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COMD 3700
Lesson 3- Measurement of Sound 1
Page 1 of 8
ACOUSTICS REVIEW
Review textbook, pgs. 30-46. Know
the concepts below:
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Waves
Vibrations
Frequency
Resonance
Sound Velocity
Wavelength
Phase
Complex Sounds
Intensity
Begin in the textbook, pg. 47, for
the rest of this lecture
PHYSICAL ATTRIBUTES OF SOUND
 Sound is a form of energy
 It is a wave disturbance that travels through any medium
 Sound in air is propagated as a longitudinal wave
 Sound waves are produced by molecular vibrations
 Sound differs from other vibrating motion in that sound waves in free air are 3-dimensional
 Three properties are necessary to produce sound waves:
1. A Force = intensity of the wave
2. A Vibrating Mass = air molecules are the mass undergoing vibration. Air molecules and their
vibratory motion seem to be connected by springs.
3. An Elastic Medium = air itself has an elastic nature.
 Compression = when the molecules are packed closely together as a result of pressure
 Rarefaction = when the molecules are spread far apart
 Successive compressions and rarefactions of air molecules are passed along a line of air particles at the
speed of sound
SOUND WAVE FORMS
 Sine Waves = waves behaving in simple periodic oscillation
 Described in terms of:
1. Frequency (pitch = cycles/sec = cps = Hertz = Hz) = how often they move from maximum
compression to maximum rarefaction and back to their origin
2. Intensity (loudness) = the force that moves the wave to its maximum amplitude
 Complex Waves = waves of different frequencies combine
 Produce the quality or tambour of a sound
MEASUREMENT OF SOUND
 Sound may be regarded objectively if we consider its waves in terms of their frequency, intensity, phase,
and spectrum.
 Sounds may also be studied subjectively, in terms of pitch, loudness, or the interactions of signals
producing masking or localization.
 In discussing sound energy it is always important to specify precisely the various aspects and appropriate
measurement references, such as hertz, decibels (IL, SPL, HL, or SL), mels, sones, or phons.
 Intensity and pressure are two different ways of looking at the same sound wave.
COMD 3700
Lesson 3- Measurement of Sound 1
Page 2 of 8
ACOUSTIC REFERENCE
 Will help to define the different thresholds (in terms of SPL, HL, SL, IL)
 Important to develop or define:
 How cold is a cold temperature?
 How soft is a soft sound?
 Beginning Point
 Zero Point
 Threshold
THE DECIBEL (dB)
 Developed by Alexander Graham Bell
 dB = 1/10 (tenth) of a Bel
 No fixed absolute value. Means nothing by itself
 Dimensionless number t = log of a ratio of 2
powers or pressures
 It is exponential
 It is a logarithmic unit of measurement that expresses
the magnitude of physical quantity relative to a
specific reference level
 Measurement unit for intensity (loudness), sound
pressure level
 Measurement of intensity used in acoustics and
audiometry, electronics
 Usually measured in a range of 0 to 140 dB
REFERENCE OF ZERO IN dB
 Beginning Point in Sound = 0 dB
 0 dB does NOT mean the absence of sound.
 Compare temperature to dB (ie: 0C is not the absence of temperature)
 There is sound at –5 dB, -10 dB, etc, just like there is still temperature at –5C, -10C
 In terms of temperature, the physical reference point for 0C is water freezing
 In terms of sound, the physical reference point for 0 dB is 20 μ Pa
 It means the amount of sound pressure you are comparing is equal to the reference level and no pressure
increase occurs
THRESHOLD
 Threshold = Lowest point in intensity that a person can perceive the stimulus
 Measured in various ways:
 Intensity Level (IL)
 Sound Pressure Level (SPL)
 Hearing Level (HL)
 Sensation Level (SL)
 Don’t get overwhelmed in understanding the mathematical calculations. Just understand the principles in
each type of measurement, how it is measured, and when each measurement is used
COMD 3700
Lesson 3- Measurement of Sound 1
Page 3 of 8
REFERENCE LEVELS
 A decibel is a ratio that does not have an absolute or fixed value, it always has a reference level.
 Reference levels are:
0
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0
dB
dB
dB
dB
IL
SPL
HL
SL
= 10-12 watt/m2 OR 10-16 watt/cm2
= 20 micropascals (μ Pa) OR 0.0002 dyne/cm2
= Audiometric Zero-ANSI standards
= Patient's threshold
INTENSITY LEVEL (IL)
 Intensity (aka magnitude, strength, amplitude) = the amount of energy transmitted per second
over an area of 1 square meter
 IL = watt per square meter (watt/m2)
 IL Strength/Power (of Sound) = measured by dB, expressed in watts
 There is an absolute measure and a relative measure
 Absolute Measure of acoustic power in watts = rate at which energy is consumed
 Intensity level only exists when there is a reference given
 Therefore, the term Intensity Level in dB IL should only be used when the reference is 10-12
watt/m2 under standard conditions
 Standard conditions = 200 Celsius/Centigrade, 760 mm mercury for Barometric Pressure
IR = Intensity Reference measured in watts
 Usually measured as:
IR
=
=
10-12 watt/m2 = (.000000000001 watt/m2) AND
10-16 watt/cm2
I0 = Intensity Output (ie: loud speaker)
 Sound Intensity is a specifically defined quantity and cannot be sensed by a simple microphone, nor
would it be valuable in those recordings if it could
 Sound Intensity = most commonly measured using the dB scale
dB = 10 x log (I0/IR)
If I0=IR , the ratio is 1:1
Assume a 1:1 ratio for this equation
dB = 10 x log (1/1)
dB = 10 x log (1)
We know that Log 1 = 0
dB = 10 x 0
dB = 0
Therefore,
dB IL = 0 OR
Intensity Level = 0 dB IL
COMD 3700
Lesson 3- Measurement of Sound 1
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SOUND PRESSURE LEVEL (SPL)
 Sound Pressure (dB SPL) is NOT the same physical quantity as Sound Intensity (db IL)
 Intensity Difference = level difference between Sound Pressure and Sound Intensity
 Hearing is sensitive to sound pressure, which is RELATED to sound intensity
 Sound Intensity can also be described in terms of Pressure or Power
 Audiologists prefer to measure sound intensity in terms of Pressure
 Most common usage of dB in reference to sound loudness = dB SPL
 Sound reference point = the quietest sound that can just be perceived by the best ear OR the nominal
threshold of human hearing
= 0 dB SPL
 Pressure = Force per unit Area
 Unit of Force = Newton (N)
 1 Newton (N) = Force that will accelerate 1 kilogram (kg) of mass (Mass) a distance of 1 meter (Distance)
per second (Time).
 Think of the example of pushing (force) a 1 kg eraser, the distance of 1 meter, from 1 side of the
desk to the other side of the desk and off of the desk, in 1 second
 Sound Pressure unit = Pascal (Pa)
 1 Pascal = 1 Newton per square meter (1 Newton applied to a meter squared) OR
1 Pa = 1 N/m2
 if you take the same force and apply it to a larger area, you will have a smaller force per unit area than if
you apply the same force to a smaller area
COMD 3700
Lesson 3- Measurement of Sound 1
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 The smallest, softest sound pressure variation required to produce a just audible sound to
healthy young ears is approximately:
0.00002 Pa = 20 μ Pa = sound of a mosquito flying 3 meters away
We know that μ = 1 millionth part of
0 dB SPL = 20 μ Pa
AUDIOGRAM
 Defines amount of hearing loss a person has based on their threshold at a given frequency
 Human ear is NOT equally sensitive to all frequencies
 In the normal ear, the hearing is less sensitive than the standard of 0 dB SPL
 Audiometer = instrument designed to test hearing sensitivity at different frequencies which are recorded
on an audiogram
AUDIOGRAM OF dB SOUND PRESSURE
LEVEL (SPL) (See Diagram to Right)
 1938 = Beasley tested a variety of
persons hearing at the frequencies or
octaves on the audiogram. He averaged
all of their thresholds onto an audiogram
and determined that the human ear is
NOT equally sensitive to all frequencies
 Additional studies were conducted.
 American National Standards
Institute (ANSI) published standards in
2004 for Audiometric Testing (aka
Audiometric Zero)
 This audiogram represents the current
ANSI standards
 In this audiogram, ANSI specified that
this be performed (or that these results
were obtained) using TDH-49
Headphones
2004 ANSI Standards are:
0 dB HL
0 dB HL
0 dB HL
0 dB HL
0 dB HL
0 dB HL
0 dB HL
at
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125 Hz =
250 Hz =
500 Hz =
1000 Hz =
2000 Hz =
4000 Hz =
8000 Hz =
X = amount of sound pressure it took for the normal hearing subject to just
perceive the sound at the different frequencies
47.5 dB SPL
26.5 dB SPL
13.5 dB SPL
7.5 dB SPL (Memorize this calculation ONLY, NOT the whole table)
11.0 dB SPL
10.5 dB SPL
13.0 dB SPL (For the example below)
OR at 8000 Hz, it takes 13.0 SPL to reach 0 dB HL
OR 13.0 SPL is the sound pressure that it takes for the normal ear to just detect the sound
stimulus
COMD 3700
Lesson 3- Measurement of Sound 1
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 For Reference:
Humans = hear from 20 Hz – 20,000 Hz
Dogs = hear from 40 Hz – 40,000 Hz
Piano: Lowest note = 27 Hz; Highest note = 4,186 Hz
AUDIOGRAM
Hearing Level (HL)
Frequency in Hertz (Hz)
125 Hz –8000 Hz in Octave Increments
HEARING LEVEL (HL)
CONVERTED HEARING LEVEL AUDIOGRAM (See Diagram Above)
 Same frequencies as before (125-8000 Hz)
 dB measured in Hearing Level
 Converted from dB SPL to db HL
 This is the audiogram/hearing chart used to test hearing
 This audiogram is referenced in HL, NOT in SPL
 Audiogram determines each patient’s individual threshold
 7.5 db SPL at 1,000 Hz = 0 db HL
 Each frequency at 0 db HL represents a different value SPL
SENSATION LEVEL (SL)
 Auditory threshold of an individual
 Term used to designate an intensity level above threshold
 Must know patient's threshold (softest sound they can hear) at a given frequency before you can identify
the SL level
COMD 3700
Lesson 3- Measurement of Sound 1
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 If audiologist wants to present a stimulus to a patient at 20 dB SL at 1000 Hz" and patient's threshold at
1000 Hz was 25 dB HL. The stimulus would be presented to the patient at 45 dB HL
 Sometimes audiologists want to deliver a stimulus at a constant level above a patient’s threshold.
For example:
Patient 1 has threshold at 20 dB at 1000 Hz
Patient 2 has threshold at 30 dB at 1000 Hz
Patient 3 has threshold at 40 dB at 1000 Hz
 However, all patients hear this at 40 dB Sensation Level
 Audiologist can present a stimulus that is equally intense or loud for all of them
 Audiologist presents a 20 dB SL to all these patients
Therefore:
Patient 1 has threshold at 20 dB at 1000 Hz + 20 dB = 40 dB. Aud presents the tone at 40 db HL
Patient 2 has threshold at 30 dB at 1000 Hz + 20 dB = 50 dB. Aud presents the tone at 50 db HL
Patient 3 has threshold at 40 dB at 1000 Hz + 20 dB = 60 dB. Aud presents the tone at 60 db HL
Therefore:
Each patient received the stimulus at a constant value of 20 dB SL above their threshold
COMD 3700
Lesson 3- Measurement of Sound 1
VOCABULARY
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Amplitude
Anechoic chamber
Aperiodic wave
Artificial ear
Artificial mastoid
Audiometer
Beats
Bel
Brownian motion
Cancellation
Complex wave
Component
Compression
Cosine wave
Cycle
Damping
Decibel (dB)
Difference tone
Dyne (d)
Elasticity
Erg (e)
Exponent
Force
Forced vibration
Formant
Fourier analysis
Free field
Free vibration
Frequency
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Fundamental frequency
Harmonic
Hearing level (HL)
Hertz (Hz)
Impedance
Intensity
Intensity level (IL)
Inverse square law
Joule (J)
Kinetic energy
Localization
Loudness level
Masking
Mass
Mass reactance
Logarithm
Longitudinal wave
Loudness
Mel
Microbar (µbar)
Newton (N)
Octave
Ohm (Ω)
Oscillation
Overtone
Pascal (Pa)
Period
Periodic wave
Phase
Phon
Pitch
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Potential energy
Power
Pressure
Pure tone
Quality
Rarefaction
Ratio
Reactance
Resistance
Resonance
Resonant frequency
Reverberation
Sensation level (SL)
Sine wave
Sinusoidal
Sone
Sound-level meter
Sound-pressure level (SPL)
Spectrum
Stiffness
Stiffness reactance
Threshold
Transverse wave
Velocity
Vibration
Watt
Wave
Wavelength
Work