Sound is the auditory perception of vibration or pressure oscillations that occur in the air around us. The pitch of a tone can be analyzed in terms of frequency and is expressed in hertz (Hz), the number of vibrations or pressure oscillations that occur per second. Humans can generally hear from 20 Hz (low pitch) to 20,000 Hz (high). While we enjoy this full spectrum when listening to a symphony, the critical frequencies for understanding speech typically occur at mid-range frequencies between 125 Hz and 4,000 Hz. Most noises contain a combination or range of frequencies occurring simultaneously.

The intensity or loudness of a sound is related to sound pressure and is expressed in decibels (dB). Because decibels are logarithmic units, a change of 3 dB will be barely noticeable but a 10 dB change will appear twice (or half) as loud. Zero dB represents the threshold of audibility and sound pressures above 100 dB are loud enough to cause deafness and pain. Ours is an increasingly noisy society, and occupational health and safety regulations limit the duration for which a person may be exposed to very loud noise. Even in less noisy environments, however, attention must be given to noise control if we are to provide optimum living, working and listening conditions.

Noise is propagated and travels in waves in much the same way as ripples spreading out from a pebble tossed in water. Visualizing this can help to understand how sound is reflected when it encounters a hard dense surface, absorbed when it enters a resilient or porous material, or transmitted through a lightweight construction. Sound waves can also be focused or dispersed depending on whether they reflect from a concave or convex surface.

When designing the acoustical environment of a room, one begins by identifying the sources and characteristics of sounds in the space and by defining the acoustical criteria (such as loudness, reverberation, speech intelligibility, etc.) that are required by its occupants. The acoustics of an open office or restaurant, for example, must enable occupants to have intelligible yet private conversations. And in auditoria and conference rooms, sound must be carefully distributed so that performers or speakers can be clearly heard throughout the room. Once these project parameters are determined, ceiling and wall systems can be designed to reinforce desirable sounds and attenuate undesirable noises.

The noise reducing potential of panels is determined with ASTM C 423 - Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method. Note that test results can vary depending upon how panels were mounted in the test chamber: a test report that replicates the installation conditions expected on the job site should be used. The percent of noise absorbed is typically calculated at six frequencies from 125 Hz to 4000 Hz. To simplify comparisons between acoustical products, the noise reduction coefficient (NRC) is the average of sound absorption tests at 250, 500, 1000, and 2000 Hz, rounded to the nearest multiple of 0.05. These mid-range frequencies are used because they cover the spectrum most important for spoken communication.

While an NRC is useful for preliminary product screening, it should not be relied upon for final product approval. When test data are plotted on a graph, they usually form a gradual curve; a peak in the curve indicates that the tested material is especially efficient at absorbing certain frequencies. This could be desirable if, for example, one needs to dampen the whine from a machine operating at that particular frequency. But if the same insulation were used in a music hall, it would have the disconcerting effect of hushing certain notes. For this reason, it is good practice to ask panel manufacturers to furnish complete product test reports prepared by qualified, independent acoustical laboratories, especially when designing spaces where high quality acoustics are essential.

It is important to match the reverberation time of a space to the type of listening environment required. Reverberation is the persistence of a sound after the source of the sound has stopped; the time it takes for the sound to "decay" to a point of inaudibility. In a room surrounded by hard, reflective materials, sound bounces from surface to surface and may continue to echo or reverberate for several seconds. These reverberations could make conversation or music difficult, since each syllable or note would echo back and forth and interfere with the ability to distinctly hear subsequent syllables and notes. Offices and recording studios, consequently, are generally designed to have short reverberation times; a reverberation time less than 1 second is generally suitable for a conference room or class room. In other spaces, however, a longer reverberation time may be desirable. Reverberation times of 2 or 3 seconds may be desirable in a symphonic hall or church to allow the music to have a "live" feel. For example, the long, sustained notes of a pipe organ or choir can help to create the proper liturgical atmosphere in a cathedral.

The following formula is used to predict a room's reverberation time:
T = 0.05 V/a

where:
T = reverberation time, the time required for a sound to decay 60 dB, in seconds
V = room volume in cubic feet
a = the total amount of sound absorbing material in a room, measured in Sabins. (A Sabin is equal to one square foot of surface that absorbs 100% of the sound falling on it. For example, a window with one square foot of open area would have one Sabin since all of the sound energy impinging on the window would leave the room.)

Noise reduction is tested at six frequencies, but only four mid-range values are averaged to obtain a Noise Reduction Coefficient (NRC). The graph shows two panels with the same NRC; however "A", has better overall performance, especially in the lower frequencies that are critical for speech.

Based on this, the reverberation time can be decreased by using perforated panels with acoustical insulation to increase the sound absorption (a) in a space. Alternatively, reverberation time can be increased by using acoustically transparent perforated panels to increase the volume (V) of the space.