Chapter 2 pt 1 Properties of Sound

Pressure

The force generated by molecules running into walls of an enclosure

Ambient pressure

force/are of surface

Faster movement= increase kinetic energy and ambient pressure

More molecules= more collisions and pressure

Sound wave creation

Rapid movement of a structure in a medium can compress medium and change pressure

Condensation

When sound waves compress the molecules in a medium causing higher than average density area

Rarefaction

The void or low pressure area that is filled by molecules after a condensation, lower than average density area

Near field

area where molecular movements are significantly greater than ambient levels

Far field

area where molecular movements are no more than normal, sound only defined as when and in what direction molecules move

Impulse sounds

A single disturbance

Periodic sounds

Repeated movements (most biological sounds)

Waveform

Pressure plotted against time

Longitudinal waves

when molecular movements parallel direction of disturbance

Transverse waves

Molecular movements perpendicular to direction of disturbance (not seen in gas or water, found in solids or dense fluids)

Waves through solid substrate

Can be either longitudinal or transverse

P waves (compressional waves)

Longitudinal, can travel through Solids, Liquids, and Gasses

S waves (sheering waves)

Transverse, can travel through solids, some liquids, but not gasses

Rayleigh waves (solid)

molecular movements increase in amplitude at surface due to lack of resistance from molecules above

Waves at water surface

Transverse waves can form only at water's surface

Lack of molecules above allows for this

Frequency (HZ)

The number of cycles passing per second

Frequency (Hz) = 1/(period (s))

Period

The time required for 1 cycle of a wave to pass

Period = 1/(Frequency (Hz))

Wavelength

•(λ) is length (m) of one cycle.

λ = (Speed of sound (m/s))/(Frequency (Hz))

Speed of sound in air

343 meters per second

Phase

is relative location of peaks in a cycle

When peaks line up (in phase) sound amplified

When peaks dont line up (out of phase) they can cancel eachother

Amplitude

measurement of disturbance to a medium from it's resting state

If you are twice as far away from the source

So 2(r) = I/4

3(r)= I/9

Converting pressure to intensity

p= √(I x Z) so I= p^2/Z

Z= the impedance of a medium (air= 413)

Pressure to decibels

20 log_10⁡(p/pref)

•Example:

20 log_10⁡((1000 µPa)/(20 µPa))

= 33.98 dB re 20 µPa

Fourier Transform

mathematical operation used to determine the amplitude (and phase) of every frequency making up any waveform

Turns wavelength into spectrogram

Long Bins or window= high frequency resolution

Short Bins or windows= high temporal resolution

In regards to Fourier transformation

power spectrum

Fourier transformation for a single bin

Harmonics

any non- sinusoidal but periodic waveform will have them at predictable intervals

multiples of the fundamental

Fundamental frequency

Rate at which waveform repeats

Frequency Modulation

when fundamental Hz change over time

Temporal modulation

when temporal components change overtime

Speed of sound

Negatively correlated with density of a medium

Positively correlated with medium stiffness (has a larger effect then density)

Speed of sound faster in solids than liquids and faster in liquids then gasses

Spherical spreading loss for pressure

reciprocal of distance from source

Ex. 5m between sender and receiver. Sound at receiver has 1/5 the pressure (and 1/25 the intensity) of sound at sender.

Heat loss

high frequency more effected by heat loss due to more movement

Heat loss in air is greater in air than water due to the wavelengths being larger in water.

acoustic impedance

resistance of a medium to being altered

How hard it is for condensation to move adjacent molecules

Acoustic impedance higher in solid than air

Scattering

when many objects with one acoustic impedance are scattered in a medium with a different impedance

Rayleigh scattering

λ > 6x size of objects, sound bends around objects with very little reflection

Mie Scattering

λ < 6x size of objects, complex mixture of reflections and diffractions