Masking in Clinical Audiometric Testing: A Tutorial



get ready as an inductor and clinical supervisor I often see students completely thrown by the concept of masking when I point out that masking is needed during testing they freeze up and get discombobulated I'm here to show you that masking is nothing to be afraid of once you break it down the first thing we need to understand is why masking is so important to demonstrate this let's meet my nieces and nephews okay so behind me a true boom that represent your right and your left auditory pathway the doorway represents two ear canal and my teenage niece in here well she represents you right cochlea Nova in the next room is my teenage nephew representing the left cochlea like most teenagers they do not want to be bothered by their siblings and those little munchkin they represent sound coming out of your earphones when testing and they are about to unleash their fury on the right cochlea go get her kids whoo this doorway represents the seal from your headphone so when using insert earphones during audiologic testing make sure it is properly placed with minimal air leaks or if you're using headphones make sure there are no leaks around the ear cushion okay so let's get a good seal here on a door / earphone here and go check on the left cochlea so the sound that is intended only for the right cochlea is being propagated through the wall and it's affecting the left cochlea how much the wall blocks out sound represents the interaural attenuation of sounds and this wall represents you get that your head and skull potato there's just no escaping them is there so why am I telling you this now that you know that sound propagates through the skull it is very important that we audiologists test the ear that we want to test for example we can't have the left cochlea knowing that we're going to throw them a party tomorrow what you guys are throwing me a party oops see what I mean excuse me a minute we're not throwing your party we're writing with sharpies duh okay I'm not gonna have you listen to this there you go okay so we can talk about him all we want right so why is that it's because I'm masking the left cochlea over there keeping them busy so I can discuss all the important party details with the right cochlear over here now a long time ago nifty people with a knack for coot that calculated the attenuation of intensity by a noggin as sound travels from one ear over to the other now there's something that you'll notice that the attenuation of sound varies as the function of dump the data that transducer now it appears that it takes less intensity to rattle the skull so to speak with the super oil headphone than it does with the insert now why is that well that answer basically comes down to two words surface area so let's say that this speaker here is the transducer we are using to test the giant ear and this vise here represents bone when you use super aura hub bone there is a greater surface area contact between the transducer and the ear slash bone so when you play a tone voilá the bone vibrates however when you use an insert earphone the speaker is housed away from the ear because there is less contact between the speaker and the ear you have to turn the intensity level up before you can rattle that right so now that we know why we masks now the question is when do we masks now this is where we refer back to the very same people with the knack for acoustics so we know the interaural attenuation value for each frequency and each headphone don't sweat it I'm not going to make you memorize all of these numbers the only thing that you have to know it absolute certainty is the minimum attenuation value for each headphone so for the headset it's 40 the insert earphones is 55 and for bone it's zero these three numbers you've got to memorize memorize this hi my name is Rachel I'm the audiology doctoral student it's nice to meet you it's nice to meet you too Rachel so when I'm testing a patient where do we start well if you answer with a thorw case history you are correct now there are many questions that need to be asked during case history but from a masking perspective the two very important questions one is which ear is better than the other and two is a middle ear disorder that could possibly cause a conductive or mixed hearing loss knowing the answer to these two questions will help you decide whether you need two masks during testing now I personally follow my case history interrogation with a Tosca P and immittance that includes tympanometry and acoustic reflexes before I put the patient into the booth for testing you see determination of masking is based on a because when you start the hearing test you start with air conduction testing and while you're testing by air conduction you are thinking about the cochlea making assumptions about the bone conduction thresholds the entire time starting with immittance allows me to get as much information about the middle ear status before we even touch the audio meter now that flat right temp and absent acoustic reflexes mean you're going to have your work cut out for you in there Oh chin up let's do this thing okay so we have them in the booth and we have chosen a transducer the trusty super aural headphone now what the interaural attenuation of the super oral head found 40 dB that's right 40 DB you can start your testing with either pure tone audiometry or the srt speech reception threshold testing I personally like to start with the SAT because I could quickly confirm the better ear and also how much of a difference there is between the two years first you get the SRT value of the reported better ear and then you switch over to the poorer ear if the difference between the SOT values is greater than 40 DB then you have the math now before we go any further I want to make sure that you understand an assumption that we are making here when we decide whether we need to or do not need to mask we are comparing the air conduction threshold of the test ear to the bone conduction threshold of the non-test ear bone conduction the bone conduction threshold that we haven't measured yet see now you understand why I like to do a mitten first because I measured normal middle ear function in the good ear and because there was no report by the patient of pain pressure drainage from that good ear I'm assuming that phone equals air conduction threshold in that ear so the difference between this SI T value and the assumed bone conduction SOT value is less than 40 dB now let's look at another scenario let's say a patient here has a severe hearing loss in his right ear due to a childhood illness but he comes to you because he has pain in his left ear accompanied by oil pressure and ten so you look in his ears and you see that it looks normal and healthy on the right side but then when you're looking at left ear it looks like the signs of a middle ear infection which is confirmed by a flat temp this is left ear still has reported better year we begin measuring the s our teeth in that ear and get a value of 40 dB then we switch over to the right ear and measure the srt of Matier and we get a value of 7 dB now the difference between the air conduction values is 30 dB but what can we assume about the bone conduction threshold of the last year well I would assume that is either normal or possibly a slight loss based on his case history and tympanum ectric results remember knowing when the mask is based on the difference between the air conduction value of the ear that you are testing and the bone conduction value of the opposite ear so as you can see the difference between these two values is greater than 40 dB now the math game rules and techniques of speech reception threshold testing are exactly the same as the masking rules and techniques for pure tone testing which we're going to get to in a second now back to reality so my wonderful student here has moved on to pure tone audiometry and I just completed testing of the good left ear and she's now ready to move on to that right ear all right we just finished with the left ear now we're going to go to the right ear you're going to hear some static in that left ear but just ignore that static and only press the button when you hear those beats okay she's starting with a thousand Hertz it should get 45 DB as the threshold the threshold in the left ear is 25 DB so the difference is 20 I'm assuming that there's no conductive component in the left ear because of normal admittance she's going to keep testing until she find the difference of 40 DB or greater ahoy now there's a big difference 65 minus 10 equals 65 DB so now the question is does he hear it in his right ear or is the sound being transferred to his left ear via bone conduction there's only one way to find out masking when masking the first question is how much narrowband noise do we play to the good ear well that's a very good Rushton there are many various rules for calculating the introductory masking level and in the end if the plateau method of masking is properly executed we will all arrive at the same outcome it's just like the different driving directions to get to a destination however like direction we want to pick the world that gets to the proper outcome in the most efficient manner so my rule of thumb for determining the appropriate minimum masking levels is death when the threshold of the non-test ear is normal or less than 15 decibels just too dense the masking noise at 30 when it is between 15 and 60 just at 15 DB to the threshold value when it is greater than 60 DB add 10 DB to account for possible recruitment so the threshold and the good ear is 10 decibels so that means the masking level would be 30 what do you think is that correct you get that 30 DB start your level in order to demonstrate the plateau method of masking we're going to have the Battle of the ears where you get to see the actual cochlea responses so Rachel here is going to prove that narrowband masking noise to the left ear at 30 decibels then presents a tone to the right ear at 65 no response so she's going to increase the level to the right ear to 70 and present the tone we have a response so which your is it raise the level of the masking noise to 35 and present 70 again no response so this means that the good left ear was responding to that home all along so now raise the level of the tone to 75 and we have a respond raise your noise to 40 now and we still have a respond 45 and we have at least fun this is the right ear the right ear Mack threshold is 75 decibels that's three responses in a row so it must be the right ear responding because we have achieved a plateau of three straight responses when the noise is increased 15 decibels now let's do another one this time keeping you in add to the cochlea responses cochlea you can meet now okay so the unmasked response is coming in at 50 decibels at 500 Hertz and the left ear is at 10 decibels a 50 minus 10 is 40 so we have two masks hmm so where would I stop my masking levels 30 dB very good who taught you to be so wise okay let's see if I can do this nasty noise on in the left ear at 30 DB present the tone to the right ear at 50 DB no response okay I'm going to turn my tone up to 55 DB present again I got a response okay turn the noise up in the left ear to 35 DB present the tone again at 55 DB to the right ear another response so I turn up the noise to 40 DB and present the tone again at 55 I got another response I turn the noise up to 45 DB and present again at 55 I got the plant oh I have never been more proud the next thing we're going to do with word recognition testing because word recognition is performed at super threshold and because speech with a broadband signal and because most hearing losses tend to be sloping configurations we generally always need a mask to determine masking level first follow this easy formula you take the presentation level of the test ear then subtract the interaural attenuation of the transducer that you are using then add 20 DB as a buffer what you'll notice here is that these two numbers are constants if you are always using the same transducer so if your clinic always uses headphones then the masking level equals the presentation level minus 20 if you use insert then the masking level equals the presentation level minus 35 decibels once you have derived your masking level you have to take a look back at the audiogram and consider your assumption you made about the bone conduction threshold of the non-test ear okay so for this example we are presenting the sentences to the left ear at 65 decibels so 65 minus 20 is 45 now looking at his audiogram he can't hear 45 DB and his right ear very well now can you know so do we say that we don't have the mask hmm so what do we assume about the bone conduction threshold of the right ear well based on is abnormal admittance we're assuming that the conductive component in that right ear so we do have to mask we have to make sure that sound is not being crossed over to that right cochlea so let's prevent the broadband masking level to the right ear at 15 DB above the SI T at 65 decibels okay so now we move on to bone conduction testing what is the interaural attenuation of bone conduction 0 DB that's right technically the interval attenuation of bone conduction can vary based on head sizes and skull thickness so it's not truly 0 DB but it's close enough that we can assume that is 0 but before we go into masking for bone conduction we have to consider one thing so mic here has normal hearing so his air conduction threshold is equal to his bone conduction threshold now my phenomenal student out there had just found out that the phone conduction threshold at 250 Hertz is 10 DB so now what I'm going to do is I'm going to put a headphone over his left ear as if I'm going to mask now Rachel find that threshold again okay so what is it so now it's negative 5 why is that it's because of the occlusion effect that happens in the low frequencies the occlusion effect occurs in people who have normal hearing or sensory neural hearing loss loss okay let me put it this way I need you to count to 3 count with me 1 2 3 come on let me hear you 1 2 3 now I want you to stick your fingers in your ears and count again 1 two three did you hear that your voice instantly got louder when you plugged up your ears now why is that well that because of the occlusion effect now if you didn't hear your voice get louder the second time you counted or you just may have a conductive loss because your middle and your system is literally acting like clubs in your ears so why am I telling you this I'm making you count to three with fingers in your ear well because that's a seclusion effect you have to add additional masking noise and the low frequencies to negate its effect the additional masking values are 20 dB at 250 Hertz 15 dB at 500 Hertz and 10 DB at a thousand Hertz so if you I'm asking for bone conduction at 250 Hertz and you start with 30 DB of masking noise you should add 20 DB and start with 50 DB of masking noise due to the occlusion effects otherwise you are under masking so memorize these correction factors for correct song conduction masking okay so back to our example so first let's get the unmasked bone conduction threshold she has placed the oscillator behind the patient's left ear and is ready to go again we're going to allow you the viewer to see the actual cochlea responses down there we have a response now which gear do you think that is it's okay to not know because sometimes it's actually both cochlea is responding so which year do you think it's likely to be the last year good why and don't say it's because the oscillator is behind the left ear because it's within 10 DB of the air conduction threshold of the left ear fair enough now the truth is we don't actually know which it responded unless we act the patient which year they heard it in the perfect the bone conduction testing is to determine the site of lesion whether the lesion is in the middle ear system the inner ear system or both because either the better of both cochlea are responding within 10 DB of the left air conduction respond we can safely assume that the left ear is a – hold and take the response at the left ear to document this decision I think a more accurate way of documenting this respond is to use the unspecified symbol but this symbol isn't widely used so maybe I should start a revolution okay so rachel has finished all the bone conduction testing and has found that the left ear has a sensor in our law good job now we're going to move on to the testing of the right bone conduction with masking okay so where do you want the noise last year and where does the bone oscillator go behind the right ear very good now I'm asking for bone conduction is just like masking for air conduction except that you have to take the occlusion effect into account so for this example given the case history the right immittance result and the good word recognition score we're going to assume that this loss in the right ear is a mixed loss so you don't have to correct for the occlusion effect because they're already occluded very good now go get those plateaus nicely done – look at that beautiful audiogram you did that like a champ I did it over masking tends to happen in conductive of mixed losses and it is what causes the masking dilemma masking dilemma occurs when there is a large conductive component in both ear so that you simply cannot mask effectively at all so let's suppose I start air conduction threshold testing and I get the values ranging between 70 and 105 across the frequencies then I move on to bone conduction testing unmasked responses of 15 to 20 DB across while 70 minus 15 is 55 decibels and that is greater than my interaural attenuation so I have the mask so I put in about 80 DB of noise in the right ear and present the tone no response increase the level of the tongue presented to the bomb by 5 DB and respond increase the noise no respond increase the tone respond noise no response what is happening the sound of the masking noise are crossing back over to the test ear as you increase the noise creating a masking dilemma so you simply take the unmasked respond label it with an asterisk and stays that you cannot mask without over masking you have to confirm this at all frequencies and you have a masking dilemma all around even with the air conduction threshold now you dress completed a crash course of review and masking the last thing you need to remember is to always validate your audio metric findings by making sure everything agrees from case history to emitting the pure tone in speech audiometry because you are about to place that audiogram into the patient's medical record alright looks good you another one okay you you

34 comments

  1. The beginning of it was quite aggressive on my ears, I even had the sound way down.  Perhaps they are trying to get more hearing clinic clients by promoting hearing loss with the intro of this video.

  2. hi amanda i am going for hearing aid dispensing and i recently took the written exam and failed. one of the main things that killed me were case study questions i.e how you would fit based off of an indivisual needs binaural vs monoaural cros bicros different diseases affecting different wrs srt scores and what hearing aid is appropriate for each loss range was wondering if you can make a video on how to properly do a case study and break it down? i would appreciate it! or at least send a link where i can learn from a website that shows how to break one down? thanks amanda!

  3. This explains why some audiologists have told me I have some hearing in my right her whilst others tell me I have none. An audiologist tried to explain to me that the other audiologists had not done proper testing and that what they thought was me hearing in my right ear was actually just conduction to my left ear.

  4. wait so
    If the unmasked bone conduction of left ear is normal for example, but the unmasked air-conduction for the right ear shows a moderate CHL and the difference was 40dB and they're wearing inter-aural headphones, would you mask bone conduction for the right ear?

  5. Wish I could have you as my proff! Great video, really helped me out. I would love if you made another video using insert headphones

  6. jay KIm u r so ignorant, anyways Amanda thank you so much for such an interesting explanation of masking, excellent job

  7. hi iam Tanu Garg intern in northern regional centre india Delhi iwant to ask how you have taken during ac testing g masking level to be 30 at 250hz acc.to which formulas

  8. This is fantastic! A brilliantly clear (and fun) explanation of a gnarly topic. Yours sincerely, a very fresh (and confused) Audiology student…

  9. Nice video Amanda! I have a few questions about the speech-in-noise test. Is it the same as a word recognition test, except that you add in background noise? But how do you decide the level of background noise? And is the noise available to both ears or just the test ear? Lastly, do you still need masking in that case? It will be great if you can help:) I can't find any proper explanation online…

  10. Amanda, this is just terrific. I hope you don't mind, I'm assigning it as a home supplement to my undergrad students to review, because you do a great job of explaining in a way that supports my mantra in class that Masking Is Fun. 🙂

  11. Hi Amanda.  I would very much like to watch this video. Unfortunately, the subtitles are not making any sense.  Do you by any chance have a transcript.   Thanks.  Dee

  12. Thank you so much. The ways you reviewed masking really helped me understand the concept. Could you make a video explaining the plateau method a little bit more? 

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