ACOUSTICAL MEASUREMENTS
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WELCOME TO OUR PRESENTATION
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BUTTERFLY
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GROUP MEMBERS
NAME ID
Rejvi Ahmed 13307132
Md. Mehedi Hasan 15107001
Md. Zahidul Islam 12307061
Md. Kurat-E-Khuda 13207109
Md. Mahadi Hasan 12307016
Md. Noman Siddiquee 11107012
Md. Iliyash 12207100
Md. Ruhul Amin 12207153
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CONTENTS
• Properties of Sound
• Basic acoustical Parameters
• Psychoacoustic Relationships
• Sound Measuring Apparatus and Techniques
• Applied Spectrum Analysis
• Measurement and Interpretation of Industrial and Environmental
Noise
• Some practical aspects of Sound Measurements
• Calibration Method
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WHAT IS SOUND?
Sound can be defined in two ways. [Physical or psychophysical]
Sound can be defined as a wave motion in air or other elastic media.
Or as excitation of the hearing mechanism that results in the perception
of sound.
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TRANSMISSION OF SOUND
• Requires a medium with elasticity and inertia
(air, water, steel, etc.)
• Movements of air molecules result in the
variation of sound pressure causing the
propagation of a sound wave
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HOW SOUND IS PRODUCED?
Energy in the form of sound is produced when a vibrating
surface or object is contact with the air.
Sources of sound
Vocal
Plucked strings
Air column
Vibrating plate
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HOW SOUND IS PERCEIVED?
Physical stimulation of the ear by the sound wave.
Physiological and psychological processing and perception in
ear and brain (psycho-acoustics) resulting from nerve
impulses stimulating the acoustic cortex of the brain.
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PROPERTIES OF SOUND
• Amplitude
• Period
• Frequency
• Speed
• Wavelength
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PROPERTIES OF SOUND (CONT..)
• Amplitude
Time
Period, T
Amplitudey
For a simple sine wave it is easy
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PROPERTIES OF SOUND (CONT..)
Period (T)
is the time it takes to complete one full cycle
Frequency (f)
is the number of times per second a complete
wave passes a point. The number of cycles per
second is termed Hertz (Hz).
The period and the frequency are simply related
by the following equation
T =
𝟏
𝐟
(seconds)
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PROPERTIES OF SOUND (CONT..)
Speed (c)
of sound in air is governed by density and air pressure
which in turn relates to temperature and elevation above
sea level.
• The speed of sound in air is approximately 343 m/s.
Sound travels about 1 kilometre in 3 seconds.
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PROPERTIES OF SOUND (CONT..)
Wavelength (λ)-
is the length of one complete cycle, and is
measured in metres (m).
It is related to the frequency (f) and speed of
sound (c) by:
Wavelength (λ) = c/f metres
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ACOUSTICAL PARAMETERS
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BASIC ACOUSTICAL PARAMETERS
 Sound pressure
 sound pressure level
 Sound Power
 Sound Power level and
 sound intensity
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SOUND PRESSURE
Sound Pressure is the force of sound on a surface area perpendicular to the direction
of sound.
Sound pressure is the difference between instantaneous absolute pressure and the
ambient pressure
In air, sound pressure can be measured using a microphone, and in water with
a hydrophone. We can define the mean-square sound pressure as
where
prms is the root mean square of acoustic pressures
T is the period of measurement
p is the instantaneous acoustic pressures
The SI unit of sound pressure is the pascal (Pa).
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SOUND PRESSURE LEVEL
Sound pressure level is the end result.
Pressure level as a ratio of sound pressure to a base level.
Sound Pressure Level (SPL) is a logarithmic measure of the RMS sound pressure
of a sound relative to a reference value, the threshold of hearing. It is measured in
decibels (dB).
for air ... water, steel, etc., are different.
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Sources at 1 m Sound Pressure Lp re 20 µPa *
Rifle 200 Pa 140 dB
Threshold of pain 20 Pa 120 dB
Pneumatic hammer 2 Pa 100 dB
6 dB = double the Pa 1 Pa 94 dB
Street traffic 0.2 Pa 80 dB
Talking 0.02 Pa 60 dB
Library 0.002 Pa 40 dB
TV Studio 0.0002 Pa 20 dB
Threshold of hearing 0.00002 Pa 0 dB
Reference Sound Pressure 𝑝0 in air= 2 x 10-5 Pa = 20 µPa = 0 dB
SOME TYPICAL SOUND PRESSURE LEVELS
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SOUND POWER
Sound power is the rate at which sound
energy is emitted, reflected, transmitted or
received, per unit time.
Sound Power Level and the Sound Power
from some common sources as fans, jet
engines, cars, humans and more ..
The SI unit of sound power is the watt (W)
Sound power passing through an area is
sometimes called sound flux or acoustic
flux through that area.
Sound power denoted by P
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SOUND POWER LEVEL
Sound power can more practically be expressed
as a relation to the threshold of hearing –
10-12 W - in a logarithmic scale named Sound
Power Level -Lw, expressed as
Lw = 10 log (N / No)
where
Lw = Sound Power Level in Decibel (dB)
N = sound power (W)
No = 10-12 - reference sound power (W).
Human hearable Sound Power spans from
10-12 W to 10 - 100 W, a range of 10/10-12 = 1013.
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SOUND INTENSITY
Sound intensity also known as acoustic intensity is defined as
the sound power per unit area.
The usual context is the noise measurement of sound intensity in the
air at a listener's location as a sound energy quantity.
Sound intensity is not the same physical quantity as sound pressure.
The SI unit of sound intensity is the watt per square meter (W/m2)
Its denoted by I.
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PSYCHOACOUSTIC RELATIONSHIP
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PSYCHOACOUSTICS
Psychoacoustics is essentially the
study of the perception of sound. This
includes how we listen, our
psychological responses, and the
physiological impact of music and
sound on the human nervous system.
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HEARING
• Hearing – the process that transforms
sound waves into neural signals that
can be interpreted by our brain
• Sound waves – fluctuations in air
pressure across time, created by the
motion or vibration of an object (e.g.
the vibration of vocal chords, oscillating
violin string) - physical properties:
frequency and amplitude.
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THE PERIPHERAL AUDITORY SYSTEM
• The peripheral auditory system consists of the outer, middle and
inner ear.
• In brief: The ear drum moves in and out in response to the pressure
changes in sound waves – transmitted through the middle to the
inner ear – transducer into neural signals that are interpreted by the
brain.
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THE BASILAR MEMBRANE RESPONSE TO SOUND
• Movement of the stapes sets the oval window in motion – causes the
BM to move.
• Response of BM to sinusoidal stimulation – travelling wave, which
moves from base to apex.
• The position of the peak in the vibration pattern on the BM depends
on the frequency of the sound – this is due to the mechanical
properties of the BM
• High (low) frequencies produce max. BM displacement near the base
(apex) – frequency analysis – each point on the BM is sharply tuned.
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THE BASILAR MEMBRANE RESPONSE TO SOUND
• Each point on the BM is sharply tuned, responding with high
sensitivity to a limited range of frequencies.
• BM vibration is nonlinear – the magnitude of its response does not
grow directly in proportion with the magnitude of the input
• Linear for low input sound levels (<20dB SPL) and very high input
sound levels (>90dB SPL)
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SOUND-MEASURING APPARATUS AND
TECHNIQUES
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SOUND-MEASURING APPARATUS AND TECHNIQUES
 Measurement of the parameters associated with sound use a basic
system made up of a detector-transducer (the microphone),
intermediate modifying devices (amplifiers and filtering systems), and
read-out means [a meter, CRO(Character Read Out), or recording
apparatus].
 Most sound-measuring systems are used to obtain
psychoacoustically related related information. Elaborate filtering
networks also provide the basis for analyzers, devices for separating
and identifying the various frequency components or ranges of
components forming a complex sound.
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pressure meter
with microphone
extension cord,
preamplifier and
microphone.
Sound level meter with
microphone extension,
Although long
discontinued, this has
been a standard of the
industry for firearm
sound measurements.
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MICROPHONES
 Most microphones incorporate a thin diagram as the primary
transducer, which is moved by the air acting against it. The
mechanical movement of the diagram is converted to an
electrical output by means of some form of secondary
transducer that provides an analogous electrical signal.
 Common microphones may be classified on the basis of the
secondary transducer, as follows:
1. Capacitor or condenser
2. Crystal
3. Electrodynamic (moving coil or ribbon)
4. Carbon
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The capacitor or condenser microphone is probably the most
respected microphone for sound measurement purposes.
We can use this formula for find out the output voltage:
E = Qd
Where,
E = The Voltage
Q = The charge provided by the polarizing voltage (relatively
constant)
d = The separation of the plates.
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SCHEMATIC OF THE CONDENSER-TYPE MICROPHONE
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MICROPHONE SELECTION FACTORS
An ideal microphone used for measurement would have the following
characteristics:
 Flat frequency response over the audible range.
 Non directivity.
 At the lowest sound level to be measured, output signal that is
several times the system’s internal noise level.
 Minimum dimensions and weight.
 Output that is unaffected by all environmental conditions except
sound pressure.
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THE SOUND LEVEL METER
The basic sound-level meter is a measuring system that senses the input sound
pressure and provides a meter read-out yielding a measure of the sound magnitude.
The sound may be wideband, it may have random frequency distribution, or it may
contain discrete tones. Each of these factors will, of course, affect the read-out.
Fig: Block Diagram of a typical sound-level meter, or sound-level recorder
Microphone
Weighting
networks
Oscillograph
MeterAmplifier
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THE SPECTRUM ANALYZER
A spectrum analyzer measures the magnitude of an input signal versus frequency within the full
frequency range of the instrument.
Spectrum analyzers usually display raw, unprocessed signal information such as voltage, power,
period, wave, shape, sidebands, and frequency. They can provide you with a clear and precise window
into the frequency spectrum.
Major blocks in a spectrum analyzer are:
1. RF input attenuator 6. Video filter
2. Mixer 7. Local oscillator
3. IF (Intermediate Frequency) gain 8. Sweep generator
4. IF filter 9. CRT display
5. Detector
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APPLIED SPECTRUM ANALYSIS
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APPLIED SPECTRUM ANALYSIS
• A spectrum analyzer measures the magnitude of an input signal
versus frequency within the full frequency range of the instrument.
primary use is to measure the power of the spectrum of known and
unknown signals. The input signal that a spectrum analyzer
measures is electrical however, spectral compositions of other
signals, such as acoustic pressure waves and optical light waves, can
be considered through the use of an appropriate transducer. Optical
spectrum analyzers also exist, which use direct optical techniques
such as a monochromator to make measurements.
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INSIDE THE SPECTRUM ANALYZER
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SPECTRAL ANALYSIS USING THE FFT
The FFT Analyzer can be broken down into
several pieces which involve the digitization,
filtering, transformation and processing of a
signal. Several items are important here:
Digitization and Sampling Quantization of
Signal Aliasing Effects Leakage Distortion
Windows Weighting Functions The Fourier
Transform Measurement Formulation
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MEASUREMENT AND INTERPRETATION OF
INDUSTRIAL AND ENVIRONMENTAL NOISE
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SOUND INTENSITY
Sound intensity known as acoustic intensity is defined as
the sound power per unit area. The SI unit of sound intensity
is the watt per square meter (W/m2).
The usual context is the noise measurement of
sound intensity in the air at a listener's location as a sound
energy quantity
Sound intensity is not the same physical quantity as sound
pressure. Sound energy passing per second through a unit
area held perpendicular to the direction of propagation of
sound waves is called intensity of sound.
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SOUND INTENSITY MEASUREMENT
One method of sound intensity measurement involves the use
of two microphones located close to each other, normal to the
direction of sound energy flow. A signal analyzer is used to
compute the cross power between the measured pressures and the
sound intensity is derived from (proportional to) the imaginary part
of the cross power.
Sound exposure is the integral, over time, of squared sound
pressure. The Si unit of sound exposure is the Pascal squared
second
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SOUND EXPOSURE LEVEL
Sound exposure level (SEL) or acoustic exposure level is
a logarithmic measure of the sound exposure of a sound
relative to a reference value. logarithmic measure
Sound exposure level, denoted LE and measured in DB.
Sound Exposure Level :
SEL is the logarithmic measure of the A-weighted sound
pressure Level squared and integrated over a stated period of
time or event, relative to a reference sound pressure value.
The units are the decibel
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WHY MEASURE NOISE IN
THE WORKPLACE?
Measuring noise levels and workers' noise exposures is the
most important part of a workplace hearing conservation and
noise control program.
It helps identify work locations where there are noise
problems, employees who may be affected, and where
additional noise measurements need to be made.
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HOW IS WORKPLACE NOISE
MEASURED?
Various instruments and techniques may
be used. The choice depends on the
workplace noise and the information
needed. However, the first step is to
determine if there is a noise problem in the
workplace.
This document briefly outlines the steps
involved in the noise measurement. For
details, you should consult the current
version of the Canadian Standard CSA Z
107.56 or the standard that applies in your
jurisdiction.
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CONTINUE…….
A sound level meter is used for acoustic (sound that travels
through air) measurements. It is commonly a hand-held
instrument with a microphone. The diaphragm of the
microphone responds to changes in air pressure caused by
sound waves.
That is why the instrument is sometimes referred to as a
Sound Pressure Level (SPL) Meter. This movement of the
diaphragm, i.e. the sound pressure deviation (Pascal Pa), is
converted into an electrical (volts V).
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CALIBRATION METHOD
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CALIBRATION TECHNIQUES
• Reciprocity calibration method
• Comparison or substitution methods
• Pistonphone (closed coupler)
• Sound pressure calibrator
• Electrostatic actuation
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RECIPROCITY CALIBRATION METHOD
• Microphone can be used
as a loudspeaker
• Three test microphones
measured against each
other alternating the
function
• As a result a set of 3
equations with
microphone sensitivities as
unknowns
• Very accurate
• Rather tedious
• Requires well-controlled
environment
• Seldom used in practical
situations
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COMPARISON/SUBSTITUTION METHODS
• Microphone measured related to a reference
microphone
• Comparison method: microphone and
reference at the same time
• Substitution method: microphone put in the
lace of the reference
• Sound source stability
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PISTONPHONE
• Closed coupler
• Well-defined sound
pressure level
• Relatively simple
mechanically, very stable
• Used often as the sound
source in
comparison/substitution
calibration
• Accuracy around 0.1 dB
• Depends on
• Volume of the coupler
• Volume displacement
• Barometric pressure
• Humidity
• Heat dissipation
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SOUND PRESSURE CALIBRATOR
• Small, self-contained
• Comparison calibrator
• Closed coupler
• Small loudspeaker
produces single-frequency
signal
• Reference microphone
gives feedback signal
• Well-defined, provided
that reference microphone
and feedback gain are
stable
• For field-calibration of
microphones
• Normally not for
laboratory calibrations
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ELECTROSTATIC CALIBRATION
• Direct use of electrostatic
actuator to drive the
diaphragm
• 800 V DC
• 50-150 V AC signal
• Generally used to measure
frequency response of
microphones
• Widely used as a
convenient and accurate
test method
• For production and final
calibration of
measurement
microphones
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THANK YOU
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