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When we talk about making a room “sound right”, we are probably dealing with room acoustics as a science acoustics has been around for about hundred years. Until then good acoustics happened by experiment, by experience, or simply by accident..
Today we know a lot about the parameters that influence the “sound” of a room.
Talking about the control room we know that basically this room should act as neutral as possible. But this is not always the case. Lets just take a short look on some of the issues, than one must be aware of.
Good acoustics
Here is a list of important parameters concerning good acoustics.

» Proper reverberation time
» Good sound distribution
» Adequate sound pressure level
» Low background noise level
» No echo (flutter echo)

Now lets have a look on these headlines that will be adequate for the most purposes concerning sound production.
» Reverberation Time
» Sabine formula
» Room modes
» How does the standing waves unfluence the sound fields?
» Standing Waves
» Absorbent Materials
» Diffusors
» Comb Filtering
» Rear wall cancellation

The reverberation time is defined by the time it takes a sound to attenuate 60 dB after the source is stopped. In the real life we can experience reverb times from approximately 0 sec. (outdoors or in anechoic chambers) to something like 10 – 12 sec. In special reverb chambers the time may exceed 20 sec. Control rooms normally should have a reverb time around 0.2 – 0.3 sec.

Why do we have reverberation? The speed of the propagating sound wave is very slow – at least compared to light: approx. 1130 ft. or 340 m per sec.

If there are no reflecting surfaces between the sound source and our ears, only the direct sound is heard and there is no reverberation.
If there is a single reflecting surface we may hear the reflected sound in one way or another, but there is still no reverberation.

If the sound is generated in a room, there are a whole lot of reflections. Each of these travels different paths with different distances on the wayto the receiver. Each time the sound hits a surface it may loose some energy if the surface is absorbing.
One sound source, one receiver and no room. Only the direct sound is received
One sound source, one receiver, and one reflecting surface. The sound is received twice. (In the control room this is normally experienced as comb filtering, see later).
One sound source and one receiver in a room. The sound impulse is reflected in many surfaces
All the reflections are melting together and heard as reverberation.

Sabine is the father of modem acoustics. He found that reverberation time is described by a relationship between the room size and the amount of absorption in the room. Larger rooms – longer reverberation. More absorption – shorter reverberation.

T = 0,161 x V / A

T: Reverberation time in seconds
V: Volume in m3
A: Absorption in m2 Sabine
0,161: Is a constant (to make the calculation right with the actual units)


Note:One square meter (1 m2) Sabine is comparable to an open window with an area of one square meter:
The sound that hits the window will disappear and never return. One square meter Sabine is one square meter with full absorption. The basic formula sounds simple, but the problem is that the materials in the room will absorb differently at different frequencies. The absorption may range from nothing (fully reflective) to total absorption.
A proper reverb time should be constant with frequency, but this is not always the case because of the behaviour of the materials in the room. The low frequencies are the most difficult to control.
This is why the reverberation time against frequency in practice may look like this:
Reverberation time measured in a control room. From 250 Hz and above the curve is nicely placed around 3 sec. But belov the reverb time rises to 75 sec. which is too much.
All materials in the room act acoustically even if they are not so-called acoustical materials. Basically we have three kinds of absorbers.

Membrane absorbers
This kind of absorber includes wooden floors, windows, doors, etc. This absorber provides absorption in the low end of the frequency range. The efficiency is normally not very high, but under normal conditions large areas are included in the basic room construction. Special designed membrane absorbers can be very effective.
Resonance Absorbers
Resonance absorbers include slit panels, perforated plate, Helmholtz resonators etc. These absorbers are normally used in the frequency midrange. The absorption is medium to high.
Holes or cracks
Mineral wool or similar

Porous absorbers
These absorbers include mineral wool, carpets, curtains, and so on. They can be very effective, but the thickness of the material has to be taken into account. Thin layers will only absorb the highest frequencies. (Think of a rehearsal room in the concrete basement where the only damping is a carpet on the door: Not good at low frequencies!).

To absorb a given frequency (and all frequencies above) the thickness of the absorber must be the quarter of the wavelength of that frequency. Or: the front of the material must be placed at a distance of one quarter of the wavelength.

A diffuser provides diffuse reflection of the sound radiated against it. It can be a very useful solution in cases where reflections are disturbing the sound image and it is not advisory to ad further absorption. So in order to reduce flutter echoes, comb filtering etc., special elements can be placed on the “disturbing” surface. These elements must have dimensions comparable to the frequencies at which diffusion is wanted.

Absorption or diffusion?

A special technique developed by Manfred Schroeder is very capable in making a smooth and controlled diffusion. These diffusers normally referred as the "Woodiffusor" can be found as prefab modules.

Cross section of one example of the ."Woodifusor" Jocavi


Standing waves exist in all kind of rooms. The shape of the room, the dimensions of the room, and the relationship between the dimensions of the room, are important parameters that will determine the frequencies around which the phenomenon exists as well as the distribution of these standing waves. But how do they occur?

Imagine a sound source. When the sound is emitted the sound is emitted the sound wave will propagate in all directions if no obstacles in sight. This will of course happen with the speed of sound. Now, if the sound source is placed inside a room the sound wave will hit the boundaries of the room.
If the boundaries consist of acoustically hard (reflective) surfaces, the sound is reflected. If the angle of incidence is 90º the sound will be reflected right back where it came from. Under certain circumstances the sound wave will meet itself again. For instance if the sound is reflected between to parallel walls.


This Becomes a problem, when the sound wave not only meets itself, but when it meets itself in phase. And this will happen when the distance between the walls is half a wavelength of the radiated sound wave. Or one whole wavelength – or 1 ½, 2 ½ and so on.

This phenomenon is called standing waves. Actually the sound wave is not standing. But it is experienced like that because the sound pressure maxima and minima are positioned in fixed places in the room.

The sound field is initially radiated having a radical wave front but within a few reflections the sound field has obtained a plane wane front.

The special frequencies are also called room modes. Standing waves between parallel walls are called axial modes. Other modes exit. For instance tangential and radical modes. (See the illustration). Normally the axial modes are the strongest.





The standing waves are characterized by having a maximum sound pressure at the boundaries of the room. Depending on the frequency there are one or more dips across the room. In a box shaped room the frequencies can be calculated as follows:

where :
f = frequency in Hz
c =speed of sound (approx. 340 m/s or 1130 ft/s)
l = length of the room
w = width of the room
h = height of the room
n = integer from 0 and up


The maxima of the standing waves are shown in the figure. The curve expresses the area of the room where the actual frequency is audible. At the minima the frequency is represented at a much lower level (sometimes – 40 dB compared to the maximum).

If the room has the same dimensions as length, width, and even height it is very problematic to obtain an even sound distribution.

How to prohibit standing waves?
Parallel walls in the room should be prevented. Then the strongest modes are suppressed. When placing the monitors it is important that as few modes as possible are excited. This is why the monitors should not be placed in a maximum of a standing wave. At low frequencies a monitor can be considered as to radiate the sound energy in all directions.

This is also called a 4 radiation

When placing the monitor close to a solid boundary – for instance a wall – the sound energy that should have been radiated in the direction of the wall instead is radiated into the free half space. Hence the sound pressure is doubled in the half space, which yields +6 dB.

This is also called a 2 radiation

When placing the monitor close to a solid boundary – for instance a wall – the sound energy that should have been radiated in the direction of the wall instead is radiated into the free half space. Hence the sound pressure is doubled in the half space, which yields +6 dB.
This is also called radiation.

Placing the monitor against two boundaries – for instance in a corner limited by two walls – it is now radiating to the quarter space. Now the sound pressure is doubled twice, which yields +12 dB.

This is also called /2 radiation..

In practice, the placing by barrier walls, or ground, influence the frequency range below 125-150 Hz.
The filtering function that arises when a signal is added to itself after having been delayed in time is called a comb filter. The resulting frequency response resembles a comb, hence the name.
Two 500 Hz sinusoidal tones added. The second tone is delayed 1 ms hence the sum is zero. Two 1 kHz sinusoidal tones added. The second tone is delayed 1 ms hence the sum is the double (+6 dB).
The comb filter function is almost never intentional, but it is heard all the time in sound productions, where it can arise both acoustically and electrically. Acoustically, it typically occurs when the sound on its way from source to recipient takes in part a direct path and in part an indirect path via a single reflective surface. The reflection must be attenuated at least 10 dB and preferably 15 dB in order for it not to have an effect on the sound field at the recipient position. Electrically, the phenomenon arises when two microphones with a certain distance between them capture the same signal and the level from each microphone is of the same order of magnitude.
Two typical situations in which comb filters arise, either acoustically or electrically.
In general: All digital signal processing takes time. This means in practice that comb filter effects can arise if you loop a signal via, for example, a compressor and combine this signal with the original.
dB Level Frequency– Hz
An example of a comb filter created by the combining of two signals with the same amplitude, but with a time delay between them of just 1 ms.

It can be seen that a dip occurs due to cancellation at 500 Hz, 1.5 kHz, 2.5 kHz, etc. It can also be seen that the two signals add to double their value (+6 dB) at low frequencies and with a full wavelength’s delay at 1 kHz, 2 kHz, 3 kHz etc.

Dip Frequency
Cancellation occurs for a comb filter at all the frequencies where the two signals are in opposite phase. This occurs when the time delay comprises duration of ½, 1 ½, 2 ½, etc. periods. At 1 kHz the period is 1 ms. Half of the period is 0.5 ms. If a time delay of precisely 0.5 ms occurs, it means that cancellation will arise, not just at 1 kHz, but also at 2 kHz, 3 kHz, 4 kHz etc

When the monitor is set up at some distance in front of a wall, reflections from the wall may occur and influence the perceived frequency response. This could result in comb filtering if all frequencies produced by the monitor were radiated in all directions. But the monitors are typically only omni- directional at low frequencies. The result of the reflection is a single or few dips in the frequency response perceived in front of the monitor. The frequency response may look like this: one cancellation at the frequency that has a wave length of four times the distance to the rear wall.

The dip – or cancellation frequency – is dependent on the distance to the wall. If the distance is 1 m, the first dip frequency with a wavelength of 4 m.

l = c/f
Where :

l = wavelength (m) (ou ft)
c = speed of sound [m/sec] (or ft./sec.)
f = frequency (Hz)


4 = 344 / f
F = 86 Hz

A closer position will result in a cancellation at a higher frequency. This is then limited by the frequency where the monitor becomes directional and does rot radiate sound to the rear.

A farther position will result in cancellation at a lower frequency. This is limited by the distance being so long, that the reflected sound is attenuated due to the long extra path travel.

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