Noise: "Undesired sound", derived from the Latin word "Nausea"Noise: is highly subjective
Many sounds are unpleasant or unwanted — these are
called noise. However, the level of annoyance depends
not only on the amplitude of the sound, but also our attitude
towards it. e.g. The sound made by a creaking door hinge or a leaf blower can be measured, but not the level of annoyance.
Sound measurements
Sound measurements permit precise, scientific
analysis. However, we must remember
that due to the physiological and psychological differences
between individuals, the degree of annoyance
cannot be scientifically measured. However, the measurements do give us an objective means of
comparing sound under different conditions.
Sound meters
Sound or Noise Meters as they are often called, measure the Sound Pressure Level (SPL) much as we measure air pressure, however Sound Meters respond a lot faster!. How this pressure measurement is expressed can be confusing. It may be in one of the many forms of expressing Decibels. Australian Noise Legislation is defined in Decibels and usually in terms of the "A"-weighted scale of Decibels..
Decibels
The decibel (abbreviated dB) is the unit used to measure the intensity of a sound. The decibel scale is a little odd because the human ear is incredibly sensitive. Your ears can hear everything from your fingertip brushing lightly over your skin to a loud jet engine. In terms of power, the sound of the jet engine is about 1,000,000,000,000 times more powerful than the smallest audible sound. That's a big difference!
On the decibel scale, the smallest audible sound (near total silence) is 0 dB. A sound 10 times more powerful is 10 dB. A sound 100 times more powerful than near total silence is 20 dB. A sound 1,000 times more powerful than near total silence is 30 dB. Here are some common sounds and their decibel ratings:
* Near total silence - 0 dB
* A whisper - 15 dB
* Normal conversation - 60 dB
* A lawnmower - 90 dB
* A car horn - 110 dB
* A rock concert or a jet engine - 120 dB
* A gunshot or firecracker - 140 dB
A tenth of a Bel?
The decibel (abbreviated dB) must be the most misunderstood measurement since the cubit. Although the term decibel always means the same thing, decibels may be calculated in several ways, and there are many confusing explanations of what they are.
The decibel is not a unit in the sense that a meter or a kilogram is. You can go to the National Standards Laboratory at Lindfield and see one of these. A decibel is a RELATIONSHIP between two values of POWER.
Decibels are designed for talking about numbers of greatly different magnitude, such as 23 vs. 4,700,000,000,000. With such vast differences between the numbers, the most difficult problem is getting the number of zeros right. We could use scientific notation, but a comparison between 2.3 X 10 and 4.7 X 10 to the 12th is still awkward. For convenience, we find the RATIO between the two numbers and convert that into a logarithm. This gives a number like 11.3. As long as we are going for simplicity, we might as well get rid of the decimal, so we multiply the number times ten. If we measured one value as 23 hp and another as 4.7 trillon hp, we say that one is 113dB greater than the other.
The usefulness of all this becomes becomes apparent when we think about how the ear perceives loudness. First of all, the ear is very sensitive. The softest audible sound has a power of about 0.000000000001 watt/sq. meter and the threshold of pain is around 1 watt/sq. meter, giving a total range of 120dB. In the second place, our judgment of relative levels of loudness is somewhat logarithmic. If a sound has 10 times the power of a reference (10dB) we hear it as twice as loud. If we merely double the power (3dB), the difference will be just noticeable.
What do we hear?
The smallest distinction in volume level or amplitude, or energy level that the human ear can hear is 3dB. This can be achieved by having the two sounds available so we can compare them.
Inside a specialised insulated sound room called an "anechoic chamber", we can hear a slightly smaller distinction as long as we can switch between the two sound sources.
Also, the human ear does not hear all frequencies the same way. A home stereo system has a "loudness control" to boost power to low frequencies or the "bass notes" of music to compensate for this characteristic of the ear.
The human ear is quite sensitive to the frequencies of speech and "noise" in this range can be really annoying.
The above graph shows the Equal Loudness Contours, as measured by Robinson and Dadson, 1956. The original measurements were carried out by Fletcher and Munson in 1933, and the curve often carries their name.
The lines represent the sound pressure required for a test tone of any frequency to sound as loud as a test tone of 1 kHz. Take the line marked "60" - at 1 kHz ("1" on the x axis), the line marked "60" is at 60dB (on the y axis). If you follow the "60" line down to 0.5 kHz (500 Hz), and look across to the y axis, the value is about 55 dB. What this means is that a 500 Hz tone at 55 dB SPL sounds as loud to a human listener as a 1 kHz tone at 60 dB SPL.
If every frequency sounded equally loud, then this graph would just be a series of horizontal lines.
Notice that low frequencies are heard poorly.
Human hearing spectrum
The A-Weighted Scale
To measure sound in a meaningful way, a "filter" is applied to a sound meter, so that the measured pressure waves are expressed in the same form as human hearing. This is what is known as the "A-weighted Scale" which is the basis for NSW Noise regulations
The A-weighted scale is the internationally recognised scale for environmental sound tests, including all vehicles.
Note that a motorcycle with a deep "clean" note will have the energy level discounted in the test, but a motorcycle with higher frequencies in the exhaust note may not only fail the test, be also be "annoying'. Here we have the reason why "trail bike noise" is marked for attention by authorities, yet ordinary street registered machines mostly pass unnoticed by the general population - unless surprised by it, or in a very quiet location where any noise would be a disturbance.
