CHAPTER 18
NOISE AND VIBRATIONS IN MECHANICAL AND ELECTRICAL SYSTEMS
OVERVIEW
This chapter introduces concepts of acoustical engineering. The objective is to develop
an awareness of the role of acoustics and vibrations in building systems. Of necessity, it
logarithms (and antilogarithms) in virtually every equation. This all takes place in very
limited print space. Thus, the instructors and students may find need to avail themselves
of outside reference material.
This very first concept to grasp is that everything we will discuss – sound
pressure, sound intensity – are all related to sound power. Sound power is measured in
CHAPTER 18
NOISE AND VIBRATIONS IN MECHANICAL AND ELECTRICAL SYSTEMS
QUESTIONS AND ANSWERS
18.1 What are the Five Senses?
18.2 From now until the next class, listen carefully to the sounds around you.
Keep a log of these sounds. Classify them as Pleasant, Neutral, Obtrusive, or
Warning. What is the origin of the sounds you have identified? Aside from
the sounds you have identified, what are some other sounds that fit each of
the above five categories?
18.3 On a scale of 1 to 10, how do you rate your classroom acoustically for your
learning experience?
18.4 What is the speed of sound at (a) 50 Hz, (b) 500 Hz, and (c) 5000 Hz?
18.5 What is the wave length of (a) a 50 Hz sound wave, (b) a 100 Hz sound wave
and (c) a 1000 Hz sound wave ?
18.6 What is the rationale for measuring sound with A-weighting?
161
18.7 In the Devils Dictionary (1911) Ambrose Bierce defined noise thus: “Noise, n,. The
chief product and authenticating sign of civilization.”
(a) Do you agree with this definition? Why or why not? (b) How would you define
noise?
18.8 How would you define noise as opposed to sound?
18.9 Noise has grown more intense over the passing years. How do you account for this?
18.10 Identify some spaces in buildings where a quiet environment is of primary
importance.
18.11 What are two primary components of sound and what are their characteristics?
18.12 A
B
” mineral ceiling tile has octave band absorption coefficients of
125 250 1000 2000 4000 8000 Hz
0.31 0.29 0.70 0.71 0.71 0.58
What is the NRC for the tile?
18.13 Four sound sources individually produce sounds of 55, 55, 58, and 64 dB at a
measurement location. What will be the sound level at the measurement location if all
sounds are active at the same time?
(a) To add sound levels in decibels, first convert the sound levels to sound power by
18.14 Define a diffuse sound field.
A diffuse sound field is one in which sound is traveling in many (theoretically all)
18.15 A point noise source measured at 300 ft in a free field is 60 dB. What is the sound
power of the source in Watts?
163
1
18.16 For the point source in Question 18.15, how close to the source would one have to
move for the sound pressure level to be 70 dB?
53.3 ft
d
1
164
18.17 For the distance in Question 18.16, what is the sound power at the closer location?
18.18 The decrement (D) by which sound level diminishes is given by the general equation
2
18.19 If in Question 18.18 the decrement ǻ = -4.5 dB/dd, what is the coefficient A?
18.20 If one was measuring sound emitted from a large source – say, a large air-handling
unit – how would one be able to estimate approximately where the near field gave
way to the far field (i.e., the distance from the unit where the sound field began to
act as if it were coming from a point source)?
PW
L
=
SPL
–
10 Log
1
–
10.3;
PWL
=
75
–
(-45.5)
–
10.3
=
110.2 dB
W
W=
1
*
0
*
10
=
1
*
10
*
10
=
0.10 Watt
s
2
ref
ref
PWL
-12
110.2
10
-12
11.02
§
¨·
¸
©
¹
18.21 A centrifugal fan operating at 38,000 cfm, 1800 rpm, 8 blades, 2-in. H20 generates
the total sound power levels in each octave band given in the following table. These
data do not contain a correction for blade passage frequency.
Center frequency 63 125 250 500 1k 2k 4k 8k
Fan PWL 92 92 91 86 81 75 71 69
What is the blade passage frequency?
18.22 The fan in Question 18.21 sits in a mechanical equipment room. The supply is
ducted from the room. The HVAC system is an open return. The inlet to the fan is
open to the room. The room absorption is given in the following table. What is the
nominal reverberant SPL in the room? Assume that half of the total sound power is
emitted to the room and that no direct sound reaches the opening.
18.23 For Question 18.23, assume that the reverberant sound in the MER is as given in the
following table and the return opening in the wall is 2 ft wide × 2 ft high. What is the
reverberant sound power level emitted through the opening?
166
Recall that for English units and standard atmospheric conditions, IL is numerically equal
to PWL.
I
IL
10
Results for the remaining octaves follow:
Center frequency 63 125 250 500 1k 2k 4k 8k
18.24 For good vibration isolation, what should be the minimum ratio of the disturbing
force frequency Ȧf to the resonant (natural) frequency Ȧn?
18.25 What is the isolation frequency for a system with a single degree of freedom whose
frequency ratio is:
Z
Z
f
n
2
67% (See Equation 18.23)
18.26 Hawaii is famous for its volcanoes and earthquakes. Are there any areas in the
United States that have a worse potential for earthquakes?