Transcript Audiology 1 - Physics of sound File

AUDIOLOGY 01
The physics of sound
based on Adam Beckman’s* lecture 2015
*Head of Audiology Services, Plymouth Hospitals NHS trust
Pedro Amarante Andrade, PhD
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BIOSCIENCES
FOR SPEECH AND LANGUAGE THERAPY
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WHY STUDY AUDIOLOGY?
Mr. Fairfield
74 yrs. old
Aphasia
? Comprehension
http://www.tamcummings.com/stages_of_dementia_11.html
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INTRODUCTION
• Physics of sound
• The ear (Anatomy and Physiology)
• Hearing
– How it works
– How we measure it
– Pathology
• The vestibular system
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WHAT IS SOUND?
HOW DO WE MEASURE AND DESCRIBE
SOUND?
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WHAT IS SOUND
Sound is:
“vibrations that travel through the air or another
medium and can be heard when they reach a
person’s ear”
Oxford dictionaries
– Examples:
Vocal folds, guitar strings, musical instruments,
loudspeaker, engines and thunder
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SOUND PROPAGATION
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SIMPLE VIBRATIONS AND SOUND
TRANSMISSIONS
• Propagation of Sound
SOURCE
• back and forth movement of air molecules
• around their position of equilibrium
• in response to the back and forth vibration of the object
http://www.acs.psu.edu/drussell/Demos/waves/wavemotion.html
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SIMPLE VIBRATIONS AND SOUND
TRANSMISSIONS
• Molecules nearest to object move first
– Molecules only move in a localised region pushing
against adjacent molecules causing the pressure
variations (sound wave) to move across space
http://www.acs.psu.edu/drussell/Demos/waves/wavemotion.html
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WHAT IS SOUND
Sound consists of combinations of different
Pressure [Pa]
Rarefaction | Compression
pure tones and is mostly found as complex
vibrations
Time [s]
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WHAT IS SOUND
Complex vibrations (pure tone 1 + pure tone 2)
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VIBRATIONS AND SOUND
TRANSMISSIONS
• For unobstructed sound waves
– Air particles move outward in spherical direction
– Air pressure peaks (amplitude) diminishes with
distance because of inertia of molecules as well as
increased surface
A = 4 π r2
http://www.acs.psu.edu/druss
ell/Demos/rad2/mdq.html
http://www.performing-musician.com/pm/apr09/images/TechNotes_02.jpg
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BASIC CONCEPTS
FOR UNDESTANDING SOUND
•
•
•
•
•
•
Frequency
Period
Amplitude
Phase and Wavelength
Reflection
Velocity
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PHYSICAL PROPERTIES OF
WAVES
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• Total number of back-and-forth
movements around the resting
position of an object. Vibrations per
second
• Unit of measurement
Hz (Hertz)
• Hearing is measured across
Frequency
Period
Amplitude
Phase
Wavelength
Reflection
Velocity
(20  “250 Hz – 8000”  20.000 Hz)
• Psychoacoustic equivalent is pitch ()
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Frequency
Period
Amplitude
Phase
Wavelength
Reflection
Velocity
Time [s]
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RELATIONSHIP BETWEEN FREQUENCY
AND PERIOD
Frequency (Hz)
Period seconds
(sec)
Period milliseconds
(ms)
250
500
1000
2000
4000
8000
0.004
0.002
0.001
0.0005
0.00025
0.000125
4.0
2.0
1.0
0.5
0.25
0.125
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Frequency
Period
• General term to describe the
Amplitude
magnitude of sound
Phase
• Maximum amplitude is related to how Wavelength
far the object (hence the air particles
Reflection
Velocity
set in motion) moves back and forth
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+
|
Amplitude
Amplitude analysis methods
• Peak-to-peak amplitude
– Amplitude change that
occurs between the positive
peak and the negative peak
• Peak amplitude
– Measure the amplitude
from baseline (zero) to one
of the peaks
• RMS (root-mean-square)
– Square root of the
arithmetic mean of the
squares of each data point
Frequency
Period
Amplitude
Phase
Wavelength
Reflection
Velocity
Time
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• Initial angle of a sinusoidal function
at its origin
• Expressed in degrees relative to the
angle around the circle
• Our hearing is not sensitive to phase
Frequency
Period
Amplitude
Phase
Wavelength
Reflection
Velocity
Wikipedia. J JMesserly.
https://commons.wikimedia.org/wiki/File:3_phase_AC_waveform.svg
Wikipedia. Peppergrower.
https://commons.wikimedia.org/wiki/File:Phase_shift.svg
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IN PHASE
vs
• 2 waves begin at the
same phase
• Their amplitude is added
resulting in a wave with
higher amplitude
OUT OF PHASE
• Two pure tones of the
same frequency and
amplitude presented
180° out of phase will
cancel each other out
http://www.ducksters.com/science/physics/wave_behavior.php
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OUT OF PHASE
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• Length of a single cycle
• Measure in meter
• λ = notation
Frequency
Period
Amplitude
Phase
Wavelength
Reflection
Velocity
Wikipedia. Brews ohare.
https://en.wikipedia.org/wiki/File:Wavelength_indeterminacy.JPG
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• Length of a single cycle
• Measure in meter
• λ = notation
Frequency
Period
Amplitude
Phase
Wavelength
Reflection
Velocity
Intensity
Distance
Time[m]
[s]
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Frequency
Period
• Determines how sound is affected when
Amplitude
travelling past objects in its path
Phase
Wavelength
• The longer the wavelength the less
Reflection
likely the object will have an affect on
Velocity
the sound
• If the wavelength is too short relative to
the size of the object, then the object
can block (and reflect) the sound
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STANDING WAVE AND
RESONANCE
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RESONANCE/REFLECTION
• Standing waves occur when the sound
wave is reflected from a solid object
• In 0° phase difference between them,
a constructive interaction occurs,
A
increasing the amplitude. This is
known as resonance (A)
• When the reflected wave is in 180°
anti-phase to the incident wave,
destruction interaction occurs,
B
producing nodes where the waves
cancel one another out (B)
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Frequency
• The elasticity and density of the medium in
Period
which it travels determine the velocity
Amplitude
(speed) of propagation of a sound wave
Phase
Wavelength
– In air the velocity is approximately
Reflection
340m/sec
Velocity
– In water the velocity is quadrupled and
steel fourteen times faster than air
• Therefore the denser the medium, the
faster the velocity.
All sounds, regardless of frequency, travel
through air at the same speed, in a
longitudinal direction
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Frequency
Period
Amplitude
Phase
Wavelength
Reflection
Velocity
The human ear hears sounds from 20Hz to 20,000Hz, which
corresponds to 17m to 17mm in wavelength.
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COMPLEX SOUNDS
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WHAT IS SOUND
Complex vibrations (pure tone 1 + pure tone 2)
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COMPLEX SOUNDS
• Complex sounds can be classified as:
– Periodic (Quasi-periodic)
– Aperiodic
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COMPLEX SOUNDS
Periodic
• Periodic:
– Vibratory pattern repeats at regular intervals
• Simple periodic vibration - Pure tone
• Complex Periodic Vibrations - Two or more pure
tones combined into a non-sinusoidal pattern
may also be considered periodic if the wave
pattern repeats itself as a function of time
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COMPLEX SOUNDS
Periodic
• Fundamental frequency (f0)
– Lowest frequency component in a
complex periodic vibration
• Harmonics
– Multiples of the fundamental
frequency
• Example: C on a cello and clarinet
Timbre 
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COMPLEX SOUNDS
Periodic
• Analysis:
– If we know the frequencies, amplitudes and starting
phases of all the individual components
– We can construct the predictable vibration pattern that
would result from their combination
FAST FOURIER TRANSFORM (FFT)
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COMPLEX SOUNDS
Periodic
POWER SPECTRUM
PHASE SPECTRUM
Kramer, S. 2008. Audiology: Science to Practice
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COMPLEX SOUNDS
Aperiodic
• Aperiodic
–Pattern of vibration that does not repeat
itself regularly
–THUS – no periodicity in the wave pattern
–Product = NOISE
• NOISE:
–Produced by a combination of many
frequencies with random starting phases
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NOISE
Kramer, S. 2008. Audiology: Science to Practice.
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SOUND MEASUREMENT
from intensity & pressure
to decibels
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SOUND MEASUREMENT
‘Quantity’ of a sound e.g. amplitude
Intensity (W/m2)
– Range of sounds we can hear
• 0.000000000001 to 100 W/m2
Pressure (Pa) or (N/m2)
– Range of sounds we can hear
• 0.00002 to 200 Pa
– (1W=1Nm/sec; 1Pa= 1N/m2)
I ~ p2
– NOT convenient units
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SOUND MEASUREMENT
• We use a logarithmic scale
– logarithm tutorial
• Unit of measurement:
– Decibel (dB)
• Other units you may come across:
– dBHL = decibel Hearing Level
– dBSPL = decibel Sound Pressure Level
•  = loudness
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SOUND MEASUREMENT
• Decibel Scale:
– Ratio scale where any measured value is
relative to some specified reference value
– The most intense sound that can be
tolerated is 1014 times greater than the
lowest sound intensity that can be heard
– The lowest audible sound intensity
expressed as 1 or 100
– Ratio: 1014 / 100 (reference value)
– Logarithmic scale
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SOUND MEASUREMENT
• Log is the power to which 10 must be raised
to produce the number defined by the ratio
• THUS
– Range in intensity (Bels) would be from
0 (least intense) to 14 (most intense)
Upper limit of intensity:
Lower limit of intensity:
= 10 log (1014 / 100 )
= 10 log (100 / 100)
= 10 log (1014)
= 10 log (1)
= 10 (14)
= 10 (1)
= 140 dB
= 0dB
RANGE of AUDIBILITY = 0 – 140 dB
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INTENSITY
(W/m2)
Ref = 10-12 W/m2*
PRESSURE
(Pa)
Ref = 20µPa*
Converted into logarithmic scale
DECIBELS
10 log (I?/I ref)
20 log (P?/P ref)
Sound
pressure
Level
(SPL)
or
Sound
Intensity
Level
(SIL)
(dB)
*Ref = Lowest intensity/pressure we can hear @1000 hz
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DECIBELS
• 0 dB SPL
– does not mean the absence of sound pressure just
that the measured sound pressure is the same as
the standard referenced sound pressure (20 µPa)
or sound intensity (10-12)
• Lowest audible intensity depends on
– Frequency & type of earphones
• If we double the intensity of a sound
– The decibel level increases by 3 dB
• If we double the pressure of a sound
– The decibel level increases by 6 dB
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DECIBELS
• IMPORTANT:
– Decibels cannot simply be added or
subtracted
– E.g. 40dB + 40 dB ≠ 80 dB
– Decibels should be converted back to
pressure or intensity before being
combined
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WHY IS THIS IMPORTANT?
http://www.nottinghamcity.gov.uk/cdpc_decibel_chart.gif
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AUDIBILITY
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AUDIBILITY - FREQUENCY AND
INTENSITY
• Human ear responsive to Frequency range:
20 to 20000Hz (20kHz)
• Human ear responsive to the intensity range:
0 dB to 140dBSPL
• However not equally sensitive across the
range
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AUDIBILITY
• Threshold of pain
(120dB – 140dB)
(tympanic membrane)
Kramer, S. 2008. Audiology: Science to Practice.
• Most people find
above 100dBSPL
uncomfortable
• Useable range for
human hearing
• At lower and higher
frequencies more
sound pressure is
required for audibility
• Most sensitive to
frequencies between
500 - 6000Hz
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HEARING RANGES FOR DIFFERENT
SPECIES
Amy Blackman Hearing ranges of a number of different land and marine mammals.
http://www.vanderbilt.edu/exploration/text/index.php?action=view_section&id=13
95&story_id=338&images=
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ACOUSTIC SIGNALS AND ROOM
ACOUSTICS
• Sound waves carry acoustic energy generated
by a source away from that source
• In a free-field condition (a space with no
boundaries)
– Sound intensity decreases inversely with
the square of the distance from the source
– This is known as the inverse square law
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ROOM ACOUSTICS
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ACOUSTIC SIGNALS AND ROOM
ACOUSTICS
•
Obstacles can alter the sound wave in a number of
ways:
− Reflection
− Reverberation
− Absorption
− Diffusion
− Transmission
WHY?
Because there is a change in the physical properties
of the medium through which a wave travels
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Reflection - shadow effect
• Short wavelength:
– high frequencies
– little penetration
• Long wavelength:
– low frequencies
– deep penetration
Reflection
Reverberation
Absorption
Diffusion
Transmission
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• Reverberation occurs within a
confined space and can be likened
to a sound wave bouncing around
the room, leaving a persistence of
sound after the sound source ceases
until eventually it decays or is
absorbed by soft furnishings
• Reverberation time is measured by
the amount of time it takes for a
tone to decay by 60dB. For a sound
proof room, this should be less that
0.25 seconds
Reflection
Reverberation
Absorption
Diffusion
Transmission
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• Opposite of reflection
– Sound is absorbed by the medium
– What are good absorbers of
sound?
Reflection
Reverberation
Absorption
Diffusion
Transmission
• 2 elements
– Conversion to other form of
energy
• Heat
– Transmission
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LET‘S CONSIDER THE FOLLOWING
EXAMPLE
Background noise
Distance
Reverberation
Slide used with permission from Phonak
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DISTANCE AND BACKGROUND
NOISE
dB
80
Teacher‘s voice
70
Background noise
60
50
40
0
1
2
4
6
Distance (meters)
Slide used with permission from Phonak
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SUMMARY
• Sound is vibration of molecules in a medium,
transmitting energy
– Air, water, solid
• Sound has a variety of properties
– Frequency
• Pitch
– Amplitude
• Loudness
– ….
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SUMMARY
• Sound transmission is affected by
environment
– Reflected
– Absorbed
• We need to hear sound to be aware of it
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HOW SOUND IS HEARD
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REFERENCES
• Bess, F.H. and Humes, L.E. 2008. Audiology: the Fundamentals. 4th ed.
Philadelphia: Lippincott, Williams and Wilkins.
• Kramer, S. 2008. Audiology: Science to Practice. San Diego: Plural
publishing.
• Smurzynski ,J. 2006. Acoustic foundations of signal enhancement and
room acoustics. In: Chermak, G.D. and Musiek, F.E. Handbook of
(Central) Auditory Processing Disorder. Comprehensive Intervention,
volume II. Pp. 51-74.
• Rosen, S and Howell, P. 2007. Signals and systems for speech and
hearing. Acedemic Press
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