ACOUSTIC DESIGN OF A MUSIC OR SPEECH RECORDING STUDIO

ACOUSTICS II

BUILDING SCIENCES AND SERVICES

AMMANI NAIR A.2022.2008 | BHAVIKA AGGARWAL A.2004.2008SPA DELHI IIIRD YEAR SECTION ‘A’

FrequencyIt 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.

WavelengthSound 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 WavelengthHigh Frequency

Long WavelengthLow Frequency

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

attenuate : to reduce in strength

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

Some energy transmitted

Sound waves from source

Some energy reflected

Some energy absorbed

ReverberationThe persistence of sound in a particular space after the original sound is removed.orThe 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 echoesto 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.

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.orThe time for the sound to die away to a level 60 decibels below its original level.

typical loudest crescendo of orchestra

100db

typical room background level

40db

60db drop used to define the standard reverberation time

Calculation of RT60Sabine equation

RT60 = 0.163 x V/A

whereV= volume of the roomandA= effective absorbing area

= α x S (surface area)

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

Optimum Reverberation Time

0s. No reverberation.

0.3s. Difficulty hearing in the back, difficulty hearing the bass

1s. Clearer articulation of speech, loss of richness and fullness

3.5s. Fuller, richer musical sound, some loss of articulation

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

1.5 to 2.5s. General purpose: both speech and music

8.5s. Notre Dame.

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.

Recording room or studio

Control room

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

where the

musicians perform

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 Width Length

1.00 1.14 1.39

1.00 1.28 1.54

1.00 1.60 2.33

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

Standing WavesIt 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.

SOUND SOURCE

STANDING WAVES

SOUND WAVES

WALLS

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

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

OUTSIDE

SOUND WAVES

SOUND-ABSORBING MATERIAL

INSIDE

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

Absorbers

Controls midrange and high frequency

reflections

Bass trap, is mainly for low frequencies

Diffusers

Live RoomA live room is a room with little sound absorptionand 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 RoomA 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 „base‟ 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

original reflected

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 cornerIn plan

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.

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

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

Quaternote, Shivalik, New DelhiBasement studio

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

RecordingRoom

ControlRoom

Lobby

Sound lock

Fender Music Academy, Shahpur Jat, New Delhi

The studio is only

acoustically treated, not

insulated.

Many bass traps and

diffusers hang on the

wall. The floors are

carpeted.

Windows and doors act

as major leaks.

diffusers

bass traps

Control Room

Recording Room

Machine Room

Bibliography:

• Acoustic Treatment and Design for Recording Studios and Listening

Rooms, Ethan Winer

www.ethanwiner.com/acoustics.htm

• Reverberation time

http://hyperphysics.phy-astr.gsu.edu/hbase/acoustic/revtim.html#c2

• An Introduction to Recording Studio Design

http://www.ahisee.com/content/rsdpart1.html#TOC16

•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: www.acousticalsurfaces.com/acoustic_IOI/101_6.htm

Rigid fiberglass photograph by Ethan Winer.

All other photographs and drawings by authors.