What is Sound?

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All machines generate sound and vibration. Sound is an energy form that propagates as longitudinal waves through the air, which is an elastic medium. The sound wave causes small changes in the ambient air pressure, which can be registered by a pressure sensitive instrument (e.g. a microphone).

What are sound power and sound pressure?

A sound source radiates sound power and this results in a sound pressure fluctuation in the air. Sound power is the cause of this. Sound pressure is the effect. Consider the following analogy: an electric heater radiates heat into a room and a temperature change occurs. The temperature change in the room is obviously dependent on the room itself. But, for the same electrical power input, the heater radiates the same power, which is almost independent of the environment. The relationship between sound power and sound pressure is similar. What we hear is sound pressure, but this pressure is caused by the sound power of the sound source. Sound power is expressed in Watts. The sound power level is expressed in decibels (dB), i.e. a logarithmic scale (dB scale) with respect to a reference value that is standardized:


LW = sound power level (dB)
W = actual sound power (W)
W0 = reference sound power (10–12 W)

he sound pressure is expressed in Pa. The sound pressure level is equally expressed in decibels (dB), i.e. a logarithmic scale (dB scale) with respect to a reference value that is standardized:

Lp = sound pressure level (dB)
p = actual sound pressure (Pa)
p0 = reference sound pressure (20 x 10-6 Pa)

The sound pressure that we observe is dependent on the distance from the source and on the acoustic environment in which the sound wave is propagated. For indoor noise propagation, it therefore depends on the size of the room and on the sound absorption of the surfaces. Consequently, the noise emitted by a machine cannot be fully quantified by exclusively measuring sound pressure. Sound power is more or less independent of the environment, whereas sound pressure is not.

Information about the sound pressure level must thus always be supplemented with additional information: the distance of the measurement position from the sound source (e.g. specified according to a certain standard) and the Room Constant for the room in which the measurement was made. Otherwise, the room is assumed to be limitless (i.e. an open field.). In a limitless room, there are no walls to reflect the sound waves, thereby impacting the measurement.

What is sound absorption?

When sound waves come into contact with a surface, a portion of the waves are reflected and another portion is absorbed into the surface material. The sound pressure at a given moment therefore always partly consists of the sound that the sound source generates, and partly of the sound that is reflected of surrounding surfaces (after one or more reflections). How effectively a surface can absorb sound depends on the material of which it is composed. This is usually expressed as an absorption factor (between 0 and 1, with 0 being completely reflecting and 1 being completely absorbing).

What is the room constant and how do we calculate it?

The impact of a room on the propagation of sound waves is determined by the Room Constant. A Room Constant for a room having several surfaces, walls and other inside surfaces can be calculated by taking into account the sizes and absorption characteristics of the various surfaces. The equation that applies is:

Reverberation

A Room Constant can also be determined using the measured reverberation time. The reverberation time T is defined as the time it takes for the sound pressure to decrease by 60 dB once the sound source has been shut off. Absorption coefficients for different surface materials are frequency-dependent and are therefore the derived reverberation time and the room constant. The average absorption factor for the room is then calculated as:

V = volume of the room (m3)
T = reverberation time (s)

The Room Constant K is then obtained from the expression:

A = total room surface area (m2)

What is the relationship between sound power level and sound pressure level?

Under some particular conditions, the relationship between sound power level and sound pressure level can be expressed in a simple manner. If sound is emitted from a point-like sound source inside a room without any reflecting surfaces or outdoors where no walls are close to the sound source, the sound is distributed equally in all directions and the measured sound intensity will therefore be the same at any point with the same distance from the sound source. Accordingly, the intensity is constant at all points on a spherical surface surrounding the sound source.

When the distance to the source is doubled, then the spherical surface at that distance will have quadrupled. From this, we can infer that the sound pressure level falls by 6 dB each time the distance to the sound source is doubled. However, this does not apply if the room has hard, reflective walls. If this is the case, the sound reflected by the walls must be taken into account.

Lp = sound pressure level (dB)
Lw = sound power level (dB)
Q = direction factor
r = distance to the sound source

For Q, the empirical values may be used (for other positions of the sound source the value of Q must be estimated):
Q=1 If the sound source is suspended in the middle of a large room.
Q=2 If the sound source is placed close to the center of a hard, reflective wall.
Q=4 If the sound source is placed close to the intersection of two walls.
Q=8 If the sound source is placed close to a corner (intersection of three walls).

If the sound source is placed in a room where the border surfaces do not absorb all the sound, the sound pressure level will increase due to the reverberation effect. This increase is inversely proportional to the Room Constant:

In the proximity of the power source, the sound pressure level drops by 6 dB each time the distance is doubled. However, at greater distances from the source, the sound pressure level is dominated by the reflected sound, and therefore, the decrease is minimal with increasing distance. Machines which transmit sound through their bodies or frames do not behave as point sources if the listener is at a distance from the center of the machine that is smaller than 2–3 times the machine's greatest dimension.

How do we measure sound?

measuring the sound that a compressor installation produces

The human ear distinguishes sound at different frequencies with different perception efficiency. Low frequencies or very high frequencies are perceived less intensely than frequencies at around 1000–2000 Hz. Different standardized filters adjust the measured levels at low and high frequencies to emulate the human ear's ability to hear sounds. When measuring occupational and industrial noise, the A-filter is commonly used and the sound level is expressed as dB(A).

What happens when several sound sources interact?

When more than one sound source emits sound toward a common receiver, the sound pressure increases. However, because sound levels are defined logarithmically, they cannot simply be added algebraically. When more than two sound sources are active, two are first added together, and the next is then added to the sum of the first, and so on. For memory, when two sound sources with the same levels must be added, the result is an increase of 3 dB.

Background sound is a special case, which require subtraction. Background sound is treated as a separate sound source and the value is deducted from the measured sound level.

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