The Science of Sound

The two main elements of sound are frequency and pressure. Frequency is a measure of the number of vibrations or cycles per second (cps). One cycle per second is defined as a hertz (HZ). The measurement of pressure is in decibels (dB). For each 20 decibel increase in sound there is a corresponding tenfold increase in sound pressure. The human ear has a unique ability to reduce it's sensitivity as the pressure increases. Therefore, while a ten decibel increase in sound results in a threefold increase in pressure, the loudness sensation to the ear is only doubled(see Figure 1). Sounds are generated by vibrating objects. These vibrations are transmitted by contact with air, or other mediums, and are carried forward in waves. The speed at which sound travels through a medium depends on the density, absorptive qualities, and stiffness of the medium.

All solid materials have a natural frequency of vibration. If the natural frequency of a solid is at or near the frequency of the sound which strikes it, the solid will vibrate in sympathy with the sound, and the sound will be re-generated on the opposite side. This is true for all solids, including walls and partitions. This transfer effect is particularly noticeable (and measurable) if the wall or partition is light or thin. Conversely, the vibration is effectively stopped if the partition is of heavy, rigid material. In dense solids the natural cycle of vibration will be relatively slow, and only sounds of low frequency will cause sympathetic vibration. An enclosure, such as a cavity wall, or a furred out wall, has it's own sound transmittance characteristics. The enclosure's resonance frequency becomes lower in proportion with the amount of air in that enclosure. Therefore, the greater the air space in a cavity wall, the less audible noise is transmitted.

The human ear can perceive sounds as low as 16 cycles per second to as high as 20,000 cycles per second. However, it is most sensitive to sounds between 500 and 5000 cycles. For human voices speaking in conversational tones, a frequency of approximately 500 cycles is assumed.

Because of it's mass and rigidity, a concrete masonry unit is especially effective in reducing the transmission of unwanted sound. Furthermore, when used in conjunction with furring /insulation/ drywall, as it is commonly used, it becomes even more effective as a sound barrier. The cavity wall, which has been proven to excel in the categories of fire protection, insulation, and water protection, also shows extremely good performance in the isolation of sound.

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