Since the goal is to program the patient's hearing aids to process speech effectively, many believe that speech signals are best suited to determine how the hearing aid will work in the real world. (Scollie, 2003)
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Frequently Asked Questions

Over the years we have found there are topics that are important to address, but may not have been covered in a formal publication. This Frequently Asked Questions section addresses as may of the topics as possible. If you have any further questions or topics to discuss, please contact us at dsl@nca.uwo.ca and we will provide a reply and post it on the website so others can access the information.

Click on a link (bullet point) below to see the answer to the FAQ.

Getting Started The Real-Ear-to-Coupler Difference (RECD) The DSL m[i/o] v5 Targets Pediatric Hearing Aid Fitting Verification Getting Started

1) How are auditory brainstem response (ABR) threshold estimates used in hearing aid fitting? [back to top]

It is possible to prescribe and fit a hearing aid to a young infant based on data obtained from an ABR assessment. However, it should be noted that the thresholds estimated from the ABR are typically higher when compared to behavioural thresholds. In fact, they can be as much as 20 to 30 dB higher, depending on frequency (Stapells 2000). This is a significant difference in terms of intervention. Therefore, a correction is applied to the ABR threshold estimates to better predict the behavioural threshold. This corrected ABR shall be referred to as Estimated Hearing Level, eHL, to distinguish it from a typical ABR reference in nHL. Some ABR systems have the correction imbedded in it as part of it's calibration. For those that do not, a correction must be applied prior to the hearing aid prescription being calculated.

In DSL v5, the clinician can choose to enter the nHL values and use default or user defined corrections. Or, the clinician can enter eHL values into applications of DSL v5. This would have an eHL reference and no further correction would be applied. For more detailed information on this topic, please refer to Bagatto et al, 2005.

2) How are the corrections applied to the ABR values? [back to top]

The frequency-specific corrections are subtracted from the ABR values. Since previous research has shown that ABR threshold estimates (nHL) are typically higher than behavioural thresholds, the correction value is subtracted to provide a better estimate of behavioural thresholds (eHL).

nHL - Frequency Specific Correction = eHL

The Real-Ear-to-Coupler Difference (RECD)

1) It can be a challenge to measure the RECD on a young child. What are some strategies for measuring an RECD with this population? [back to top]

  1. Premeasure the probe tube (15 - 25mm from intertragal notch) and measure the coupler portion of the RECD before approaching the child for probe tube insertion. This will save time and cooperation needed from the child.
    2. figure 1
  2. If the child has a personal earmold, use it to obtain a more customized insertion depth by running the probe tube along the bottom part of the earmold. Be sure the medial end of the probe tube extends approximately 3 to 5mm from the sound bore of the earmold. Mark the probe tube where the most lateral portion of the earmold meets the tube. Figure 1. Insert the tube until the mark meets the intertragal notch.
  3. If you have some clear wrap or soft surgical tape, you can connect the probe tube to the earmold and insert the unit simultaneously. To the child, this may not even seem like a different procedure, but that you are merely inserting his earmold.
  4. Toddlers may be curious about what you are doing at their ear. Having a helper hold a mirror in front of the child while you insert the probe tube will help reduce the child's head movements. He/she will be able to see what you are doing by looking into the mirror.
    5. figure 2
  5. If there is a cord clip on the probe microphone module, clip it to the shoulder opposite to the ear you are working on. This will allow the probe module to lie snugly against the child's cheek. Movement of the module will be reduced and the child will be less tempted to reach up and grab it. Figure 2.
2) Strategies for measuring RECDs on toddlers may not work for young infants. Infants are usually being cradled in the caregiver's arms or in a stroller. Access to the ear is a challenge due to this positioning and the close proximity of the ear to the shoulder. Are there modified strategies for inserting the probe tube into a young infant's ear for measuring the real-ear portion of the RECD? [back to top]

A variation of Section ii in Question 1 above has been evaluated and shown to be feasible (Bagatto et al, 2006). The strategy involves connecting the probe tube to a small eartip or personal earmold using plastic wrap or soft surgical tape. The probe tube should extend approximately 2 to 4mm beyond the sound bore. Plastic wrap used for protecting hearing aids from moisture (Moisture Guard) is no longer available. Therefore, soft surgical or first aid tape is recommended. Soft surgical tape should be used as opposed to regular tape as this may cause a sharp edge when applied.

figure 3

If inserting the probe tube separately from the eartip is more suitable, the insertion depth guideline determined in the Bagatto et al, 2006 study is 10 mm from the opening of the ear canal.

3) What is the difference between a foam tip and an earmold RECD? [back to top]

The main difference will be noted in the high frequency region. An RECD measured with a foam tip will show a reduction in values starting around 3 or 4 kHz. When an earmold from a toddler or adult is used in place of a standard foam tip, the increased length of tubing on the earmold will cause a reduction in RECD values around 2 kHz (see Figure 4). This change in values may not be present when measuring with a young infant's earmold as the tubing may be the same length as a standard foam tip.

figure 4

It is important to be able to distinguish that the change in high frequency values is due to increased tubing length and not shallow probe tube insertion. However, if the clinician used the customized probe tube insertion depth strategy outlined in Section ii), Question 1, then a roll-off in high frequency values will likely be due to increased tubing length from the earmold.

4) In many clinical situations, audiometry is conducted using foam tips coupled to insert earphones and an RECD is required for the hearing aid fitting. The RECD is then measured using the child's personal earmold. What are the implications of this strategy? [back to top]

RECDs are used in several stages of the hearing aid fitting process. One is to convert audiometry measured in dB HL or dB eHL to dB SPL at the eardrum. This provides a more accurate description of hearing thresholds and the values are used to calculate the prescription in the DSL Method. Another use is in the verification stage. For the pediatric population, many times it is not feasible to conduct actual real-ear measures of the hearing instrument. With the RECD, clinicians can obtain a simulation of hearing instrument performance in the coupler. Therefore, it would be ideal to use RECD measures with different coupling (ie. Foam tip and earmold) in the DSL Method. This would provide the correct RECD measure at the appropriate stage in the process. Unfortunately, this application has not yet been implemented in manufacturer's implementations of DSL v5.

In the meantime, if the RECD was measured with the earmold and audiometry was conducted with a foam tip, there will be some error in the converted SPL thresholds, but only in the high frequency region. This error will transfer to the calculation of targets since they are calculated from the SPL thresholds. The discrepancy should only be present at the initial stages of the child's hearing aid use because subsequent audiometry can be conducted with the child's personal earmolds.

5) How often should the RECD be measured? [back to top]

It is recommended that the RECD be measured whenever a new earmold is obtained. Infants and young children require earmolds frequently due to the rapid growth of the outer ear. Since the size of the ear canal has changed, an RECD measurement is needed to capture the associated changes in ear canal acoustics so that they can be applied to the hearing instrument fitting.

**Please note that with new earmolds, it is important to trim the tubing to the length that it will be when worn with the hearing aid prior to measuring the RECD.

In addition, the RECD should be measured any time there is a change in middle ear status in one or both ears.

6) Is it necessary to measure the RECD on both ears? [back to top]

If the physical appearance of the outer ear and the middle ear status is the same for both ears, an RECD measurement on one ear can be used for the other ear. There is some data to suggest that if these conditions are met, the RECD values should be similar for both ears (Tharpe, Munro).

7) What modifications to the procedures, if any, need to be made when the earmold has a vent? [back to top]

For many pediatric hearing aid fittings, the earmold will be too small to accommodate a vent. However, a vent larger than about 1 mm will affect the low frequency region. There are two stages in the fitting process where venting can have an affect. In the assessment stage, when conducting audiometry or measuring an RECD with a vented earmold, the vent should be plugged on the medial end. A fully occluded ear canal will ensure that the behavioural thresholds and RECD values are accurately represented at 250 and 500 Hz. This will ensure accurate conversion from HL to SPL for the purposes of calculating the hearing aid prescription.

In the verification stage, the full effects of the vent cannot be fully accounted for unless the performance of the hearing aid is measured in the real ear. With many pediatric patients, coupler-based or simulated real-ear measurements are the preferred method for verification. With this strategy, the impact of the vent will not be completely accounted for. Attaching the earmold to the BTE and coupling the earmold to the HA-1 coupler for simulated verification is a way to account for some of the impact of the vent. However, it may be a challenge to securely attach a soft earmold to putty. Therefore, there are some modifications that need to be made when fitting a hearing aid with a vented earmold.

8) How is a myringotomy tube or eardrum perforation accounted for when applying RECD? [back to top]

Myringotomy tubes or eardrum perforations will cause negative values in the low frequency region (i.e. -5 to -10 dB). If this is the case, the negative RECD values should be used for the conversion of audiometry to SPL and for verification. This is because the status of the eardrum impacts the conduction of sound for both audiometry and hearing aid use. In addition, the hearing thresholds in the low frequencies are likely to be worse due to the abnormal status of the eardrum. The combination of higher hearing thresholds and negative RECD values will result in more prescribed gain for average speech, which is what is appropriate for the patient at this time.

9) What if measuring the RECD is not possible? [back to top]

In applications of DSL v5, there are age-appropriate predicted RECD values. These have been updated since DSL v4.1. The updated values are for foam tip and earmold coupling and are provided in one-month age increments. However, it is important to note that while these values are more accurate than using average adult RECD predictions, large variability exists in the data making the predictions less than ideal. For example, at some frequencies, the range of error can be up to ±12 dB. Therefore, it is recommended that the RECD be measured whenever possible. In cases where it is unobtainable, more precise RECD predictions are now available. See Bagatto et al, 2002 and Bagatto et al, 2005 for more details.

The DSL m[i/o] v5 Targets

1) Are the DSL v5 targets different from the DSL v4.1 targets? [back to top]

In the latest version of DSL, targets are available for use with those who have congenital hearing loss (i.e. Child) and those who have acquired hearing loss (i.e. Adult). The Adult targets are approximately 7 dB lower than the targets for children. For the child targets, the goal was to keep the DSL v5 targets as close to the DSL v4.1 targets as possible. However, for more severe hearing losses, the targets for average conversational speech are lower. This is due to a correction that aims to ensure that the peaks of speech are within the patient's auditory area and not saturating the hearing aid. Overall, when the binaural correction is not applied for the fitting, the DSL v5 and DSL v4.1 targets are within 3 dB.

2) Should the binaural correction be applied for infants and children? [back to top]

At this point, the literature is not conclusive on whether a gain reduction is needed for binaural fittings in children. This element of DSL v5 needs further investigation. Until conclusive results are available, it is recommended that clinicians do NOT apply the binaural correction even for those children fitted binaurally.

3) How are conductive hearing losses accounted for with DSL v5? [back to top]

DSL v5 has a conductive correction that the clinician can choose to apply. Since the literature reports that listeners with conductive and/or mixed hearing losses have higher loudness discomfort levels and prefer a higher level of use gain than listeners with sensorineural hearing loss, the strategy applied in DSL v5 is to increase the predicted upper limits of comfort. This results in an increase in the amount of gain prescribed. The calculation is based on increasing the upper limit of comfort by 25% of the uncorrected air-bone gap, averaged across the frequencies of 500, 1000, 2000, and 4000 Hz, to a maximum of at 60 dB gap. With this correction, however, the upper limit targets will not exceed 140 dB SPL.

4) What targets should be selected for an adult with congenital hearing loss who has been aided since childhood? [back to top]

An adult that has a congenital hearing loss has likely been provided with amplified levels of sound that are more consistent with what the DSL Child target criteria provide. Although the literature remains inconclusive on this topic, some studies suggest that children prefer higher levels of amplified sound, like those prescribed by DSL v5 Child targets. This patient has not recently acquired the hearing loss and likely prefers more gain than the DSL Adult prescription would provide. Therefore, this patient should be fitted to DSL Child, regardless of his/her age.

Pediatric Hearing Aid Fitting

1) Other than prescriptive targets, what else should be considered when selecting hearing aid for an infant or young child? [back to top]

There are several non-electroacoustic items that should be considered when selecting a hearing aid for an infant or young child. One is to ensure the behind-the-ear (BTE) hearing instrument has a pediatric-sized filtered earhook. This will allow for better retention on the child's ear and the filter will reduce peaks in the hearing instrument response that could compromise the fit to targets. Direct audio input is also an important feature as use of an FM system is likely to occur. Also, locking mechanisms for the volume control and battery door are crucial. For most situations, the volume control should remain in one position in the hearing aid. Locking it will ensure that accidental increases or decreases in the amount of volume do not occur while the child is wearing the aids. This could result in over- or under- amplification for a period of time. Locking the battery door compartment keeps the battery from inadvertently coming out and being swallowed by the child. Hearing aid batteries are toxic, therefore, this is a critical safety precaution. Finally, deactivation of advanced features (i.e. directional microphones, noise reduction, multiple memories) is useful, especially at the early stages where advanced technologies are not required.

2) When should advanced signal processing be used / not used for infants or children? [back to top]

At this point, there is very little research regarding the application of advanced technologies with infants or children. One main thing to consider is whether or not the caregiver and/or the child can reliably apply the technology in the appropriate situations. Since incidental learning is a large part of how the child learns to attach meaning to sound, reducing noise or sound from certain locations may not be appropriate.

3) Should the gain of the hearing aid be turned down to match DSL v5 targets for children with more severe hearing losses who have been previously fit with DSL v4.1? [back to top]

Verification

1) What is the most accurate and practical way to verify hearing instrument performance for infants and young children? [back to top]

Performing simulated or coupler-based real-ear measurements has been shown to be an accurate and practical way to verify hearing instruments for the pediatric population (Scolle et al, 1999). Children are not likely to sit still and quiet while facing a speaker for the duration of time it takes to make adjustments to the hearing aid at various levels and frequencies. By applying the RECD, the performance of the hearing aid can be predicted in the real-ear by making measurements in a controlled test-box environment. And when compared to actual real-ear measurements, it is accurate to within ±2 dB on average.

2) Why are some targets below threshold? [back to top]

3) Why are there additional MPO targets displayed on the Verifit? [back to top]

The upper limit of comfort target, which is represented by the '*', is really intended to be matched by fully saturated hearing aid responses. Therefore a slightly lower target, shown by the '+', may be more appropriate for use with the MPO test signal. For this reason, the target input/output function within DSL v5 can be used to compute a level-dependent target for either 85 dB SPL (in the real ear) or 90 dB SPL (in the coupler, using simulated real ear measurement). This new target will be somewhat lower than ULC for most hearing losses.

4) What stimulus levels should be used to verify average conversational speech? [back to top]

Typically 60 or 65 dB SPL.  For an explanation, please see this article.

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