1. Properties of sound

2. Frequency and wavelength
Wavelength of a sound is the distance traveled in one cycle (or period).
Frequency (in cps or Hertz) = 1/period, (f =1/T)

3. Pressure vs particle movements
Close to sound source, pressure and particle displacement
are out of phase, but one wavelength from source they are
in phase

4. Near field vs far field
Near field - molecular displacements are greater than normal, usually extends only to 1-2 wavelengths from source
Far field - pressure displacement which travels as a wave away from source

5. Frequencies and wavelengths in air

6. Acoustical property comparisons
Property Air Water
Speed (m/s)
Density (g/cm3)
Impedance (rayls) ,000
Particle velocity at 50 8
1 W/cm2 (cm/s)

7. Wavelength problem
Which sound has a shorter wavelength: kHz in air or 3 kHz in water?
Wavelength = speed of sound / frequency
Air: 340 m/s / 1000 cycle/s = 0.34 m/cycle
Water: 1500 m/s / 3000 cycle/s = 0.5 m/cycle
Therefore, the answer is 1 kHz in air

8. Source movement
When the sound source is moving, the frequency of the sound will be altered. This is known as the Doppler shift
Approaching sounds are higher in frequency
Departing sounds are lower in frequency

9. Phase shifts
Sounds that arrive out of phase cancel each other out (negative interference)
Sounds that arrive in phase increase in amplitude (positive interference)
Sounds partially out of phase create varying amplitudes (beats)

10. Amplitude measurement
Peak, peak-to-peak, RMS (root-mean-squared)
Sound pressure is measured in decibels (dB) on a log10 scale relative to a reference level
dB = 20 log10 P1/Pr where Pr is a reference pressure level, usually the threshold of human hearing at 4 kHz. This is referred to as sound pressure level (SPL)
A sound with twice the SPL is 6 dB louder

11. Sample sound pressure levels
soft whisper dB
nearby songbird, office hum 50 dB
barking dog dB
roaring lion , heavy truck 90 dB
echolocating bat dB
jet take-off dB

12. Amplitude problems
If sound A has 10 times the SPL of sound B, how much louder is A than B in dB?
dB = 20 log10 10 = 20 dB louder
If sound A is 100 db and sound B is 80 db, how much louder is A than B?
20 db
If an 80 db sound is combined with a 40 db sound, how loud is the sound (approximately)?
80 db

13. Sound attenuation Spherical spreading Absorption Scattering
Temperature and humidity effects
Scattering
Reflection, refraction, diffraction
Masking by background noise

14. Spherical spreading
Loss in sound intensity follows the inverse square law: pressure halves for each doubling of distance, i.e. - 6 dB for each doubling of distance

15. Atmospheric attenuation
Nonlinear with humidity
Increases with temp. &
square of frequency

16. Acoustic impedence
The degree to which a medium is compressible, expressed as the product of speed of sound and density of medium (air < water < earth)
Transmission between media with different impedences is difficult
Dictates efficiency of sound production and reception by organisms
Sounds reflect off animals in air, but can pass through them in water

17. Reflection and refraction
If the speed of sound in medium 1 < medium 2, then the refracted wave is bent toward the boundary

18. Diffraction Reflected wave is out of phase with creeping wave.
Occurs when wavelength is similar to object diameter

19. Scattering and wavelengths

20. Forests attenuate sound
Excess attenuation depends on frequency due
to effects of scattering and absorbtion

21. Background noise can mask sounds
Pasture
Forest
Open ocean
Coastal ocean