Noise Transmission Paths
NOISE TRANSMISSION PATHS
Noise, or unwanted sound, disturbs our peace and quiet in all aspects of our lives. Sound energy can be transmitted through several different media, including gases, liquids and solids. This enables noise to leak into our homes, vehicles, workplaces and classrooms through numerous “noise transmission paths.” A critical first step in developing an effective soundproofing solution is to identify the offending noises and their sources, and to determine the noise transmission paths through which the noise is transmitted to our ears. One effective means of soundproofing is to interrupt or intercept this noise along the noise transmission paths through the addition of sound barriers and sound absorbent materials.
The proper identification of the noise transmission paths can be complicated for a variety of reasons. The human auditory system identifies the direction or source of noise (called sound localization) by using small differences in loudness levels between your ears and/or determining the time delay in which an acoustic wave arrives at one ear verses the other. At higher frequencies we can normally determine the source and direction of noise using both time delay and level differences. At frequencies below about 200 Hz, however, it becomes impossible to determine loudness level differences, and at frequencies below about 80 Hz, it becomes impossible to determine the time delay differences. It is therefore necessary to determine the acoustic characteristics of the noise in order to successfully identify noise transmission paths.
Can sound level meters identify noise transmission paths? Acoustic characteristics include the loudness, duration and frequency content of the noise, which can be measured with a sound level meter (SLM). While there are several low-cost “noise level meters” on the market that report overall noise levels (in decibels), these may not be adequate since it is necessary to examine the frequency content of the sound. A typical SLM will report noise (in decibels) as a function of frequency, usually measured in octave bands. In recent years, several sound level meter apps have become available for download onto mobile devices; however, they may not be accurate due to limitations of their built-in microphones. More information about sound level meters can be found in our section entitled “Sound Level Meters.”
Since sound is readily converted into vibration (and vice versa), it is sometimes difficult to determine the root cause of the noise. For example, when sound encounters a wall, sound waves are reflected off of the wall, are absorbed by the wall and/or leak around the wall (through so-called flanking paths). Airborne acoustical energy can also create structure borne vibration in the wall itself, effectively transferring sound through the wall. This phenomenon is very similar to the tin can and string telephones we experimented with as children. When a structure borne sound wave reaches the opposite side of the wall, it is converted once again to the airborne sound that we hear.
In other situations, vibrations of mechanical components may “short circuit” against a wall, and transmit vibrations to the wall which radiates as sound on the other side of the wall. Such is the case with plumbing that is firmly attached to a wall, transmitting vibrations of water in the pipes into airborne noise. Depending on the noise transmission paths and the noise acoustical characteristics, an effective soundproofing solution will isolate against mechanical vibrations, block the sound with a barrier material and/or dissipate the energy with a sound absorbent material. You can see that noise transmission paths are as varied as the noise sources themselves.
Sonic-Shield noise and vibration engineers are available to assist you in identifying noise transmission paths. For certain problems, we have already developed optimum solutions that can be readily adapted to your situation. For other more complex problems, we can use sophisticated, state-of-the-art acoustical testing equipment and analysis software (such as an acoustic camera) to rapidly gather data and perform simulations in order to recommend and design the most cost-effective soundproofing solutions.