Transcript Chapter 14 – Noise Pollution and Control

Chapter 14 – Noise Pollution and Control
Hwk#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.fi.edu/fellows/fellow2/apr99/sound
vib.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).
v  xf
where, v= speed of sound, m/s or ft/s
 = wavelength, m/cycle or ft./cycle
f = frequency, Hz or cps
The human ear can detect sounds in the
frequency range of about 20 to 20,000 Hz.
(The average is 200 - 10,000 Hz)
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)
P
SPL = 20 X log ( )
P0
where, SPL= sound pressure level, dB
P= rms sound pressure,  bar
P0 = reference pressure,  bar
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)
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)
DB
SLB  S A  10X log
DA