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1,086

Alcohol

1,213

3,980

Lead

1,220

4,003

Hydrogen, 32° F (0°C)

1,269

4,164

Water, fresh

1,480

4,856

Water, salt, 69.8°F (21°C), at 3.5% salinity

1,520

4,987

Human body

1,558

5,112

Plexiglas

1,800

5,906

Wood, soft

3,350

10,991

Concrete

3,400

11,155

Fir timber

3,800

12,468

Mild steel

5,050

16,570

Aluminum

5,150

16,897

Glass

5,200

17,061

Gypsum board

6,800

22,310

Copper

3,901

12,800

Brick

4,176

13,700

Source: A. J. Schneider, Noise and Vibration Rocket Handbook, Bruel & Kjaer, Cleveland. OH. p. 18; IAC Noise Control Handbook, Industrial Acoustics Co., New York, 1982, p. A-6.

Source: A. J. Schneider, Noise and Vibration Rocket Handbook, Bruel & Kjaer, Cleveland. OH. p. 18; IAC Noise Control Handbook, Industrial Acoustics Co., New York, 1982, p. A-6.

For every doubling of distance, the intensity is reduced by a factor of four, or 6dB. The sound from an infinite line source spreads geometrically in two dimensions so that energy is halved, or loses 3dB, when the source distance doubles. When reflecting objects are near, a more complex sound field results.

Noise Noise is unwanted sound. It may be unwanted for a variety of reasons: causing hearing loss, interfering with communication, causing loss of sleep, adversely effecting human physiology, or causing just plain annoyance.

Noise Pollution Noise pollution is the condition in which noise has characteristics and duration injurious to public health and welfare or unreasonably interferes with the comfortable enjoyment of life and property in such areas as are affected by the noise.

Ambient Noise Ambient noise is the total noise in a given situation or environment.

Noise Level Noise level is the weighted sound pressure level in dBA* obtained by the use of an approved type [American National Standards Institute (ANSI)]

* The A-weighted scale approximates the frequency response of the human ear.

FIGURE 4.13 Absolute auditory threshold for a typical group of Americans. Curves are labeled by percent of group that could hear tones below the indicated level. (Source: Toward a Quieter City, A report of the Mayor's Task Force on Noise Control, New York, 1970.)

sound-level meter. See "Decibel" and "Sound Pressure," as well as "Sound-Level Meter," later in this chapter.

Frequency Frequency of sound is the number of times a complete cycle of pressure variation occurs in 1 second, both an elevation and a depression below atmospheric pressure. The frequency of a sound determines its pitch. Frequency is expressed in hertz (Hz), which is the metric unit for cycles per second (cps). For example, sounds with a frequency of 30 Hz are considered very low pitch; sounds with a frequency of 15,000 Hz are very high pitch. A young, healthy ear can detect frequencies over a range of about 20 to 20,000 Hz, but the most common sensitive hearing range is between 1,000 and 6,000 Hz. Normal speech is in the range of 250 to 3,000 Hz. However, the audibility of sound is dependent on both frequency and sound pressure level. This is illustrated in Figure 4.13 for a typical group of Americans. Since most sounds are made up of several frequencies, a narrow-band analyzer is used to determine the various frequencies in a sound. Most sounds are in the sonic frequency range of 20 to 20,000 Hz. Ultrasonic range is 20,000 Hz and above; infrasonic range is 20 Hz and below. See "Sound Analyzer" and "Octave-Band Analyzer" later in this chapter.

Decibel Decibel (dB) is a dimensionless unit to express physical intensity or sound pressure levels. The starting or reference point for noise-level measurement is 0 dBA, the threshold of hearing for a young person with very good hearing. The threshold of pain is 120 dBA. The decibel is one-tenth of the bel, a unit using common logarithms named for Alexander Graham Bell.

Sound Pressure The sound pressure level of a noise source is expressed by this relationship:

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.

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