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Agenda
Sound Waves
Room Acoustics
Sound Wave Reflections
Absorption Coefficients
The Sabine Equation
Reverb Calculation Example
Estimating the Reverberation Time
Reverb Calculation Example 2
Correcting the Reverberation Time
Absorbers
Reverberation
Sound Waves
Sound Propagation
The Recording Environment
What is Sound?
Sound is produced when an object (the source) vibrates and causes the air around it to move.
The Recording Environment
Sound Propagation
• Sound travels in air as a lengthwise or longitudinal wave. This sort of density wave is the way in
which sound is transmitted through air, gases and liquids.
• In solids we also find transverse waves.
The Recording Environment
Sound Propagation
Longitudinal Wave
The Recording Environment
Snell's Illustration of Sound Waves
http://www.youtube.com/watch?v=OQPI5Ng-3vI&list=PL689066FDE3D631CE
Disturbance takes place perpendicular to the direction which it is moving
Sound Propagation
Transverse Wave
The Recording Environment
Sound Propagation
Transverse Wave Machine
The Recording Environment http://www.youtube.com/watch?v=tihcRFWeZlQ&list=PL689066FDE3D631CE
Clip taken from - Sound Waves and their Sources (1933)
The Recording Environment
The Recording Environment
Sound Waves
Sound Wave Properties
The Recording Environment
Sound Waves: Fundamental Mathematical Relationships
The Recording Environment
Time
Am
plit
ude
0
+
-
t
The rate at which the source oscillates is the frequency of the sound wave it produces, and is
quoted in hertz (Hz) or cycles per second (cps). 1000 hertz is termed 1 kilohertz (1kHz)
The Recording Environment
In air, the speed of sound is approximately 340 meters per second
The frequency and wavelength of a sound wave are related very simply if the speed of the wave
(usually denoted by the letter c) is known.
c = fλ
(speed of the wave = frequency x wavelength)
or
λ = c/f
(wavelength = speed of the wave x frequency)
(λ = The Greek letter lambda is often used to represent wavelength)
The Recording Environment
Relationship between elapsed time and traversed distance for the propagation of a sound wave in air
time (ms)
2 4 6 8 10 12 14 16 18 20 22 24 ms
0.7 1.4 2.0 2.7 3.4 4.1 4.8 5.4 6.1 6.8 7.5 8.2 m
distance (m)
The Recording Environment
Room Acoustics
The Recording Environment
Influence of Acoustical Space on the Sound Event
The Recording Environment
In enclosed performance spaces, a new phenomenon appears: reverberation, which is caused by sounds
being repeatedly reflected from all surfaces and objects in the room.
The Recording Environment
Room influences may be described in two ways:
1. Objectively through measurement of the sound events and their variation with time (room
acoustics.
2. Subjectively through verbal description of the audible experience (aural acoustics)
Both methods are necessary depending on the question at hand; either the objective or subjective
one may assume the greater importance.
The Recording Environment
The Recording Environment
Fundamentals of Aural Acoustics
The Recording Environment
Fundamentals of Aural Acoustics
The Recording Environment
1. The Listenability of a Room - Generally describes its suitability for certain sound events.e.g
Good listenability of a room for speech means that we hear speech well at every seat in the
house without the need for reenforcement.
2. The Transparency of a Room - The ability to differentiate between simultaneously played
instruments or instrument groups in spite of superimposed room reverberation. Transparency
is a basic requirement when we hear complex musical structures.
Fundamentals of Room Acoustics
The Recording Environment
Sound propagation in an enclosed room (the rays show the propagation direction and intensity)
The Recording Environment
Direct Sound
First Reflections
Reverberation
Sound Level
Time
Direct Sound and diffuse sound (reverb build-up, early reflections and decay) in an enclosed space
The Recording Environment
The term RT60 refers to the time it takes the reverb to decay by 60dB. RT is measured at the point at
which the reverb decays to -60dB of its peak level.
RT60
The Recording Environment
The Recording Environment
• Direct sound arrival is followed by reflections from room surfaces.
• Overlapping reflections are heard as reverberation.
• Direct-to-Reverberant ratio gives cues to size of room, type of room surfaces, and distance from source.
The Recording Environment
Significance of Room Tone for Microphone Placement and the Listening Experience
The Recording Environment
• The microphone generally picks up both direct and diffuse sound.
• While the direct sound is influenced little by the nature of the room, the diffuse room
tone transmits information about the room size and the nature of the wall treatment.
• The acoustical attributes of the room tone provide information about the cultural and
social environment into which a musical performance has been placed.
Significance of Room Tone for Microphone Placement and the Listening Experience
The Recording Environment
• Thus church music requires the acoustics of a large church for which it generally is
written; symphonic music is written for concert halls, chamber music for the small,
private room in a castle or home.
• Folk music needs the intimate atmosphere of a pub.
• In pop music and other similar musical forms, we see the creation, through the use of
artificial reverberation of new acoustical surroundings which really do not exist in real
life.
Significance of Room Tone for Microphone Placement and the Listening Experience
The Recording Environment
Sound Wave Reflections
The Recording Environment
Sound Wave Reflections
Creation of Sound Reflections
The Recording Environment
Sound Wave Reflections
Creation of Sound Reflections
The Recording Environment
1. Sound reflections within a room occur when sound reaches a boundary surface
without too much absorption.
2. Dimensions and nature of the surface determine how the diffused sound is
scattered.
Sound Wave Reflections
The Haas Effect
The Recording Environment
The Haas effect can be summarised as follows:
• The ear will attend to the direction of the sound that arrives
first and will not attend to the reflections providing they arrive
within 30 ms of the first sound.
• The reflections arriving before 30ms are fused into the
perception of the first arrival. However, if they arrive after
30ms they will be perceived as echoes.
Angus, J & Howard, D (2009) Acoustics and Psychoacoustics. UK, Elsevier.
Comb Filtering
The Recording Environment
direct path
Comb Filtering
Diagram showing an example of a comb
filter created by the combining of two signals
with the same amplitude, but with a time
delay between them of just 1 ms.
A.C.M.E Mixing Desk
The Recording Environment
speaker
Mic
Reflected Sound
Sound Source
Direct Sound
Surface
Resulting Frequency Response
dB
The Recording Environment
An affected waveform shows lots of sharp peaks and troughs that look not unlike the teeth of a comb
Ceiling reflections cause acoustic
interference at the listeners position
Splayed Ceiling Design
The Recording Environment
Splayed ceiling reduces
unwanted reflections
The Recording Environment
Standing Waves
Max Min Max Min Max Min Max.
Pressure
Bo
un
dary
Maximum
Minimum
Pre
ssure
Reflection
The formation of a standing wave by reflection at a boundary
Room Resonances, Natural Frequencies & Modes
The Recording Environment
Image showing room resonances, natural frequencies & modes
Bo
un
dary
Bo
un
dary
F
2F
3F
11.3 Feet
100Hz
200Hz
300Hz
Image showing room resonances, natural frequencies & modes
100Hz
The Recording Environment
The same effect will
occur at frequencies that
are multiples of 100Hz
(200Hz, 300Hz, etc)
Node
Anti Node
The Recording Environment
