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Frequency It is the measurement of the number of times that a repeated event occurs per unit time. All sound waves are travelling at about the same speed. In the case of sound waves, the number of wave peaks that goes by per unit time is the frequency or pitch. Wavelength Sound is actually "compression" waves in a medium. When something makes a sound, the air is compressed or rarified in waves that travel out from that source in all directions. The wavelength is the distance between repeating units of a wave pattern.

Short Wavelength High Frequency

Long Wavelength Low Frequency

Absorption In acoustics, the reduction in sound pressure levels through the conversion of sound energy to heat, captured within an acoustic attenuator. The opposite of reflection.

Sound waves from source

Some energy absorbed

attenuate : to reduce in strength

Reflection As relates to sound, a return of residual sound, after striking a surface within a room or space. The opposite of absorption.

Some energy transmitted

Some energy reflected

Reverberation The persistence of sound in a particular space after the original sound is removed. or The repetition of a sound resulting from reflection of the sound waves.

A reverberation, or reverb, is created when a sound is produced in an enclosed space causing a large number of echoes to build up and then slowly decay as the sound is absorbed by the walls and air.


The length of this sound decay is the reverberation time.


In a more reflective room, it will take longer for the sound to die away. In a very absorbent room, the sound will die away quickly. But the time for reverberation to completely die away will depend upon •how loud the sound was to begin with •the acuity of the hearing of the observer.

typical loudest crescendo of orchestra 60db drop used to define the standard reverberation time typical room background level

Standard reverberation time, RT60 The time required for reflections of a direct sound to decay by 60 dB below the level of the direct sound. or The time for the sound to die away to a level 60 decibels below its original level.

Calculation of RT60 Sabine equation

RT60 = 0.163 x V/A where V= volume of the room and A= effective absorbing area = Îą x S (surface area)

The fractional loss of sound waves due to absorption and transmittance is characterized by an absorption coefficient Îą, which can take values between 0 and 1, 1 being a perfect absorber.

Optimum Reverberation Time

8.5s. Notre Dame. 5.5s. Muddy, severe loss of articulation, can’t understand speech

3.5s. Fuller, richer musical sound, some loss of articulation 1.5 to 2.5s. General purpose: both speech and music

1s. Clearer articulation of speech, loss of richness and fullness 0.3s. Difficulty hearing in the back, difficulty hearing the bass 0s. No reverberation.

Recording Studio an assemblage of equipment, spaces and persons such that a performance in sound may be created and recorded onto a medium for later reproduction. Control room

where the musicians perform

Recording room or studio Machine room

containing the equipment for recording, editing and mixing music

high-volume instruments like drums are played to separate their sounds from those the microphones in the main room are picking up

The size and shape of a room determines its natural resonances - often called room modes. Every rectangular room has three sets of primary modes, with one each for the length, width, and height. The fundamental wavelength for each of these modes is half the dimension. For example, a 6m wide room will have a fundamental mode wavelength of 3m and so a fundamental mode frequency of 14Hz. Even though this creates many little resonant peaks in the response, the peaks are close together, so the average response is fairly flat.

Larger the dimension, smaller the frequency. Hence, larger rooms are better acoustically than smaller rooms because the modes are spaced more closely, yielding an overall flatter response.

Another important factor in the design of studios and listening rooms is the ratio between the length, width, and height. The worst shape is a cube having all three dimensions the same. A cube has the fewest number of peaks, and therefore the greatest distance between peaks, because all three dimensions resonate at the same frequencies. In an ideal room, each dimension will contribute peaks at different frequencies, thus creating more peaks having a smaller distance between them. Height 1.00 1.00 1.00

Width 1.14 1.28 1.60

Length 1.39 1.54 2.33

A few "ideal" ratios of room height, width, and length that professional studio designers agree should be used if possible.

Standing Waves It is a wave characterized by lack of vibration at certain points. When sound waves bounce off the surrounding walls and create a pressure front that makes them "stand still" within the space itself, they are called standing waves. It occurs when your loudspeakers play a sustained bass tone.





Studios should ideally have no parallel walls, as these create standing waves in the space inside. It is also always better to create angles and chamfers instead of rights angled corners.

Acoustic Insulation The reduction of the sound transmission from one space to another especially significant through walls and floors between separate buildings and from external sources.




Acoustic Treatment The use of sound-absorbing materials to give a room a desired degree of freedom from echo and reverberation.


Acoustic insulation or reduction due to sound transmission and leakage is accommodated for in construction by: •using thick massive walls •isolating the building structures, generally by floating the walls and floors •hanging the ceilings with shock mounts. Basement studios are preferred, because they are inherently insulated. Walls can be insulated with glass wool and than covered with canvas cloth. Ceilings are tiled with bhusa board, which have both insulation and acoustic properties. Floating floors and floating ceilings are often employed because the air gap insulates the room. They are also used to variably change the dimensions of the room.

Doors and windows also act as leaks and it is important to insulate these openings. Doors are a minimum of 80mm thick, wood, sandwiched with foam in between. Normally studio designers employ a sound lock, that is, a double door system.

The door frame also has a rubber beading running the entire length. External windows are avoided. Internal windows, like the one between the recording room and the control room, are carefully designed so as to minimize sound loss. Double panes with an insulating air gap are preferred.

Why do we need acoustic treatment? All rooms sound differently, both in their amount of sound wave reflection and their frequency response. A mix that sounds good in the room it was created in (which has its own particular frequency response) is likely to sound very different in other rooms. Therefore, the only practical solution is to make the room as accurate as possible so any variation others experience is due solely to the response of their room.

There are four primary goals of acoustic treatment: • To prevent standing waves and acoustic interference from affecting the frequency response of recording studios and listening rooms • To reduce modal ringing in small rooms and lower the reverb time in larger studios, churches, and auditoriums • to absorb or diffuse sound in the room to avoid ringing and flutter echoes, and improve stereo imaging • to keep sound from leaking into or out of a room. That is, to prevent your music from disturbing the neighbours, and to keep the sound of passing trucks from getting into your microphones.

Acoustic Treatment


Controls midrange and high frequency reflections


Bass trap, is mainly for low frequencies

Live Room A live room is a room with little sound absorption and a lot of reflectivity. It has a long RT-60. A live room is generally where the recording happens, but the “liveliness” or reflectivity changes from studio to studio.

Dead Room A dead room is a room with very thick sound absorbers, causing a very dull sound with no reverberation. It ensure there is no reflection and the sound heard is only direct sound wave. The control room is required to be a dead room. It is impossible to make a completely dead room “Live" and "dead" as described here concern only the mid and upper frequencies. Separate low frequency treatment is required.

A dead room is good for solo vocal tracks but not for instrumental because that produces an eerie and unnatural sound. A hard (reflective) floor gives a nice ambience when miking drums, guitar amps, and acoustic instruments. Reflective floor helps achieve a natural sound when recording acoustic instruments. The acoustics of the room should be a combination of both, absorption and reflection. The amount of each will determine how live the room is.

There is no one correct way to treat every room because different engineers prefer a different amount of liveness, though smaller rooms require more absorptive surfaces while large studios can have all reflective surfaces. Now days, a completely dead room is also adequate since computer softwares can be used to create an ambience, but a good „baseâ€&#x; track with ambient sounds is always preferred and is considered better.

Diffusers are used to reduce or eliminate repetitive echoes that occur in rooms having parallel walls and a flat ceiling. Diffusion is often used in addition to absorption to tame sound reflections. Such treatment is universally accepted as better than making the room completely dead by covering all of the walls with absorbent material.

The simplest type of diffuser is one or more sheets of plywood attached to a wall at a slight angle, to prevent sound from bouncing repeatedly between the same two walls. Alternatively, the plywood can be bent into a curved shape. For diffusion to be effective, you need to treat more than just a few small areas. When walls are parallel, adding diffusion to only a small percentage of the surface area will not reduce objectionable echoes as well as treating one or both walls more completely.

Like diffusion, midrange and high frequency absorption helps minimize echoes and ringing. But unlike diffusion, absorption also reduces a room's reverb time. The most effective absorber for midrange and high frequencies is rigid fiberglass. Rigid fiberglass is not really rigid like a piece of wood or hard plastic. Rigid fiberglass is made of the same material as regular fiberglass, but it is woven and compressed to reduce its size and increase its density, i.e. It is more rigid than the fiberglass used for home insulation. As with all absorbent materials, the thicker it is, lower the frequency it will absorb to. or If fiberglass one inch thick absorbs reasonably well down to 500 Hz, when two inches thick, it is equally absorbent down to 250 Hz.

Acoustic interference occurs inside a room when sound waves bounce off the floor, walls, and ceiling, and collide with each other and with waves still coming from the loudspeaker or other sound source. Left untreated, this creates severe peaks and dips in the frequency response that change as you move around in the room.

The only way to get rid of these is to avoid or reduce the reflections that cause them. This is done by applying treatment that absorbs low frequencies to the corners, walls, and other surfaces so the surfaces do not reflect the waves back into the room. A device that absorbs low frequencies is called a bass trap. Bass traps are also used to reduce modal ringing, that causes some bass notes to sustain longer than others point of collision



There are a number of ways to create a bass trap. The simplest and least expensive is to install a large amount of thick rigid fiberglass, spacing it well away from the wall or ceiling. When the rigid fiberglass is mounted in a corner like this, the large air gap helps it absorb to fairly low frequencies.

A bass trap fixed onto a corner In plan

Quaternote, Shivalik, New Delhi Basement studio


Wood paneled doors with foam insulation inside and rubber beading along the edges. Door Thickness: 80mm

Glass wool covered with canvas cloth panels on all walls. In some places dado wood on bottom half of wall.

Sound lock Carpeted flooring with wooden floor beneath in both the recording and control room.

Control Room

Recording Room

The window between the control recording room consists of 2 layers of double glazed glass tilted towards the inside. The vacuum gap has a layer of stones with silica below.

Fender Music Academy, Shahpur Jat, New Delhi The studio is only acoustically treated, not insulated. Machine Room

Recording Room

Many bass traps and diffusers hang on the wall. The floors are carpeted. Windows and doors act as major leaks. diffusers

Control Room

bass traps

Bibliography: • Acoustic Treatment and Design for Recording Studios and Listening Rooms, Ethan Winer • Reverberation time • An Introduction to Recording Studio Design •The Architecture Of Sound: Designing Places Of Assembly; Peter Lord, Duncan Templeton; Architectural Press, 1986 Case studies: • Quaternote, Shivalik, New Delhi • Fender Music Academy, Shahpur Jat, New Delhi Special thanks to Gaurav and Nikhil for showing us around their studios, and to Akshay for helping us with the technical bits. Reverberation graphic: Rigid fiberglass photograph by Ethan Winer. All other photographs and drawings by authors.

Acoustic Design of a Music or Speech Recording Studio  

Building Sciences and Services project with Ammani Nair.