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Creating effective acoustic solutions

Because the perception of sounds can be as unique as the appearance of colors in a rainbow, it is important to understand the source of the particular noise to be attenuated prior to designing the acoustical solution. Sounds that are often the target of acoustical engineering include machinery, traffic (trucks and automobiles), trains (including urban rail systems) and aircraft. The differences in the peak frequency of each of these sources of noise create unique requirements for their attenuation. Acoustical engineers have identified sound reduction targets for various sources of noise based on specific frequencies within a sound source. Different targets exist, for example, for diesel vs. non-diesel trains, heavy vs. medium trucks and even multiple noise sources. Glass laminated with polyvinyl butyral (PVB) interlayers is one of the most effective ways to reduce noise in all these situations because PVB reduces sound transmission through the glazing across a range of frequencies.

 

Noise pollution can originate in many ways, but increasing noise pollution is the result of growing populations; additional road, rail and air traffic; and greater industrial, neighborhood and recreational noise.

Sound transmission loss

Just as glass lets in light, windows can also transmit noise and they need to have damping added in order to provide sound control. A building material’s resistance to the transmission of sound is known as the transmission loss (TL) and is expressed at a given frequency in decibel (dB or dBA) units. Transmission loss of a glazing material is typically measured between 80 and 5,000 hertz (Hz) in one-third frequency bands. The material’s ability to minimize the passage of sound is quantified and reported as sound transmission loss (STL) at each defined frequency.

The STL of a material is measured by sending a known amount of frequency specific sound from one room (source room) through a wall-mounted specimen and recording how much of that sound gets through the other side (receiving room). The difference between the sound levels in the source room and the other room (the receiving room) is defined as the noise reduction (NR).

After obtaining the STL of the mounted panel the data is analyzed to determine the effectiveness of the material based on applications. For the most accurate reduction of noise, specific frequencies are reviewed to determine the level of sound reduction needed to dampen the offending noise. Because a thorough study of the surrounding noise can not always be obtained, it is common for the industry to use a single number rating system as a means of evaluating acoustical performance of materials.

For glazing, there are two methods of calculation used in the U.S. Sound transmission class (STC) uses the STL gathered in one-third octave bands from 125 – 4,000 Hz and a contour fitting principle to determine the single number rating. STC was designed to be used for sound transmission rating in interior compartments of buildings. Outside-Inside Transmission Class (OITC) is another single number rating that uses STL gathered from 80 – 4,000 Hz and a mathematical equation designed to depict human hearing to develop a single number rating. OITC was developed to assist in the determination of how much unwanted external noise will reach the inside of buildings. Although OITC is the better single number rating to use for outside noise penetration into a given facility, STC is more commonly referenced. It should be noted however that STC is weighted to “conversation” type noise and that certain frequencies of outside noise may not be damped to an acceptable level if sole reliance on STC is used during the acoustical design of a facility.