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Hearing Science


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Transducer properties
- each transducer has a specific physical structure and so also has specific impedances--> in turen specific frequency responses
- a large object wil respond better to low frequency & resist high frequency changes
- smaller object will respond better to higher frequencies

Acoustic transducers convert ?
- sound energy to electrical energy (microphones)

Electromagnetic transducers convert?
variations in an electromagnetic field into electrical energy (telecoil)
What properties should a good transducer have?
-frequency response should be adequate for the measurements that are being made (recording music vs voice)
-sensitivity - should be able to follow small changes in the signal as well as large changes in the signal
-Dynamic range - should be large enough to capture the signal of interest (the dynamic range must equal or exceed the dynamic range of the applied signal)

Other desirable properties of transducers?
- Minimal internal noise - should be as low as possible
-can be omnidirectional (equal response from all directions)
-or directional (more sensitive to sound from the front)

Some input transducers are?
Strain Gauges

these take signal in and convert

Some output transducers?
insert phones
loud speakers
bone oscillators

these generate a signal from input

Output transducers specifications
(headphones, insert phones, sennheiser phones) all have different frequency response
- minimum power rating
-maximum SPL

What is linearity with regard to acoustic transducers?
a measure of the change in output as a function of change in input
Distortion of acoustic transducer?
Measure of the change in the output relative to the change in input

usually less than 1%

How does a microphone work?
movement of the microphone diaphragm in response to applied sound pressure is converted to electrical energy
Types of microphones
Crystal mics
dynamic mics
condenser mics
electret mics
probe tube mics

With probe mics it is important to remember the response characteristics of the probe mics relies on?
the length of the attached tube
- if the length of the tube is altered, the response of the mic is also altered.
Pressure mics do not compensate for ?

Free field mics ?

their effects on the soundfield so have to be held at 90 degrees to the source

Do compensate and so can be used at 0 degrees to the source

Acoustic transducers (headphones)
usually only 1 diaphragm, no need to generate high levels of power
- output is measured as a function of freq and can be measured in 6 cc coupler
-insert phones measured in a 2cc coupler the accuracy and validity of the measurement depend on the use of a tube that is the correct length
-calibration depends on the interaction of the properties of the tube & the diaphragm of the insert phone

Vibratory output transducer (bone oscillator)
-The case of the bone oscillator is set into vibration so an electrical signal is transduced into mechanical movement of the transducer
Analog signals are signals that are____ _____ ?
continuously varying
- the sounds we hear can have pressure values that exist over a wide range.
-if sound impinges on a microphone, then the sound pressure is transduced into voltage
-this electrical signal is still analog

How an analog HA works
-input comes into the microphone
-the microphone output is amplified & shaped according to the needs of the patient
-the amplifier output is directed to the reciever
-the reciever creates sound pressure that is delivered to the patients ear
-signal has never been changed in terms of continuity

How digital HAs work
-input signal voltages at the microphone are converted to digits
-the digits are processed using a mathematical procedure
-the digits are converted, back to voltages at the reciever
-the receiver then creates pressure according to the voltages applied and soundpressure is now delivered to the patient's ear

Integers have to be converted to ?
1s and 0s
binary system (base 2)
If 1 bit can only be a 1 or 0 then...
only 2 numbers can be represented by one bit
2 bits you can get 4 numbers

The number of values that can be represented in the binary system is a factor of?
1 bit = 2 levels
2 bits = 4 levels 2to second power
3 bits = 8 levels or 2 to the 3 power
CDs typically use 16 bits
cell phones use 8 bits
Movies typically use 24 bits

Changing a waveform into a series of digits stored as ones and zeros =
digitizing to a binary code
An analog event is a physical event that can consist of any value and is said to be represented by a ___ ____
continuous signal
Digital signals have values only at specific times and are undefined at all other times. are said to be discrete representation of ___ ____ & _____
Analog events and signals
The more bits ....
The better the amplitude representation of the analog signal
If you want to faithfully reproduce analog signals you need...
A lot of bits

16 bits = 65,000 levels .. not much error

The number of bits represents ?

how do we get the best representation of time?

amplitude resolution of the analog signal

-taking samples at regular intervals
-if there is a regular interval, then there is a constant amount of time between samples
Best resolution = fast and regular sampling

The more samples gotten per unit of time... the
better the signal representation
The nyquist theorem gives what guideline?
"In order to correctly capture the signal, it must be sampled with a frequency that is at least twice as high as the maximum frequency?
If the bandwidth of a signalis wide then the sampling requency has to be
- if signal is 60,000 Hz then have to sample 120,000 Hz.
12o,ooo samples for every second.

When you have less time between samples the sampling frequency goes?
A noise that contains a limited set of frequency components?
NB noise
A continuous spectrum represents ?
white noise
-amplitude is constatn as a function of frequency
-energy is present at all frequencies

A resonating system that responds differentially to frequency is?
the definition of a filter
Describe the threshold of audibility curve
at each freqency the average normal hearing person will need a certain amount of sound pressure to hear the sound
so... 125 Hz = 45 dBSPL
250 hz = 25.5
500Hz = 11.5
1000Hz = 7.7
2000Hz = 9
4000Hz = 9.5
8000Hz = 13
so more power needed in low frequencies to hear

The human ear is sensitive to a range of about?
20-140 dB

In terms of Frequency we can actually hear as low as and as high as?
2 hz (but won't sound tonal until 20 Hz)
20,000 Hz
Critical bands do vary with ?
they are smallest at?
and increase with frequ above ?

low freq
increase with freq above 1000 hz

Measuring the frequency selectivity of the ear, critical band notion is based on the assumption that
there is a filter in the ear that allows certain frequencies to pass through
Critical bandwidth is a point where
decreasing the stimulus BW will result in a lower threshold
There is an increase of ~8-10 dB of power required for every ____fold decrease in duration down to about ____ msec
10 fold
10 msec
So as the duration decreases below 1 sec, the power of the tone must
increase in order for the energy to remain constant
For durations less than 10 msec there is
not a constant effect of duration on threshold
The intensity required to "hear" different frequencies ____.
Stimulation duration plays an important role

once the duration exceeds ____, further increases do not matter



What is the absolute sensititivy of the ear?

also called?

the smallest level or intensity needed for detection of sound

Thresholds of audibility

What is MAPC?
when earphones are calibrated in a 6cc soupler
- not a true map measure, referred as MAP coupler
Each earphone-coupler combination generates?
a different set of values for RETSPLs
[the standardized RETSPL measurements indicate the level in decibels required for threshold detection of tones of diff freq presented via different earphones and calibrated with the appropriate coupler]
MAF thresholds are consistantly about ___ dB lower than MAP

how do we account for the missing dBs

6dB lower
-MAP measures at the TM& MAF measure pressure at the entrance to the EAC, so you lose canal resonance which serves to enhance sound pressure reaching the TM
-also MAF get 3dB from binaural advantage

Thresholds of discomfort are at around what range?
110-120 dB SPL
Threshold of Pain =
120-140 dBSPL
Methods for measuring frequency discrimination
DLF = discrimination of successive tones with slightly different frequency
-keep frequ of 1 tone constatn
-vary frequency of other tone to find JND
FMDL - frequency modulation difference limen
-task is to discriminate a tone that is freq modulated from one that is not
-use low rate of modulation to find out amount of modulation needed for detection

Equal loudness contours are obtained by?
Asking listeners to equate loudness of various frequencies to the loudness of a 1000 Hz pure tone

the C-scale is preactically linear in the auditory range

What are Sound level meters used to measure?
The SPL of a signal in a soundfield
-can be used to calibrate equipment or earphones
Purpose of filter networks
-used to break the sound of interest into bands to do a spectral analysis
What is rise time?
What is Fall time?

how are they measured?

Rise = the amount of time after the presentation switch is pressed and the signal is between -20dB and -1dB of steady state value
Fall = time required for a tone to decrease from a steady state to a turned off state

The measurement is made with the attenuator dial set to 60dBHL or less
-the rise/fall times cannot be less than 20 ms and not longer than 200 ms
measurement is made from direct output of audiometer. Transducer is not used but is measured with storage oscilloscope

Measurement process of sound level meter
- set meter to the appropriate time constant (fast or slow)
-set filter network as needed for the measurement (A,B,C or linear) or select the external filtering network (octave or 1/3 octave)
-select microphone type (free field or pressure
-measure signal
-recheck calibration

To make measurement in the soundfield do what?
-Hold mic at arms length away or use a remote mic to reduce body baffle
-stay away from other reflecting surfaces
-make sure nothing is obstructing the sound from the SLM
-make sure SLM doesn't overload

What are the different type of response characteristics of microphones?
Free field mics(most in europe compensate for their effects in soundfield - held at 0 deg relative to sound source

pressure mics - must be held at 90 deg relative to sound source

random incidence - type used mostly in US

SLMs have ____ response integration times

what are they good for

two (fast and slow)
Slow - (1sec rise time)good for measureing signals that have level fluctuations typically >4dB ie masking signals or ambiant noise
Fast - (.2sec rise time) good for measuring signals having slight or no level fluctuation like puretones.

To check output levels of the speech circuit do what?
Play the 1000 Hz calibration tone on cd through the speech circuit
-adjust the VU meter so that the needle peaks at 0dB
-set the attenuator dial to 70dBHL & measure the output levels

An octave has a range such that...
the highest frequency is twice s high as the lowest frequency in the band

a 1/3 octave is 1.26 times as high as the lowest freqency

If the measurements using the A scale (on SLM) are less than the measurement obtained using the linear scale what can you conclude about the spectral quality of the sound you measured?
measurements in linear scale include all sound levels so broad range of spectral quality
measurements in A scale limit sounds measured to 40 phon contour or sounds between 24dB to 55 dB. Most of the spectral quality would be below 500 Hz
What is missed on an electrical only calibration?
-measurement of signals are below ambient noise levels
-so unless there is a voltage to SPL conversion , the absolute sound level value is unknown

the usual conversion factor is 1 V = 130 dBSPL

In order to use calibration forms the error must be rounded to?
the nearest 5dB
-If the output of the audiometer is less than the expected value the audiometer = weak therefore the threshold must be shifte by subtracting the correction factor form the measured threshold
-if the output from the audiometer is more than the expected value, the audiometer is considered to be too strong. In this case the threshold is shifted by adding the correction factory to the measured threshold

Why are Impulsive sounds important to measure accurately?
(measured at 35ms)
because while a short duration sound does not sound as loud as a continuous sound, the potential damage is still there
Not only is it important to evaluate the equipment, but it is also important to determine______ _____ ______
the level of noise in the environment
How does cross talk occur?
-occurs between earphones- not good
To test:
-one ear phone is disconnected and a signal is directed to that disconnected earphone. If the signal is heard in the connected earphone then "crosstalk" has occurred.
-attenuator is set to 70dBHL ...this must be corrected!

The closeness of the measurement to the actual value
Being able to make repeated measurements and get the same outcome each time

(a broken thermometer could be precise but not accurate)

the number of waves that pass a certain point in each second
The amount of energy in a sound wave

the larger the amplitude the louder the sound

Distance from one crest of a wave to another
In a complex wave, the repeat rate is determined by
the fundamental frequency or the lowest frequency or the wave with the largest period
Frequency vs Period
frequency = number of time a complete cycle is completed in 1 sec. 1 cycle per sec = 1 hz

Period = the time it takes to complete 1 complete cycle

these are inverses of each other

Pitch =
Frequency how low or high a sound seems
the X axis is called the
The Y axis is called the
Transients are?
Brief sounds or clicks
What is threshold?
the lowest intensity at which a person is able to respond to a stimulus
Sound wave =
wave in which the energy is transported in the same direction as the applied force

applied force causes compression and rarefaction

Natural resonant frequency
-determined by Mass, Stiffness & resistance of the open system
-is the frequency at which a vibrating object has the greatest magnitude when the system is forced into vibration
What is noise?
a non-repetitive sound wave or sound waveform for which we can percieve no fundamental frequency
any number to the zero power =
negative exponents =
the number of decimal places before 1
Kilo -thousand
Deci - tenth
centi - one hundreth
milli - one thousandth
micro - on millionth
mega - million
nano =

10 to the 3rd power
10 to the neg 1
10 to the neg 2
10 to the neg 3
10 to the neg 6
10 to the 6th
10 to the 9th

Positive exponent means
number of 0s after 1 so
10 to the 4th = 10,000
If a person has a hearing threshold of 25dB at 1000 Hz what does that mean?
it means that it takes 25 dB more than a group of young normal hearing listeners

the Sound pressure = 25dB plus 7.7 = 32.7dBSPL

If a patient has a threshold of 45dBSL and is presented a signal at 20dBHL
45 plus 20 = 65dBHL
If a sound is 60 dBSPL at 3 meters then what is it at 6 meters
it is 54 dBSPL

60 dBSPL ia approximately conversational speech level so at 2 times the distance the level (54) is clearly below conversational speech level

sound treated rooms are designed to ...
minimize reflection (absorb sound)
Carpet will absorb more ____ frequency sounds

A dB is an
expression of the relative difference in power or pressure
a measure of a physical property

Phonetic aspects of speech broken down into...
Acoustic phonetics = sound characteristics

Articulatory phonetics = the movement characteristics

Phonetic description of consonants have 3 features...
-presence or absence of voicing
-place of articulation (bilabial, labiodental, dental, alveolar, palatal, velar, glottal)
-Manner of articulation - stops fricatives nasals approximants

What is co articulation?
Simultaneously articulating more than one phoneme
- either in anticipation of a following sound
-or as a result of carry over from a previous sound

Speech production is aerodymanic. Air flow is ?
egressive - moves from lungs through the system to output
Forming consonants
- use one or more areas of constriction to vocal tract
-can be voiced or unvoiced
-turbulant air flow
-less overall energy than vowels
-contain greater meaning

Forming vowels
-produced relatively open vocal tract
-sounds are always voiced
-have greater energy
-contain less meaning

Theoretically the vocal tract has how many formants?

For specification of a vowel how many formants are important?

an infinite number but the reality is that only the first 4 or 5 are relevant for speech

only the 1st three formants are relevant to vowels

Formant frequencies increase as
the overall length of the vocal tract shortens (as resonating cavities within the vocal tract become smaller)
Acoustic theory of speech
-vocal tract filter is freq dependent
-the vocal tract resonates the source signal by allowing certain frequencies to pass through a filter with greater amplitude than other frequencies
-the characteristic resonances of the vocal tract are those frequencies that are selected
-the characteristic resonances are called formants

Source Filter Theory
P = U *T *R
P = spectrum of sound pressure wave exiting lips (output
U - the glottal volume velocity
T - transfer function of vocal tract (filter specs)
R - radiation characteristics at the lips

speech production system can be broken down into?
- the sound source
-the filter (resonator )

also called the source/filter theory of vowel production

Utterances that are created by constrictions along the vocal tract are?
consonants (articulatory processes involved)
Utterances that are created with relatively open vocal tract are?
Vowels (the tongue does not close off the vocal tract)
The characteristics of the sound at the lips are determined by?
-the characteristics of the vocal chord vibration
-the vocal tract configuration

The vibration of the vocal folds stimulates
-a tube which resonates at its own natural frequency
Air comes from the lungs to vocal folds...
if they are open?

if they are closed?

- open = no phonation

- closed -they are blown open& set into vibration

Breathing for speech consists of
-active inhalation

-active & precise control of exhalation

Breathing for life consists of
active inhalation

passive exhalation

The lowest resonant frequency of the vocal tract =
485.7 Hz
The lowest freq is 485.7 Hz which is F 1
so F2 and F3 are?
485.7 times 3 = 1457

485.7 times 5 = 2428

Vocal tract length affects forman ___?
so Formant 1 for:
males = 485.7
females = 566.67
child = 944.44

What freq sounds are more likely to penetrate walls?
low freq

the decay in sound energy can be characterized by ?
the reverberation time (of the room)

= the time that it takes for a sound to decrease by 1 millionth or 60dB

(RT 60)

Generally how much reverb is needed for
an auditorium
a classroom
recording studio
hearing test suites

-auditorium= 2 sec -classroom < 1 sec -Rec. Studio = low -hearing test suite should approach zero
how much reverb is good?
for general purpose speech and music...

1,5 -2.5 sec

Reverberation times decrease as
frequency increases
(so low frequency sounds persist longer than high frequency sounds)
which is good because hearing sensitivity is poorer in the low freq

Where a sound drops by 60dB =
the characteristic reverb time of the room
What is the sound pressure that is present at a patients ear if threshold at 2000 Hz is 40dBHL
What is dBSL?
with dBSL - the reference is the individuals own threshold
so if their threshold is 30dBHL and he listens to a sound of 40dBSL the presentation level is 70dBHL
a resonating system can be described as
a system that responds differentially to frequency.

this is the definition of a filter

Other terminology for filter =
transfer function
resonance curve
system frequency resopnse
amplitude response

To describe the relative intensities of 2 sounds use?
the decibel
What is an aperiodic wave?
-there is no orderly arrangement of the components
-random vibratory motion
-results in "noise"

What is fourier analysis
This is a formal approach that is based on the notion that a complex wave can be described as the sum of simple waves
Complex wave definition
If there is more than one component in the sound then the wave is considered complex
a wave that has 2 cycles in .002 sec = what Hz
means 1 cycle in .001sec
means in 1 sec have 1/.001 = 1000 so 1000 Hz
What is the period of a wave?
how long it takes to complete 1 cycle = seconds/cycle
The amount of energy transferred in a wave =
the amplitude
Frequency vs pitch
Frequency is a description of a physical property

pitch is a description of the perception of frequency (non physical)

Intensity vs loudness
Intensity is a description of physical property

Loudness = description of perception of intensity (non physical)

Waves that can travel unimpeded =
traveling waves
Impedance can be in the form of
a physical boundary (a wall)
a change in characteristic of medium air -->liquid

For a fixed setting of the hearing-level dial of a puretone audiometer, which of the following is true of the actual sound-pressure level output of the audiometer earphone?
a)It is constant across all frequencies
b)it is lowest at midfrequencies
b) it is lowest at mid frequencies
-the sound pressure level necessary to achieve 0dB hearing level is highest at low frequencies lessens in the middle frequencies and then increases at high frequencies
Which of the following is the primary reason investigators have given for limiting a hearing aid's SSPL 90 to a lower level when it is worn by a child than when worn by an adult?
a) The volume of the external auditory meatus in children is less than th
The SPL at the eardrum is a function of the volume of the external auditory meatus: the smaller the volume, the greater the pressure. An SSPL 90 set to a level appropriate for adults may produce an undesirably high maximum SPL in a child
In the measurement of real-ear sound-pressure levels with a probe-tube microphone system, insufficient probe-tube depth will tend to ?
a)increase the high freq response
b)decrease the high freq response
c)decrease the response at all frequencies<
Probe tubes for measuring real ear SPL should be inserted as close to the tympanic membrane as possible since it is the SPL at the tympanic membrane that is being measured. If the probe tube is too far from the tympanic membrand, high freq sound waves bounced off the eardrum will dissipate before reaching the probe, but low frequency sound waves , which do no dissipate as easily , will be essentially unaffected. The overall effect will thus be a decrease only in the high freq response.
An audiologist is using a tracking procedure to assess a cochlear implant patient's progress in communicating under audition plus vision conditions. The materials used are articles from popular magazines. Data are colleced for 15 minutes every working d
Tracking involves having the client repeat phrases read by the audiologist from different sources. Because each author has a different style, there will be considerable variation in how familiar the vocabulary is to the client and in the number and degree of redundancy of syntactic clues; since those factors affect word-per-minute scores C is the correct answer

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