Chapter 14 – Noise Pollution and ControlHwk#7 review questions – pp.472-473 #6,8,15,19 practice prob. – p.474 -#2,7,9,15•Noise – is an undesirable and unwanted sound, and noise is a form of waste energy.[Note: Not all sound is noise]

•Sound Waves - Is the form of transport for sound and it cannot be transmitted in a vacuum due to a lack of medium to carry the vibration.

Chapter 14 – Noise Pollution and Control • Wave Lengths-distance between pressure peaks•Frequency - The number of wavelengths that appear to pass a fixed point in 1 second. [cps or Hz]•Amplitude-heights of the peak which represents the pressure intensity and is related to the volume or loudness.•Cycle - is a single wave length

http://www.umanitoba.ca/faculties/arts/linguistics/russell/138/sec4/acoust1.htm

http://www.fi.edu/fellows/fellow2/apr99/soundvib.html

Chapter 14 – Noise Pollution and Control • Speed of Sound- 1500 m/s in water and 5000 m/s in steel, 340 m/s in air (1100 ft/s).

[Note: Speed of sound is constant so the frequency and wavelength is inversely proportional.]

The human ear can detect sounds in the frequency range of about 20 to 20,000 Hz.(The average is 200 - 10,000 Hz)

x f

where, v= speed of sound, m/s or ft/s

= wavelength, m/cycle or ft./cycle

f = frequency, Hz or cps

v

Sin500.wav Sin300.wav

Chapter 14 – Noise Pollution and Control • microbars- 1 millionth of a bar, where a bar is equal to 100 kPa or 14.7 psi (atmospheric pressure at sea level)

The reference pressure generally used for Po is the hearing threshold or lowest audible sound pressure of .0002 microbars. (1000 microbars is the highest sound pressure w/o pain)

0

0

PSPL = 20 X log ( )

P

where, SPL= sound pressure level, dB

P= rms sound pressure, bar

P = reference pressure, bar

Chapter 14 – Noise Pollution and Control • decibel scale (dB) - a ratio of two pressures (Note : 10,000 microbars can cause immediate physical damage.)•Sound Pressure Level - expresses the magnitude of volume or a sound (dB) [Refer to p.461 in text fig. 14.3]Note: The avg. person will perceive a high-pitch sound to be louder than a low pitch sound with the same SPL. (see fig. 14.5 under sound-level measurements in phons)

http://www.jimprice.com/prosound/db.htm

Chapter 14 – Noise Pollution and Control • Sound Intensity (W/m2) - is proportional to the square of the root mean square (rms) value of a sound pressure or SPL (Note :for every 10 dB increase in SPL, there is a 10 fold increase in sound intensity.)•Frequency Weighting Networks- noise is broken down to bands of low-medium-high frequency. “A-weighted” network filters out low and high frequency where human ears is less efficient. [dBA is a.k.a A-weighted decibels][Refer to p.464 in text fig. 14.3]Sound Level (SL)- Sound Level measurement over a period of time. i.e. L90=75 dBA means 90% of the time sound level exceeeded 75 dBA.

Chapter 14 – Noise Pollution and Control • Sound Exposure Level (SEL) - provides a basis for computing noise events of variable durations which matches a person’s impression of noise. It is standardized to 1 sec. (Note: refer to figure 14.7, p. 465)•Equivalent Sound Level (Leq)- is is the average or constant SPL over the period of interest. [i.e. Leq(8) is the average for an 8 hr. period.][Refer to p.464 in text fig. 14.3]•Day - Night Sound Level (DNL)- Leq(24) with a 10-dBA penalty for night time which is more annoying. see fig. 14.8

[Note: Noise can cause damage to our irritability, anxiety, stress and other emotional symptoms.]

Temporary Threshold Shift (TTS)- Temporary hearing loss (approx. 1 month)Noise -induced permanent threshold shift (NIPTS) - loss of hearing with no chance of recovery

Chapter 14 – Noise Pollution and Control • less than 80 dBA -no hearing loss•80 to 130 dBA - TTS hearing loss is noticeable•50 % of people exposed to 95-dBA will experience NIPTS (permanent hearing loss)•150 dBA or greater- can physically rupture the human eardrum.

(Note: 1 hr. of 100 dBA can produce TTS whereas 8 hours of 95 dBA over 10 years may cause NIPTS)

BB

A

D10X log

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