As the wave passes, the disturbance of particles
is in the direction of the wave travel.
The displacement of particles of the medium results in alternate regions of high particle density and low particle density. Regions of
high particle density are called compressions. Regions of low
particle density are called rarefactions.
Rarefactions and compressions both move in the direction of the wave travel. The the particles of the medium do not move bodily in the direction of the wave movement; they vibrate about their normal positions. Each complete vibration of a particle is called a cycle ( i.e. from its starting position, to a maximum distance in one direction, back through the starting position, then to a maximum displacement in the opposite direction and back to the starting place).
The number of cycles completed in one second is
called the frequency of the vibration. One of the most noticeable
differences between two sounds is a difference in pitch. It is the
frequency of a sound that mostly determines its pitch.
Frequency is
measured in hertz, one hertz (Hz) being one cycle per second.
(One thousand hertz = 1 kilohertz = 1 kHz.) A high frequency vibration
produces a high pitched note; a low frequency vibration gives a low
pitched note.
The human hearing range (audible range) is about 16Hz to
16kHz. The frequencies of notes that can be played on a piano range
from 27.5 Hz to just over 4kHz.
Any note played on a piano will sound
different to a note of the same pitch produced by another type of instrument, e.g. a tuning fork.
The
musical note produced by a tuning fork is called a pure tone because it
consists of a tone of one frequency. A note played on a piano, or most
other instruments, consists of several such tones all sounding together
at different frequencies. These frequencies are related to the frequency
(usually the lowest one) which gives the note its characteristic
pitch.
The tone with the lowest frequency is called the fundamental.
The other tones are called overtones If the overtones have
frequencies that are whole number multiples (x2, x3...up to x14) of the
fundamental frequency they are called harmonics.
It is the difference in the harmonic content of notes that gives each musical
instrument its characteristic sound or timbre ("tam-brah"). Therefore although the highest note of a piano has a fundamental frequency of just over 4kHz, equipment used to record music must be able to handle much higher
frequencies to preserve the harmonics associated with each note.
Sounds produced by percussive effects are particularly rich in high harmonics. These occur mainly at the start of a sound, e.g. when a stringed instrument is plucked or a cymbal is struck. These starting transients are also characteristic of the instrument producing them. Sound equipment must be able to cope with these high frequencies otherwise the tonal quality of the sounds will be altered. Cymbals, for example, can produce frequencies around 20kHz to 25kHz.
There are two types of sound; sound of definite pitch, which musicians call a note and sound with no definite pitch, which is a noise. "Music" involves not only notes (of definite pitch) but also much noise as well, e.g. percussion. Noise is an integral part of much music. The difference between a note and a noise is obvious to the ear but what causes the difference? A note consists of regular vibrations; a noise consists of irregular vibrations.
Periodic motion is movement that is regular and repeating eg. the to-and-fro or oscillation of a pendulum. A musical note is produced by periodic motion at an audible frequency. Non-periodic motion is perceived as a noise.
In music, the range of sounds is divided into sections known as octaves. Each octave consists of seven different notes and eight intervals or spaces between the notes. The seven notes are named A to G and the eighth note is the A of the next octave. Each note sounds very similar to the note baring the same name in another octave. The reason for this "sameness" is that a note an octave higher than its namesake has twice the frequency (e.g. A 440Hz and A 880Hz). So, if one vibrating system oscillates at twice the frequency of another system, the two notes produced are said to differ in pitch by an octave.
Particles vibrating due to the passage of a wave are said to be in
phase if they are moving in the same direction and have the same
displacement (i.e. are the same distances from their starting positions).
A wavefront is a surface on which all the particles are in the same
phase of vibration. The distance between successive wavefronts is
called the wavelength of the sound. Wavelength is given the Greek
symbol lambda. Looked at in terms of compressions and rarefactions
the wavelength is the distance between adjacent centres of compression
or adjacent centres of rarefaction.
Sound travels at a constant speed (called the speed of sound) if the temperature and pressure of the air are constant. This is about 330 metres per second. Wavelength and frequency are related to each other and to the speed of sound. The relationship is: v=f x (lambda)
Wavelength is important to a sound engineer. The dimensions of a room or even a microphone can affect sound waves, if the wavelength of the sound is similar to those dimensions.
A loud sound (note or noise) is produced by vibrations more violent than those producing a soft sound. The more violent vibration is said to have a greater amplitude. The amplitude of a wave is the maximum displacement of the vibrating particles from their undisturbed positions.
The greater the amplitude of a wave, the greater will be the energy of the vibrating
particles and the sound will be more intense. As a sound wave
travels out from the source, energy is transferred from one vibrating
particle to the next. Energy is gradually dissipated in the form of
heat so the intensity of the sound decreases as the distance from the
source increases.
The intensity of the sound is said to be inversely proportional to the distance from the source. The decrease obeys an inverse law (the inverse square law).
At twice the distance from a source the intensity of sound drops to one quarter. At four times the
distance it drops to one sixteenth, e.t.c. So moving a microphone may have
a greater effect than might be supposed. When a listener moves away
from a sound source the sound level does not appear to drop by such
proportions. This is because the ear has a built in compensating
mechanism. The ear drum is connected to the inner ear by a system of
three levers. The positions of the pivots of these levers can be
changed to provide greater or lesser leverage. Adjustment allows weak
sounds to reach the inner ear with maximum strength while loud sounds
are reduced to prevent damage to the inner ear. A listener moving twice
as far away from a loudspeaker would not experience a decrease in
loudness to one quarter. However the (measurable) intensity of the
sound would have decreased to that extent. Loudness then, is the
magnitude of the sensation experienced by someone hearing a sound.
Intensity is a measurable, physical quantity. Loudness depends not
only on the intensity of a sound but also the sensitivity of the
listener's ears.
The crowding of particles together causes the pressure of the air to be greater than normal. The pressure is therefore a maximum in the regions of compression and a minimum in the regions of rarefaction. Still air, undisturbed by a sound wave, has a fairly uniform pressure.
If two identical sound waves arrive at the same place at the same time the regions of high pressure will coincide and so will the regions of low pressure. This will result in a single wave of greater intensity. The two waves are said to reinforce each other and the phenomenon (observable event) is called reinforcement.
If one wave arrives half a wavelength late, the high pressure region of one wave will be cancelled by the low pressure region of the other. The result is no sound. This is called cancellation.
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Where reinforcement occurs the waves are said to be in phase with each other. Elsewhere the waves are out of phase and partial or complete cancellation occurs.
A flat sound source (eg. a sheet of metal) radiates sound waves from the front and back. The areas of high and low pressure directly in front (or behind) the sheet spread sideways into the undisturbed regions. Sound waves therefore radiate out at a divergent angle.
It takes time for the pressure to spread sideways into the undisturbed areas. If waves follow each other slowly (have long wavelengths and low frequencies) there is time for the sideways spread to occur before the next half cycle occurs. If the wave has a high frequency the next half cycle arrives before the sideways spread can occur. The pressure is reversed before it can spread to the undisturbed side regions. The result is a wide spreading of sound at low frequencies, reducing in angle as the frequency increases until at high frequencies the beam of sound is a narrow one with parallel sides. With any source of high frequency sound a microphone must therefore be positioned directly in front of the source. If a source has a big range of frequencies (a wide frequency spectrum) the lower frequencies will be overemphasized by an "off axis" microphone. The sideways spread of sound waves also means that a small object placed in the path of a long wavelength (low frequency) wave will have little effect. The waves will merge around it, leaving no "shadow" area.
If the wavelength of a sound is decreased until it is about the same as the dimension of an obstacle then a "shadow" will be cast and sound intensity will drop behind the obstacle. Again it is the short wavelength (high frequency) sounds that are most affected. Even the body components of a microphone can create obstacles to high frequency sound waves.
Distortion is any unwanted change in the waveform (shape of a wave).
When waves that are odd fractions of a wavelength out of phase meet,
the actual shape of the sound wave is altered. This results in one kind
of distortion. (Distortion may also be caused electronic equipment but
here we are just considering sound itself.)
When sound meets with a
large surface the sound may be absorbed or reflected
depending on the nature of the surface. Hard, glossy surfaces such as
glass, bricks and ceramic tiles are efficient reflectors; porous surfaces
such as carpets and curtains are good absorbers.
Refraction is the change of direction of a wave as it passes from a medium of one density to a medium of another density. Different temperatures can cause layers of air to have different densities. If the upper layer of air is warmer (less dense) sound will be bent or refracted downwards. This often happens out of doors at sunset. The result is that sounds travel farther. When the upper layer of air is cooler (and therefore denser) sound waves are refracted upwards. This is why sounds do not travel so far on a hot summer's day.
Noise may be defined as an unwanted sound.
A sound engineer is concerned with two sorts of noise
A sound engineer is also concerned with resonance. All objects that can be made to vibrate have a certain frequency at which they will vibrate most strongly (ie. with maximum amplitude). If a body is excited with a whole range of frequencies it will vibrate approximately equally in response to them all except those frequencies nearest to its own natural frequency. At one frequency it will vibrate most strongly. This frequency is called the resonant frequency and the condition is called resonance. A string of a musical instrument will vibrate at one frequency, its resonant frequency, whether the string is plucked, bowed or struck. Air particles have mass so any enclosed body of air (eg. the pipe or "tube" of a clarinet) has a resonant frequency. The air contained in a hall or studio can have resonance too, resulting in certain sounds being unduly emphasized.
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