Sound is transmitted as wave motion through a medium such as air, water or metal.
Waves are divided into types according to the direction of displacement of the medium in relation to the direction of the motion of the wave itself.
Two basic classifications are transverse and longitudinal waves.
Transverse Waves
An example of a transverse wave is the ripples on the surface of water. The vibrations of the water molecules are at right angles (up and down) to the direction of motion (out from the disturbance).
Longitudinal Waves
If the vibration is parallel to the direction of motion, as it is with sound, the wave is known as a longitudinal wave. As the sound wave is propagated outward from the centre of disturbance, the individual air molecules move back and forth, parallel to the direction of the wave motion.
Each individual molecule passes the energy on to neighbouring molecules, but after the sound wave has passed, each molecule remains in about the same location in space.
Compression and Rarefaction
Thus, a sound wave is a series of alternate compression (increase in density) and rarefaction (decrease in density) events in the medium e.g. air.
Wavelength and Amplitude
For a transverse wave, the wavelength is the distance between two successive crests or troughs. For longitudinal waves, it is the shortest distance between two peak compressions.
Velocity
Sound in steel moves at a speed of just under 5000 m/sec. (For example, faster than the speed of a bullet)
The speed of sound in water, is roughly 1500 m/sec (For example, faster than a fighter jet) at ordinary temperatures but increases greatly with an increase in temperature.
The speed of sound in the air around us is roughly 1/3 km/s i.e. 333m/s
Thus to travel the 3m lenght of an average living room sound will take t=3/333s =0.009s
Frequency, Velocity and Wavelength
The frequency of the wave is the number of vibrations per second. The unit is Hertz (Hz).
The velocity of the wave, which is the speed at which it advances, is equal to the wavelength times the frequency.
Velocity = wavelength x frequency
Standing Waves
Standing waves disturb, but do not travel through the transmission medium. They are present e.g. in the vibration strings of musical instruments and other places.
A violin string when bowed or plucked, vibrates as a whole with a node (minimum) at each end and an anti-node (maximum) in the middle.
It also vibrates in halves, with a node at the centre, in thirds and in various other fractions, all simultaneously.
Such vibrational modes also occur within cavities e.g. a room or the bore of a flute. A room can support a standing wave with a node at each pair of opposing walls.
Harmonics
The vibration as a whole produces the fundamental tone, and the other vibrations produce the various tones.
A harmonic is an integer multiple of the fundamental frequency e.g. 2x fundamental, 3x fundamental, etc.
So a sound consisting of components at frequencies of 1000Hz and 3000Hz would contain a 3rd harmonic of the 1 kHz fundamental.
Amplitude
The amplitude of a sound wave is the degree of motion of air molecules within the wave, which corresponds to the extent of rarefaction and compression that accompanies the wave.
The greater the amplitude of the wave, the harder the molecules strike the ear drum or microphone diaphragm and the louder the sound that is transduced.
The amplitude of a sound wave can be expressed in absolute units by measuring the actual distance moved by the air molecules, or the pressure difference in the compression and rarefaction, or the energy involved.
Ordinary speech, for example, produces sound energy at a power level of about one hundred-thousandth of a watt. (Very very small amount of power)
Sound Intensity & Level
These measurements are extremely difficult to make, so the intensity of sounds is generally expressed as an equivalent sound level.
Normal atmospheric pressure is 100,000 Pa.
Sound Level and the Decibel
The sounds intensity of normal conversational speech is around 100,000 times that of whispered speech.
So that we can conveniently discuss and graph such a huge range of values, sound intensity level is defined using a logarithm and is measured in decibels dB.
The Inverse-Square Law
The intensity of the sound received varies inversely as the square of the distance R from the source.
In open air, sound will be roughly nine times less intense at a distance of 3m from its origin, as at a distance of 1m.
Echos and Reverberation
An echo is the perceived reflection of sound from a surface. The fraction of sound level reflected is known as the reflection coefficient. The time difference between the echo and the direct sound depends on the distances traveled and the speed of sound. The difference must be greater than about 100ms to be perceived as an echo.
Spectrum
Since many sounds contain various frequency components it is often useful to display a sound spectrum, that is a graph of sound level against frequency over a short period of time.
Spectogram
The variation of intensity with time and frequency can be displayed as a Spectogram by representing intensity by colour or brightness on a Frequency vs Time axis.
Useful links:
http://www.mediacollege.com/audio/01/sound-waves.html
http://www.mediacollege.com/audio/01/wave-properties.html
http://www.mediacollege.com/audio/01/wave-interaction.html
http://www.mediacollege.com/audio/01/sound-systems.html
http://science.howstuffworks.com/sound-info.htm
No comments:
Post a Comment