03/03/2016
Session 7
Acoustics in the
Environment
Rachel Edwards &
Nick Roberts
& Gavin, a bit
Today’s session
• Wave spreading
• Amphitheatres and reflections
• Reverberation time / absorption and
their effect on room acoustics
• Anechoic chambers
• Acoustics in the environment
• Testing of rooms!
1
03/03/2016
Wave spreading
Sphere surface area
A  4r 2
Area of an ear
Aear  12cm 2
Energy of sound collected
 Aear (m 2 ) 

E  E0 
2
 4r 
Reflecting walls


Architectural acoustics, Marshall Long
2
03/03/2016
Amphitheatres
Amphitheatres: Epidaurus
The Origins of Building Acoustics for Theatre
and Music Performances – John Mourjopoulos
169th ASA Meeting
Architectural acoustics, Marshall Long
3
03/03/2016
Amphitheatres: Epidaurus
The Origins of Building Acoustics for Theatre
and Music Performances – John Mourjopoulos
169th ASA Meeting
Acoustic response measurement for the
Epidaurus theatre, assuming that the source
emits a short pulse and the microphone is at
a seat at 15 meters.
Reverberation
• Sound waves can take many
paths
• Use a sharp sound (clap, or
gun firing) and listen for
echoes
Figures from hyperphysics website
4
03/03/2016
Sound intensity
Reverberation
• Or, generate a steady sound and look
at decay when it’s switched off
From Measured
Tones
Time (s)
Intensity
Reverberation time;
I (T60 )  I (0) 10 6
time
Absorption
From Measured Tones
From hyperphysics website
Effective absorbing area = real area x absorption coefficient
5
03/03/2016
Absorption
Treble absorber
Soundonsound.com
•
•
Rough, porous surface
Long wavelength ignored, small wavelength scattered
Absorption
Bass absorber
•
•
gikacoustics.co.uk
E.g. wood panels
Large surface that can vibrate and trap energy for low frequencies
6
03/03/2016
Anechoic chamber
University of Salford
DTU, Denmark
(photo: Alastair Philip Wiper)
Acoustic cloaking?
Metamaterials: control, direct and manipulate waves
Víctor Manuel García-Chocano
Steven Cummer, Duke University
7
03/03/2016
Absorption
Effective absorbing area, Sa = real area x absorption coefficient
Measured in Sabines
From Measured Tones
Absorption
Effective absorbing area, Sa = real area x absorption coefficient
Measured in Sabines
Volume of the
room
Reverberation
time
T60 
24 ln 10 V
V
 0.1611
c20 Sa
Sa
Speed of sound
in air at 20C
8
03/03/2016
Reverberation time
Absorption
z
x
y
Reverberation time
From Measured Tones
9
03/03/2016
Butterworth Hall
Acoustics in the environment
࢜ ൌ ࢊȀ࢚
10
03/03/2016
Acoustics in the environment
Video and image © Aaron Corcoran - www.sonarjamming.com
Non-destructive testing
How to make sure this doesn’t happen;
Broken axle on
second car
11
03/03/2016
Non-destructive testing
• Look at reflection or transmission of sound
Non-destructive testing
Hole!
Track
12
03/03/2016
Non-destructive testing
Hole!
Track
Ultrasound in the environment
13
03/03/2016
Ultrasound in the environment
Adding reverb
Live (amplified) performance: example…
• Laura Moody @ the concert
• Song: Satellite
• Request: “Can I have the reverb we discussed please!”
Studio recording:
• Record “dry” (but not “flat”, in my experience).
• Add reverb in a controlled way.
• Often a necessity rather than a choice, e.g. overdubs,
small studio spaces.
• Rock band vs. string quartet?
14
03/03/2016
Adding reverb: rooms
Nice speaker, nice microphone, nice room…
Adding reverb: springs
Speaker 
transducer
Room  springs
and plate
Microphone 
transducer
15
03/03/2016
Adding reverb: digital
Early
reflections
Reverb tails
Easy: copy sound and add a little later  early reflections
But: reflections of reflections of reflections….
Computationally horrible! Algorithms to approximate tails.
16
03/03/2016
Adding reverb: convolution
Measure the impulse response of an actual acoustic
space.
Microphones and sudden sound in a real space, e.g.
Sydney Opera House or BandSoc Room.
Then “mix” the impulse response with dry sound
source. Mathematical operation: convolution.
17