Sound

بسم الله الرحمن الرحيم

الْحَمْدُ لِلَّهِ رَبِّ الْعَالَمِينَ

"Reciting Salawath on our Prophet Muhammad (peace be upon him) is an activity that will be accepted by Allah, even if we don't have Ikhlas (piety)".

Sound is generally known as vibration transmission of mechanical energy that propagates through matter as a wave that can be audibly perceived by a living organism through its sense of hearing. For humans, hearing is limited to frequencies between about 20 Hz and 200 Hz, with the lower limit generally decreasing with age. Other species have a different range of hearing. For example, dogs can perceive vibrations higher than 20 kHz. Sound perception doesn't always require an external source: sometimes the brain of an organism can synthesize a perception of sound on its own.

Sound is further characterized by the generic properties of waves, which are frequency, wavelength, period, amplitude, speed, and direction (sometimes speed and direction are combined as a velocity vector, or wavelength and direction are combined as a wave vector).

The mechanical vibrations that can be interpreted as sound can travel through all forms of matter, gases, liquids, solids, and plasmas. However, sound cannot propagate through vacuum. The matter that supports the sound is called the medium. Sound propagates as waves of alternating pressure deviations from the equilibrium pressure (or, for transverse waves in solids, as waves of alternating shear stress), causing local regions of compression and rarefaction. Matter in the medium is periodically displaced by the wave, and thus oscillates. The energy carried by the sound wave is split equally between the potential energy of the extra compression of the matter and the kinetic energy of the oscillations of the medium. The scientific study of the propagation, absorption, and reflection of sound waves is called acoustics.

Noise is often used to refer to an unwanted sound. In science and engineering, noise is an undesirable component that obscures a wanted signal.

Speed of sound

A Aircraft breaking the Speed of Sound

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 343 m/s, in water 1482 m/s (both at 20 °C, or 68 °F), and in steel about 5960 m/s. 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.

Sound pressure level

Sound pressure is defined as the difference between the actual pressure (at a given point and a given time) in the medium and the average, or equilibrium, pressure of the medium at that location. 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 -\sqrt{2} Pa) and (1 atm +\sqrt{2} 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 L_p 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 Electro technical 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.