As we know from our high school science classes, sound waves are created when an object moves or vibrates. When these waves reach our ears, they cause our eardrums to vibrate, sending signals to the brain that we interpret as sound.
| Frequency
[number of wave crests per unit time that pass a fixed location | measured in Hz- the number of waves per second]
measures the tone or pitch of a sound.
For example, a bass guitar plays lower frequencies than a violin.
Human beings usually hear 15 to 20,000 [Hz] frequency sound.
| Amplitude
[the wave height | measured in dB] used to measure the intensity of a sound
A measurement of the wave traveling through the air is used as an indication of the intensity of sound or its volume, and is described in terms of a scale called the decibel (dB). Noise measurements made by filtering high- and low-pitched sounds—approximating the way an average person hears sounds—is called the A-weighted level or dBA [1924].
0 dB | the faintest sound we can hear;
30 dB | a quiet library or in a quiet location in the country;
45 dB | typical office space or ambience in the city at night;
60 dB | a restaurant at lunch time;
70 dB | the sound of a car passing on the street;
80 dB | loud music played at home;
90 dB | the sound of a truck passing on the street;
100 dB | the sound of a rock band;
115 dB | limit of sound permitted in industry; and
120 dB | deafening.
The dB(A) scale begins at zero, which represents the faintest sound that can be heard by humans with very good hearing. Conversations take place in the 50 dB(A) range and a chainsaw whines at about 100 dB(A). Normal highway traffic sounds rank about 75 dB(A) and jet airliners around 90 dB(A). For most people, discomfort starts in the 70 to 80 dB(A) range, with the threshold of pain around 140 dB(A). The Federal Highway Administration (FHWA) has chosen 67 decibels as the point where state and federal agencies must consider reducing the noise level.
www.quietpavement.com
| Sound behavior
Because the sound has the properties as wave, it has properties of “reflection” and “transmission”, and attenuates in accordance with the distance. These properties are illustrated below for reference.
| Atmospheric effects
Winds will increase sounds downwind from a source and reduce them upwind. This is not solely a result of the velocity effect, but also because the spherical wave-front is deformed by the prevailing wind.
As discussed previously, the speed of sound is dependant on temperature, the higher the temperature, the higher the speed. This means that when the temperature near the ground is higher than that of the upper air, sound rays tend to arc upwards slightly. Thus less energy will reach a listener some distance away at ground level. (For a given amount of sound energy, the distribution area is increased).
At night, when the ground surface is cooler than the upper air, the inverse occurs: sound energy tends to arc downwards. (For a given amount of sound energy, the distribution area is reduced).
| Barrier Design
Barriers, such as walls or screens, will act to create an acoustic shadow. The reduction in sound level within this shadow zone is dedendant on frequency (as we discussed earlier). At high frequencies the effect of the barrier is most pronounced whereas at low frequencies much diffraction occurs at the edges, so the shadow effect is diminished.
| Type of Road and Average Speed
Rural roads – 110 km/h speed limit – use 108
Urban Freeway – 90 km/h speed limit – use 92
Urban Highway – 70 km/h speed limit – use 65
Urban Street – Dual Carriageway – use 60
Urban Street – Single Carriageway – use 55
Urban Street – Single Congested – use 50
Filed under: weight Points, Research
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