Physics of Sound
Part 2
Basic Acoustics
Inverse square law
Reinforcement/cancellation
Interference
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Phase
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measurement of where the amplitude of a wave is
relative to another wave
A cycle can start at any point in a waveform
Two waves with the same frequency can start at
different times
Measured as an angle in degrees
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Related to the sine wave representation of the wave
Interference
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Constructive of
destructive interference
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Waveforms will add by
summing their signed
amplitude at each instant
in time
Beats
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Happens when two
slightly different
frequencies interfere
Often used in tuning
Standing waves
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When sound waves bounce off
of obstructions, they can
interfere with themselves
Tends to reinforce some
frequencies and attenuate
others
Prevented by using
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Non- Parallel walls, ceilings
Convex surfaces
Multi-level ceiling sections
Reverberance (Reverb)
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Consisting of multiple, blended sound images caused by
reflections from walls, ceilings and other structures which do not
absorb sound
NOT echo
 Echo consists of individual, non-blended sound images
Reverb time is related to
 The time it takes for a sound to reduce to an inaudible level
 Loudness of sound relative to background noise
 Ratio of loudness of reverberant to direct sound
Short reverb time (less than 1.5 sec) is better for speech or
drama
Long reverb time (more than 1.5 sec.) is better for music
Absorption
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Controlling reflections can reduce or increase reverb
time
Air tends to absorb frequencies above 2K Hz
Sight line obstructions
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Frequencies above 10 kHz tend to not bend around
corners well or other obstructions
 l=1.3 inches for 10 kHz tone
Frequencies below 1kHz do very well
 l=5.65 feet for 200 Hz tone
Specialists are often hired to “tune” a space
acoustically
Acoustic attributes
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Defined by Leo Beranek after a 6 year study
of 54 concert halls
Used to define acoustic properties in terms
that other trained professionals can
understand
Acoustic attributes
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Intimacy – Indicates the size of a room
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How it sounds to the listener, not actual size
Determined by the initial-time-delay-gap (ITDG)
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Interval between the sound that arrives directly at the
ear and the first reflection
Usually considered to be the most important
attribute
Acoustic attributes
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Liveness
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Related to Reverberance
Room size is related
More reflections is live. Less reflections is dry or
dead
Warmth
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More low frequency sound relative to mid
frequency
Too much low frequency sound is said to be
“Boomy”
Acoustic attributes
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Loudness of direct sound
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Inverse square law
Loudness of sound will decrease by one quarter
every time the distance from the source is
doubled
Definition or Clarity
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Good definition when sound is clear.
Related to intimacy, liveness, loudness of direct
and reverberant sound
Acoustic attributes
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Brilliance
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A hall that has liveness, clarity and intimacy
Diffusion
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Relates to the orientation of reverberant sound
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Where is the reflected sound coming from
It is preferable to have reverb sound coming from
all directions
Intensity
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Like pitch, loudness is a sensation in the
consciousness of a listener
To produce a sound twice as loud requires 10
times the power
Inverse square law
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Sound level is reduced by a factor of the square
of the distance away from the source
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If you move double the distance from the source, the
sound intensity will by one quarter
Intensity
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Intensity is a measurable quantity
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SPL – Sound Pressure Level
dB – deciBel
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A system of measuring a ratio between two powers
dB (spl)=20log(P1/p0)
p0 is a reference level, usually taken to be the
minimum intensity audible to an average person –
“Threshold of Hearing”
3 dB change is hardly noticed as a change
6 dB changes is heard as twice as loud
dB SPL
Sound
150 dB
Jet engine at 1m
140 dB
Rock and Roll stack at 1m
130 dB
Thunderclap, Air Raid Siren 1 Meter
120 dB
Jet takeoff (200 ft)
110 dB
Rock Concert
100 dB
Train passing up close
90 dB
Heavy traffic
80 dB
Hair Dryer
70 dB
City street
60 dB
Noisy bar or restaurant
50 dB
Open plan office environment
40 dB
Normal conversation level
30 dB
Library, Soft Whisper (5 Meter)
20 dB
Quiet domestic environment
10 dB
Broadcasting Studio, Rustling Leaves
0 dB
Threshold of hearing in young adult
Sound Envelope
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Listener does not hear individual cycles of sound waves
Attack – Time it takes for sound to rise from nothing to its
greatest intensity. Usually short.
Decay – Time it takes for a sound to fall from its attack level to its
sustaining level. Decay time is usually short
Sustain – The time during which the initial vibrating source
continues to supply energy to the sound. Usually perceived as
the duration and intensity of the sound
Release – Time it takes for the sound to drop from its sustain
level to inaudibility after vibrating object stops supplying energy
For Next Class
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Read
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Basic Electricity, Kai’s Sound Handbook
P. 23-27, Leonard