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What is sound ?
Sound waves are in fact variations in the air pressure. The term Sound Pressure Level (SPL) is used to define the loudness or intensity of sound. The intensity of a sound wave is defined as the average rate per unit area at which force is transmitted by the sound wave. The SI unit for intensity is thus newtons per meter square.
For a normal human ear, the displacement amplitude of sound wave ranges from approximately 10-5m for the loudest tolerable sound to 10-11m for the faintest detectable sound. Since a large ratio, 106, is formed from these two limits of human detectable displacement amplitude of sound wave; instead of speaking of the intensity of a sound wave, it is much more convenient to speak of a sound pressure level in terms of dB. We define dB as the Sound Pressure Level in logarithmic ratio to the sound pressure, Po = 0.0002 m bar or 20 m Pa, which is the minimum sound pressure that a normal human can detect at 1 KHz.
The mathematical relationship between two sound pressure levels expressed in dB is shown below:
SPL in dB = 20 log10(P/Po)
where P is the unspecified sound pressure. The range of detection for a normal human ear is from 0 to 120 dB. To have a physical understanding of what SPL means in dB, the following table ( Figure 1.) illustrates the Sound Pressure Levels in dB of different environments:
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Environment | SPL in dB |
Threshold of Hearing | 0 |
Rustle of Leaves | 10 |
Whispering | 20 |
No Traffic Midnight City Street | 30 |
Small Talk in Office | 60 |
Jackhammer (1m) | 90 |
Heavy Metal Rock Concert | 110 |
Threshold of Pain | 120 |
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A - Weighting Curve
The rate of change in Sound Pressure Level is measured in frequency. A normal human ear can perceive sound frequencies from the range of 20 Hz to 20 KHz. However, our ears respond better at mid range frequencies and also, they have higher sensitivity on certain frequencies than others.
This explains why a given sound pressure level seems "louder" at 5000 Hz than the same sound pressure level at 50 Hz. In order to provide a balancing effect due to this phenomena, an A - Weighting curve is employed, so that the measured sound pressure level is adjusted to a way similar to how we hear. Figure 2. shows the A - Weighting curve.
Figure 2. A - Weighting Curve
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Determination of Sound Pressure Level
There are many external factors that can influence the measurement of the Sound Pressure Level. The size and shape of the room, the constructional materials of the ceiling, wall, and floor, background noise, and even testing methods and instruments can all affect the result of the measured Sound Pressure Level. Therefore, in order to minimize these influences, the testing of Sound Generators should always be carried out in relatively quiet open space or even better, in anechoic chambers.
Sometimes, the distance ( 10 cm) that we use for testing the output of sound pressure level from the sound level meter may not be useful. Below is a simple guide for the determination of SPL from different distances. The mathematical relationship is as follows:
S1 = Sound Pressure Level output at distance D1
S2 = Sound Pressure Level output at distance D2
R = D2/ D1
Se = Estimated change in Sound Pressure Level = 20log(R)
S1 = S2 + Se
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Figure 3. points out the value change in SPL against measuring distances from sound level meter.
For example, if users require a sound generator that can produce min. 85 dB from a distance 30 cm away from the sound level meter, then the appropriate type of JL sound generator which satisfies this requirement should have at least 94 dB output of Sound Pressure Level at 10 cm.
Figure 3. Estimated Change in SPL VS Ratio of Distances
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Application and Selection of JL World Sound Generators
JL World Sound Generators can be applied in almost any areas that need sound including:
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Computer Systems
personal computer, computer terminal, printer, modem.
Communication
cellular phone, home phone, pager, PDA.
Office and Home
portable game, copying machine, telephone, POS system, timer, electrical and electronic appliances, security system for home and vehicle.
Automotive
Dashboard equipment, door alarms, chimes, alerts.
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To select the appropriate type of JL World Sound Generators, users should consider the following factors:
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- Size
- Application Voltage
- Application Sound Frequency
- Required Sound Pressure Level
- Power Consumption
- Frequency Range
- Assembly Process Employed
- Cost and Availability
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