Sound is a longitudinal, mechanical wave.

Sound can travel through any medium, but it cannot travel through a vacuum. There is no sound in outer space.

Sound is a variation in pressure. A region of increased pressure on a sound wave is called acompression (or condensation). A region of decreased pressure on a sound wave is called ararefaction (or dilation).


The sources of sound

  • vibrating solids
  • rapid expansion or compression (explosions and implositons)
  • Smooth (laminar) air flow around blunt obstacles may result in the formation of vorticies (the plural of vortex) that snap off or shed with a characteristic frequency. This process is called vortex shedding and is another means by which sound waves are formed. This is how a whistle or flute produces sound. Aslo the aeolian harp effect of singing power lines and fluttering venetian blinds.

What are the different characteristics of a wave? What are the things that can be measured about waves? Amplitude, frequency (and period), wavelength, speed, and maybe phase. Deal with each one in that order.

amplitude, intensity, loudness, volume

  • Unlike our ears and hydrophones, fish ears don’t detect sound pressure, which is the compression of molecules. Instead, they perceive something called particle motion, the tiny back-and-forth movements of particles in response to sound waves.

speed of sound

The speed of sound depends upon the type of medium and its state. It is generally affected by two things: elasticity and inertia. This is the Newton-Laplace equation. Laplace added the γ (gamma) correction factor for ideal gases.


v = √ E
E =  young's modulus
ρ =  density


v = √ K
K =  bulk modulus
ρ =  density

ideal gases

v = √ K  = √ γP  = √ γRT  = √ γkT
ρ ρ M m
K =  bulk modulus
γ =  cP/cV (specific heat ratio)
P =  absolute pressure
ρ =  density
T =  absolute temperature
R =  gas constant
M =  molar mass
k =  boltzmann's constant
m =  molecular mass

Acoustic Thermometry of Ocean Climates (ATOC)

  • in water, sounds below 1 kHz travel much farther than higher frequencies
  • "shipping noise is loudest in the 30 to 200 Hz range [lowest piano note to middle of cello]"
  • "blue and fin wales are the loudest sound in the 17 to 30 Hz range"
  • "In pre-industrial times, the low frequency range of 15 to 300 Hz in which most of the baleen whales sing was the quietest part of the sound spectrum, nestled between the subsonic ramblings of earthquakes and the higher pitched rattle of wind, waves and rain." Bob Holmes. "Noises Off." New Scientist. 1 March 1997: 30–33.



  • As with any wave the speed of sound depends on the medium in which it is propagating.
  • Sound generally travels faster in solids and liquids than in gases.
  • The speed of sound is faster in materials that have some stiffness like steel and slower in softer materials like rubber.
  • Factors Which Affect the Speed of Sound in Air.
  • The speed of sound in air is approximately 330 m/s (about 1,200 kph or 700 mph).
  • The speed of sound in air is nearly the same for all frequencies and amplitudes.
  • It increases with temperature.
  • Determining the Distance to a Lightning Bolt: Sound waves take approximately 5 seconds to travel 1 mile. Using this information, it is possible to measure one's distance from a lightning bolt. Begin counting immediately after you see the flash. Every five seconds counted is roughly equivalent to one mile of distance.

Sound Page 2

    • Interaural Time Difference (ITD)
    • Interaural Phase Difference (IPD) Phase differences are one way we localize sounds. Only effective for wavelengths greater than 2 head diameters (ear-to-ear distances).
    • Interaural Level Difference (ILD) Sound waves diffract easily at wavelengths larger than the diameter of the human head (around 500 Hz wavelength equals 69 cm). At higher frequencies the head casts a "shadow". Sounds in one ear will be louder than the other.
  • The human ear can distinguish some…
    • 1400 different pitches
  • three (four?) vocal registers
    • (whistle register?)
    • falsetto
    • modal — the usual speaking register
    • vocal fry — the lowest of the three vocal registers

More in the next section.


  • avalanches: location, depth, duration
  • meteors: altitude, direction, type, size, location
  • ocean waves: storms at sea, magnitude, spectra
  • severe weather: location, intensity
  • tornadoes: detection, location, warning, core radius, funnel shape, precursors
  • turbulence: aircraft avoidance, altitude, strength, extent
  • earthquakes: precursors, seismic-acoustic coupling
  • volcanoes: location, intensity
  • Elephants, whales, hippos, rhinoceros, giraffe, okapi, and alligator are just a few examples of animals that create infrasound.
  • Some migratory birds are able to hear the infrasonic sounds produced when ocean waves break. This allows them to orient themselves with coastlines.
  • An elephant is capable of hearing sound waves well below our the human hearing limitation (approximately 30 Hertz). Typically, an elephant's numerous different rumbles will span between 14 and 35 Hertz. The far reaching use of high pressure infrasound opens the elephant's spatial experience far beyond our limited capabilities.
  • Silent Thunder, Katy Payne


  • animal echolocation
    • microchiropterans a.k.a. microbats: carnivorous bats (not fruit bats or flying foxes)
    • cetaceans: dolphins, porpoises, orcas, whales
    • two bird species: swiftlets and oilbirds
    • some visually impared humans have learned this technique
  • sonar (an acronym for sound navigation and ranging) including
    • bathymetry
    • echo sounding
    • fish finders
  • medical ultrasonography (the images generated are called sonograms).
Typical parameters used for medical ultrasound Source: Physics Today
imaging, echo 1 20 0.05 1.75 0.2 1 μs
imaging, doppler 1 20 0.15 15.7 0.3 10 μs
physiotherapy 0.5 3 < 3 2.5 continuous
surgery 0.5 10 ~ 200 1,500 1 16 s
fish actinopterygii frequency range (Hz)
american shad Alosa sapidissima 200 180,000
goldfish Carassius auratus 5 2,000
atlantic cod Gadus morhua 2 500
tuna Thunnus … 50 1,100
catfish … … 50 4,000
amphibians amphibia frequency range (Hz)
tree frog … … 50 4,000
bullfrog Lithobates catesbeianus 100
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