The Nature of Sound



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 a compression (or condensation). A region of decreased pressure on a sound wave is called a rarefaction (or dilation).


The sources of sound

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

Amplitude goes with intensity, loudness, or volume. That's the basic idea. The details go in a separate section.

[ISO 226:2003]

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.

gases liquids
v = √  K  = √  γP  = √  γkT
ρ ρ M
v = √  K
K =  bulk modulus K =  bulk modulus
ρ =  density ρ =  density
γ =  cP/cV (specific heat ratio) solids
P =  absolute pressure
v = √  E
k =  boltzmann's constant
T =  absolute temperature E =  young's modulus
M =  molecular mass ρ =  density

Acoustic Thermometry of Ocean Climates (ATOC)



solids v (m/s)   liquids v (m/s)
aluminum 6420   alcohol, ethyl 1207
beryllium 12,890   alcohol, methyl 1103
brass 4700   mercury 1450
brick 3650   water, distilled 1497
copper 4760   water, sea 1531
cork 500   gases (STP) v (m/s)
glass, crown 5100   air, 000 ℃ 331
glass, flint 3980   air, 020 ℃ 343
glass, pyrex 5640   argon 319
gold 3240   carbon dioxide 259
granite 5950   helium 965
iron 5950   hydrogen (H2) 1284
lead 2160   neon 435
lucite 2680   nitrogen 334
marble 3810   nitrous oxide 263
rubber, butyl 1830   oxygen (O2) 316
rubber, vulcanized 54   water vapor, 134 ℃ 494
silver 3650   biological materials v (m/s)
steel, mild 5960   soft tissues 1540
steel, stainless 5790      
titanium 6070      
wood, ash 4670      
wood, elm 4120      
wood, maple 4110      
wood, oak 3850      
Speed of sound in various materials Source: probably an old version of the CRC

frequency, pitch, tone

The frequency of a sound wave is called it pitch. High frequency sounds are said to be "high pitched" or just "high"; low frequency sounds are said to be "low pitched" or just "low".

f (THz) device, event, phenomena, process
0.1–2 SASER (sound laser)
f (MHz) device, event, phenomena, process
1–20 medical ultrasound
f (kHz) device, event, phenomena, process
25–80 bat sonar clicks
40–50 ultrasonic cleaning
32.768 quartz timing crystal
18–20 upper limit of human hearing
4–5 field cricket (Teleogryllus oceanicus)
2.2–2.8 clapping
2–5 maximum sensitivity of the human hear
f (Hz) device, event, phenomena, process
300–3000 voice frequency (VF), important for understanding speech
2048 C7 scientific scale, highest note of a soprano singer (approximate)
440 A4 american standard pitch, tv test pattern tone
435 A4 international pitch
426.67 A4 scientific scale
261.63 C4 american standard pitch
258.65 C4 international pitch
256 C4 scientific scale, typical fundamental frequency for female vocal cords
128 C3 scientific scale, typical fundamental frequency for male vocal cords
64 C2 scientific scale, lowest note of a bass singer (approximate)
90 ruby-throated hummingbird in flight
60 alternating current hum (US and Japan)
50 alternating current hum (Europe)
8–20 lower limit of human hearing
17–30 blue and fin wales are the loudest marine sounds in this range
1–5 tornadoes
Frequency of Selected Sounds

human hearing and speech

Humans are generally capable of hearing sounds between 20 Hz and 20 kHz (although I can't hear sounds above 13 kHz). Sounds with frequencies above the range of human hearing are called ultrasound. Sounds with frequencies below the range of human hearing are called infrasound.

More in the next section.



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
Typical parameters used for medical ultrasound Source: Physics Today