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Sound

2007 Schools Wikipedia Selection. Related subjects: General Physics

   Sound is a disturbance of mechanical energy that propagates through
   matter as a wave. Sound is characterized by the properties of sound
   waves, which are frequency, wavelength, period, amplitude and velocity
   or speed.

Explanation

   Humans perceive sound by the sense of hearing. By sound, we commonly
   mean the vibrations that travel through air and can be heard by humans.
   However, scientists and engineers use a wider definition of sound that
   includes low and high frequency vibrations in air that cannot be heard
   by humans, and vibrations that travel through all forms of matter,
   gases, liquids and solids. The matter that supports the sound is called
   the medium. Sound propagates as waves of alternating pressure, causing
   local regions of compression and rarefaction. Particles in the medium
   are displaced by the wave and oscillate. The scientific study of sound
   is called acoustics.

   Noise and sound often mean the same thing; but noise is often used to
   refer to an unwanted sound. In science and engineering, noise is an
   undesirable component that obscures a signal.

Perception of sound

   A schematic representation of hearing. (Blue: sound waves. Red:
   eardrum. Yellow: cochlea. Green: auditory receptor cells. Purple:
   frequency spectrum of hearing response. Orange: nerve impulse.)
   Enlarge
   A schematic representation of hearing. (Blue: sound waves. Red:
   eardrum. Yellow: cochlea. Green: auditory receptor cells. Purple:
   frequency spectrum of hearing response. Orange: nerve impulse.)

   Sound is perceived through the sense of hearing. Humans and many
   animals use their ears to hear sound, but loud sounds and low-frequency
   sounds can be perceived by other parts of the body through the sense of
   touch as vibrations. Sounds are used in several ways, notably for
   communication through speech and music. They can also be used to
   acquire information about properties of the surrounding environment
   such as spatial characteristics and presence of other animals or
   objects. For example, bats use echolocation, ships and submarines use
   sonar and humans can determine spatial information by the way in which
   they perceive sounds.

   Humans can generally hear sounds with frequencies between 20 Hz and 20
   kHz although this range varies significantly with age, occupational
   hearing damage, and gender; the majority of people can no longer hear
   20,000 Hz by the time they are teenagers, and progressively lose the
   ability to hear higher frequencies as they get older. Most human speech
   communication takes place between 200 and 8,000 Hz and the human ear is
   most sensitive to frequencies around 1000-3,500 Hz. Sound above the
   hearing range is known as ultrasound, and that below the hearing range
   as infrasound.

   The amplitude of a soundwave is specified in terms of its pressure. The
   human ear can detect sounds with a very wide range of amplitudes and so
   a logarithmic decibel amplitude scale is used. The quietest sounds that
   humans can hear have an amplitude of approximately 20 µPa (
   micropascals) or a sound pressure level (SPL) of 0 dB re 20 µPa (often
   incorrectly abbreviated as 0 dB SPL). Prolonged exposure to a sound
   pressure level exceeding 85 dB can permanently damage the ear,
   resulting in tinnitus and hearing impairment. Sound levels in excess of
   130 dB are more than the human ear can safely withstand and can result
   in serious pain and permanent damage. At very high amplitudes,
   soundwaves exhibit nonlinear effects, including shock.

Speed of sound

   The speed at which sound travels 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 stiffness of the medium and its
   density. Those physical properties and the speed of sound change with
   ambient conditions. For example, the speed of sound in air and other
   gases depends on temperature. In air, the speed of sound is
   approximately 345 m/s, in water 1500 m/s and in a bar of steel 5000
   m/s. The speed of sound is also slightly sensitive (to second order) to
   the sound amplitude, resulting in nonlinear propagation effects, such
   as the weak production of harmonics and the mixing of tones (see
   parametric array).

Sound pressure

   Sound pressure is the pressure deviation from the local ambient
   pressure caused by a sound wave. Sound pressure can be measured using a
   microphone in air and a hydrophone in water. The SI unit for sound
   pressure is the pascal (symbol: Pa). The instantaneous sound pressure
   is the deviation from the local ambient pressure caused by a sound wave
   at a given location and given instant in time. The effective sound
   pressure is the root mean square of the instantaneous sound pressure
   over a given interval of time. In a soundwave, the complementary
   variable to sound pressure is the acoustic particle velocity. For small
   amplitudes, sound pressure and particle velocity are linearly related
   and their ratio is the acoustic impedance. The acoustic impedance
   depends on both the characteristics of the wave and the medium. The
   local instantaneous sound intensity is the product of the sound
   pressure and the acoustic particle velocity and is, therefore, a vector
   quantity in time.

Sound pressure level

   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 L[p] is defined as

          L_\mathrm{p}=10\, \log_{10}\left(\frac{{p}^2}{{p_0}^2}\right)
          =20\, \log_{10}\left(\frac{p}{p_0}\right)\mbox{ dB}
          where p is the root-mean-square sound pressure and p[0] is a
          reference sound pressure. (When using sound pressure levels, it
          may be important to quote the reference sound pressure used.)
          Commonly used reference sound pressures, defined in the standard
          ANSI S1.1-1994, are 20 µPa in air and 1 µPa in water.

   Since the human ear does not have a flat spectral response, sound
   pressure levels 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.

Examples of sound pressure and sound pressure levels

              Source of sound              sound pressure   sound pressure level
                                               pascal           dB re 20 µPa
  threshold of pain                                     100                  134
  hearing damage during short-term effect                20          approx. 120
  jet, 100 m distant                                6 - 200            110 - 140
  jack hammer, 1 m distant / discotheque                  2          approx. 100
  hearing damage during long-term effect            6×10^−1           approx. 90
  major road, 10 m distant                2×10^−1 - 6×10^−1              80 - 90
  passenger car, 10 m distant             2×10^−2 - 2×10^−1              60 - 80
  TV set at home level, 1 m distant                 2×10^−2               ca. 60
  normal talking, 1 m distant             2×10^−3 - 2×10^−2              40 - 60
  very calm room                          2×10^−4 - 6×10^−4              20 - 30
  leaves noise, calm breathing                      6×10^−5                   10
  auditory threshold at 2 kHz                       2×10^−5                    0

Measurement of sound

     * Decibel, sone, mel, phon
     * sound pressure level
     * Particle velocity, acoustic velocity, sound velocity
     * Particle displacement, particle amplitude, particle acceleration
     * Sound power, acoustic power, sound power level
     * Sound intensity, acoustic intensity, sound intensity level
     * Acoustic impedance, sound impedance, characteristic impedance
     * Speed of sound, amplitude
     * Sound energy flux
     * See also Template:Sound measurements

   Retrieved from " http://en.wikipedia.org/wiki/Sound"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
