Chapter 14

1

• Chapter 14 Noise Pollution and Control
• Hwk7 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.

2

• 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.h
tml
3

• 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)
4

• 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)
5

• 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
6

• 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. L9075 dBA means 90 of the
  time sound level exceeeded 75 dBA.

7

• Chapter 14 Noise Pollution and Control
• Sound Exposure Level (SEL) - provides a basis
  for computing noise events of variable durations
  which matches a persons 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

8

• 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)