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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.