The level of pressure variation (amplitude) determines the loudness; the number of pressure variations per second determines the frequency / (CPS. Hz)

Sound travels through air at a speed (speed of sound) dependent on temperature. The speed of sound at 70°F is approximately 1130(FPS) feet per second. Higher temperatures generate faster speeds of sound. The frequency (CPS, Hz) of sound can be measured. The human ear hearing range is 20 Hz to 20,000 Hz, and is most sensitive in the 1000 to 4000 Hz range.

Wavelength is an essential variable in describing the sound characteristics, and the resulting silencer design. For example, the wavelength of 20 Hz at 70°F is 56.5 feet, at 2000 Hz at 70°F it is 7 inches. Low frequencies (Hz) have long wavelengths; high frequencies have short wavelengths. Most sound sources in industry are referred to as broad band, a mixture of low and high frequency sounds.

The human ear is remarkably sensitive to low energy sound, as a consequence, exposure to excessive high levels of sound is a safety and comfort concern.

Decibels (dBA) (A. Weighted Sound Pressure)

Sound amplification or loudness is measured in decibels (dB). The threshold of human hearing is approximately 6 dB at 1000 Hz and increases with decreasing frequency. (approximately 25 dB at 63Hz)

Noise is any unwanted sound. Noise may be continuous or intermittent based on the source.

Noise measurements are taken in the audible sound spectrum and defined as center octave bands consisting of:

A decibel (dB) level is measured in each octave band at a known distance, the decibel levels are combined logarithmically on an energy basis to give an overall level. The decibel level is typically measured in sound pressure level – (SPL), or sound power level – (PWL).

Sound Power Level (PWL) is a measure of total acoustic power or potential, similar to the watt rating of a light bulb.

Sound Pressure Level (SPL) is a measure of the actual acoustic effect due to distance and other mitigating factors. Sound Pressure (SPL) is what we hear, caused by Sound Power (PWL) emitted from the source.

NOISE TRANSMISSION AND PROPAGATION

Most noise sources radiate uniformly in all directions, commonly referred to as hemispherical radiation. However many factors can affect the noise level such as walls, buildings, trees or any object in the noise path. Hard surfaces tend to be reflective, soft surfaces tend to be absorptive. The noise level will decrease with distance, as a rule of thumb the noise level will decrease by 6dB each time the distance is doubled. This is referred to as distance divergence. Direction also affects the noise level, (0°) direct line of sight noise level will be higher than (90°) direction line of sight to a stack emission point.

Background noise, that is the ambient existing noise level, is important, a noise level cannot be reduced further than +3 dB of the background noise level.

Existing OSHA requirements restrict worker exposure to a maximum of 90dBA for 8 hours. Local ordinances of codes may impose different standards and criteria. Engineering, and/or administrative noise control measures are required if the worker exposure exceeds 90 dBA. 

Engineering includes the application of silencers; administrative includes limiting exposure time.

In part,  noise or continuous loud noise exposure above 90dBA can result in permanent hearing loss.

Noise levels above 80 dBA interfere with normal speech making communication difficult.

NOISE CONTROL / SILENCER TYPES

Engineering solutions to noise control typically can involve isolation of the source through enclosure, or absorption in the noise path, or a combination of the two. Silencers are noise path treatments and are available in three types:

There are no national or international codes that govern the design, application, or fabrication of Industrial Silencers. Frequently silencer performance can be misinterpreted if not defined by a specific "Dynamic Insertion Loss," not industry grade.