hearing science unit 2

What is sound

An invisible energy that evokes the sensation of hearing

In acoustics, what is sound?

A disturbance of an elastic medium that causes an auditory sensation

What 3 things are needed for sound to occur?

Vibrating object + elastic medium + force

Describe how sound is generated

Apply a force to a vibrating object —> creates a disturbance in an elastic medium —> creates the sensation of sound

Speech example

Vocal folds = vibrating object, air pressure from lungs = force, air = elastic medium

What 2 things are needed for sound to be heard?

A listener + sound transmission

What 2 properties must a medium have?

mass + elasticity

How does energy travel through a medium?

Air particles collide with neighboring particles and transfer their energy

Do the air particles travel from the source to the listener?

No. Energy is transferred from particle to particle

How does sound travel through air?

Longitudinally

Longitudinal wave motion

Air particles are disturbed in the same direction or parallel to the directionof the sound source

Compression

High density of air molecules —> peak

Rarefaction

Air molecules spread apart —> trough

Transverse wave motion

Particles are disturbed perpendicular to the direction of the sound source

Which wave best represents sound travel?

Longitudinal

What is propagation velocity?

The speed at which sound travels

What determines the speed of sound?

Properties of the medium, including mass/density and elasticity

wavelength

The distance between 2 identical points on adjacent cycles

High frequency

short period, short wavelength

Low frequency

longer period, longer wavelength

Frequency and period/wavelength relationship?

inverse

Period and wavelength relationship

Direct

What is sound pressure?

The amount of pressure being put on the air particles

What happens to sound pressure as intensity increases?

Sound pressure increases

Where is sound pressure greatest?

Closest to the sound source

What happens to sound pressure as distance increases?

It decreases as energy disperses into the environment

What are the sound taxonomy at categories?

Audibility —> complexity —> behavior —> duration

Complexity classifications

simple and complex

simple sound

one pure frequency

complex sound

multiple frequency components

behavior classifications

deterministic + stochastic

deterministic

periodic —> predictable, repeatable pattern

stochastic

aperiodic—> not predictable or repeatable

Duration classifications

impulse + continuous

Impulse sound

very short duration + loud

Impulse example

gunshot

Continuous sound

longer duration in the environment

Continuous subclassifications

stationary + nonstationary

Stationary

frequency and intensity remain constant over time

Nonstationary

frequency and intensiry fluctuate over time

Sound propagation

Sound traveling within an acoustic environment

What influences sound propagation?

Boundaries + obstacles + acoustic phenomena

boundary

a sizable object that defines a space

Boundary example

walls

What 2 types of spaces can boundaries define?

open + closed

obstacle

a smaller object that gets in the way of sound travel

acoustic phenomena

how sound behaves when it encounters a boundary or obstacle

internal friction

energy being used by the object itself

external friction

when an object collides or makes contact with another object or surface

what happens to energy during external friction

energy is absorbed by the new surfaces or object

absorption coefficient

the ability of a surface/object to absorb energy

what properties increase absorption

softer + more porous + denser

Does absorption always occur when sound hits a boundary/obstacle?

yes, there will always be some amount of absorption

reflection

Sound encounters a surface and bounces back into the original acoustic environment

reflection coefficient

ability of a medium/boundary to reflect sound energy

what surfaces have high reflection?

hard + smooth

reflective example

glass

refraction

sound encounters a boundary and travels through the boundary

What happens to the boundary during refraction?

It temporarily becomes a new medium

what affects sound traveling though the new medium

physical properties of the medium + frequency of the sound

which frequencies are least vulnerable to absorption?

low frequencies

which frequencies are more vulnerable to absorption?

high frequencies

Why does speech through a wall sound lower/muffled?

High frequencies are absorbed more; low frequencies pass through more easily.

Refraction example?

Hearing neighbors through a wall.

Reverberation?

A product of reflection.

How is reverberation time measured?

Time it takes for reflection to stop after the original sound has stopped.

Why should reverberation time be short?

Extra noise distorts the original message.

What does reverberation time depend on?

Reflective surfaces + absorptive surfaces + intensity + room size.

Where is reverberation most likely?

Closed space with more reflective than absorptive surfaces.

How does reverberation affect speech?

It can mask speech sounds, making listening and understanding harder.

Reverberation

resonance

echo

repetition

Why is an echo heard as a repetition?

More time occurs between the original signal stopping and the reflection returning.

Where is echo most likely?

Large open spaces with significant distance to a boundary.

Diffraction?

Sound energy encounters an obstacle that impedes its pathway.

High frequency = what wavelength?

short

Low frequency = what wavelength?

long

Obstacle larger than wavelength →

Creates a sound shadow

What happens in a sound shadow?

No sound occurs in the shadow; sound picks back up after it.

Head shadow effect applies mainly to what?

High-frequency sounds.

High-frequency localization cue?

interaura intensity differences

wavelength larger than obstacle —>

sound bends/wraps around the obstacle

Low-frequency localization cue?

Interaural timing differences

High-frequency/head-shadow example?

Determining the direction of an emergency vehicle siren.

Low-frequency diffraction example?

Not knowing which nearby car is playing bass because it sounds all around you.

Doppler effect

change in perceived pitch when the sound source or listener moves at a significant speed

Sound source moves toward you

Higher perceived pitch.

Why does approaching sound seem higher?

Air compresses → shorter wavelength → higher perceived frequency/pitch

Sound source moves away →

Lower perceived pitch

Does the actual frequency of the source change?

No. It is a change in perception.

What is pitch?

Our perception of frequency.

What is required for a noticeable Doppler effect?

Significant speed.

Shock wave?

Created by a sound source when it is moving at the speed of sound.

Sonic boom?

The noise the listener hears when the object breaks through the shock wave/sound barrier.

When is the sonic boom heard?

After the object has passed the listener.