Sound: Physics, Perception, Acoustics

acoustics

the study of physical properties of sounds in the environment, how they travel through air, and how they are affected by objects in their environment

psychoacoustics

the study of how we perceive sounds

pitch and frequency

sound

is a pressure wave moving through a medium (substance like air or water)

movement of air molecules from sound source

sound pressure level

0 dB SPL

other elastic mediums sound can travel through

water/liquid, metal, bone/skin/tissue, and wood

velocity

speed of a sound wave

impacted by elasticity and density of medium its going through

wavelength

distance a sound wave travels in one cycle

pure tones

one single isolated pitch also known as a “simple sound”

in tuning forks and audiometers

complex sounds

two or more pitches combined

everything other than pure/simple sounds like music, speech, ringtones, etc

transverse wave

a wave where the particles of the medium vibrate perpendicularly to the direction the wave is traveling

longitudinal wave

a wave where the particles of the medium oscillate parallel (back and forth) to the direction that the wave’s energy is traveling (movement in one direction)

forms compressions and rarefactions

longitudinal waves

are sound waves transverse waves or longitudinal waves?

properties of sound

four physical parameters describing simple harmonic motion (pure tone) as a function of time: frequency, amplitude, period, phase

frequency

the number of complete vibratory cycles per unit of time

amplitude

a derived unit of measurement describing an object’s distance from rest to maximal displacement

period

amount of time needed to complete one cycle of vibration (B and E in the picture)

phase

describes the relative timing of compressions and rarefactions of waves (i.e., how cycles relate to each other)

compression and rarefaction

are the points of maximum displacement in a sound wave

pitch/frequency

the number of cycles per second expressed in hertz

includes subsonic, supersonic, infrasonic, and ultrasonic

loudness

amplitude of the cycles expressed in decibels

larger amplitude = louder sounds

subsonic

type of pitch/frequency wave that travels slower than the speed of sound

supersonic

type of pitch/frequency wave that travels faster than speed of sound

infrasonic

type of pitch/frequency that is below frequencies that humans can hear

ultrasonic

type of pitch/frequency that is above frequencies that humans can hear

cycle of a wave

compression

when the vibrating object moves in one direction, the air molecules are pushed together, causing an increase in the density of air molecules (more molecules per volume)

rarefaction

when the vibrating object moves in the opposite direction, the air molecules are pulled apart, causing a decrease in the density of air molecules

compressions

are the peaks on a sound wave compressions or rarefactions?

rarefactions

are the troughs in a sound wave compressions or rarefactions?

pitch/frequency

equivalent to cycles per second (cps)

low frequency sounds = longer period and high frequency sounds = shorter periods

perceived as pitch

20 Hz to 20,000 Hz

what frequency range is it that humans can hear?

125—8000 Hz

what frequency range are human ears most sensitive to? which is also the range tested on audiograms

intensity

is a physics measure and is objective information

loudness

is a perceptual phenomenon and is very subjective

is perceived (what is for someone could be different for someone else)

decibel (dB)

units of measurement based on the physical properties of sound waves

1/10 Bel

dB SPL (Sound Pressure Level)

used in measuring hearing aid performance/function

dB IL (Intensity Level)

used in electrical measurements/calibration of audiometers

dB HL (Hearing Level)

used on hearing tests/audiograms

each frequency calibrated to “human 0”

logarithmic/exponential scale

dB SL (Sensation Level)

refers to the level above threshold, used for speech perception testing

no

does 0 dB mean the absence of sound?

yes

can some people hear at negative dB levels?

makes numbers more manageable

why is a logarithmic scale used versus a linear scale when measuring relative intensity?

ratio scale

a scale in which each value is related to a specified reference value so that equal intervals on the scale are equal ratios

linear scales are turned into these

audiograms

inverse graph that measures frequency vs intensity

normal hearing is at the top and deafness is at the bottom