Sound
Sound is produced when an object vibrates. Trains, planes, automobiles and other tools of modern society produce sounds that can be undesirable in our work and home environments. The human ear can perceive sound as either pleasurable or annoying, depending on the circumstances surrounding the event.

While the roar of a jet engine may seem acceptable as you travel to a vacation destination, it may be an extreme annoyance to office workers located close to the airport. Consequently, glass selection for commercial building applications becomes very important.

Understanding the relationship of sound and glass allows us to maximize the acoustical performance of glass in building constructions. Sound that comes from one particular source may be a combination of sound transmitted at various frequencies. Therefore, it is imperative to select a certain glass type that is capable of reducing sound over a range of different frequencies.

For example, a jet engine may produce sound at very low frequencies as it starts up, but as it attains greater power it produces sound at higher frequencies. The glass selection to reduce sound transmission will need to have a broader range of sound reduction potential. The figure below illustrates the distribution of typical sounds and their peak frequencies.

Since sound is produced by an object when it vibrates, the movement of jet engine parts causes the jet to vibrate. The engine vibration creates a disturbance in the air, which spreads out in all directions-much like ripples from a rock dropped in still water. When air particles are disturbed by a vibrating source, the layer of air nearest the source follows the back and forth motion of the source. It then causes an air pressure disturbance.

The initial disturbance gradually spreads out to air particles further away and at a certain speed, which is known as the speed of sound. The speed of sound varies slightly with the temperature and humidity, but is independent of frequency.

The chain reaction of air pressure disturbance, which is a sequence of air pressure changes (compression and rarefraction) is called a sound wave. The number of air pressure changes spreading out from the vibrating source is measured in cycles per second (cps). For example, 500 ripples of air pressure from the vibration is actually 500 cps, which is the sound frequency. The basic unit of frequency is Hertz (Hz).


Sound Intensity
Sound intensity is the amount of acoustical energy in a sound wave and is proportional to sound pressure. The most common measure of sound pressure is sound pressure level (SPL), which is expressed in decibels, using a compressed or logarithmic scale.

The human ear is capable of detecting very faint and loud sounds. Differences between sounds can be dramatic and are described as intensity. The differences in intensity between faint sounds and loud sounds is similar to the difference in weight of a paper airplane and a 747 jet. The ear is not equally sensitive at all frequencies. For example, the sound pressure level of two different noises may be the same. One noise may be perceived as being louder if the sound power is concentrated in a single frequency or a range of frequencies where the ear is more sensitive. The second noise may have a single frequency or a range of frequencies where the ear is less sensitive. The sensitivity of hearing is generally limited to a range of 10 Hz to 20,000 Hz; however, the human ear is most sensitive to sound within a range of 500 Hz to 8,000 Hz. Beyond this range, our hearing capability gradually becomes less sensitive.

To accommodate for this sensitivity, sound level meters incorporate a filtering device. The filtering is designed to correspond to the varying sensitivity of the human ear to sounds within the audible range of frequencies. This is referred to as A-weighting. Sound pressure levels, using the A-weighted meter scale, are identified as dBA.

Sound Pressure Levels
Figure 2 illustrates the correlation between sound intensity or pressure (cps) and sound pressure level. The sound pressure level is an easy way of classifying sound intensity. Sound pressure levels are listed in decibels (dB). Notice that a sound pressure level of 0 dB does not mean there is no sound (see Figure 2). Instead, it means that there is no sound detectable by a person with normal hearing. Whenever the sound pressure level increases 10 dB, the sound intensity increases by ten.

Under typical conditions, an individual with normal hearing cannot detect a change in sound pressure of 1-2 dB. A difference in sound pressure of 3 dB is barely perceptible if the change is sustained and no time lapse occurs. A change of 5 dB is clearly detected and a change of 10 dB is perceived as twice or half the noise level.


Sound Transmission Loss
To determine the acoustical performance of glass, it is important to consider the application in which it will be used, as well as the framing system that supports the glass. For each sound frequency, the reduction in sound produced by a sound barrier is called the sound transmission loss (STL) at that frequency. When glass is used on an exterior wall, its STL at various frequencies is used to determine the effectiveness of the glazing.

Viracon has tested various glass configurations to determine the sound transmission loss over a range of 100 Hz to 5,000 Hz. These tests help designers evaluate and select the best glass to provide greater sound transmission loss at those frequencies where the greatest amount of noise potential exists.

As indicated earlier, the selection of window framing systems is important when reducing sound transmission. Window framing systems are evaluated for thermal characteristics, as well as air and water infiltration. Certain window framing systems may perform better acoustically than others as a design function. One important attribute to consider is the air tightness of the system. Window framing systems that allow greater amounts of air infiltration also allow greater sound transmission.

Dry glazed window systems, which use rubber gaskets as weather seals, may not be as effective at reducing sound transmission as systems that use wet seals (gunable sealants). The combination of wet seals with butyl or open cell foam dramatically reduces the potential for air infiltration; thus, flanking sound transmission.

In addition, sound pressure impinging on the window framing will cause it to vibrate, transmitting sound to the building interior. Consequently, the window glass performance cannot solely be relied upon to reduce sound transmission to the building interior. The sound transmission of the window framing will result in higher levels of sound transmission through the glass and wall.

STC RATING
Sound Transmission Class Rating
When glass is used on the building interior, the sound transmission classification (STC) value can be used to categorize the glass performance. The STC rating is a single-number rating system for interior building partitions and viewing windows.

The STC rating is derived by testing in accordance with ASTM E90, "Laboratory Measurement of Airborne Sound Transmission of Building Partitions". The STC value is achieved by applying the Transmission Loss (TL) values to the STC reference contour of ASTM E413, "Determination of Sound Transmission Class". The STC rating is a basis for glass selection. Its original intent was to quantify interior building partitions, not exterior wall components. As a result, it is not recommended for glass selection of exterior wall applications, since the single-number rating was achieved under a specific set of laboratory conditions.

Laboratory measurements of sound transmission loss and subsequent STC ratings are dependent on a number of factors present at the time of testing. The laboratory test is an "ideal test condition" used to minimize extraneous factors from the test results. Cautious consideration must be given to the laboratory "test results" versus actual job conditions.

The test frame aperture size available at most testing laboratories may also be limited and standardized to facilitate the installation of popular products. As a result, the standardized aperture size may be inappropriate for all products tested nor representative of actual building conditions.

It is not recommended to estimate STC ratings based on the performance of tested products in comparison to "new" configurations. This is because of the critical relationship of glass construction and its reaction to sound at various frequencies.

Minor changes to the glass construction and air-space thickness may increase sound transmission loss at some frequencies and decrease it in others. Depending on where the critical frequencies exist for a particular construction, the STC rating could actually be lower even though the glass construction was thought to have been improved with minor modifications.

OITC RATING
Outside-Inside Transmission Class Rating
This rating is used to classify the performance of glazing in exterior applications. This is based on ASTM E-1332 Standard Classification for the Determination of Outdoor-Indoor Transmission Class. While STC rating is based on a 'White ' noise spectrum, this standard utilizes a source noise spectrum that combines Aircraft/Rail/Truck traffic and is weighted more to lower frequencies.