CLASS 1: APPLICATIONS OF ARCHITECTURAL ACOUSTICS, Sep. 10, 2015
Sound Proofing vs. Acoustical Treatment
We use different materials to achieve both of these goals, and in most cases, these
materials don’t provide both soundproofing and acoustical treatment; they are separate
issues studied and addressed by the same people.
Sound Proofing = Reducing sound transmission between spaces
Soundwaves move like an assembly line, not air molecules themselves moving across a
room with the sound. Sound leakage between rooms is not air molecules escaping; it’s
air molecules on either side of a wall or a floor or whatever getting moved by acoustical
energy on the other side. Sound proofing tries to limit the transfer of acoustical energy.
Sound Proofing is achieved with mass, isolated construction and tight closures
-Room within a room construction
-Neoprene isolation
-Mounting speakers on cinder blocks (sometimes filled with sand) to decouple them from
the floor.
-Door and wall seals and thickness can compromise soundproofing significantly.
-White (chapter 18) goes into more detail on room within a room construction, isolated
wall construction and door upgrading
Acoustical Treatment = Tailoring the sonic quality and character of a space
-This is often corrective, where we are trying to compensate for undesirable resonances,
reflections, and imbalances present in a space that negatively affect the sound in some
way
-Some designs and treatments are constructed to achieve a specific character (Distler
Hall, Symphony Hall, Jordan Hall, Mechanics Hall, A recording studio).
-When we talk about recording, we often make different considerations for control rooms
and ‘live rooms’ where we would record acoustic instruments, which will cover later
Issues we attempt to address with acoustical treatment
REFLECTIONS
Any hard, flat surface, will reflect sound just like a mirror will reflect light, at any
frequency whose wavelength equals one dimension of the surface.
Triangle of acoustic approximations:
Frequency of 1 kHz = Wavelength of 1 ft = Period of 1 ms
THIS PRESUMES that the speed of sound is 1126 ft/second
Analogy for a room with a lot of reflections
 Imagine a room full of mirrors, with two bright lights shining into those mirrors at
the same time.
Echo = distinct single reflections where we can pick out the individual occurrences
Acoustically distinct delays will require distances of at least 20’ because we begin
hearing distinct delays around 20 ms. The classic canyon echo presents a large, full
spectrum reflector a long distance away. A sound projected against that surface will
likely reflect some time later, and present a single distinct echo.
Flutter echos and focuses are a concern for small rooms and studios. These occur
when high-frequencies are reflected multiple times.
Reflections of less that 20 ms can result in comb filtering
 Desks
o Play examples of a microphone on a desk stand
 Music stands
o Have them speak into a music stand, then add foam
 Hard surfaces around speakers
o Take a portable studio monitor, set it up in a corner, then move it away
 Control room glass
Frequency Specific Concerns
Nodes = A point of destructive interference resulting in zero pressure aka: no vibrations
at a certain frequency.
A node occurs when direct and reflected waves that are out of phase with each other
collide in the air. It is a place in the room where a null or dip in the frequency response
occurs.
source
Modes = wave pattern associated with a particular standing wave
A mode occurs when direct and reflected waves that are in phase with each other collide
in the air. It is a natural resonance that occurs in a room, and the frequency of the
resonance depends on the room dimensions.
source
Echos and Reverberation
Reverb = dense collection of many indistinct reflections heard within 100 ms of the
original articulation
(source)
Reverb is often measured using:
RT60 = measurement of time it takes for a sound to decay below -60 dB
 measure RT60 with a impulse (swept sine, balloon, hand clap, etc.) and look at
the waveform in a DAW.
 Calculate using Sabine’s Reverberation Calculation calculator available here
o takes the room’s volume and divides it by the absorptive coefficients *
area of all the surfaces of the room
Additional references: http://hyperphysics.phy-astr.gsu.edu/hbase/acoustic/revtim.html
While concert halls and even studios like Abbey Road studio 2 are optimized for acoustic
music (RT60 > 1 second), they are compromised for amplified music, speech
intelligibility, critical listening, or objective measurement
Critical Distance = location where direct and reflected sound are equal
MATERIALS USED IN ACOUSTIC TREATMENT
Absorption = conversion of acoustical (mechanical) energy to heat
 the amount of heat generated is marginal
Absorbers
 Acoustic foam, draperies, soft furniture, porous people
Absorption coefficients of materials
 measured across the spectrum
 0 = perfect reflector 1 = perfect absorber
 Noise Reduction Coefficient = average of four octaves (250, 500, 1k, 2k)
 We are generally looking to absorb equal across the spectrum
find common absorption coefficients in White: pg.169
Anechoic Spaces
 Used measurement, but not appropriate for critical listening or recording because
they absorb a great deal of the acoustical energy and would require a very
powerful monitoring system or acoustic source, and are unnatural to listen in
 Measuring outdoors or in large spaces to minimize early reflections
 Some recordings made in anechoic spaces:
http://www.openairlib.net/anechoicdb
Diffusors
 Quadratic, Fractal, phase grating, formulas depend on what part of the spectrum
you aim to diffuse. Free construction plans at http://arqen.com/
 Book cases and interrupted walls without large parallel surfaces help
Reflectors
 Hard flat surfaces (floor, ceiling, walls, desks, music stands)
 Concave shapes
o Dome shaped ceilings
o Coliseums/amphitheaters
o Whisper galleries (Mapparium in Boston)
o Whisper benches (Philadelphia, Central Park, Tanglewood)
Intentional sources of reflection:
 Acoustical Clouds and Shells: Kennedy Center Before/After
 Non-parallel walls
 Surfaces meant to enhance reflections in certain frequency ranges
MEASUREMENT TOOLS
 EASE
 SMAART
 REW http://www.roomeqwizard.com/
 http://www.faberacoustical.com/
Building a small studio
Are you building a recording studio for acoustic instruments, or control/listening room
optimized for monitoring systems and a little voice recording?
 for monitoring, mixing, mastering, we want a space that will not color the sound
o T of 0.2-0.5 mS, occasionally with extended decay in low frequencies
 for recording acoustic instruments, we may not want a space that colors the
sound, or we may want something that does!
o think about the space while you’re tracking and how you’ll eventually want
this instrument to sound in the mix
o use microphone and instrument placement to minimize or maximize the
role of the room in your recording
60
Priority list of acoustical concerns for a control room/home studio:
 speaker placement
o decoupling from the room with mass (heavy stands, cinder blocks)
 mix position
 major reflections
 sound proofing
 isolation
o ‘acoustically sealed’ room within a room


fire proofing is a thing
if it sounds good, is it good?
Special concerns for a studio where you’re recording acoustic instruments
 sound proofing
 ambient noise floor
 noisy devices
 HVAC
 sonic character
Everything else to consider for the small studio:
 neighbors & sound proofing
 how long will you be there? is this a remote space? Make it work?
 do you need a critical mixing/mastering/acoustic recording space, or just a place
to be creative without distraction?
Working in less-than-accurate recording spaces
 use good headphones to monitor low-end
 look for consistent and drastic gestures across your work
Next Steps:
Careers, internships and graduate programs in musical and architectural acoustics:
Bose
Cavanaugh Tucci - Senior Acoustical Engineer
Other acoustics consultants near Boston:
http://www.acentech.com/
http://perkinswill.com/
http://www.sonic-space.com/
http://www.fmdesign.com/
Graduate programs
ASA Directory: http://asa.aip.org/asagrad/gpdir.cm.html
ASA Resource:
http://acousticalsociety.org/education_outreach/careers_in_acoustics
RPI: http://symphony.arch.rpi.edu/acoustics/
Penn State: http://www.acs.psu.edu/
University of Florida: http://gsoa.dcp.ufl.edu/master-of-science-in-acoustics/
ARUP:
http://www.arup.com/Careers/Graduates_and_interns/Americas/GraduateOpport
unities/Acoustics.aspx
Try to optimize your own recording or listening space:
-6’ equilateral triangle speaker placement, away from walls
-Homebrew adsorption and diffusion (bookcases, couches, large mammals)
-Calculate/analyze for and beware of modes & nodes
-Download REW (free!) and play!
-Check out other free resources
http://www.xix-acoustics.com/
http://arqen.com/