Noise Control

One of the most important tasks of architects, builders, acoustic engineers, urban planners, industrial hygiene engineers, equipment manufacturers, and public health personnel is to ensure that noise and vibration are kept to an acceptable level in the general environment, in the workplace, and inside dwellings. Noise is of special concern in occupational health where hearing loss has been documented.

The discussion that follows will touch on some of the fundamentals of noise and its effects, measurement, reduction, and control. Special problems should involve experts such as acoustical consultants.

Definitions and Explanation of Selected Terms and Properties of Sound

Sound Sound and, therefore, all noise, is physically a rapid alteration of air pressure above and below atmospheric pressure. Basically, all sounds travel as sound pressure waves from a vibrating body such as a human larynx, radio, TV, record player speaker, or vibrating machine.

A sound that contains only one frequency is a pure tone, which is expressed in Figure 4.12 as a sine curve. Most sounds contain many frequencies. In general, the waves travel outward from the source in three dimensions. The pitch of a sound is determined primarily by frequency: vibrations per second. The amplitude or magnitude of sound is the sound pressure.

FIGURE 4.12 Pure tone, sine wave.

The distance that a sound wave travels in one cycle or period is the wavelength of the sound. This is illustrated in Figure 4.12. Wavelength is given by the equation c

X = wavelength, ft f = frequency, Hz (cycles/sec) c = speed of sound, ft/sec

Sound travels through gases, liquids, and solids but not through a vacuum. The speed with which sound travels through a particular medium is dependent on the compressibility and density of the medium. Our own voice reaches us primarily through the bony structures in our head. Most sound reaches us through the air and less frequently through solids and liquids. The speed of sound through various media is given in Table 4.9.

As sound travels through a medium, it loses energy or amplitude in two ways: molecular heating and geometric spreading. For example, drapes absorb sound, releasing the energy as heat to the surrounding air. Air itself also absorbs sound to a smaller degree because it is not perfectly elastic. Plane waves emitted from a large distant source travel in a plane or front perpendicular to their direction of travel. There is no geometric spreading or energy loss in plane waves, neglecting molecular heating. Spherical waves, resulting from a small vibrating sphere in close proximity, spread in three dimensions. They lose energy according to the inverse square law, given by

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

Get My Free Ebook


Post a comment