The term “ampclusion” denotes the occlusion effect experienced during hearing aid use when the physical occlusion of the ear canal and the hearing aid amplifier gain are included in the wearers’ perception. Part 1 of this article series provided a summary of the factors that can contribute to the wearer’s complaint of ampclusion. Because these factors can work singly or in combination, ampclusion management requires a clear understanding of the relevant factor(s) that may be responsible for this complaint. A systematic approach to remediation is necessary so that the complaint can be prevented, and if still present, its origin can be identified quickly and a solution applied effectively.

In Part 1 of this 2-part article (see the August 2002 HR), potential reasons for the ampclusion complaint by hearing aid wearers were reviewed.1 These include various issues originating from shell occlusion of the ear canal, sub-optimal amplification provided by the hearing aids (including some digital hearing aids with long group delays), and unrealistic expectations for the performance of the aids. Figure 1 summarizes the various factors that may contribute to the ampclusion complaint.

figure 1
Figure 1. Factors affecting the ampclusion complaint.

Because these factors can work singly or in combination, ampclusion management requires a clear understanding of the relevant factor(s) that may be responsible for this complaint. A systematic approach to remediation is necessary so that the complaint can be prevented, and if still present, its origin can be identified quickly and a solution applied effectively. Kuk2 offered a flow chart approach as a systematic way to handle hearing aid wearer complaints. In this paper, we integrate the essence of that approach and present a step-by-step protocol for the systematic management of the ampclusion effect.

While the following may represent the first published protocol for the management of ampclusion, the authors are quick to acknowledge the work of many clinicians and researchers cited in the previous article (and many who shared their experiences with us) that inspired the methods in this article. In this paper, the text will provide the background, and a checklist is included to enable clinicians to follow the protocol. To facilitate ease of reading, the steps listed in the following sections refer to the corresponding steps in the checklist.

Step 1: Set the Right Expectations The best defense is a good offense.
If wearers are warned ahead of the potential ampclusion effect, they are more psychologically prepared for such an occurrence, and will know that the unnatural voice perception is a necessary part of the hearing aid experience. Consequently, hearing care professionals need to counsel wearers and set expectations prior to the recommendation of hearing aids. This way, the wearers are less likely to question the performance of the selected hearing aids or the competence of the dispensing clinicians when they experience ampclusion during fitting.

The manner in which one uses to set the right expectations is important. Too little warning of the potential ampclusion problem could surprise the wearers. However, over-emphasizing the normal nature of ampclusion could negatively portray the hearing aid experience and may discourage the wearers from trying amplification. Education by demonstration of the ampclusion effect during the initial visit is an effective approach in setting realistic expectations for the wearers.

The mode and the stimulus for the demonstration should be carefully chosen. Because the ampclusion complaint is in response to wearers’ perception of their own voice, they should be instructed to judge their own vocalization. Because the ampclusion effect is primarily a low frequency phenomenon, the materials that are used during vocalization should include significant low frequency content. Many investigators3,4 showed that narrow vowels like /i/ and /u/ have the highest low frequency content to illustrate this effect. These vowels have their first formant frequency between 200-300 Hz, while the more open vowels like /a/ tend to have their first formant frequencies at and above 500 Hz. If the purpose is to measure the ampclusion effect using real-ear measures, sustained /i/ and /u/ may be the materials of choice.

On the other hand, if the task is to form a subjective impression of the potential ampclusion effect during conversational speech, sentence materials with an emphasis on low frequency vowels may be more desirable. We have found that asking wearers to repeat the phrase, “Baby Jeannie is teeny tiny,” is effective in allowing them to judge the quality of their own voice.

To demonstrate the potential ampclusion effect, instruct the wearers to plug their ears tightly with their fingers (or press firmly at the tragus) to close off the ear canal. Instruct them to repeat the above phrase, and ask that they pay attention to the quality of their own voice. The specific instructions might be as follows:

“Hearing aids allow you to hear sounds that you may not have heard before. At the same time, they may alter the way some sounds sound to you, even with the hearing aids turned off. To demonstrate what I mean, I want you to repeat the phrase “Baby Jeannie is teeny tiny” twice. The first time I want you to stick your fingers into your ears tightly while you are repeating the phrase. Then I want you to repeat the phrase with your fingers pulled out slightly (or barely touching the entrance) of the ear canals. I want you to tell me how your voice sounds in each situation.”

For many wearers, they will report that their voice sounds “hollow,” “echoic,” or “unnatural” when their fingers are tightly inserted in their ear canals. At the same time, many of these wearers will report a significant reduction in the “hollowness” of their voice when their fingers are slightly pulled out of their ear canals.

This simple demonstration illustrates several important aspects of the ampclusion effect. First, it demonstrates that the physical occlusion of the ear canal itself is one source of unnatural sound perception. Its natural occurrence is the fault of neither the clinicians nor the hearing aids. This recognition helps set the stage for any potential ampclusion effect that may occur as a result of hearing aid wear.

Second, the demonstration that the amount of unnaturalness decreases as one’s fingers are pulled out of the ears slightly sends the message that such unnaturalness can be decreased. The clinician should point out that such observation is similar to having a large vent in the hearing aid or wearing a loosely fit hearing aid. Especially for a potential wearer who desires a CIC style hearing aid against the clinician’s recommendation, this will be an opportunity for the clinician to point out the difficulty of venting that style of hearing aid to minimize potential ampclusion effect. If the wearers are insistent on obtaining a CIC against the clinician’s recommendation, they are at least prepared for what may potentially happen to their voice. The specific instruction/explanation might be as follows:

“What you have just experienced is common to new hearing aid use. When we talk, our voice escapes through the ear canals. When you have your fingers in your ears, your voice is trapped inside your ears, giving you the unnatural sound quality. As you loosen the seal by pulling your fingers out slightly, some of that trapped sounds escape, and your voice becomes more natural. This could happen in hearing aid use.

When you wear hearing aids the first time, your voice may sound unnatural because the hearing aids make it louder, and some of your voice is trapped inside your ears. Some people will get used to it immediately; many will adapt to it over time. In some cases, adjustment is necessary on the hearing aids. In addition, we can improve your perception by placing a vent in the hearing aids or earmolds to let some of the sounds out. This requires available space on the hearing aids. That may be limited for a small hearing aid like the CIC style. So, I want you to be aware of such potential perception when the hearing aids are fitted on you. In any case, please be assured that I will be working with you to give you the best hearing available today.”

Some clinicians may further wish to predict a priori—which wearer is most likely to experience the ampclusion effect with the recommended hearing aids. Such a prediction may alert clinicians to make special provisions for the potential wearers (eg, ordering a larger vent, or specifying a deep canal fitting.).

Indeed, there have been many attempts to correlate or predict the clinical ampclusion effect. For example, Carle et al5 showed that wearers’ middle ear compliance correlated highly with the minimum vent size they selected, suggesting that a higher compliance may be related to a higher likelihood of ampclusion effect. Staab5 suggested the BING-TOO (Bing Test of Occlusion) as a tool to predict potential occlusion problems in wearers. On the other hand, Kampe and Wynne4 reported no correlation between measured occlusion effect (with real ear measures) and reported ampclusion.

Considering that the compliance of the middle ear and the depth of hearing aid insertion, together with other hearing aid factors, may change the perceived ampclusion effect, setting wearers’ expectation may be a more fruitful use of clinical time.

Step 2: Choose the Optimal Hearing Aid/Earmold
Although voices inevitably sound unnatural with initial hearing aid use, such perceptual difficulty may be minimized through the choice of appropriate hearing aids. In the previous article, we identified several hearing aid factors that may help improve the ampclusion effect. For example, if ampclusion is related to the amount of low frequency gain in the hearing aids, the selected hearing aids must have enough electroacoustic flexibility in the low frequency so that gain adjustment can be made easily to alleviate such perception. In addition, the choice of multichannel compression hearing aids that use steep filters with narrow bandwidth would seem desirable. The steep, narrow bandwidth filters allow specificity of gain adjustment in the low frequency without affecting nearby frequencies. Compression allows automatic gain reduction at the high input level typical of the wearers’ voice levels. It also allows gain adjustment at discrete input levels (in contrast to linear hearing aids where gain adjustment affects all input levels). Furthermore, a hearing aid that minimizes saturation distortion at both the input and output stages may help optimize sound quality.

If digital signal processing hearing aids are to be chosen, those that have the least group delay (less than 10 ms) and those that have an active feedback cancellation system may be more desirable. A small group delay (less than 10 ms) minimizes the “echoic” sensation. An active feedback cancellation system allows one to achieve the target gain more easily before feedback, even with a larger vent diameter.

To minimize the ampclusion effect that originates from shell occlusion, a hearing aid that allows selective vent size may be desirable. As indicated in the previous paper, while a larger vent can reduce the occlusion effect more, it can also impact the real-world effectiveness of directional microphones7, noise reduction algorithms, and compression ratios8 of the hearing aids. Consequently, the proper vent size must be chosen based on a careful consideration of all the available features. A select-a-vent system, for example, allows the flexibility to try out different vent sizes. If the selected hearing aids include all the aforementioned features, one may start with a conservative vent diameter, and gradually increase its size if the other means of minimizing ampclusion described previously are not sufficient.

It was mentioned earlier that the depth of hearing aid insertion could affect the measured occlusion effect, implying that hearing aid shells (or earmolds) may be made longer (or shorter) to minimize that effect. Furthermore, the shell/earmold may need to provide a total seal in the bony portion of the ear canal. However, if one examines Pirzanski’s9 data on the magnitude of the occlusion effect as a function of insertion depth, one can see that the hearing aids must be inserted at least 4-5 mm beyond the second bend, and the canal tip must provide a total seal to minimize the occlusion effect. This shell configuration may cause the wearers to reject the hearing aids because of discomfort and difficulties with insertion. In view of Bongiovanni et al’s10 finding that the canal lengths of CIC hearing aids did not affect the perceived ampclusion effect, one may start by ordering the conventional canal lengths (ie, hearing aid terminates before the second bend) and have them remade if they need to be longer. The specific approach to determine if they need to be re-made will be described in a later section.

Ampclusion Checklist

Step 1: Set the right expectations (prior to hearing aid selection).

  • Ask wearer for subjective response to sound. Say, “Baby Jeannie is teeny tiny” while patient has fingers in his/her ears (or pressing tragus).
  • Explain and counsel the wearer.

Step 2: Choose optimal hearing aids  (at hearing aid selection). Considerations may include:

  • Multiple channels, preferably with WDRC and steep, narrow filters.
  • Mechanisms to minimize potential of input and output saturation distortion.
  • If digital, consider DSP that utilizes shorter group delays (ie, less than 10 ms).
  • Flexible and discrete gain adjustment in low frequencies.
  • Feedback cancellation system.
  • Select-A-Vent or similar options for flexible venting.
  • Canal length for comfort and ease of insertion.

Step 3: Ensure adequacy to external sounds/fit.

  1. Check to ensure output of hearing aids meets insertion gain targets for soft, normal, and loud sounds.
  2. Ask wearer: “How do your hearing aids sound relative to external sounds?”
            • Loudness: Too loud, comfortably loud, or too soft?
            • Quality/naturalness: Boomy, clear and natural, or too tinny?
            • Distortion: crackling/raspy, crisp and bright, or muffled?
  3. Ask wearer: How is the physical fit of the hearing aids (earmolds) in your ear (tight, just right, or loose)?

Step 4: Establish the need for intervention.

  1. With the hearing aid(s) in the wearer’s ears, ask wearer: “How does your own voice sound to you when you say, ‘Baby Jeanie is teeny tiny?’ ”
            • Loudness: too loud, comfortably loud, or too soft?
            • Quality/naturalness: Too boomy, clear and natural, or too tinny?
            • Distortion: Crackling/raspy, crisp and bright, or muffled?
  2. Ask: “Which of the following adjectives describe your perception of your own voice?”
            • Fine, louder, hollow, echoic, raspy, distorted, muffled, talking under water, talking through tunnel, dull, talking through my nose/head , stuffed up, or others (specify).
  3. Ask: “On a rating scale of 1-10, with ‘1’ the most unnatural and ‘10’ the most natural, how you would rate your voice problem with the hearing aids?” (Rate 1-10)
  4. Ask wearer: “Rate (1-5) how much your voice problem affects you (if appropriate)?”
            1 – It is very noticeable and very distracting;
                I cannot adapt to it.
            2 – It is moderately noticeable and distracting, but I can
                adapt to it.
            3 – It is noticeable but not distracting.
            4 – It is hardly noticeable unless I focus my attention to it.
            5 – It is not there (very natural).

Step 5: Diagnosis of ampclusion origin

  1. Ask wearer: “Turn off your hearing aids. With the hearing aids in your ears, repeat the phrase, “Baby Jeanie is teeny tiny.” Compare the quality of your voice between the hearing aids’ “on” and “off” positions. Is there a difference?”
            • "Off" sounded better (indicates amplifier origin)
            • "Off" sounded worse (shell origin or insufficient gain)
            • No difference (shell origin)
  2. Ask wearer: Is your voice worse with one hearing aid or both hearing aids?
            • Worse with one
            • Worse with both
            • No difference between one and two
  3. Ask wearer: “Does your voice problem change if you speak louder or softer?”
            • Worse when speak louder (saturation/excessive gain)
            • Worse when speak softer (insufficient gain)
            • No difference (shell origin)
  4. Ask wearer: “Does your voice problem change if you push the hearing aids in deeper or pull them out slightly?”
            • Pushing them in sounds better (need to increase length/seal)
            • Pulling them out sounds better (vent/leak/shorten length)
            • No difference (possible amplifier origin)
  5. Ask wearer: How would you rate the hearing aids (Questions 4c and 4d) after the adjustments? How do they sound to external sounds (Question 3b)?

Step 3: Ensure Optimal Fit for Listening to External Sounds
While the ampclusion effect relates primarily to the wearers’ perception of their own voice, sub-optimal adjustment of the wearers’ hearing aids for external sounds may also lead to wearer complaints that resemble the ampclusion effect. For example, an excessive gain setting for loud sounds would lead to complaints of loudness discomfort for external sounds, and a complaint of “boomy” or “hollow” sounds relative to the wearers’ own voice. In these cases, a reduction in gain for loud sounds would resolve the loudness discomfort and the ampclusion complaints. This demonstrates why the hearing aids should be fitted optimally for external sounds prior to any ampclusion adjustment.

The most common approach to ensure optimal gain is to match the hearing aid output to some predefined real-ear (or coupler) targets. While the use of generic gain targets is widely accepted, targets need to include considerations for the power summation effect seen in modern multichannel compression hearing aids.11 Otherwise, the real-world output may be significantly higher than what the matched output would predict. Fortunately, many manufacturers of multichannel compression hearing aids also offer their recommended gain targets for the clinicians to evaluate the adequacy of their fitting.

In addition to matching the gain targets, one may also evaluate the appropriateness of fit by asking wearers to judge the sound quality of recorded female speech presented at 65 dB SPL in quiet. In addition, tolerance to loud sounds should also be evaluated using real-life sound sources (eg, banging of doors or shaking of keys). Any negative responses to external sounds should be corrected prior to an evaluation of ampclusion complaints.

The timing of this evaluation to external sounds (and to the ampclusion adjustment) is important. Because of the novelty of the hearing aid settings to the wearers, it may not be appropriate to make fine-tuning (including ampclusion) adjustment during the initial fit. Rather, the wearers should use the hearing aids at the recommended settings for some time before the settings are fine-tuned. An adjustment period of 2-4 weeks is recommended (shorter if the wearer refuses to adjust to the new sensation).

Step 4: Establish the Need for Intervention
If the reason for the ampclusion complaint is unfamiliarity with the sound picture provided by the new hearing aids, any manipulation of the shell or settings on the hearing aids will not improve wearer satisfaction. Worse yet, one may compromise the effectiveness of the settings for listening to external sounds. Consequently, the decision to intervene (ie, make adjustments on the hearing aids) must be taken only when the wearers indicate a problem that could compromise their use of the aids.

To establish the need to intervene, the hearing care professional needs to provide the condition for the ampclusion effect to be evaluated. This would involve asking the wearers to repeat the phrase, “Baby Jeannie is teeny tiny” with the hearing aids turned on and in-situ. A description of their perception should be recorded because it may shed light on the origin of the ampclusion complaint and the direction for its management (Steps 4a-4b in the Amplusion Checklist on page 42).

If a problem is indicated, it is also important to ask the wearers to rate the severity of their problem (Step 4c). This would be useful for pre- and post-intervention comparison. Use of descriptive categories may also help the clinicians understand the wearers’ tolerance for the ampclusion complaint (Step 4d). In this case, one may not want to intervene if the wearers’ impression of their ampclusion is only “noticeable but not distracting.” A decision to intervene should be a category rating of 1 or 2, where the problem is at least moderately noticeable and distracting to the wearers. Counseling may be the best approach for an ampclusion category rating that is higher than 3. (See Step 4 of the ampclusion checklist for the pertinent questions).

Some may argue that, if the wearers do not mention the ampclusion effect as their primary issue during fine-tuning, one should not even alert the wearers to such a possibility. This approach could minimize the amount of work for the clinicians. Furthermore, it would not make a non-issue an issue. However, this approach could potentially let some cases of dissatisfied wearers slip through your care and decrease wearer satisfaction for hearing aids. For example, an introverted wearer who experiences ampclusion effect may not know that such effect can be minimized. If one does not question the wearer directly, such a problem may go undetected.

Step 5: Determine the Origin of the Ampclusion Effect
It was indicated earlier that the complaint of ampclusion could stem from an amplifier origin with either too much or too little gain in the low frequencies12, or a shell origin with either insufficient venting (leakage) or insufficient depth of insertion. These origins require different remediation approaches.

Unless the origin (or at least the predominant origin) of the ampclusion complaint is identified reliably, the resulting solutions for the problem may be spotty. In some cases, one may worsen the ampclusion. Thus, a systematic approach is needed to provide differential diagnosis of the origin of the ampclusion complaint and to offer specific treatment approaches.

The first step in differential diagnosis is to ask the wearers to repeat the phrase, “Baby Jeannie is teeny tiny,” with the hearing aids in-situ but turned off. If the major contributor to the ampclusion complaint has an amplifier origin, turning off the hearing aids while they are in-situ should deplete any output from the hearing aid. If the perception of “hollowness” or “echoic” remains the same while the wearers repeat the phrase, shell occlusion is the likely cause of the complaint. If the perception of “hollowness” is changed, it may suggest an amplifier origin to the complaint.

Amplifier origin: Saturation distortion of the hearing aids, too much output, and insufficient output (especially in lows) from the hearing aids can contribute to the ampclusion complaint. A report of “better” or “worse” to the ampclusion complaint when the hearing aids are turned off could indicate if the hearing aid gain settings should be increased or decreased. Additionally, the adjectives that the wearers use to describe their complaints may also shed light on the possible etiology of the complaint. The list of potential adjectives that hearing aid wearers used to describe their complaints is included in the ampclusion checklist.

If the “hollowness” perception is improved when the hearing aids are turned off, or if the adjectives that are used to describe the perception are “boomy,” “echoic,” “hollow,” or “too loud,” it probably suggests too much output (or an output level that is unfamiliar to the wearer) during vocalization. In such a case, gain reduction for high input level sounds in the low frequencies should improve the perception.

If the “hollowness” perception is worse when the hearing aids are turned off, or if the adjectives that are used to describe the perception are “stuffed up,” “closed,” or “cannot hear my own voice,” it may suggest insufficient gain for the wearers’ voice in the first place. Although it is intuitive to increase the low frequency gain for loud sounds only, often it is found that the gain for soft and normal sounds should also be increased. Consequently, an increase in low frequency gain across all input levels should improve the perception.

If the ampclusion perception improves with the hearing aids turned off, and if the adjectives that are used to describe the perception are “distorted,” “crackling,” or “hisses,” the input or output of the hearing aids may have exceeded the saturation limits at the input/output stages. Such complaints are not common in hearing aids that use input stage and output stage compression limiting systems. The speculation on saturation would be further confirmed if a change in the ampclusion effect is seen as the wearers change their vocal efforts (ie, louder or softer voice). If the wearers’ hearing aids cannot be exchanged for models that include input and output compression limiting mechanisms, lowering the gain for high inputs across all frequencies may reduce the amount of output saturation distortion. However, this could compromise gain for listening to external sounds. In addition, this maneuver will still be ineffective for input stage saturation.

Shell origin: Because changing the depth of hearing aid insertion and increasing the vent size (or leakage) have been reported to be effective in managing shell occlusion by many clinicians, one cannot rely on just one approach to solve shell occlusion for all wearers. A way to distinguish between the two possibilities is needed.

Many clinicians use the push-pull maneuver. With the hearing aids in the wearers’ ears and turned off, the wearers report any improvement of occlusion if pushing the hearing aids (or earmolds) into the ears or pulling them slightly out of the ears while they repeat “Baby Jeannie is teeny tiny.”

If pushing the hearing aids inward improves the occlusion perception, it suggests that a tighter seal may be necessary. In many cases, increasing the length of the canal portion of the hearing aids has also been reported to be beneficial. On the other hand, if pushing results in increased occlusion perception, this suggests that more leakage/venting may be in order. In both cases, the hearing aids should be sent to the manufacturers for a remake of the shell. Make sure an ear impression that extends significantly beyond the second bend of the ear canal is included for the remake. If possible, indicate how much longer/shorter the new canal should be in the instructions.

If pulling the hearing aids outward improves the occlusion perception, it suggests that an increase in leakage of the hearing aid may offer a potential solution. This possibility can be evaluated first by increasing the diameter of the vent in a stepwise manner until the largest vent size is tried. Tapering of the canal would increase the leakage and may also improve the occlusion effect. As indicated earlier, increasing the vent size or leakage could reduce the effectiveness of some special features in some of today’s hearing aids.

Frequently, the perceived ampclusion effect that wearers report is a combination of shell and amplifier origins. Consequently, the hearing care professional may need to attempt amplifier readjustment after shell maneuvers (and vice versa) to completely resolve the ampclusion complaint. It is also necessary to reconfirm that the modifications made to resolve the ampclusion complaint do not compromise the performance of the hearing aids for external sounds. Post-adjustment evaluation of the hearing aids for external sounds and the wearer’s voice is necessary.

Example of a Digital Occlusion Management System

Several advanced digital instruments have built-in features that are designed to reduce or eliminate perception of the ampclusion effect by the patient. The following provides an example of a feature found in the Senso Diva hearing instrument, a 15-channel digital signal processing (DSP) hearing aid that uses an Infinite Impulse Response (IIR) filter design, Extended Input Dynamic Range (EIDR) at its input stage, and active feedback cancellation with short group delays to minimize ampclusion (detailed descriptions of the aid can be found in Ludvigsen & Kuk13, 14).

Figure 2. Input-gain curves showing the extent of gain
adjustment in the Occlusion Manager.

The features cited above are an example of how one hearing instrument manufacturer applies occlusion management “inside” a digital hearing aid. In some cases, the “outside” fitting software associated with the hearing aid is also used to help fight occlusion. In the Diva hearing aid, an Occlusion Manager (OcM) allows for specific adjustment of the low frequency gain. The OcM is normally deactivated when the hearing aid is fitted to the wearers. Once the OcM is activated, the four frequency channels below 500 Hz (125, 250, 350, and 500 Hz) are de-coupled from the rest of the higher frequencies for discrete adjustment. In addition, gain for high input sounds is automatically reduced in all four channels.

Assuming that the ampclusion effect is the result of an abundance of low frequency output to a high input, this action would automatically minimize the ampclusion effect. However, if this action is insufficient to reduce the ampclusion effect, one can further adjust gain in the four frequency channels in either a positive or negative direction. When these parameters are adjusted positively, gain at all input levels can be increased by as much as 20 dB relative to the recommended settings. It assumes that the reported ampclusion complaint originates at least partially from insufficient gain. When these parameters are adjusted negatively, one could decrease gain for loud sounds by as much as 10 dB. It assumes that the ampclusion complaint originates from too much gain at a high input.

Figure 2 shows the input-gain curves of the four channels where adjustments in both the positive and negative directions are made.

The ampclusion complaint that hearing aid wearers report may have multiple origins. As such, this complaint must be approached systematically and methodically. Setting the right expectations will minimize future surprises when the wearers receive their first set of hearing aids. Choosing the right hearing aids will also minimize any amplifier-related ampclusion complaint. If these preventive measures are not sufficient, knowing how to diagnose the origin of the complaint is important in helping find the most effective solution.

Correspondence can be addressed to HR or Francis Kuk, PhD, Widex Office of Research in Clinical Amplification, 2300 Cabot Dr, Ste 415, Lisle, IL 60532; email: [email protected].

f05a.jpg (6206 bytes) f05b.jpg (7141 bytes) Francis Kuk, PhD, left, is director of audiological services at the Widex Office of Research in Clinical Amplification, Lisle, Ill, and Carl Ludvigsen, MS, is director of audiology at Widex APS, Vaerloese, Denmark.

1. Kuk F, Ludvigsen C. Occlusion management 101: understanding variables. Hearing Review. 2002;9(8):22-32.
2. Kuk F. How flow charts can help you troubleshoot hearing aid problems. Hear Jour. 1999;52(10):46-52.
3. Killion M, Wilber L, Gudmundsen G. Zwislocki was right: a potential solution to the “hollow voice” problem (amplified occlusion effect) with deeply sealed earmolds. Hear Instrum. 1988;39(1):14-18.
4. Kampe S, Wynne M. The influence of venting on the occlusion effect. Hear Jour. 1996;49(4):59-66.
5. Carle R, Laugesen S, Nielsen C. Observations on the relations among occlusion effect, compliance, and vent size. J Amer Acad Audiol. 2002;13:25-37.
6. Staab W. Solving challenges in deep canal fittings. Hear Jour. 1995;48(1):34-40, 49.
7. Ricketts T. Directional hearing aids. Trends Amplif. 2002;5(4):139-176.
8. Fortune T. Real ear compression ratios: the effects of venting and adaptive release time. Amer J Audiol. 1997;6(2):55-63.
9. Pirzanski C. Diminishing the occlusion effect: clinician/manufacturer factors. Hear Jour. 1998;51(4):66-78.
10. Bongiovanni R, Auriemmo J, Kuk F. Is a longer canal length always better when fitting CIC hearing instruments? Hearing Review. 2001;8(9):44-48,77.
11. Kuk F, Ludvigsen C. Variables affecting the use of prescriptive formulae to fit modern nonlinear hearing aids. J Amer Acad Audiol. 1999;10(8):458-465.
12. Sweetow R, Valla A. Effect of electroacoustic parameters on ampclusion in CIC hearing instruments. Hearing Review. 1997;4(9):8-12;16-18, 22.
13. Ludvigsen C, Kuk F. New solutions for age-old hearing aid problems. Hearing Review. 2001;8(11):32-36, 55.
14. Kuk F, Ludvigsen C. The real-world benefits and limitations of active digital feedback cancellation. Hearing Review. 2002;9(4):64-68.