Introduction to noise
The standard unit of measurement of noise is the decibel (dB). The term decibel was developed by Graham Alexander Bell to simplify the calculations involved when measuring the resistance of a piece of wire. The decibel scale is used to calculate noise levels and counts ratios in powers of ten. For example, a noise level recorded at 10 dB which then increases to 40 dB has multiplied by a factor of 10,000 (10 x 10 x 10 x 10 or 104). The decibel scale is ideal for acoustics because of the large range of sound levels the human ear is able to hear and the comparatively large numbers we would otherwise have to use had this system not been adopted.
Acoustic parameters such as the sound pressure level or the sound absorption coefficient are usually expressed in increments of octaves and one-third octaves. For room acoustics the relevant octave frequencies are 125 Hz, 250 Hz, 500 Hz, 1000 Hz, 2000 Hz and 4000 Hz. The octave increments are obtained by doubling the previous frequency. Each octave comprises three one-third octave values.
The number of cycles per second, for example if an object vibrates 500 times per second, the frequency of the noise it produces is 500 cycles per second or 500 Hertz (Hz).
The human ear is able to hear frequencies ranging from 20 Hz to 20,000 Hz. The ear interprets the frequency of a sound as pitch. Noises with low frequencies such as thunder are heard as low-pitched noise. Noise with high frequencies such as mosquitoes buzzing are heard as high-pitched noise.
A-weighting is applied to instrument-measured sound levels in effort to account for the relative loudness perceived by the human ear, as the ear is less sensitive to low audio frequencies.
The A-weighting curve has been widely adopted for environmental noise measurement, and is standard in most sound level meters. This curve shows that sounds at 50 Hz would have to be amplified by 30 dB to be perceived equally as loud as a sound at 1000 Hz at normal sound levels.
A sound source radiates power and this results in a sound pressure. Sound power is the cause. Sound pressure is the effect. Consider the following example: an electric heater radiates heat into a room and temperature is the effect. Temperature is also the physical quantity that makes us feel hot or cold. The temperature in the room is obviously dependent on the room itself, the insulation, and whether other sources of heat are present. But for the same electrical power input, the heater radiates the same power, practically independent of the environment. The relationship between sound power and sound pressure is similar. What we hear is sound pressure but it is caused by the sound power emitted from the source.
Sound pressure that we hear, or measure with a microphone is dependent on the distance from the source and the acoustic environment (or sound field) in which sound waves are present. This in turn depends on the size of the room and the sound absorption of the surfaces. So by measuring sound pressure we cannot necessarily quantify how much noise a machine makes. We have to find the sound power because this quantity is more or less independent of the environment and is the unique descriptor of the noisiness of a sound source. (From Bruel & Kjaer)
Sound power is the rate at which energy is radiated (energy per unit time). Sound intensity describes the rate of energy flow through a unit area in m2, therefore the units for sound intensity are watts per square metre.
Sound intensity also gives a measure of direction as there will be energy flow in some directions but not in others. Therefore sound intensity is a vector quantity as it has both magnitude and direction.
Outdoors, sound waves travel in a continuously expanding spherical wavefront from the source. For a point source that emits a certain sound energy, this energy is concentrated in a single point at the source. At a distance from the source, the same energy is distributed over a sphere. The greater the distance from the source, the larger the surface over which the energy is dispersed. This may be illustrated by studying a segment of the expanding sphere. The sound energy is dispersed over an imaginary sphere with a surface that grows in proportion to the square of the distance from a point source.
The surface of the sphere grows four times with each doubling of the distance from the source. The sound hence rapidly declines with the distance from the source. Each doubling of the distance from the point source yields a 6 dB reduction of the sound level.
Adding noise sources
When adding two sound sources together the total sound level does not double, this is because decibels are expressed in logarithmic values so simple addition and subtraction cannot be used.
Sound loudness is a subjective term describing the strength of the ear’s perception of a sound. A widely used ‘rule of thumb’ for the loudness of a particular sound is that the sound must be increased in intensity by a factor of ten for the sound to be perceived as twice as loud.