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XXXHearingSciFinal

XXXHearing Science Dr. Milner Final

QuestionAnswer
Assessment of Auditory Sensitivity method of assessment can influence results!
What kind of methods are used for assessment of auditory sensitivity? psychophysical methods
Psychophysical methods are used for what? assessing auditory sensitivity
What do psychophysical methods of auditory sensitivity link? they link the physical properties of a stimulus to the perception and response to that stimulus
Before a listener responds to a stimulus what happens first the stimulus is processed psychologically by the listener
What are the three main methods used for response when testing auditory sensitivity? Method of LIMITS Method of ADJUSTMENT and Method of CONSTANT STIMULI
Method of Limits: one parameter is changed to obtain threshold subject is asked to indicate if stimulus was heard and one approach is used (either ascending or descending)
With regard to Method of Limits one parameter is changed such as Intensity
When one parameter is changed by the examiner such as intensity with regard to Method of Limits what is being obtained? threshold
With regard to Method of Limits when the subject is asked to indicate whether the stimulus was heard or not how might this be indicated raise hand or button push or some other behavioral response
What are the two approaches that relate to Method of Limits ascending and descending
Method of Limits describe Descending Approach: Sound starts off at an AUDIBLE LEVEL and then is decreased until it is inaudible…
With the descending approach the listener has a clear mental representation of the sound but time above threshold is wasted.
What is the positive point of using descending approach to test hearing limits? the listener has a clear mental representation of the sound because the sound starts off at an audible level. YIELDS LOWER THRESHOLDS TOO.
What is the negative point of using descending approach to test hearing limits? time testing above threshold is wasted because the hearing test starts at an audible level and then the sound is decreased until it is inaudible.
Which has lower thresholds when testing hearing limits descending or ascending approach and why? Descending yields a lower threshold by 3-4dB possibly due to listener having a clear mental representation of the sound they are trying to hear.
With regard to Method of Limits how is an ascending method of limits different from a descending one? In ascending method of limits the sound starts out at a low level and is increased until it is audible to the listener.
What is the positive point of using ascending as a method of limits to test hearing? if hearing is NORMAL this method is a relatively quick method – otherwise it can be time consuming…
In a hearing test blinking could be a behavioral response indicating the stimulus was heard
In a hearing test with a very small child like a baby in the booth the cessation of sucking on a pacifier could be a behavioral response indicating that the stimulus was heard.
How do you determine if a behavior is an indicator of a stimulus being heard? the behavior has to be a consistent and clear response.
Visual Reinforcement Audiometry child hears a sound looks at box box opens and there is a bear in there playing the drums – need two people test one has to keep child from just randomly staring at the box waiting for the bear to appear instead of responding to a sound stimulus
Visual Reinforcement Audiometry relates to Conditioned Response by Skinner
The parameter that changes most often is intensity
Why is intensity the parameter that changes most often because it is useful to test for the softest intensity whereat a person can detect a stimulus
Descending: if the stimulus starts at 30dB what is the next dB level 20dB – go down by 10dB and continue until the person stops responding.
We like using descending approach because of two reasons the person has a clear mental representation of what they are listening for and they are REALLY LISTENING and the test yields lower thresholds.
With Ascending Method of Limits test you start off at a low level (zero) and work your way up. If there is hearing loss you may be wasting quite a bit of time going up and up until you reach the person’s threshold…
If hearing is normal using the ascending method of limits is relatively quick
If the person doesn’t respond at Zero in an ascending method of limits hearing exam the next dB level to test is 15dB then 30dB then 45dB then to 60dB until they HEAR it. Keep going up in Ascending by increments of 15dB.
In an ascending method of limits hearing test you start at zero and keep going up by increments of 15dB until the person can hear it.
If you have a person in the booth that is 75 years old and has a history of noise exposure and the person seemed to have trouble hearing you prior to the test you might want to start at 50 and go up or down from there.
Ascending Method of Limits test is nice if the person has normal hearing.
When you get an affirmative response on a Method of Limits hearing test you have to hone in on the threshold
If the person in the booth gives you an affirmative response at 30dB and they raise their hand next you go down by 10dB – if they didn’t hear it at 10dB then go UP by 5dB. You go down 10 and you go up 5.
The Down 10 Up 5 incremental adjustments in a hearing test is to utilize the clearing of the auditory memory. We go UP from not hearing to hearing it.
It is important to present a stimulus in a hearing test in a non-predictable pattern and do not provide any cues of when the stimulus was presented.
Varying the time between stimulus presentation is needed Sometimes present close together or further apart so that the person cannot predict the signal and provide a false response.
Tell the person in the booth to raise their hand when they HEAR it and not when they ‘think’ they hear it because otherwise you can have them overwork and get a false positive result.
To see a response to a hearing test use peripheral vision or the audiometer light
If you go from 30 to 20 to 10 to 0 and they hear it at zero what do you do next wait a while and re-present it at 0 – if they respond again at zero this is a valid response.
Do not test lower than zero because no one cares if someone can hear lower than 0dB it is a ratio if there is no clinical value to doing something you shouldn’t be doing it.
Signal Detection Theory examines what? Signal detection theory examines a listener’s ability to respond to the presence of a stimulus outlines the possible outcomes/subject responses and there are 4.
What are the 4 possible outcomes for Signal Detection Theory? Hit Miss False Positive/Alarm and Correct Rejection
A psychophysical theory that examines a listener’s ability to respond to the presence of a stimulus with four possible outcomes/subject responses Signal Detection Theory
What are the four possible outcomes to any test in which the listener must determine the presence or absence of a stimulus Hit Miss False Positive/Alarm and Correct Rejection
Hit A sound was presented and the listener indicates he/she heard it
Miss A sound was presented and the listener indicates that she/he did not hear it
False Positive/Alarm No sound was presented and the listener indicates that he/she heard it
Correct Rejection No sound was presented and the listener indicates that he/she did not hear it
Outcomes #1 and #4 are accurate responses
This theory is used extensively in medical testing with emphasis on reducing the false positive rate as much as possible Signal Detection Theory
Method of Adjustment is subject controlled
With Method of Adjustment one parameter of the auditory stimulus is changed such as intensity by the listener in order to obtain threshold
With Method of Adjustment The subject keeps the stimulus at a barely audible level by adjusting the correct button on a tracking audiometer The intensity variations are recorded and the tracking thresholds or record of sensitivity across frequencies are obtained
Method of Limit: Examiner controlled!
Method of Adjustment Subject/Participant controlled!
Which method is most often used in an experimental setting Method of Adjustment
In Method of Adjustment in an experimental setting patient becomes a participant
Method of Adjustment is Participant Controlled the listener is the one changing the stimulus and adjusting the know to the point where the stimulus is ‘just audible’
With Method of Adjustment even though it is participant controlled you are still looking for thresholds while they turn the know down down down until it is just audible you use a tracking audiometer and you get tracking thresholds.
Method of Limits testing ability to hear intensities but during that test you are also going to test frequencies.
During Method of Limits you test ability to hear intensities and at varying frequencies and you do it from 250 – 8000 Hz in both ears.
During a Method of Limits exam we always start with 1000Hz because that is a place where we anticipate success a place where most people can hear the stimulus.
During a method of limits exam go from 1000 – 8000 then same ear again 1000 2000 etc to 8000 then test 500 and 250 last you have to get the threshold between + or – 5ddB to have it be valid results.
Auditory memory lingers there like closing your eyes and you can still see something it lingers there. Same thing with hearing. So when you get an affirmative you go down 10 and up 5 to ‘erase’ the auditory memory.
Method of Constant Stimuli Uses a technique called a two-alternative forced choice procedure
Method of limits or method of adjustment is used to approximate hearing sensitivity for a certain stimulus. Several levels above and below this approximation are then selected
In Method of Constant Stimuli The stimulus is then presented a certain amount of times such as 100 and the listener is cued to a time period when the stimulus will be presented The listener indicates “yes” or “no” to hearing the stimulus during this time period
In Method of Constant Stimuli Results are plotted as the percentage of time each level of stimulus was detected in the correct time frame “threshold” is whatever definition the examiner chooses This is a dynamic measure that changes as parameters change
You have Choice A or Choice B forced choice procedure you choose method of limits or method of adjustment to hone in on thresholds.
You present the stimulus a certain number of times but in clinic we limit it or we’d be there forever.
Every time they hear the stimulus they hit a button or raise their hand and then out of 100 times we look at how many yeses to if they heard it and depending on the percentage set by examiner 50% is threshold or 75% is threshold
In Method of Constant Stimuli we use 50% because that is where the odds ‘shift’
dBHL is only done in the clinic
Subject Variables A listener’s own predispositions to respond will influence test outcome
– Some people are more willing to respond than others due to many reasons and this will influence thresholds
– The predisposition to respond a certain way is called a prepatory set - which can be altered by examiner instructions
– Instructions can be altered throughout testing based on observation of the subject’s behaviors and facial expressions
• Different listener variables like anxiety can influence results. Can relate to personality. Can alter instructions if you perceive a hindrance.
• I am going to present it to them at 10 and over mike say yes that is the tone if they have a behavioral signal that indicates they likely heard it and then go over your results by representing it several other to ensure it is valid data.
• Hearing sensitivity begins to decrease at around ages 25-30 years of age
– Loss begins in the high frequencies (4000 Hz and up) and then expands in frequency and degree throughout life
– Loss is greater in men than women
– Loss due to aging is in many cases exacerbated by noise exposure dietary differences life factors etc.
• Not everybody who ages gets a hearing loss it either stays where it is or it gets worse. Outer hair cells high frequencies.
• Men more apt to be in environments where there is potential noise damage (professions.) (construction machinery etc.) This is a generalization and ‘not true’ but there is some truth to it years ago only men were allowed into military.
• Loss due to aging is presbycusis.
What is normal hearing? • It is difficult to create a satisfactory definition
–• It is difficult to create a satisfactory definition Hearing sensitivity varies across frequency: normal hearing and hearing loss must be defined differently at each frequency in dBSPL
• Extensive research in this area began in the 1930s at Bell Telephone Laboratories
• Large amounts of young adults (18-25 years old) with no auditory pathology were tested and the median threshold for this group was obtained This median became the “zero” decibel hearing level (0 dBHL)
• The frequencies that must be tested were determined by physiology Equal distances along the BM separate points most responsive to frequencies one octave apart
– Thus hearing is tested at octave frequencies between 250-8000 Hz
• Audiogram record of thresholds at these frequencies
• 0 dBHL = audiometric zero. 0 dBHL is arbitrary not a scientific measurement.
• We looked a physiology of the ear and determined where we need to hear the best we found equal points along the basilar membrane and found the hearing range where the basilar membrane was
• We tested them at octaves they are doublings. An octave is a doubling in frequency. Half octaves between 2000 and 4000 is 3000. 3000 is the half octave and sometimes we test those. To fit a hearing aid it is useful to test at half-octaves because at 3000 there is a lot of amplification.
Sound Localization The ability to determine the location of a sound source in space: extracranial image
• The brain uses multiple pieces of information for localizing sound This information must first be detected and then synthesized by the brain
Terms of Sound Localization: Azimuth and Head Shadow Effect
– 1. Azimuth: angle of incidence of the sound as it reaches the head measured in degrees
– 2. Head Shadow Effect: the loss of intensity of a sound as the sound travels around the head
When a sound encounters an object longer than its wavelength it is reflected or absorbed by the object
Head Shadow Effect Effect is greatest for high frequencies
Minimum Audible Angle : the smallest separation of angles of incidence that can be perceived
• Sounds are localized when the brain perceives differences in the time of arrival and intensity of the sound arriving at each ear From peripheral and central level:
• Extracranial : from outside in the world and brain uses multiple pieces of information for localizing sound.
• Azimuth: an angle in degrees of incidence. 0 degrees azimuth directly in front directly behind = 180 degrees
• Counterclockwise or clockwise depending on how you and the sound are oriented
• Head Shadow: sound travels around the head and you lose intensity because your head is in the way – an object in space that causes damping in sound as it travels around it.
• Lower intensity on opposite side No head shadow at 0 or 180.
• High frequency sounds lose their intensity because they have a shorter wavelength.
• High frequency sounds have a shorter wavelength. Long wavelength
• Low frequency sounds wrap around things and can wrap around head.
• Shorter wavelength high frequency just crash into things.
• Head shadow causes a time difference cue.
• Minimal audible angle the smallest separation of angles that can be perceived.
• Two people sitting close together asking a question but it would be harder to detect who said it.
• Two sound sources close together how close can they be and still be heard as two separate sources.
• It is the degree in between those two sources that help identify the sounds as two sources.
• 2 sound sources close together in space how close can they be to have it be a minimum audible angle.
• At some point they are so close they sound like ONE SOUND LOCATION. Detect 2 separate sound sources that are actually close together – sounds like one source.
• When brain perceives a difference in intensity or angle it helps with localization of the sound.
• If a sound source is at 0 or 180 degrees azimuth there is no difference in intensity
• But at 0 degree azimuth it is in front of you and you can see the sound source.
No difference from one ear to the other in 180 degrees because input comes in same intensity on both sides. Same with 0
• When the sound is coming at you head on there is a slight difference in intensity as compared to 180 but the input is equal when you consider it from a left to right intensity perspective.
Intensity Differences If a sound source is directly in front of (0o azimuth) or behind (180o azimuth) the listener there is no difference in the intensity of the sound arriving at each ear
– One ear is inevitably closer to the sound source
– The head shadow effect means gain or loss of intensity
• Sound is more intense on the side toward the sound source (near side) than on the side away from the sound source (far side)
• When the azimuth of the sound is 90o the head shadow effect is greatest but can be seen at smaller angles as well
– The result is an interaural intensity difference (IID)
• Azimuth of the source can also effect ear canal resonance seen when someone “cocks” the head to hear
• The IID is largest for tones 2000 and greater therefore this is the cue used mostly for localization of high frequency sounds
• When you are not at 0 or 180 one ear is unavoidably closer to the sound source even if only incrementally and brain can detect these variations.
• Intensity difference brain recognizes from head shadow is what the brain uses for localization and to make the person’s head turn.
• Head shadow is a LOSS in intensity. When the azimuth of the sound is 45 degrees sound will still get to ear with diff in intensity but if 2 sounds present alternately at 45 degrees sound still comes in right to left only slightly different but brain can still pick up the angle variation and look right or left depending on which side sound occurred.
• The IID is greatest for 2000 Hz and greater and so the intensity cue is used most frequently to identify where the sound is coming from.
• High frequency sounds have shorter wavelengths and so therefore high frequencies generate a higher intensity difference from one ear to the other because there is no wrapping only crashing and this lends to intensity cues.
Time Difference Cues used primarily for the localization of lower frequencies
The distance the sound travels to the far ear is greater than to the near ear when the azimuth if either 45 degrees or 90 degrees
What is the time difference for a 45 degree azimuth near ear = .4 msec sooner
What is the time difference for a 90 degree azimuth near ear .65 msec sooner
The brain uses time of arrival and phase differences ear to ear to localize sound
Below 1800 Hz pure tone wavelength exceeds the 7'' difference between the ears resulting in a phase difference of a sound at each ear
ITD Interauraltime difference
The ITD and phase differene of the sound between the ears can be enhanced by head movements
Minimal Audible Angle the smallest angular separation that can be perceived
MAA depends on the azimuth of the sound source
A much smaller MAA is achieved when the azimuth is 0 degrees
When the azimuth is 90 degrees 180 degrees or 270 degrees the sound source must be a greater distance apart in order to perceive the sounds as two separate sounds.
Most sounds we encounter in everyday life are complex rather than pure tones
Because most sounds we encounter in life are complex rather than pure tones we use these siimultaneously to localize sounds IIDs and ITDs
IID interaural intensity difference
ITD interaural time difference
The auditory system is part of a complex system of perception that includes these systems visual tactile olfactory and cognitive systems
Which four systems comprise the complex system of which the auditory system is a part visual tactile olfactory and cognitive systems
Both of these can be changed by moving the head IID and ITD
Determining the sound source under headphones requires use of an intracranial image
If you present identical pure tones under headphones listener perceives the sound as in the middle of the head
If you increase the sound of a tone in the right ear relative to the left listener perceives sound as moving to the right ear.
When the difference in intensity of sound at each ear reaches 30 - 40 dB the listener only perceives the sound in the ear where it is being presented with the greatest intensity.
Very small differences (approximately 10 microseconds) in the time of arrival at each ear under headphones can result in te perception of the sound reaching one ear sooner than the other.
How does the brain localize low frequency sounds? Below 2,000 Hz, the brain uses a time difference cue for localization.
If a sound is at 90 degrees azimuth to the right of you the sound arrives at which ear sooner it arrives at the right ear sooner than the left ear.
Time difference is greatest at which 2 degrees 90 degree and 45 degrees
Perception localization and comprehension all take place very fast
45 degree azimuth sound arrives at near ear how much faster on average than far ear? .4 seconds faster
90 degree asimuth sound arrives at near ear how much faster on average than far ear? .65 seconds faster
Brain looks for differences in time of arrival at each ear and phase difference at each ear
Average distance between ears is 7 inches so we know what the wavelength is for sounds below 1800
for 1800 Hz and below we use ITD and phase difference for localization cues
For 1900 Hz we use time and intensity differences for localization of sounc
MAA is smaller when you have an azimuth of 0 degrees
As most sounds are complex we are using IIDs and ITDs all time, to localize sound
We are not usually localizing by auditory stimuli alone, we use other senses such as visual, tactile, olfactory and cognitive systems too.
lateralization under headphones Intracranial image
Localization in sound field Extracranial image
Differential Sensitivity how much is the smallest change in a parameter of a stimulus such as frequency or intensity that must be made in order to be recognized by a listener
Differential Sensitivity also known as Difference Limen DL
Difference Limen DL- the differential sensitivity – smallest detectable change in a parameter of a stimulus
DL or Difference Limen is also known as JND just noticeable difference
JND = DL
Detection of small changes in stimulus magnitude is a crucial auditory skill for sound localization and speech understanding
Most studies of differential sensitivity DL or JND use two tones the standard tone and a comparison tone
The standard tone and the comparison tone are tones that differ just slightly in frequency intensity or duration
What are the three parameters that may be changed to detect JND or DL frequency
Why is it important to detect small differences in intensity frequency and duration these differences are used in speech sounds
When you have two phonemes you need to be able to perceive the formants in one vowel as different from formants in other vowels in order to perceive them as two different sounds
If you couldn’t tell the difference between /s/ and /sh/ it could lead to communication breakdown in speech understanding and sound localization so JND and DL are important.
You need differential sensitivity for all parameters in order to understand speech.
Weber found that the difference between 2 objects that could be detected was proportional to the value of the smaller object.
The amount of change in magnitude necessary for the change to be noticeable depends on the initial magnitude of the stimulus
If there were 10 items in a basket and you changed one the change would be noticed
If there were 100 items in a basket you would need to change 10 in order for the change to be noticed.
17th Century Fechner and Weber’s method of looking at differential sensitivity as a proportional change relative to initial magnitude of a stimulus S/S = constant(k)
In S/S = constant(k) what is K K = constant
In S/S = constant(k) what is S S = stimulus
In S/S = constant(k) what is   = change
In S/S = constant(k) what is this known as Weber’s Law or the Weber/Fechner law
In S/S = constant(k) Weber’s Law or Weber/Fechner Law holds best at mid frequencies NOT low ones (750 Hz - 2000 Hz)
Descrimination task you decide if two tones are the same or different
Differential sensitivity can be detected as a proportional change relative to the initial magnitude of stimulus
Delta S change in the stimulus divided by the stimulus
Delta S used to determine what is the smallest change in frequency intensity or duration that we can detect.
The smallest change in the frequency intensity or duration of a stimulus that we can detect depends on the initial magnitude of the intensity
DL or JND a.k.a the difference limen as it applies to frequency is known as DLF (subscript F)
DLsubscriptF is the smallest difference in frequency that can be judged by the listener.
DLF is a frequency discrimination task using same/different of standard tone and comparison tone presented at the same intensity for the same duration
DLF becomes smaller as the stimulus intensity increases
DLF becomes larger as the stimulus frequency increases especially above 1kHz
Most listeners are relatively insensitive to frequency changes at low LOUDNESS levels.
Overall sensitivity to some frequencies is better than others DLF values remain constant at 500 Hz – 4000 Hz with best sensitivity at 1000 Hz – 2000 Hz.
Best sensitivity 1000 Hz – 2000 Hz
At low frequencies the DLF can be as small as 1Hz.
DLF is the smallest difference in frequency that can be detected by a listener when presented 100 times
With louder sounds we can detect small differences in frequencies
As stimulus frequency increases the DLF gets larger
You want a small difference limen
With a louder or more intense sound our ability to distinguish frequency improves
Low loudness is low intensity
Most listeners are relatively insensitive to frequency changes as low intensity levels
There are two assessment methods for DLsubscriptI standard same/different comparisons and present a steady tone and vary its intensity
DLI is difference limen for intensity
Intensity discrimination DL subscript I DLI
DLI discrimination tasks standard same/different comparisons and present a steady tone and vary its intensity
Standard Same/Different discrimination task uses a standard tone and comparison tone of the same frequency
Steady Tone varied in intensity present a steady tone and occasionally vary its intensity for brief periods and see if the listener can detect the change
Research has show that the auditory system is sensitive to changes of .5 to 1.0 dB across a broad range of frequencies
Logarithms are used for measuring sound
A progressively larger change in sound pressure is required to increase SPL at higher pressures than at lower pressures a logarithmic concept
The change in sound pressure required to increase SPL from 80 to 81 is 1000 times greater than to increase it from 20 to 21 log function.
You can detect a 1dB change in 80 dBSPL signal but not in a 20 dBSPL signal
You can detect a 1dB change in 80 dBSPL signal but not in a 20 dBSPL signal due to the larger change in pressure required at 80. IT IS NOT THE SAME PRESSURE CHANGE.
Temporal Discrimination DLsubscriptT
DLT is a measure of temporal discrimination
In DLT the subject listens to two stimuli one of fixed duration and loudness level and a comparison tone of different duration and equal loudness level.
In DLT the subject listens to two tones of different duration and determines which was longest.
DLT is the smallest difference in duration that can be deteremined between the standard tone and comparison tone that results in correct judgements with what percentage of accuracy 75% accuracy.
Temporal Resolution = Temporal Auditory Ability
Temporal Auditory Ability = Temporal Resolution
The shortest time necessary for discriminating between signals = Temporal Resolution
Gap Detection Threshold = the shortest interruption of a signal that can be detected
To determine Gap Detection Threshold listener is presented with a very brief gap between stimuli that is varied in time and indicates if they hear one or two tones as the durations shortens.
Generally sensitivity to time differences improves as the length of the stimulus increases.
Temporal Resolution is the shortest gap after which two tones can be perceived as separate stimuli
Gap detection is best in mid frequencies 750 Hz – 2000 Hz
Masking is when the threshold of one sound (the signal) is raised by the presentation of another sound (the masker) in order to preclude the listener from hearing the signal in that ear.
Masking is keeping one ear busy so that it cannot participate in the test and the true abilities of the test ear are measured.
There is no universally accepted signal that serves as a masker
Maskers can be tones or noise depends on what needs to be masked
Threshold shift hearing is 20dB without water running in the background – water goes on and the hearing threshold increases to 50 dB creating a 30 dB shift meaning the water produced 30 dB of masking.
The test ear is the ear that is referred to as masked
You put the masking into the good ear you put the noise in the good ear to keep it busy
When you talk about the results you say that you tested the left ear for the test and talk about Left Masked Air Conduction. and talk about Left Masked Air Conduction.
Can do masking by both air and bone conduction
Bone conduction results are always non-ear specific because you can’t vibrate only one side of the skull.
Music masks the speech at a party unless you want to hear the music then the speech is the problem signal and masker can change.
Masking requires a narrow band noise use a filter to get rid of select frequencies.
A narrow band noise is used for masking
The ear that gets the tone is the masked ear
The results are ‘left masked’ if you are testing the left ear and putting noise in the right ear
Listen to music with a quiet fan in the background no interference of the fan noise with the music
Raise the level of the fan while holding the level of the music constant eventually the fan will interfere with the music
Distraction of the fan leads to masking of some of the weaker musical notes leading to the music becoming inaudible.
Present a pure tone at threshold and present noise at low level often hear both noise and tone
Gradually increase the level of the noise while holding the tone constant and eventually all the listener perceives is noise
The lowest level of NOISE that renders the tone inaudible = 0dB effective masking level or kneepoint
Level of the tone would have to be increased in order to make it audible of noise is at kneepoint
Straight lines that meet at a bend can introduce a noise with no effect on the one until a certain point no change in the threshold of a tone.
At some point the SPL of the noise reaches some critical value and the threshold of the tone will shift
At levels above the knee increases in noise will produce equivalent increases in threshold 1:1 relationship between noise and the masking effect each dB increase in noise produces an equivalent shift in the threshold of the tone
Kneepoint that level of noise above which any additional noise increase will produce an equivalent threshold shift.
A noise that is 10dB of effective masking will fall 10dB avove the kneepoint and produce a 10dB threshold shift in the tone
Threshold shift the numerical difference in dB between a threshold in quiet and a threshold in the presence of a masker.
The most efficient masker is the signal that provides the most masking for the least amount of energy
The most logical masker of a pure tone is another pure tone but then the listener cannot distinguish between the two tones
If with two pure tones the masker is of a different frequency this is called beats sensation
The frequency of the masker should be close to that of the signal but not too close
When the level of the masker tone is 60 dB and above its masking effect is much greater above the masker frequency than below it.
The masking effect spreads rapidly upward to higher frequencies as the intensity level of the masker is reached.
Upward Spread of Masking the masking effect spreads rapidly upward to higher frequencies as the intensity level of the masker is reached.
Narrow Band Noise NBN
Narrow band noise noise within a certain frequency range
The Narrow Band Noise is shaped through the use of a band-pass filter with a high and low cut off frequency
Frequencies inside of a narrow band noise are attenuated gradually as frequency becomes more remote from the cut off
The gradual attenuation in a narrow band pass noise is called roll off
Roll off is measured in dB/octave
Using NBN to mask pure tones uses the concept of a critical band
The concept of a critical band is that there is a range of frequencies around the signal or test frequency that contribute most to masking that frequency.
Only a narrow band of frequencies around a pure tone contribute to the masking of that tone
Frequencies outside the critical band do not Frequencies outside the critical band do not contribute to masking they only add on to the overall noise level.
When a tone is barely masked by noise the energy in the critical band is equal to the energy of the tone.
NBN reaches the 0 dB effective masking level at a lower SPL than white noise making it a more efficient masker.
NBN has energy distributed equally over a narrow restricted frequency range white noise has energy distributed over a broad range.
NBN is the preferred masker for pure tones
If the noise energy within the critical band is less than that of the energy level of the tone being masked then there is no threshold shift.
The 0 dB effective masking point (or kneepoint) of the noise is where the energy falling within the critical band first equals that of the tone.
From the 0 dB effective masking level on each dB increase in noise within the critical band will produce an equivalent increase in the pure tone threshold
NBN works well for masking tones but not for speech for masking speech a broader range of frequencies is needed.
Masking Special Cases simultaneous masking forward masking backward masking
Simultaneous masking the scenario where the masker and maskee are presented together
When the masker and the maskee are presented together as in simultaneous masking the time between the two must be brief less than 50 msec
Forward masking the masker is presented and ended just before the signal is presented
Backward masking when the signal is presented and terminated just before the masker is presented
Masking level difference until now masking has been discussed as presenting the noise and tone in the same ear
To find the 0 dB effective masking in that ear the level of the tone is adjusted until the tone is just inaudible (ipsilateral masking)
Take a tone below 500 Hz and add noise to the other ear (contralateral masking) and the tone becomes inaudible in the test ear without changing the signal or noise in that ear.
Add stimulus tone to the other ear now both ears have tone and noise (masker) and the tone is not heard by either ear.
The phase of the signal or masker must be changed by 180 degrees in either ear for the stimulus to become audible again.
The MLD (masking level difference) is determined by the amount of reduction in masking provided by a particular condition referenced to the condition that provides maximum masking.
This condition is almost always when the signal and masker are in phase binaurally. (SoNo)