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The speed of sound depends on the medium through which the waves are passing, and is often quoted as a fundamental property of the material. In general, the speed of sound is proportional to the square root of the ratio of the elastic modulus (stiffness) of the medium to its density. Those physical properties and the speed of sound change with ambient conditions. For example, the speed of sound in gases depends on temperature. In air at sea level, the speed of sound is approximately 767.3 mph, in fresh water 3,315.1 mph (both at 20 °C, or 68 °F), and in steel about 13,332.1 mph.[2] The speed of sound is also slightly sensitive (a second-order effect) to the sound amplitude, which means that there are nonlinear propagation effects, such as the production of harmonics and mixed tones not present in the original sound (see parametric array).
[edit] Acoustics and noise
The scientific study of the propagation, absorption, and reflection of sound waves is called acoustics. Noise is a term often used to refer to an unwanted sound. In science and engineering, noise is an undesirable component that obscures a wanted signal.
[edit] Sound pressure level
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Main article: Sound pressure
Sound pressure is defined as the difference between the average local pressure of the medium outside of the sound wave in which it is traveling through (at a given point and a given time) and the pressure found within the sound wave itself within that same medium. A square of this difference (i.e. a square of the deviation from the equilibrium pressure) is usually averaged over time and/or space, and a square root of such average is taken to obtain a root mean square (RMS) value. For example, 1 Pa RMS sound pressure in atmospheric air implies that the actual pressure in the sound wave oscillates between (1 atm  Pa) and (1 atm  Pa), that is between 101323.6 and 101326.4 Pa. Such a tiny (relative to atmospheric) variation in air pressure at an audio frequency will be perceived as quite a deafening sound, and can cause hearing damage, according to the table below.
As the human ear can detect sounds with a very wide range of amplitudes, sound pressure is often measured as a level on a logarithmic decibel scale. The sound pressure level (SPL) or Lp is defined as
 - where p is the root-mean-square sound pressure and pref is a reference sound pressure. Commonly used reference sound pressures, defined in the standard ANSI S1.1-1994, are 20 µPa in air and 1 µPa in water. Without a specified reference sound pressure, a value expressed in decibels cannot represent a sound pressure level.
Since the human ear does not have a flat spectral response, sound pressures are often frequency weighted so that the measured level will match perceived levels more closely. The International Electrotechnical Commission (IEC) has defined several weighting schemes. A-weighting attempts to match the response of the human ear to noise and A-weighted sound pressure levels are labeled dBA. C-weighting is used to measure peak levels.
[edit] Examples of sound pressure and sound pressure levels
| Source of sound |
RMS sound pressure |
sound pressure level |
| |
Pa |
dB re 20 µPa |
| Nuclear Weapon explosion |
|
approx 248 |
| 1883 Krakatoa eruption |
|
approx 180 |
| rocket launch equipment acoustic tests |
|
approx. 165 |
| threshold of pain |
100 |
134 |
| hearing damage during short-term effect |
20 |
approx. 120 |
| jet engine, 100 m distant |
6–200 |
110–140 |
| jackhammer, 1 m distant / discotheque |
2 |
approx. 100 |
| hearing damage from long-term exposure |
0.6 |
approx. 85 |
| traffic noise on major road, 10 m distant |
0.2–0.6 |
80–90 |
| moving automobile, 10 m distant |
0.02–0.2 |
60–80 |
| TV set – typical home level, 1 m distant |
0.02 |
approx. 60 |
| normal talking, 1 m distant |
0.002–0.02 |
40–60 |
| very calm room |
0.0002–0.0006 |
20–30 |
| quiet rustling leaves, calm human breathing |
0.00006 |
10 |
| auditory threshold at 2 kHz – undamaged human ears |
0.00002 |
0 |
[edit] Equipment for dealing with sound
Equipment for generating or using sound includes musical instruments, hearing aids, sonar systems and sound reproduction and broadcasting equipment. Many of these use electro-acoustic transducers such as microphones and loudspeakers.
[edit] References
- , (2006) The American Heritage Dictionary of the English Language, Fourth Edition. Houghton Mifflin Company.
- , The Soundry: The Physics of Sound
[edit] Sound measurement
- Decibel, sone, mel, phon, hertz
- Sound pressure level
- Particle velocity, acoustic velocity
- Particle displacement, particle amplitude, particle acceleration
- Sound power, acoustic power, sound power level
- Sound energy flux
- Sound intensity, acoustic intensity, sound intensity level
- Acoustic impedance, sound impedance, characteristic impedance
- Speed of sound, amplitude
[edit] See also
Pitch Acoustics | Auditory imagery | Audio bit depth | Audio signal processing | Beats | Cycles | Diffraction | Doppler effect | Echo | Music | Note | Phonons | Physics of music | Pitch | Psychoacoustics | Resonance | Rijke tube | Reflection | Reverberation | Sonic weaponry | Sound localization | Soundproofing | Timbre |
[edit] External links
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