Acoustics Review Flashcards
Importance of understanding acoustics
Reading Audiograms
Understanding Hearing loss
Environmental Acoustics
What is sound?
An object makes a sound by
vibrating
The vibrations pass through air
making air molecules vibrate
When the sound pressure waves
reach the ear, hearing may take
place
Physical Acoustics:
Propagation of pressure waves through an elastic medium, without permanent
displacement of the particles
Three prerequisites:
◦ Source of energy (eg a force)
◦ Vibrating object
◦ Medium of transmission
A receiver of these prerequisites is optional!
Characteristics of Sound
Mass
◦ form of matter capable of vibratory
motion – air molecules
Inertia
◦ tendency to resist change in motion
Elasticity
◦ Restorative property of an object
Simple Harmonic Motion
◦ Compressions and rarefaction
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Simple Harmonic Motion
Pure tone
◦ A sound source sets air molecules into vibration.
◦ Air molecules bunch together (compression) and then spread out (rarefaction) at
a steady rate.
◦ Elasticity restores the displaced air molecules to its original resting place
◦ Inertia causes the air molecules to swing past the rest position when returning to
their original state.
◦ This chain reaction of air disturbances create pressure waves that reach our ear
and vibrates the tympanic membrane which transfers the sound to the brain.
Properties of sound
Frequency
◦Period
◦Wavelength
Phase
Intensity
(Amplitude)
Frequency
Range of human hearing HZ?
of cycles completed in 1 sec
Unit of measurement: Hertz (Hz)
Perceptual correlate: pitch
Human range of hearing: 20 – 20,000 Hz
Wavelength (λ)
Distance sound wave travels in
one cycle of vibration
Length of the wave
λ = v/f (v=1130 ft/sec or
340m/sec)
Inverse relationship between f & λ
Wavelength (λ) and Distance
Low vs high frequency
Wavelength = velocity/frequency
λ250Hz =1130ft/s/ 250Hz = 4.52 ft
λ8000Hz = 1130ft/s / 8000Hz = .14ft or 1.68 inches
Low frequency sounds have longer wavelengths
that refract or bend more easily around objects
than high frequencies.
Phase
Position in time of the displacement
of the air molecules during simple
harmonic motion (how cycles relate
to each other)
Unit of Measurement: Degrees
Standard is 0 degrees
Resonance and frequency
All physical objects, if disturbed by another object will begin to
vibrate.
All objects possess a natural resonant frequency
Resonant frequency – frequency rate at which an object will
vibrate with the least amount of externally applied energy,
depending on its specific density and stiffness (elasticity)
For example, a tuning fork with a specific intensity/stiffness will
vibrate at a specific frequency when struck.
Resonance and speech and hearing
◦Fundamental frequency of the vocal cords determines the
pitch of the speaker’s voice
◦Length and shape of the vocal tract and articulators provide
clues regarding the identity of the specific sound (formant
frequencies)
◦Length and shape of external auditory meatus resonate at
high frequencies and enhances speech understanding
Frequency and sound localization
Diffracton - Sound bends around a barrier
Reflection - Sound bounces off of a barrier
Head Shadow Effect
◦ Low frequencies bend around the head. This creates a phase or time
difference at the opposite ear.
◦ High frequency sounds will not wrap around the head as readily. This
creates a lowering of sound levels at the opposite ear.
Sound localization is dependent on the ear picking up these
phase (time) and intensity cues.
Intensity
Intensity is force over unit area
Amplitude of the displacement
Unit of Measurement: decibel (dB)
Perceptual correlate: loudness
Normal human ear can
hear from about 0dB – 140 dB SPL
Micropascals (μPa)
Sound waves travel through air and create pressure
waves
Pressure is force / area (Relates to amplitude of the pure
tone)
The greater the change in pressure, the greater the
intensity of sound.
The unit of measurement of air pressure change that we
hear is micropascal (μPa)
Micropascal (μPa) is an absolute unit for measuring
sound pressure
