PSYC 215 Chapter Notes - Chapter 4: Rarefaction, Sound Energy, Acoustic Impedance
20 Nov 2018
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Chapter 4: Audition
Introduction
• Sound can be sensed in all directions around the body and through opaque occluding
objects.
• Social and cultural communication in human relies heavily on sound stimuli
• Hearing as an inverse problem i.e. perception of sound
− World distal stimulus (the sound stimulates the body’s sense organ)
− Sound waves = proximal stimulus - the raw pattern of neural activity (sensory organ
transforms the input energy from distal stimulus into neural activity i.e. transduction)
− Brain reconstruction (neural signals are transmitted and processed in the brain to be
perceived as the sound)
Sound as a Physical Stimulus
• Sound consists of pressure waves carried by vibrating air molecules
− Usually produced by a vibrating surface
→ Gong struck vibration push the surrounding air molecules pressure change
passed on by collisions between air molecules sound waves travel away
(335m/sec)
→ vibrating surface creates waves of fluctuating air pressure waves consisting of
alternating ‘phases of compression and rarefaction.
− Compressions : parts of the wave where air pressure is increased
− Rarefaction: parts where pressure is decreased
• 2 types of waves: longitudinal and transverse
− Transverse waves are used in light
→ The movement of particles perpendicular to the direction of propagation
− Longitudinal waves are used in sound
→ The movement of particles in the same direction as the propagation of the signal
→ Pressure vibrations: compression and rarefaction
• Water is an example of both waves together
Simple Sounds
• An air pressure wave is called longitudinal
− Particles vibrate back and forth in the same direction as the wave
• Sine wave: A wave whose height varies smoothly so that it conforms to a mathematical sine
function of time or distance (sinusoidal)
− Simplest sound wave (i.e. pure tone by a tuning fork)
− Repetitive alteration between compression and rarefaction as a function of distance
from the sound source has the same shape as the variation in sin(θ) as a function of an
angle θ.
− Waveform plot -> represents the rate of alternations
• Sound waves have three important features:
amplitude
− Frequency (perceived pitch)
→ The number of cycles of a wave per unit of time or distance
▪ Cycle = a single alternation between compression and rarefaction
▪ Number of alternations between compression and rarefaction generated in 1
second
→ Unit is Hertz
▪ 1 Hz = one cycle per second
→ Rate of vibration determine frequency of resulting sound pressure wave
→ frequency = perceived pitch
▪ High frequency -> high, treble pitch
▪ Low frequency -> deep, bass pitch
− Amplitude (perceived loudness)
→ The maximum height of a wave, measured from its mean value to its maximum
value
▪ Amount of change in pressure created by it
→ Expressed in decibel
▪ A measure of difference between two quantities, based on the logarithm of their
ratio (equal ratios between the quantities correspond to equal dB differences)
→ dB scale has two important properties
▪ dB scale is relative
db Sound Pressure level (SPL)
A decibel measure of sound pressure relative to a fixed reference pressure
This is chosen because it is close to the minimum sound pressure
detectable by humans (1000 Hz)
Specifies that amplitude of a particular sound is a certain number of times
higher or lower than the standard pressure
▪ the dB scale is logarithmic
Frequency
Amplitude
Phase
Equal ratios of pressure level correspond to equal increments in dB level
each tenfold change = dB change of 20dB
Each doubling of pressure adds 6 dB to sound’s decibel level. (quadruple
intensity)
Doubling of intensity adds 3dB
Logarithmic scale is used because it allows a wide range of pressure levels (1
– 10 million) to be expressed in a compact range of dB values (0 to 140)
SPL roughly corresponds with apparent loudness of a sound
Relation is complicated
SPL of normal conversation - 60dB
Loud thunder/ amplified rock music - 120 dB
Because of the logarithmic nature, the pressure amplitude of rock is
1000x greater than amplitude of normal conversation
SPL of 140 dB cause pain and hearing loss
− Phase
→ A measure of the timing or position of a wave relative to a fixed point of reference
or to another wave (measured in degrees)
→ Part of the cycle that a sound wave has reached at a given point in time.
→ Often used to compare timing of two sound waves
▪ 0 - resting
▪ 90 - maximum pressure
▪ 270 - minimum pressure
Complex Sounds
• Natural sounds never produce sounds that conform to the simple waveforms.
− Plots of sound pressure reveal more complex variation
− Any complex sounds are a large collection of simple sine waves added together
• Complex wave: A wave that can be decomposed into a collection of simple, sinusoidal
waves
− frequencies, amplitudes and phases of sine waves determine the overall form
Document Summary
Introduction: sound can be sensed in all directions around the body and through opaque occluding objects, social and cultural communication in human relies heavily on sound stimuli, hearing as an inverse problem i. e. perception of sound. World distal stimulus (the sound stimulates the body"s sense organ) Sound waves = proximal stimulus - the raw pattern of neural activity (sensory organ transforms the input energy from distal stimulus into neural activity i. e. transduction) Brain reconstruction (neural signals are transmitted and processed in the brain to be perceived as the sound) Sound as a physical stimulus: sound consists of pressure waves carried by vibrating air molecules. Gong struck vibration push the surrounding air molecules pressure change passed on by collisions between air molecules sound waves travel away (335m/sec) Vibrating surface creates waves of fluctuating air pressure waves consisting of alternating phases of compression and rarefaction. Compressions : parts of the wave where air pressure is increased.
