may become necessary.&
It should not be overdone, though, because a certain amount of lateral
reflection is subjectively desirable to not destroy the impression of a real
- Loudspeaker and
listener placement
It is often assumed that a study of room acoustics can
lead to highly specific loudspeaker and listener placement locations, down to
within an inch. Other proponents are not as optimistic and recommend a 1/3rd
rule (). I have come to the conclusion that real
rooms are acoustically far too complex to predict the transmission of sound from
speaker to listener, where the sound paths are in three dimensions, have
direction and frequency dependent attenuation and diffusion, and can excite the
inherent resonance modes of a room to unknown degrees.&
From practical experience I recommend the following setups
as starting points. They are for , a
dipole or bi-directional loudspeaker, and for ,
a monopole or omni-directional speaker. Three room sizes are considered. The 180
ft2 (17 m2) room with 8 ft (2.4 m) ceiling would seem like
absolute minimum for quality sound reproduction with the ORION. A 400 ft2 (37 m2)
or larger room with 10' (3 m) ceiling should be perfect.
D1 - Dipole setup
ORION separation is 8'. They
are slightly towed in. The listener is at the apex of an equilateral
triangle. Distance to the wall behind the speakers is 4', and to the side
The listener is only 4' from the wall behind, and
this might require some heavy curtains and other absorbing material on
that wall. As the room gets larger it expands around this triangular setup
and especially behind the listener. Sound should just wash by the listener
and disappear.&
The wall behind the speakers should be diffusive.
The rear radiation from a dipole must not be absorbed or it is no longer a
dipole. Similarly, the side walls should not absorb sound at the
reflection points but diffuse it. A dipole can even be towed in so that
the listener sees the radiation null axis in a wall reflection mirror.
D2 - Monopole setup
PLUTO setup differs from
ORION. The listener sits closer to the speakers. The distance to the wall
behind the speakers can be slightly less, because of the uniform acoustic
illumination of the room. It should not be less than 3' (&6 ms) to
separate reflection from direct sound psychoacoustically, and to preserve
phantom imaging.&
Sidewall reflections should be diffused if treated
at all. Absorbing them is like turning down the tweeter. Absorbers are not
broadband and ineffective below a few hundred Hz.. Besides, lateral
reflections are important for sound scene recognition.&
Again, larger rooms expand around the triangle and
increase& the space more behind the listener than in front of him.
Listening to
revealing test of electrical and acoustic performance of any system setup. Pink
noise must emanate from both speakers simultaneously in dual mono fashion. A
tightly confined phantom image should be heard half-way between the two
loudspeakers. As you move your head left or right the sound should become
brighter sounding and increasingly so with about a 2 inch (5 cm) periodicity as
the lateral head displacement is increased in D1. The image also becomes
significantly more diffuse and moves towards the nearest speaker. Pink noise
should sound neutral and uncolored, though what that exactly means is hard to
define. Moving around in the room the character of the noise sound should not
change significantly with speakers like ORION and PLUTO, holding up even when
you leave the room and listen from outside. This is not the case with
loudspeakers that have a greatly varying polar response.&
Listening to pink noise does not give a reliable
indication of system performance at frequencies below 100 Hz and above 10 kHz.
Even when pink noise is measured in 1/3rd-octave bands, the response graph is
not a reliable indicator of speaker performance and should not be used as the
basis for equalization. It seems so obvious that one only needs to have a flat
frequency response at the listening position and be done. But, room response
equalization is a very complex subject because it deals with sound in three
dimensions of space, with time, with frequency, and with a highly evolved
auditory stimulus processor between the two ears that is not easily fooled
long-term. The response should not be optimized merely at the listening
position. Few
deal with this adequately.
D4 - Room analysis
spreadsheet that was discussed under C above can be used to analyze the three
hypothetical rooms and to gain some general insights. Depending upon their
structural rigidity, their wall surface textures, floors, floor coverings and
objects in different locations, each room will have its own unique acoustic
signature. Broadly speaking, a room may sound live or dead. The extremes of this
would be an unfurnished room with hard walls versus a cocktail lounge full of
overstuffed armchairs and soft leather. Neither one would be suited for sound
reproduction. The descriptive parameter is the average absorption coefficient of
all surfaces and leakage paths. By definition an open window has a 100%
absorption coefficient and if that open window covered 20% of a room's total
surface area, then the average absorption coefficient for the room would be 20%.
For the 180 ft2 room example this would be an open window of 169 ft2
area out of a total surface of 847 ft2. Since we usually listen with
closed windows and very few surfaces have 100% absorption, it takes much more
than 169 ft2 to obtain an overall 20% absorption.&
D5 - Lively rooms
Absorption
F-Schroeder
Reverb-dist
monopole - m
Reverb-dist
dipole - m
dipole - dB
Direct/Reverb
2.4m from dipole
D6 - Fairly dead rooms
Absorption
F-Schroeder
Reverb-dist
monopole - m
Reverb-dist
dipole - m
dipole - dB
rel to 20% Absrp
Direct/Reverb
2.4m from dipole
The numbers in tables D5 and D6 are for hypothetical rooms
and based on a very simple rigid rectangular room model. Though the numbers look
precise they should only be taken as trend indicators. Note the relatively
narrow range from 99 Hz to 200 Hz covered by the Schroeder frequency for the
different rooms and absorptions. Below this
frequency specific room modes can dominate, down to the 1st mode. Above that frequency the
mode density becomes so high that a room is better described statistically by
its reverberation time.&For the typical home& listening rooms with
relatively large objects and different materials in them, reverberation time
usually changes with frequency regions and is not as solid a descriptor as for
concert halls. Below the first room mode the sound level becomes independent of
location in the room and is a function of the lumped mechanical properties of
the room. Similar to the modal region the level can be attenuated or amplified
depending on wall surface flexures and leaky openings. The room adds to and
subtracts from the loudspeaker's direct sound to varying degrees and in a very
complex manner over the whole frequency range of the speaker. Thus the tables
can only show trends above the Schroeder frequency. &
It can be seen in D5 that the reverberant SPL in the 400
ft2 room is 3.1 dB below that of the 180 ft2 room and when
the absorption is increased to 40% in D6, it drops by another 3 dB for the same
direct sound level. Since we judge loudness by the reverberant sound field this
means that the volume control setting has to be increased 3.1 dB for the volume
in the 400 ft2 room to be as loud as in the 180 ft2 room
in D5, and by 6.1 dB for the more absorptive room in D6. Still, this is not much
of an increase between the small and the large room. It confirms that ORION and
PLUTO can be used in a wide range of room sizes, if volume levels are set for
critical listening in the triangle seat and not for sound reinforcement at a
large party.
Under D6 the ratio of direct to reverberant sound level is
3 dB better than for the more lively rooms under D5 with half the absorption.
These numbers are for the dipole which inherently is 4.8 dB (3x) better than a
monopole. But the monopole in D2 is closer to the listener than the dipole in
D1. Thus, in all cases the direct-to-reverberant sound ratio for this monopole
at 6.4' (1.92 m) listening distance is only 2.8 dB worse than that for the
dipole at 8' (2.4 m).
Despite the poorer signal-to-reverberant ratio I find more
lively rooms preferable for music and voice reproduction. Home Theater
installers, though, try to get rooms down to the 200 ms T60 region, which is
difficult to accomplish for low frequencies.
Reverberation time of a listening room can be measured
rather easily with the , but a loud hand clap can tell already whether a room is live
or dead. Rather than special products for acoustic treatment of a room I prefer
the normal stuff of life - books, curtains, pictures, rugs, wall hangings,
shelves, cabinets, chairs, sofas, etc. - to establish the acoustics of my living
spaces. ORION and PLUTO+ are well adapted to such spaces which also convey a
friendly atmosphere to most people.
See also the
as a merging of room acoustics and auditory
perception
What you hear is not the air pressure
variation in itself&
but what has drawn your attention
in the two streams of superimposed air
pressure variations at your eardrums
acoustic event has dimensions of Time, Tone, Loudness and Space
Have they been recorded and rendered sensibly?
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Last revised: 03/10/2015
LINKWITZ LAB, All Rights Reserved