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/second Frequency 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 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 (> Hz)  Dogs: 50 Hz and as high as Hz. ultrasound!  Cats: 45 Hz and as high as Hz. ultrasound!  Bats: who are essentially blind and must rely on sound echolation for navigation and hunting, can detect frequencies as high as Hz. ultrasound!  Dolphins: as high as Hz. ultrasound!  Elephant : possesses the unusual ability to detect infrasound, 5 Hz to 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. IntervalFrequency RatioExamples Octave2:1512 Hz and 256 Hz Third5:4320 Hz and 256 Hz Fourth4:3342 Hz and 256 Hz Fifth3:2384 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* W/m 2

Intensity - Decibel Scale SourceIntensityIntensity Level# Times Greater Than TOH Threshold of Hearing (TOH) 1* W/m 2 0 dB10 0 Rustling Leaves1* W/m 2 10 dB10 1 Whisper1* W/m 2 20 dB10 2 Conversation1*10 -6 W/m 2 60 dB10 6 Busy Street Traffic1*10 -5 W/m 2 70 dB10 7 Vacuum Cleaner1*10 -4 W/m 2 80 dB10 8 Large Orchestra6.3*10 -3 W/m 2 98 dB Ipod at Maximum Level1*10 -2 W/m dB10 Front Rows of Rock Concert 1*10 -1 W/m dB10 11 Threshold of Pain1*10 1 W/m dB10 13 Military Jet Takeoff1*10 2 W/m dB10 14 Instant Perforation (popping) of Eardrum 1*10 4 W/m dB10 16

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.