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Note: This page does not relate to electromagnetic (Radio waves) which can transfer energy in a vacuum..
Waves
Waves in the context of this page are waves which propagate through a medium (solid, liquid, or gas) at a wave speed which
depends on the elastic and inertial properties of that medium. There are
two basic types of wave motion : longitudinal waves and transverse waves .
In a longitudonal wave the medium motion is parallel to the direction of wave propagation
In a transverse wave the medium movement is perpendicular to the direction of
wave propagation.
Examples of periodic wave motions are
- A long rope, string or pipe shakes at one end results in traverse waves to move to the other end
- Waves in the sea is a combination of longtitudonal and transverse waves.
- Sound waves are essentially waves
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Waves generally have a number of properties.
- Period (T) is the time for a wave to complete one cycle
- Frequency (f) is the number of cycles per unit time : f = 1 /T
- Amplitude (A) The maximum value of the periodic wave on either side of the equilibrium position
- Crest of a wave is the highest value of the wave above its equilibrium point.
- Trough of a wave is the lowest value of the wave below its equilibrium point.
- Wavelength g The distance between adjacent crests or troughs
- Intensity (I) The rate of which the wave conveys energy
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Sound
Sound waves are longitudonal waves which travel in solid liquids and gases. The transfer of
energy is by the cyclic compression and expansion of the medium at the source. The velocity
of sound in different mediums is listed below;
Velocities of sound (approximate) in various mediums at 0o C
- Velocity of Sound in dry air = 331.4 + 0,6.t m/s .....t= Temperature (Celsius)
- Velocity of Sound in water = 1,540 m/s
- Velocity of Sound in Pine Timber = 3,340 m/s
- Velocity of Sound in Brick = 3660 m /s
- Velocity of Sound in Steel = 5400 m /s
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The loudness of sound is a perception of the comparative strength of the sensation
received by the human ear. The useful measurement of sound is simple as sound is
is largely a biological effect depending on the human ear. Sound mostly depends on the energy
transferred to the human ear. The amount of energy in ergs which passes in one second
through an area of 1 cm2 is a measure of the intensity of sound
The loudness of sound varies as the square of the amplitude and inversely as the square of the distance from the
source. This does not apply in an enclosed space or if the sound source is large compared to the distance of the listener
. The ear perceives sound on a logarithmic level and is attuned to a small range of sound frequencies.
Sound level measurements in decibels are generally referenced to an accepted standard threshold
of hearing at 1000 Hz for the human ear which can be stated in terms of sound
intensity:
Threshold Level for human hearing Io = 10 -16 Watts / cm2 = 10 -9 ergs / s / cm2
The most accepted method of measuring relative sound level is by the use of the
decibel scale.
I (dB) = 10 . log 10. (I / Io )
At the threshold level defined above the dB value is 0.
A sound energy level increase of 10 result is a db value of 10.
A sound energy level increas of 100 results in a dB value of 20
| Common sound |
Sound Level dB |
Effect |
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|
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| Rocket launching pad (no ear protection) |
180 |
Irreversible hearing loss |
| Carrier deck jet operation |
|
Painfully loud |
| Air raid siren |
130 |
Painfully loud |
| Thunderclap |
|
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| Jet takeoff (200 ft) |
120 |
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| Auto horn (3 ft) |
|
Extremely Loud |
| Rock concert |
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| Fireworks |
100 |
Very Loud |
| Heavy Lorry (50 ft) |
90 |
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| City traffic |
|
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| Hearing damage (8 Hrs) |
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| Alarm clock (2 ft) |
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| Hair dryer |
80 |
Annoying |
| Road traffic |
70 |
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| Business office |
|
Intrusive |
| Air conditioning unit |
60 |
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| Conversational speech |
50 |
Quiet |
| Light auto traffic (100 ft) |
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| Quiet |
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| Living room |
50 |
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| Bedroom |
40 |
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| Quiet office |
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| Library |
30 |
Very quiet |
| Soft whisper (15 ft) |
20 |
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| Broadcasting studio |
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10 |
Just Audible |
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0 |
Hearing begins |
Phons
The sound level measured in decibels does not take into accound the effect of sound frequency
or pitch which in practice has a important effect on the perceived sound level.
A method of measuring the relativ sound level at a fixed frequency (1000 HZ) result in a more
representative value. The use of phons as a measurement unit relates to the relative
sound level in dB at 1000HZ. e.g 70 phons = 70 dB at 1000 Hz.
Sones
The use of the phon as a unit of loudness is an improvement over just quoting the
level in decibels, but it is still not a measurement which is directly proportional
to loudness as heard by the human ear. Using the rule of thumb for loudness,
the sone scale was created to provide such a linear scale of loudness.
The table below provides equivalent phon and sone values
| Phons |
20 |
30 |
40 |
50 |
60 |
70 |
80 |
90 |
100 |
120 |
130 |
| Sones |
0.25 |
0.5 |
1 |
2 |
4 |
8 |
16 |
32 |
64 |
128 |
256 |
Standard for Sound Level Measurement
ISO 226:2003. Acoustics -- Normal equal-loudness-level contours .....
This International Standard specifies combinations of sound pressure levels and
frequencies of pure continuous tones which are perceived as equally loud by human
listeners. The specifications are based on the following conditions:
the sound field in the absence of the listener consists of a free progressive plane
wave; the source of sound is directly in front of the listener; the sound signals are
pure tones; the sound pressure level is measured at the position where the centre of
the listener's head would be, but in the absence of the listener; listening is binaural;
the listeners are otologically normal persons in the age range from 18 years to 25 years
inclusive.
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