Download Week 2 - Acoustics - Anderson Sound Recording

Acoustics in Twenty Words or Less
What is Acoustics?
The Science of Sound!
What is Sound?
 Minute changes in air pressure
 Longitudinal waves traveling outwards from their source. These
waves consist of alternating areas of high and low pressure
(compression and rarefaction)
 Sound travels at 1130 feet per second (345 meters per second or
770 miles per hour) in dry air at room temperature. As
temperature increases, so does the speed of sound.
 Sound can reflect off of some surfaces and be absorbed or
blocked by others. This behaviour is completely predictable!
 Sound has two major components - frequency and intensity which happen over time
What is frequency?
 Harmonic waveforms have a repetitive cycle or pattern (called a
period)
 Frequency refers to the number of times this pattern is repeated
over a certain length of time
 Frequency is generally measured in Hertz (Hz) - cycles
(repetitions) per second.
 Frequency gives us Pitch and Timbre
Pitch
 The “highness” or “lowness” of a sound
 Pitch is how our brain interprets the “fundamental frequency,”
usually the lowest frequency in the spectrum of a sound.
 Higher frequency generally means higher pitch
 Doubling the frequency gives us the musical interval of an octave
 Humans interpret frequency logarithmically: the distance between
200 and 400 Hz sounds the same as the distance between 2000
and 4000 Hz.
Timbre
 Almost every sound is made up of a combination of different individual
“pitches” (frequencies) which are happening simultaneously
 These multiple frequencies are referred to as partials, harmonics or
overtones.
 Sine wave is made of 1 pure frequency (no overtones)
 This combination is often referred to as a “frequency spectrum” (white
light/prism analogy)
 Each instrument and voice has a unique frequency spectrum which our
ears interpret as “timbre” (i.e. the spectrum of a violin is different than the
spectrum of a flute)
 “the quality of a sound that that distinguishes it from other sounds of the
same pitch and volume”

– Bruce Swedien
Intensity = loudness or volume
Intensity - Sound Pressure Level
 The intensity of a sound is directly related to the
amount of pressure it generates (measured in Watts
per meter2).
 Higher pressure = more intense = “louder”
 Not all frequencies sound equally loud to us, even
though the actual intensity may be the same.
 Our ears are much more sensitive in the range
between 100 Hz and 10 kHz and especially so
between about 1000 and 5000 Hz.
 The “loudness” of sound is expressed in decibels (dB
SPL).
Analyzing an instrument’s intensity
over time
 A instrument’s sound can be analyzed by how it behaves over
time.
 Attack - the onset of the sound, sometimes called an attack
transient
 Decay - after the initial transient, there is often a drop-off in the
amount of energy in the sound.
 Sustain - After the energy from the attack is spent, the instrument
may ring on for some time
 Release - the behaviour of the instrument after the note is no
longer being held by the musician
 These terms are derived from synthesis and are not always
easily applicable to acoustic instruments, but can be useful if one
would attempt to duplicate the sound electronically
A bit about the decibel
 A decibel is a logarithmic ratio.
 Ratio: “a relationship between the amounts or sizes of two things”
- Webster’s II New Riverside Dictionary. Houghton Mifflin Co. 1984
 Zero dB means that there is no difference between the levels
that are being compared - not that there are no levels.
 This type of ratio is used to compare many things - voltages,
volume units, etc. including sound pressure levels
 With SPL, 0 dB = the threshold of human hearing - the softest
sound the average human can hear at 1000 Hz (measured as 1012 W/m2).
 The intensity of every sound is measured against this specific
intensity and the ratio is expressed in dB SPL.
Turning Sound Waves into Brain Waves
 The ear is a transducer
 Sound waves reflect off of the outer ear (pinna) into the ear
canal. They set the ear drum into motion which is attached to
three small bones - sound energy is transformed into mechanical
energy.
 These three bones transmit their motion to fluid in the cochlea.
 Thousands of tiny hair cells, which are hard wired to the brain,
pick up the fluid motion. The response of these hair cells is
frequency dependant. Their location along the cochlea
determines their frequency response.
 Depending on where in the cochlea the waves are strongest,
those hair cells will transmit the strongest signal to the brain.
The Human Ear
How loud is too loud?
 Level and duration of exposure are related.
As SPL increases from 85 dB(A-weighted) in 3
dB increments, safe exposure time is cut in
half.
 “Exposure to 115 dB for any duration may
pose a serious health risk.” - HEI
 If you must raise your voice to be heard in
normal conversation, you may be risking
permanent hearing loss.
Save your ears!
 Damage to your ears is permanent. There is
no cure for hearing loss!!!!
 Avoid excessive exposure
 Wear ear plugs
 Take 15 minute “quiet breaks” every hour or
two
 Have your hearing checked annually
NIOSH-Recommended SPL
Exposure Guidelines
Level (dB(A) SPL)
Maximum Exposure
85 dB
8 hrs.
88 dB
4 hrs.
91 dB
2 hrs.
94 dB
1 hr.
97 dB
1/2 hr.
100 dB
15 minutes
103 dB
7.5 minutes
106 dB
3.8 Minutes
Source: House Ear Institute
Your room as an instrument
Room Resonance
 Speed of sound = frequency * wavelength
 Since the speed of sound is constant, as
frequency increases wavelength must decrease
(and vice versa)
 The dimensions of your environment have a
large impact on the sound
 Those frequencies whose wavelengths are exact
multiples of the dimensions of the room are
known as room resonances or room modes