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.
| 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.
When a wave reaches the boundary between one medium another medium, a portion of the wave undergoes reflection and a portion of the wave undergoes transmission across the boundary. The reflected wave may or may not undergo a phase change (i.e., be inverted) depending on the relative densities of the two media. It was also mentioned that the amount of reflection is dependent upon the dissimilarity of the two medium. For this reason, acousticly minded builders of auditoriums and concert halls avoid the use of hard, smooth materials in the construction of their inside halls. A hard material such as concrete is as dissimilar as can be to the air through which the sound moves; subsequently, most of the sound wave is reflected by the walls and little is absorbed.
Reflection of sound waves off of surfaces can lead to one of two phenomenon – an echo or a reverberation. Reverberation | often occurs in a small room with height, width, and length dimensions of approximately 17 meters or less. Why the magical 17 meters? The effect of a particular sound wave upon the brain endures for more than a tiny fraction of a second; the human brain keeps a sound in memory for up to 0.1 seconds. If a reflected sound wave reaches the ear within 0.1 seconds of the initial sound, then it seems to the person that the sound is prolonged. The reception of multiple reflections off of walls and ceilings within 0.1 seconds of each other causes reverberations – the prolonging of a sound. Since sound waves travel at about 340 m/s at room temperature, it will take approximately 0.1 s for a sound to travel the length of a 17 meter room and back, thus causing a reverberation (t = v/d = (340 m/s)/(34 m) = 0.1 s). This is why reverberations is common in rooms with dimensions of approximately 17 meters or less. Perhaps you have observed reverberations when talking in an empty room, when honking the horn while driving through a highway tunnel or underpass, or when singing in the shower. In auditoriums and concert halls, reverberations occasionally occur and lead to the displeasing garbling of a sound.
Echo | Reflection of sound waves also lead to echoes. Echoes are different than reverberations. Echoes occur when a reflected sound wave reaches the ear more than 0.1 seconds after the original sound wave was heard. If the elapsed time between the arrival of the two sound waves is more than 0.1 seconds, then the sensation of the first sound will have died out . In this case, the arrival of the second sound wave will be perceived as a second sound rather than the prolonging of the first sound. There will be an echo instead of a reverberation.
| Geometry & Reflection
The way sound rays reflect off plane (flat) mirrors or surfaces is very simple: the angle of incidence is equal to the angle of reflection.
The angles in question are measured with respect to the normal to the reflecting surface. The angle of incidence is the angle the incident light ray makes with the normal, and the angle of reflection is the angle the reflected ray makes with the normal.
This is exactly the same behavior seen in billiard balls bouncing off a rail on a pool table. (We neglect friction and other minor effects.) Even sound waves bouncing off a wall at an angle follow this same law of reflection.
| 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 1
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: Uncategorized , Barriers, Highway, Materials, Shimrit cohen, Sound Wave, Tal levy
