03 – PITCH, FREQUENCY &

INTENSITY

Unit 4 - Sound

Frequency of Sound

how often the particles of the medium vibrate when a wave passes through the medium.

the number of complete back-and-forth vibrations of a particle of the medium per unit of time.

Frequency of Sound Example

If a particle of air undergoes 1000 longitudinal vibrations in 2 seconds, then the frequency of the wave would be 500 vibrations per second.

1 Hertz = 1 vibration/secondFrequency of the Source = Frequency in the Medium

= Frequency at the Ear Example

A guitar string vibrating at 500 Hz will set the air particles in the room vibrating at the same frequency of 500 Hz which carries a sound signal to the ear of a listener which is detected as a 500 Hz sound wave.

Frequency of Sound Since a sound wave is a pressure wave, a

detector could be used to detect oscillations in pressure from a compression (high pressure) to a rarefaction (low pressure) disturbance.

Pressure Time Plot:

Frequency of Sound High Frequency

Low Frequency

Frequency Detection Human Ear:

a wide range of frequencies 20 Hz to 20 000 Hz. Infrasound:

Any sound with a frequency below the audible range of hearing (< 20 Hz) Ultrasound:

Any sound with a frequency above the audible range of hearing (> 20 000 Hz) Dogs:

50 Hz and as high as 45 000 Hz. ultrasound! Cats:

45 Hz and as high as 85 000 Hz. ultrasound! Bats:

who are essentially blind and must rely on sound echolation for navigation and hunting, can detect frequencies as high as 120 000 Hz. ultrasound!

Dolphins: as high as 200 000 Hz. ultrasound!

Elephant : possesses the unusual ability to detect infrasound, 5 Hz to 10 000 Hz.

Pitch The sensations of frequencies are commonly

referred to as the pitch of a sound. high pitch sound corresponds to a high frequency low pitch sound corresponds to a low frequency.

Musically trained people, can detect a difference in frequency as little as 2 Hz.

Most people can detect a frequency difference of greater than 7 Hz.

When 2 or more waves of different frequencies: interference and superposition of the two sound waves.

Pitch

Dissonant - when 2 sounds are played together and they sound unpleasant

Consonant – when 2 sounds are played together and they sound pleasant

Such sound waves form the basis of intervals in music.

Interval Frequency Ratio Examples

Octave 2:1 512 Hz and 256 Hz

Third 5:4 320 Hz and 256 Hz

Fourth 4:3 342 Hz and 256 Hz

Fifth 3:2 384 Hz and 256 Hz

Intensity The amount of energy which is transferred to the

medium is dependent upon the amplitude of vibrations. Example: plucked guitar string. If more energy is put

into the plucking of the string (that is, more work is done to displace the string a greater amount from its rest position), then the string vibrates with a wider amplitude.

Intensity Intensity of the sound wave:

The amount of energy which is transported past a given area of the medium per unit of time

As a sound wave moves through a medium, the intensity of the sound wave decreases with increasing distance from the source.

This is explained by the fact that the wave is spreading out over a circular (2 dimensions) or spherical (3 dimensions) surface and thus the energy of the sound wave is being distributed over a greater surface area.

Intensity Threshold of Hearing:

faintest sound which the human ear can detect The range of intensities which the human ear

can detect is very large Decibel Scale:

The scale which is frequently used to measure intensity is a scale based on multiples of 10. (logarithmic scale)

threshold of hearing is assigned a sound level of 0 decibels (abbreviated 0 dB); this sound corresponds to an intensity of 1*10-12 W/m2

Intensity - Decibel ScaleSource Intensity Intensity Level # Times Greater Than TOH

Threshold of Hearing (TOH)

1*10-12 W/m2 0 dB 100

Rustling Leaves 1*10-11 W/m2 10 dB 101

Whisper 1*10-10 W/m2 20 dB 102

Conversation 1*10-6 W/m2 60 dB 106

Busy Street Traffic 1*10-5 W/m2 70 dB 107

Vacuum Cleaner 1*10-4 W/m2 80 dB 108

Large Orchestra 6.3*10-3 W/m2 98 dB 109.8

Ipod at Maximum Level 1*10-2 W/m2 100 dB 1010

Front Rows of Rock Concert

1*10-1 W/m2 110 dB 1011

Threshold of Pain 1*101 W/m2 130 dB 1013

Military Jet Takeoff 1*102 W/m2 140 dB 1014

Instant Perforation (popping)

of Eardrum1*104 W/m2 160 dB 1016

Intensity

Intensity of a sound is a very objective quantity - can be measured.

Loudness of a sound is more of a subjective response which will vary with a number of factors such as age

Furthermore, two sounds with the same intensity but different frequencies will not be perceived to have the same loudness.