Sound Intensity and Power
A
TERM PAPER
ON
SOUND NOISE AND SOUND INTENSITY
BY
OLA MOYINOLUWA AANUOLUWA
ARC/07/0985
IJATOYE OLUWATOYIN GIFT
ARC/07/0965
COURSE CODE
ARC 507
SUBMITTED TO
THE DEPARMENT OF ARCHITECTURE
FEDERAL UNIVERSITY OF TECHNOLOGY,
P.M.B 704, AKURE, ONDO STATE, NIGERIA.
MAY, 2012
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Ijatoye O.G.
Ola M.A.
Sound Intensity and Power
TABLE OF CONTENTS
1.0.
INTRODUCTION
2.0.
SOUND INTENSITY
2.1.
3.0.
RELATIONSHIP BETWEEN SOUND INTENSITY AND LOUDNESS
SOUND POWER
3.1.
RELATIONSHIP BETWEEN SOUND POWER AND SOUND PRESSURE.
LIST OF TABLES AND FIGURES
1.
TYPICAL FREE FIELD SOUND PRESSURE VERSUS SOUND POWER LEVELS – IN
DECIBEL (dB)
2.
THE DECIBEL SCALE
3.
THE SOUND PRESSURE SCALE.
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Sound Intensity and Power
1.0.
INTRODUCTION
Wave is a disturbance, which passes through a medium and transfers energy from one place
to another without causing any permanent displacement for the medium itself. Sound is a
disturbance which passes through a physical medium and stimulates the sense of hearing.
Sound as a wave is produced by the movement of an object and it travels through a medium
which could be in gaseous, liquid or a solid form. This movement or vibration causes slight
changes in air pressure that are detected by the ear. When a sound wave is propagated, its
frequency (f) is the number of vibrations or cycles produced by an object in one second
measured in Hertz (Hz) and the wavelength () as the distance travelled by the wave in the
time taken for it to complete one cycle known as its period (t). The amplitude is the product
of the wavelength and the frequency. Wave velocity is the speed with which sound travels
through the medium.
2.0.
SOUND INTENSITY
Sound intensity (I) is defined as the average rate of transfer of energy per unit area, the area
been perpendicular to the direction of propagated sound. The intensity of a sound wave
depends on how far the listener is from a source i.e. the farther away from source of sound
the decrease in the intensity, since the intensity decreases as the square of the distance
from the source (known as the inverse square law), therefore sound becomes weak, feeble
and feeble with increase in distance between a source and a listener. This explains why
sound intensity is a vector quantity having both magnitude and direction. Its fundamental
units are watts per square meter (W/m2).
Knowing the sound power of the individual machine in a factory and ranking such in the
order of highest sound power is very important as it helps in avoiding the risk of exposing
the workers to the risk of hearing damage or a sound which poses health hazards on the
listeners in a factory. Once the machine making the most noise has been located, the sound
could then be reduced by locating the individual components radiating noise. This can be
done with intensity measurements. Because sound intensity gives a measure of direction as
well as magnitude it is also very useful when locating sources of sound.
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Ijatoye O.G.
Ola M.A.
Sound Intensity and Power
For a sound source of constant power radiating equally in all directions, the sound intensity
must fall with the square of the distance to the source (since the radiated energy is spread
out through the surface of a sphere centred on the source, and the surface area of the
sphere increases with the square of its radius).
I = w/4πr2
2.1.
RELATIONSHIP BETWEEN SOUND INTENSITY AND LOUDNESS
Loudness is intimately related to sound intensity but can by no means be considered
identical to intensity. Sound loudness is a subjective term describing the perception of
sound by the ear, more also it’s just an aural sensation, a physiological phenomenon rather
than a physical phenomenon. The sound intensity must be factored by the ear's sensitivity
to the particular frequencies contained in the sound. As level of intensity cannot be felt, it is
perceived by the human ear in form of loudness as either been high of low. The intensity of
sound refers to external or objective measurement while loudness is the internal or
subjective aspect. According to Weber-fetcher’s law loudness produced is proportional to
the log of intensity.
S ∝log I
3.0.
SOUND POWER
Sound power can be defined as energy transferred per seconds from the source of sound to
the air. It is the acoustical energy emitted by the sound source and is an absolute value.
Since sound always has a source, the source could be people talking, musical instrument
(violin, piano et cetera), a stereo system or a jet taking off. Different sources produce
sounds with different qualities such as pitch and timbre. Sound power states another
difference between sources of sound: the amount of sound energy they produce. Sound
power is not affected by the environment.
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Ola M.A.
Sound Intensity and Power
A sound source has a given constant sound power that does not change when source is
placed in a different environment. Most especially, industrial machines have their sound
power specified by their manufacturers, and this differs from one to another. Sound power
is connected to sound source and is independent of distances.
Power as a physical quantity, is the time rate of change of energy and has units of Joules per
second, expressed in units called Watts (W). That is,
1 Watt = 1 Joule/second
Thus, the unit for sound power is Watts. Sound power levels is measured in decibels; the
ratio of the sound power to some reference power level. Sound power levels (Lw) are
connected to the sound source and independent of distance.
Lw = 10 log (W / W0)
Where:
W0 = reference power (W)
The normal reference level is 10-12 W, which is the lowest sound that persons of excellent
hearing can discern. Hence, it is derived that sound power is measured as the total sound
power emitted by a source in all directions in watts (joules / second).
3.1.
RELATIONSHIP BETWEEN SOUND POWER AND SOUND PRESSURE.
Sound pressure and Sound power are two distinct and commonly confused characteristics
of sound. Both share the same unit of measure; the decibel (dB), and the term ‘sound level’
is commonly substituted for each. Sound power is the acoustical energy emitted by the
sound source and is an absolute value; not affected by the environment while Sound
pressure is a pressure disturbance in the atmosphere whose intensity is influenced not only
by the strength of the source, but also by the surroundings and the distance from the source
to the receiver/listener.
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Sound pressure is what we hear, what sound meters measure and what ultimately
determines whether a design achieves quality sound while sound power is the acoustical
energy emitted by the sound source. The pressure at the point of hearing is related to the
distance from the sound source, on the other hand, sound power allow for the calculation of
the resulting sound pressure in the space.
Sound pressure levels (SPL) qualify in decibels the intensity of given sound sources while
Sound power levels (Lw) are connected to the sound source and independent of distance.
Sound pressure levels vary substantially with distance from source, and also diminish as a
result of intervening obstacles and barriers, air absorption, wind and other factors. Thus,
Sound pressure levels (SPL) is given as:
20log P/po = 10log (P/po)2
Where:
Po = 2 x 10-5 N/m2
P= root mean square pressure (N/m2)
TYPICAL FREE FIELD SOUND PRESSURE VERSUS SOUND POWER LEVELS – IN dB
FRAME SERIES
POWER LEVEL
140
180
210
250
280
320
360
400
440
5000
5800
6800
8000
X
X
X
X
X
X
X
X
X
X
X
X
x
PRESSURE LEVEL @ 3 PRESSURE LEVEL @ 5
FT
FT
X - 7.8
X - 10.6
X - 8.0
X - 10.8
X - 8.2
X - 10.9
X - 8.4
X - 11.1
X - 8.8
X - 11.4
X - 9.0
X - 11.6
X - 9.2
X - 11.8
X - 9.5
X - 12.0
X - 10.9
X - 12.4
X - 10.6
X - 12.8
X - 11.6
X - 13.7
X - 11.9
X - 13.9
X - 12.5
X - 14.7
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Sound Intensity and Power
THE DECIBEL SCALE
(Extract from UCL; Psychology and Language Sciences.)
The difference in decibels between two levels of intensity (or power) L1 and L2 is 10
log10(L1/L2); a difference of 1 dB thus corresponds to a change of about 25 percent. For two
voltages V1 and V2, the difference in decibels is 20 log10(V1/V2); 1 dB corresponding in this
case to a change of about 12 percent.
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Ijatoye O.G.
Ola M.A.
Sound Intensity and Power
THE SOUND PRESSURE SCALE.
(Extract from UCL; Psychology and Language Sciences.)
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Ijatoye O.G.
Ola M.A.
Sound Intensity and Power
CONCLUSION
Sound Intensity is the average rate of transfer of energy per unit area, the area been
perpendicular to the direction of propagated sound. It is difference from sound loudness as
Sound loudness is a subjective term describing the perception of sound by the ear, more
also it’s just an aural sensation, a physiological phenomenon rather than a physical
phenomenon. Sound power is the acoustical energy emitted by the sound source. It is
difference from sound pressure as the latter is a pressure disturbance in the atmosphere
whose intensity is influenced not only by the strength of the source, but also by the
surroundings and the distance from the source to the receiver/listener.
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