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(BQ) Part 2 book “Audiology science to practice” has contents: Masking for pure-tone and speech audiometry, outer and middle ear assessment, evoked physiologic responses, disorders of the auditory system, screening for hearing loss, hearing aids, implantable devices, vestibular system.
9 Masking for Pure-Tone and Speech Audiometry After reading this chapter, you should be able to: Understand why the non-test ear (NTE) needs to be masked in some cases in order to obtain true thresholds in the test ear (TE) Know what is meant by interaural attenuation (IA) and the minimum IA values used for each transducer when making decisions about the need to obtain masked thresholds Recognize, from the unmasked thresholds, when masked thresholds must be obtained; apply the decision-making rules for masking when testing by air conduction (AC) using supra-aural earphones or insert earphones and by bone conduction (BC) Describe the types of maskers used for pure-tone and speech testing Dene effective masking (EM) and how the maskers are calibrated and used with the audiometer Describe the occlusion effect (OE) and why this needs to be considered when masking for BC Describe two advantages of insert earphones over supra-aural earphones as they relate to masking Dene what is meant by a masking plateau and how much of a plateau is appropriate Discuss why the width of the plateau is smaller when there is a potential bilateral moderate conductive loss Dene overmasking and masking dilemma, and recognize situations in which these may occur 10 Apply the specic steps for AC and BC masking using the plateau method for a variety of unmasked audiograms 11 Apply the rules for determining if masking is needed for speech testing, and select adequate amounts of maskers for speech testing 177 178 AUDIOLOGY: SCIENCE TO PRACTICE The process of putting noise, called a masker, into the non-test ear (NTE), while measuring responses from the test ear (TE), is called masking (or clinical masking) The threshold obtained in the TE is called the masked threshold, and implies that the masker was delivered to the NTE In order to be able to deliver a masker into the NTE, a two-channel audiometer is needed so that the test sound (tones or speech) can be routed to the TE through one channel, and the masker can be routed to the NTE through the second channel Most clinical audiometers automatically route the masker to the NTE when masking is selected In Chapter 7, some basic principles of masking were presented so that you would understand why unmasked or masked symbols are used on an audiogram to represent a patient’s pure-tone thresholds To be clinically useful, audiometric measures are expected to be true representations of the TE and not a reflection of hearing by the NTE In Chapter 7, the principles of masking were presented as they pertained to thresholds for pure tones; however, as you will see in a later section of this chapter, when doing speech testing you must also be cognizant of the possible need for masking to prevent the speech signals from being heard in the NTE This chapter provides details on when masking is needed and how to perform masking The first part of the chapter will focus on masking for pure-tone thresholds and the second part of the chapter will focus on masking for speech tests There are many testing situations in which the sound presented to the TE can set up vibrations in the skull that potentially could be picked up by the NTE: When testing by bone conduction (BC) at any intensity level or when testing by air conduction (AC) at moderate and higher intensity levels, the sound vibrations can occur in the bones of the skull and, therefore, are able to be received by both cochleae through bone conduction This becomes especially problematic when the NTE has better hearing than the TE, since the patient’s response to the sound delivered to the TE could actually be a result of the patient hearing the sound through bone conduction in the NTE When the signal delivered to the patient’s TE is audible in the NTE, it is referred to as cross-hearing Keep in mind that crosshearing to the NTE (during AC or BC testing) always occurs by bone conduction (Studebaker, 1962; Zwislocki, 1953) Whenever cross-hearing could occur, masking of the NTE will be needed To prevent the patient from hearing the sound that may be heard through cross-hearing in the NTE, a masker (noise) is delivered to the NTE The patient is instructed to respond only to the pure tones or speech signals in the ear being tested, and to ignore the noise that he or she will hear in the other ear INTERAURAL ATTENUATION It is fairly easy to understand that when the bone vibrator is on the mastoid of one ear, the other cochlea is also being stimulated because it is also imbedded in the skull However, are both ears receiving the sound at the same intensity? In other words, is there some attenuation of the sound in the NTE compared to the TE? Interaural attenuation (IA) is a term that is used to quantify the difference in the level of the signal presented in the TE (by AC or BC) to the level of the signal that occurs in the NTE (by BC) Another way of thinking about this is to ask how much does the level of the signal in the TE have to be before it is capable of being heard in the NTE (by BC)? Furthermore, if the NTE is capable of hearing the sound presented to the TE (i.e., cross-hearing occurs), masking the NTE would be needed in order to establish the true thresholds in the TE Ranges of IA values have been determined for different transducers by several studies (e.g., Chaiklin, 1967; Coles, 1970; Sanders & Rintleman, 1964; Sklare & Denenberg, 1987; Studebaker, 1967) For BC testing, the IA is considered to be dB, that is, the BC sound is the same level in both ears For AC testing, the level of the pure tone presented to the TE that can cause vibrations of the skull are different for supra-aural earphones and insert earphones; insert earphones have a higher IA The difference is primarily dependent on the relative surface area of the skull that is exposed to the sound from the different AC transducers; supra-aural earphones have a larger area of exposure to the skull than Masking for Pure-Tone and Speech Audiometry insert earphones Figure 9–1 shows a comparison of the averages and ranges of IA values for supra-aural earphones and insert earphones The IA varies somewhat across frequency, and will also vary across patients; however, for clinical purposes, minimum IA values are adopted instead of mean IA values to ensure that you not miss masking someone with an IA below the average The minimum IA for supra-aural earphones has been widely accepted as 40 dB This means that, when testing with supra-aural earphones, vibrations of the skull can occur at levels greater than or equal to ( > ) 40 dB HL For insert earphones, a single minimum IA value has not yet been universally accepted or described in any standards As you can see in Figure 9–1, the IA values for insert earphones are greater in the lower frequencies than in the higher frequencies The IA values for insert earphones can also vary depending on depth of the earphone insertion; if not inserted deep enough, the IA may be less Some audiologists choose to use a different minimum IA depending on the frequency when using insert earphones However, the authors of this textbook have adopted a minimum IA for insert earphones of 55 dB for all frequencies This is a conservative, yet reasonable value, and simplifies the concept of masking with insert earphones by adopting one minimum IA value for all frequencies.1 The following are the minimum IA values adopted for this textbook for the different transducers: IA for bone vibrator = dB FIGURE 9–1. Comparison of interaural attenuation val ues for supra-aural earphones and insert earphones Source: From Sklare and Denneberg, 1987, p 298 Copyright 1987 by Lippincott Williams & Wilkins will have an IA higher than the minimum, but you not know, nor have the time to measure the IA for each patient However, in many cases, you can see from the unmasked thresholds on an audiogram that the patient’s IA is higher than the minimum when you compare the unmasked AC threshold in the TE to the BC threshold in the NTE For example, if a patient has an unmasked AC threshold in the TE of 65 dB HL and a BC threshold in the NTE of dB HL, that patient’s IA is at least 60 dB (and may even be more than 60 dB) However, you would still make the decision to use masking because 60 dB is greater than the minimum IA for either of the AC transducers IA for supra-aural earphones = 40 dB IA for insert earphones = 55 dB The reliance on minimum IA values allows you to decide if masking is necessary, but does not necessarily mean that the patient’s actual IA is at the minimum level In fact, most patients This conservative minimum is based on the lowest IA, which occurs at 2000 to 4000 Hz For lower frequencies, the minimum IA is at least 65 dB Some audiologists may use IAs higher than the 55 dB minimum IA adopted for this textbook MASKERS It is important to remember that the masker is always delivered by an AC transducer If the masker was to be delivered by a BC transducer, then the masker would always be heard in both ears, making it impossible to get a true response from the TE But by presenting the masking noise with an earphone, there is a range of masker levels (at least 40 dB HL with supra-aural earphones and at least 55 dB HL with insert earphones) that 179 180 AUDIOLOGY: SCIENCE TO PRACTICE can be applied before the cochlea of the TE is stimulated by the noise In other words, using insert earphones to present the masker to the NTE would allow at least 55 dB HL of noise to be used before there is any possibility of crossing over to the TE The masking noises used in pure-tone threshold audiometry are called narrowband maskers (or narrowband noises) For each of the audiometric test frequencies there is a corresponding band of noise (one-third octave wide) centered around the test frequency Depending on the frequency being tested, you would select the appropriate narrowband masker For example, if a 1000 Hz pure tone is being presented to the TE, a 1000 Hz narrowband masker would be presented in the NTE When masking is used for speech testing, a speech masker is used instead of a narrowband masker The speech masker is a broader spectrum noise that encompasses a range of frequencies important for speech recognition Most audiometers automatically set the masker to the selected test stimulus The maskers are calibrated in terms of their effective masking levels Effective masking is a calibrated amount of noise that will provide a threshold shift to a corresponding dB HL for the stimulus centered within the noise (Sanders & Rintleman, 1964; Yacullo, 1996, 2009) For example, 30 dB HL of effective masking for a pure tone will elevate the AC threshold of the corresponding pure tone to 30 dB HL In practice, effective masking makes the signal no longer audible The required amounts of noise that correspond to dB HL of effective masking for each audiometric test frequency and speech are specified by the American National Standards Institute (American National Standards Institute [ANSI], 1996, 2010) The ANSI effective masker levels are built into the audiometer (just as for dB HL for the pure tones) In this way, the attenuator dial of the audiometer channel used to deliver the maskers corresponds to the number on the dB HL dial for each needed level of effective masking that is presented to the NTE To illustrate, if the attenuator dial for the masker is set to 40 dB HL, it means that the masker can effectively elevate/ mask the AC threshold for the test signal (pure tone or speech) to 40 dB HL when presented in the same ear The actual amount of the threshold change that occurs with the masker will depend on the patient’s threshold For example, if the patient’s AC threshold is 30 dB HL, then putting in a 40 dB HL effective masker will elevate the patient’s threshold to 40 dB HL, but the threshold change is only increased by 10 dB (40 dB HL effective masking minus 30 dB HL threshold) As you will come to see, it is very important to keep in mind that when you increase (elevate) the AC threshold in the NTE with masking, you also increase the BC threshold by the same amount, but not necessarily to the same dB HL For instance, in cases where there is an air–bone gap in the NTE, the air–bone gap will remain As an example, suppose the AC pure-tone threshold in the NTE is 50 dB HL and the BC threshold in the NTE is 30 dB HL (20 dB air–bone gap) When a masker is presented to the NTE by AC with an effective masking level of 60 dB HL, the AC threshold (in the presence of the masker) in the NTE will be elevated to 60 dB HL (a 10 dB increase in threshold) and, therefore, the BC threshold in the NTE will also increase by 10 dB to 40 dB HL (still a 20 dB air–bone gap) As mentioned earlier, the masker is always presented to the NTE by an AC transducer When testing for AC thresholds with insert earphones or supra-aural earphones, the sound is presented to the TE through one of the earphones and the masker is presented to the NTE by the other earphone When testing for BC thresholds, the bone vibrator is placed on the mastoid of the TE and the masker is presented to the NTE by an insert earphone or a supra-aural earphone If the masker is presented using a supra-aural earphone during BC testing, the other earphone on the headset is placed on the temple next to the eye on the side of the TE CENTRAL MASKING Central masking refers to a small elevation in the threshold of a signal in the TE that occurs when masking noise is presented to the NTE Central masking may occur even though the level of the noise, either narrowband noise or speech noise, is considerably less than any IA and, therefore, not audible in the TE The source of this small masking effect is unknown, but is assumed to be due Masking for Pure-Tone and Speech Audiometry SYNOPSIS 9–1 ll ll ll ll ll ll ll ll ll The process of putting noise into the non-test ear (NTE), while measuring responses from the test ear (TE), is called masking The threshold obtained in the TE is called the masked threshold In order to deliver a masker into the NTE, a two-channel audiometer is needed so that the test tones or speech can be routed to the TE through one channel, and the masker routed to the NTE through the second channel Testing anytime by bone conduction (BC), and testing at moderate to high levels by air conduction (AC) produces vibrations in the skull that can stimulate, through BC, both cochleae Interaural attenuation (IA) is the difference in the level of the signal (by AC or BC) presented to the TE, compared to the level of the signal that occurs in the NTE (by BC) The recommended minimum IA values for the different transducers are: {{BC IA (with bone conduction vibrator) = dB {{AC IA (with supra-aural earphone) = 40 dB {{AC IA (with insert earphone) = 55 dB Cross-hearing can occur when the difference between the presentation level of the sound in the TE (by AC or BC) and the BC threshold of the NTE is equal to or greater than the minimum IA A noise masker is a sound that is delivered to the NTE that covers/obscures a sound that may cross over to the NTE, thus making it inaudible (masked) Maskers used in audiometry are either narrowband noises when masking for pure-tone thresholds or speech spectrum noises when masking for speech tests Central masking is a small (5 dB) threshold shift in the pure-tone or speech threshold that can occur in the TE when masking is presented to the NTE Central masking is due to some (unknown) effects within the central auditory system The small threshold shift is not of any real clinical signicance to some central nervous system reaction to the masker (Konkle & Berry, 1983; Liden, Nilsson, & Anderson, 1959; Yacullo, 2009) The amount of threshold elevation in the TE due to central masking is only about dB HL for pure tones or speech testing The small effect of central masking can be expected during the masking process, but is generally not of any significance WHEN TO MASK FOR AIR CONDUCTION PURE-TONE THRESHOLDS For AC pure-tone threshold testing, cross-hearing will occur when the IA is exceeded and the pure tone reaching the NTE is greater than the BC threshold of the NTE The decision on whether to obtain masked thresholds can be determined by comparing the AC presentation level in the TE to the unmasked BC threshold in the NTE; if the difference is greater than the minimum IA for the specific transducer, then masking would be needed In clinical practice, however, masking for AC is often done before obtaining the BC thresholds because it is more efficient to complete the testing of both ears while the earphones are in place, instead of switching back and forth between AC and BC for each ear In that case, you can often make your decision to mask for AC testing based on an “assumed” BC threshold of the NTE In many cases, your assumed BC 181 182 AUDIOLOGY: SCIENCE TO PRACTICE thresholds of the NTE can be based on other information/test results (e.g., immittance measures) In cases where the difference between the AC thresholds between the two ears is greater than or equal to the minimum IA, you can assume that the BC threshold of the NTE would be at the same or better level than the AC threshold of the NTE, and the decision to mask would still hold However, it is important to keep in mind that your assumed BC threshold in the NTE may turn out to be incorrect, and you may need to go back and find the masked AC thresholds after the BC thresholds are obtained: For example, if the difference between the AC thresholds of the two ears (AC TE compared to AC NTE) is less than the minimum IA you may decide that masking is not needed; however, after testing by BC, you may find that there is enough of an air–bone gap in the NTE so that the AC threshold in the TE compared to the measured BC threshold in the NTE exceeds the minimum IA, and retesting the AC threshold with masking would be needed Decisions to mask based on comparing AC to AC of the two ears is only appropriate if the difference is equal to or greater than the minimum IA; if the difference is less than the minimum IA, the decision to mask may need to be delayed until the actual BC thresholds of the NTE are known Figure 9–2 shows some situations to illustrate when AC masking thresholds would be needed or not (see figure legend for explanation) The general rule for deciding that masking is needed for AC testing is: Whenever the difference between the unmasked AC threshold of the TE and the assumed or measured BC threshold of the NTE is > 55 dB for insert earphones (or 40 dB for supra-aural earphones), masking is needed to rule out the possibility that the AC threshold is coming from the NTE (by BC) WHEN TO MASK FOR BONE CONDUCTION PURE-TONE THRESHOLDS For BC pure-tone threshold testing, cross-hearing to the NTE is a frequent problem, and can occur in the following two conditions: (1) The AC threshold in one ear is >15 compared to the AC threshold in the other ear; and (2) there is the appearance of a potential air–bone gap for both ears As discussed earlier, a 10 dB air–bone gap is typically not considered clinically significant, so masking would not be needed In both of the above conditions, since the IA for BC is dB, you will not know which ear is represented by the unmasked BC threshold In fact, the unmasked BC symbol only represents the side on which the bone conduction vibrator was placed Figure 9–3 shows some situations that illustrate when BC masked thresholds would be needed or not (see figure legend for explanation) The general rule for deciding that masking is needed for BC testing is: Whenever there is >10 dB difference between the unmasked BC threshold and the AC threshold of the TE (an apparent air–bone gap), masking is needed to rule out the possibility that the BC threshold is coming from the NTE APPLYING THE RULES FOR PURE-TONE MASKING Figure 9–4 shows three examples of audiograms with unmasked thresholds Each example has a table that indicates (+) where masked thresholds would be needed In these examples, you should be able to see where the above rules were applied to decide which thresholds for AC or BC would have to be reestablished using masking (masked thresholds) Try covering up the tables and see if you come up with the same answers In Figure 9–4A, you not know if the true right ear AC thresholds are the same as those shown by the unmasked AC thresholds or whether they are worse as a result of cross-hearing to the BC of the NTE Of course, the answers depend on which transducer: For example, from 2000 to 8000 Hz, masking would be needed for supraaurals but not for inserts because of the different IA values Applying the rule for BC masking in Figure 9–4A, you can see that the differences between the right ear unmasked AC thresholds and the unmasked BC thresholds are each >10 dB Supra-aural earphones O O