AMP Week 4B

Impedance

the total opposition to motion

Propagation of waves encountering media of different impedances…

will be modified

Impedance mismatch between media influences…

the amount of sound energy that will be reflected and absorbed by the media

Sound Reflection

When a wave reaches the boundary between one medium another medium, a portion of it will bounce off the obstacle/ reflect back

Sound reflection leads to…

reverberation and echo

Sound refraction

involves a change in the direction of waves as they pass from one medium to another

Sound absorption

measure of the amount of energy removed from the sound wave as the wave passes through a given thickness of material

Sound diffraction

involves a change in direction of waves as they pass through an opening or around a

barrier in their path

Sound absorption is _____ dependent

frequency

absorption increases with ________

frequency

Sound absorption is the ability of…

the medium and its boundaries to absorb the energy of sound waves

Sound reverberation

in an enclosed or semi-enclosed environment, a sound wave will be broken up as it is bounced back and forth among the reflecting surfaces

Reverberation is the persistence of…

sound in a particular space after the original sound is stopped

Result of multiple reflections of sound

Reverberation

Reverberation will increase________

loudness and ambient noise levels

Reverberation Time

duration of time needed for a sound pressure to decrease 1000 times (60 dB) after the sound source ceases its operation

time required for sound to “fade away” or decay in closed spaces, can vary from practically 0 to several seconds

reverberation time

Echo

a repetition or a partial repetition of a sound due to REFLECTION

Reverberation is also due to sound reflection but in reverberations,

separate repetitions of the original sound are not distinguishable, Your brain perceives it as one continuous sound

when the sound pulse can be heard twice

echo

anechoic chamber

A SOUNDPROOF room specially constructed so that its walls, ceiling and floor effectively absorb all sounds striking them, and which is thereby essentially free from REVERBERATION.

Anechoic chambers are generally used for…

testing microphones and loudspeakers or other sound-producing devices when the absence of

reflection is desirable.

the phenomenon of sound waves bending around objects and through openings in boundaries, can lead to sound shadows is called ________

sound diffraction

Acoustic Shadow

area behind an obstacle that the direct sound wave does not enter

Amount of Diffraction increases as the _______

wavelength increases

More diffraction by

lower frequencies

Interaural Intensity Differences (IIDs)

Shorter wavelengths/higher frequencies

• Don’t diffract around the head, i.e., don’t bend much

• Enters opposite ear with lower intensity

• Different intensities across both ears

Interaural Timing Differences (ITDs)

• Longer wavelengths/lower frequencies

• Arrives binaurally with same intensity

• Different arrival times across both ears

Refraction of waves involves a change in the

direction of waves as they pass from one medium to another

• This is because speed of sound varies across different mediums (densities of mediums is different)

The speed of sound in air is affected by the…

temperature of the medium (Fast at higher temperatures; slower at cool

temperatures)

Constructive Interference

interaction that results in increased pressure (displacement)

Destructive Interference

interaction that results in reduced pressure (displacement)

Subtraction of the waves or a cancellation

Note: displacements of original waves are out-of-phase

Beats

slow periodic amplitude fluctuations caused when two sine waves (close in

frequency) interfere with each other

The frequency difference for beats must be…

less than 30 Hz

Beat Frequency

number of beats (or pulses) per second

• Equal to the difference between the two frequency

• fbeats = f2 – f1

Doppler Effect

The change in PITCH of a sound heard by a listener when the source and observer are in relative motion to each other.

• It is perceived change in frequency of a sound wave resulting from the movement of a sound source, the movement of a listener, changes in the medium, or a combination

• Heard as an increase in the frequency of an approaching sound source

• Followed by a decrease in frequency when the sound source passes

Shock waves occur when

a moving object travels at (or faster than) the speed of sound

• Deformation of the air density

• Results in loud “cracking” noise

Direct Sound Field

sound field created by a sound source in an unbound space (free field)

Free field

a sound environment in which there are no appreciable effects on sound propagation

from obstacles, boundaries, or reflecting surfaces.

Under free-field conditions, the loss of energy with distance may be predicted by

the…

INVERSE-SQUARE LAW.

Indirect sound field

sound field with direct and reflected waves mixed

• Closed spaces

Critical Distance

distance where intensity of both fields is equal