# Survey of Sound class 3

## Speed of Sound

The Speed of sound is measured in meters per second.The speed of sound is affected by several factors: temperature, humidity, and elasticity.

SOS ~ 344 m/s or 1130 f/s

NOTE: for class we will assume the speed of sound in air @ 20 degrees Celsius an 75% humidity is 344m/s

Speed of sound Calculator http://www.measure.demon.co.uk/Acoustics_Software/speed.html

## Frequency/Period

Sound is a periodic event. This means that there is a fixed amount of time between different events.The Frequency component of a sound wave affects the pitch of the sound.

Frequency if the measure of how many cycles a wave complete per second.

Frequency (f) is measures in Hertz

Period (T) in the inverse of infrequency is is measured in seconds

Frequency = Cycles/Second Period = Seconds/Cycle

f = 1/T T = 1/f

Different types of frequency components periodic / aperiodic.

Periodic

sound that has a measurable frequency

Aperiodic

sound that is mostly random

It is assumed the humans can hear frequencies from 20Hz to 20,000 Hz.

20-20KHz Sweep

http://iam.colum.edu/sound/media/sweep20-20.wav 41kb,16 bit 3.7megs

# Wavelength

Wavelength is measured in distance (ft. of m)

The actual physical length of a single cycle of a wave

Wavelength = speed of sound / Frequency in Hz

λ = SOS/f

wavelength = speed of sound * period

λ = SOS*T

The wavelength of a sound wave varies greatly with the frequency of the wave. A 20 kHz wave has a wavelength of 1.72 (0.7 in)cm while a 20 Hz wave has a wavelength of 17.2 m (~56 ft.).

## Perception of Amplitude and Frequency

Humans do not hear all frequencies equally. We perceive different frequencies with equal energy to have different amplitudes. The Fletcher Munson Curves show the phons scale, how amplitude is perceived by frequency

Fletcher Munson Curves 1 http://arts.ucsc.edu/EMS/Music/tech_background/TE-02/AcNumbers/AcNumbers.html

Fletcher Munson Curves 2 http://www.allchurchsound.com/ACS/edart/fmelc.html

Fletcher Munson Curves 3 http://hyperphysics.phy-astr.gsu.edu/hbase/sound/eqloud.html