San Jose State University
Department of Mechanical and Aerospace Engineering
ME 120 Experimental Methods
Sound Measurement
BJ Furman
12OCT02
BJ Furman
SJSU MAE
Sound and Noise
Sound is: “…any pressure variation (in air, water or other
medium) that the human ear can detect.”
(Measuring Sound, Bruel & Kjaer, Denmark, BR0047-13, 1984)
❖
Longitudinal motion of the particles of the transmitting medium
produces pressure oscillations (compressions and rarefactions) about
the ambient pressure
• Sources
Vibrating bodies
Aerodynamic phenomena
Explosions
❖
Range of human hearing
• 20 – 20,000 Hz
• Most sensitive to sound around 4 kHz
Noise is: “…unwanted sound.” (Beckwith, et. al., 1993)
BJ Furman
SJSU MAE
1
Frequency, Wavelength, and Speed of Sound
Frequency, f = how often the the pressure oscillation
occurs (in Hz = cycles per second)
Wavelength, λ = the distance between successive
identical parts of the oscillation
Wave propagation speed, c in gaseous media (speed of
sound):
γ p0
c=λ f
c=
ρ
γ is
where
c ≅ 331.4 + 0.6T m/s (T in deg. C)
cp
cv
p0 is the static pressure of the gas
ρ is the density of the gas
BJ Furman
SJSU MAE
Sound Pressure, Power, and Intensity
Sound pressure, Lp = total pressure - static pressure, but
typically expressed as an root-mean-square (rms)
value:
For a pure tone,
prms =
∫
T
0
p(t ) 2 dt
prms =
P
2
Sound power , Lw = the rate at which sound energy is
radiated from a source (units of Watts)
❖
1
T
Increases as the square of the sound pressure
Sound intensity , Li = the power per unit area, W/m2
❖
Falls off as the square of the distance from the source
BJ Furman
SJSU MAE
2
Sound Pressure Level (SPL)
Level in dBA /Source
220 12 feet in front of a cannon, below muzzle
194 Theoretic maximum for pure tones
rms 0
188 Rocket launching pad
150 Jet engine test cell
0
140 Gunshot
Pain Threshold
130 Air-raid siren
120 Live rock music, thunderclap, propeller aircraft, auto horn (3 feet)
Discomfort Threshold
110 Pile driver, snowmobile (from driver's seat), sandblaster
100 Subway train, pneumatic drill, diesel truck, police siren (100 feet)
95 Ride in convertible on freeway
90 Electric lawn mower, motorcycle (25 feet), heavy truck (50 feet), city traffic
85 Average factory, electric shaver
Hearing Loss Risk Threshold
80 Hair dryer, alarm clock (2 feet), garbage disposal
70 Freeway traffic, noisy restaurant, vacuum cleaner
60 Conversation, air conditioner (20 feet)
50 Light auto traffic (100 feet)
40 Quiet office, quiet home
30 Audible whisper
20 Rustling leaves, broadcasting studio, soft whisper
10 Barely audible
http://hyperphysics.phy-astr.gsu.edu/hbase/sound/acont.html#c3
0 Threshold of hearing
SPL=20log(p /p )
where p =20µPa (about the threshold
of human hearing at 1000 Hz)
Sound level filter contours
(Source: Audio Magazine, January 1989 via http://www.locationsound.com/psreport/hearing.html)
BJ Furman
SJSU MAE
Leq and SEL
equivalent sound level, Leq, is the
continuous SPL that would have produced the
same sound energy over the same time
❖
Provides a means to assign
a single value to a time history
for comparison purposes
Leq = 10log10 (
1
T
∫
T
0
SPL, dB
The
SPL
Leq
10SPL /10dt )
Time
The
Sound Exposure Level (SEL)
(or LAE for A-weighted measurements) is the
Leq where the integration time is 1 s
BJ Furman
SJSU MAE
3
Influence of the Sound Field on Sound Measurement
Sound pressure measurements depends on the distance from the
source and on the acoustic environment surrounding the source:
❖
Free-field = the space around the source where there are no
reflections or disturbance of the sound waves
• SPL and intensity drop by 6 dB as distance from the source doubles
• Ex. Anechoic chamber, or the top of a flag pole above a quiet field
• SPL measurements should be made in the free-field
❖
Near-field = the space close (the smaller of: 2x largest dimension of
source or the wavelength of lowest frequency component) to the source
• SPL measurements in the near field are not reliable, because small
changes in position can result in big differences in readings
❖
Reverberant field = the space around the source where reflections
from walls and other objects significantly disturb the sound waves
from the source
BJ Furman
SJSU MAE
Sound Intensity Measurement
power (hence intensity) is largely
independent of the acoustic environment and
can provide a better quantitative measure of
how much noise a given source produces
Sound
❖
❖
Useful to determine how much noise a particular
source (among others) is radiating
Allows noise measurements to be made in situ
Sound
intensity is a vector quantity (magnitude
and direction), whereas SPL is essentially a
scalar quantity
❖
Sound intensity measurements are also good for
locating noise sources
BJ Furman
SJSU MAE
4
Sound Intensity Measurement, cont.
Sound
intensity is determined by measuring the
pressure and air particle velocity
Sound Intensity = pressure × particle velocity
Force distance
energy
Power
=
i
=
=
Area
time
Area ⋅ time
Area
Bruel & Kjaer, “Sound Intensity,” http://www.bksv.com/bksv/pdf/Sound
BJ Furman
SJSU MAE
Microphones for Sound Measurement
The
condenser microphone is most often used
in sound measurement instruments
❖
❖
Excellent frequency response
over a broad range
Requires a dc voltage
source to charge the
capacitor plates
Q = CV =
ε0 A
d
dQ
= I (t ) → V (t ) = I (t ) R
dt
BJ Furman
http://hyperphysics.phy-astr.gsu.edu/hbase/audio/imgaud/conm3.gif
SJSU MAE
5
Microphones for Sound Measurement, cont.
Electret
condenser microphone has a
prepolarized diaphragm
❖
Polymeric film with metallized
backing
B&K 2236 SPL Meter MICROPHONE:
Type 4188 prepolarized free-field 1/ 2″ condenser
microphone
Sensitivity: –30dB re 1V/Pa ±2dB
Frequency Range: 8Hz to 12.5kHz ±2dB
http://hyperphysics.phy-astr.gsu.edu/hbase/audio/imgaud/etret.gif
Capacitance: 12pF
BJ Furman
SJSU MAE
Types of Microphones
The microphone will disturb the sound field!
Need to use the right one depending on what you want
to measure
❖
Free-field microphone
• Compensates for the disturbance it causes
• Normally to be used pointed at source (0° incidence)
• If used in reverberant (diffuse)-field, will underestimate SPL
❖
Pressure microphone
• Does not compensate for the disturbance it causes
• Responds uniformly to the actual SPL
❖
Random incidence microphone
• Has uniform response to sound waves reaching it at all angles of
incidence
• Best for difuse-field measurements
• If used for free-field measurements, should be pointed 70-80°
from source or will overestimate SPL
BJ Furman
SJSU MAE
6
Background Noise
It
is important to separate the background
noise from the noise produced by the source
itself
❖
❖
❖
Measure SPL with source on
Measure SPL without source
Subtract readings
• If less than 3 dB, questionable
accuracy
• If 3 db < ∆SPL < 10 dB,
correct the reading
• If ∆SPL > 10 dB,
no correction is needed
Bruel & Kjaer, “Measuring Sound,” http://www.bksv.com/bksv/pdf/Measuring
BJ Furman
SJSU MAE
References
Beckwith, T. G., Marangoni, R. D., Lienhard, J. H.,
Mechanical Measurements, Addison-Wesley, Reading,
MA, 1995.
Bruel & Kjaer, “Sound Intensity,”
http://www.bksv.com/bksv/pdf/Sound
Bruel & Kjaer, “Measuring Sound,”
http://www.bksv.com/bksv/pdf/Measuring
Nave, C. R., Hyperphysics, http://hyperphysics.phyastr.gsu.edu/hbase/hph.html, 2002.
BJ Furman
SJSU MAE
7