Acoustical 101

Acoustics is defined as the scientific study of sound especially of its generation, transmission, reception and reduction. Due to differing legislation, no one standard fits all geographical regions. Therefore, the focus in this report is mainly an overview of acoustics in the area of noise, noise pollution and noise reduction

Environmental noise is a worldwide problem. Common sources for noise are automotive and truck traffic, airplanes, trains, sirens, generators, air compressors and air conditioning units. Noise can have adverse health effects such as diminish hearing, disturb sleep, disrupt conversation, and diminish one’s quality of life. Immediate safety factors should also be considered. The fact that you can’t see, taste or smell noise may help explain why it has not received as much attention as other types of pollution, such as air pollution or water pollution.

Noise is typically defined as “unwanted sound” or the human sensation of pressure fluctuations in the air. For noise to exist, there must be three components: Source, Path and Receiver.

The frequency of sound is the number of times in a period of one second that the pressure changes from zero to maximum to minimum to zero, thus completing a cycle. Amplitude refers to the loudness level of the noise and is measured in decibels (dB). Decibels (dB) are logarithmic units, where 5 dB is nominally the threshold of hearing” and 140 dB is nominally the “threshold of pain”.

Since decibels are logarithmic, multiple dB cannot be added by ordinary arithmetic means. For example, if one automobile generates 70 dB when it passes an individual, two cars passing simultaneously would not produce 140 dB. In fact, they would combine to produce 73 dB. The following shows how to add two decibel values:

When Two Decibel Values Differ By: Add this Amount of the Higher Value: Example
0 or 1 dB3 dB70+69=73
2 or 3 dB2 dB74+71=76
4 to 9 dB1 dB66+60=67
10 dB or more0 dB65+55=65

Both the frequency and amplitude of sound are significantly altered by the physical variables in the path to the receivers. For example, walls, structures, ground absorption, atmospheric conditions such as temperature, humidity, wind and rain all contribute to changes in source noise levels before it reaches the receiver

Our main concern with sound is the effect it has on the receiver…..when a sound becomes an unwanted annoyance. Variations between sound power levels from the source and the characteristics of the path between the source and the receiver determine the sound levels upon the receiver. The most critical element, however, in determining the sound levels upon the receiver is the individual sensitivity of the human receiver.

Using Sound Walls to Control Unwanted Noise

Noise levels can be lowered by eliminating or reducing the noise at the source with the use of noise-control walls and enclosures. Sound walls are most effective when built close to the source or close to the receiver. The height of the wall should interrupt line-of-sight between the source and the receiver.

Sound walls are classified as reflective or absorptive. Hard surfaces such as masonry or concrete are reflective. This means most of the noise is reflected back towards the noise source and beyond.

A barrier wall with a surface material that is porous with many voids is said to be absorptive. This means little or no noise is reflected back towards the source. Sound-absorptive walls installed between the noise source and the receivers are effective in reducing reflective noise.

In evaluating the effectiveness of a barrier or enclosure, criteria such as local, state, and federal requirements must be considered. These codes cover a large range of situations from vehicle, industrial and residential boundary noise. In order to be in compliance with these criteria, octave band frequency analysis is typically required to investigate the noise source in order to calculate the amount of reduction that will be required in order to merit the criteria that is trying to be achieved. Individual octave band measurements are a detail procedure that requires precautions from false measurements such as wind and increases in ambient noise levels. The procedure itself is more in-depth than this introduction allows.

Sound walls are performance rated in two categories Sound Transmission Class (STC) and Noise Reduction Coefficient (NRC). The STC is a single number rating of a material’s or assembly’s barrier effect. A higher STC rating blocks more noise from transmitting through a wall. The NRC is a single number index for rating how absorptive a material is. A material with an NRC of .8 will absorb 80% of the sound that comes into contact with it and will reflect 20% of the sound back into space. A good sound wall is a sound-absorbing wall with a STC rating of 30 or more and a minimum NRC rating of .85.