Open Ear Hearing Aid Fittings

Donald J. Schum, Ph.D./CCC-A
Vice President, Audiology & Professional Relations
Oticon, Inc.

Douglas L. Beck Au.D.
Director of Professional Relations
Oticon, Inc

Introduction:

Digital Signal Processing (DSP) technology has allowed, and indeed facilitated, the creation of effective digital feedback control and management systems within modern state-of-the-art hearing aids. Figure One (below) provides photographs of various hearing aid systems incorporating open ear hearing aid fittings.

Figure 1: Photos of three types of modern DSP devices with open fittings: an ITC with a large collection vent (left), a BTE with a thin tube and open dome (middle) and a receiver-in-the-ear device with an open dome (right).

Prior to modern "real-time" digital feedback management protocols, the non-negotiable laws of physics required physical separation of the hearing aid's microphone and receiver to reduce/avoid acoustic feedback. Previously, if acoustic energy exiting the receiver found its way to the microphone, an acoustic feedback loop occurred, creating the familiar whistling noise which patients and professionals find highly objectionable. Closing off the ear canal to physically block the flow of sound from the receiver to the microphone became an acceptable and standard operating procedure. However, closing off the ear canal to fit hearing aid amplification was never ideal.

It is unnatural and unhealthy to close off the ear canal. When air is trapped in the external auditory canal (EAC), the warm, moist, dark environment quickly becomes a breeding ground for fungal and bacteria-based microbes (see Kemp and Bankaitis, 2000). Closing off the ear canal also interrupts the normal function and flow of cerumen, potentially initiating cerumen occlusion and impaction. As audiologists, we know that closing off the ear canal also creates the "occlusion effect," which is the single most annoying side effect experienced by hearing aid wearers. Therefore, closing off the ear canal was never an ideal solution. Nonetheless, until 2001 (see Flynn, 2003), closing off the ear canal was often the only option for the majority of hearing aid fittings.

Candidacy for Open Ear Fittings:

Occlusion is most likely to be experienced by those with normal or mild hearing loss in the low frequencies. As hearing loss progresses past approximately 40 dB HL in the low frequencies (250 and 500 Hz), amplified sound from the hearing aid is usually sufficient to overcome the perception of occlusion (Dillon, 2001). Nonetheless, open fittings are not only desirable for those with good hearing in the low frequencies. Rather, open ear hearing aid fittings are beneficial for all people requiring hearing aid amplification.

Figure 2: Audiogram of 46 year old female fit with ITCs with 2.4 mm collection vents without feedback.

Previously, as hearing loss increased, hearing aid gain increased. As gain increased, tighter fitting earmolds and hearing aids were required to avoid acoustic feedback. Sealed ear canals, the occlusion effect and tight
fitting hearing instruments were previously assumed to be a standard part of the hearing aid experience. However, with modern DSP based highly effective feedback cancellation algorithms, the hearing aid experience has changed. Figure 2 provides the audiogram of a 46 year old female who had worn tightly-fitting ITCs for many years. This patient previously tolerated a closed ear canal and the occlusion effect to achieve her desired cosmetic result. Recently, she was fit with DSP ITCs with 2.4 mm collection vents, using the same gain levels, without feedback.

Patient Selection
Open Ear Acoustics: Technical Pre-requisites:

Two DSP-based pre-requisites are required to achieve non-occluding open ear hearing aid fittings. These are: An effective feedback cancellation algorithm, and a fast "throughput" time ("throughput time" is the time difference between when a sound enters and exits the hearing aid).

Cancellation Algorithms:

Feedback cancellation via digital architecture was eagerly anticipated well before these circuits were commercially viable (see Levitt, 1993). In 2001, digital feedback cancellation algorithms and management were able to nearly instantly cancel acoustic feedback without sound quality degradation.

Figure 3: Spectrogram of an activated DSP hearing aid circuit.
Prior to Time 1, circuit off.
Time 1, 0.05 secs, three on-circuit beeps activated.
Time 2, 0.45 secs, circuit on, feedback
present at 3260 Hz.
Time 3, 0.80 secs, feedback identified and cancelled.

Figure 4: Spectrum of hearing aid circuit (from figure 3) showing feedback present (upper panel) and feedback cancelled (lower panel).

Figure 3 demonstrates a spectrogram made on KEMAR as a DSP hearing aid circuit is activated. As one reviews the bottom of figure 3 (from left to right), it can be seen that the hearing aid is turned off prior to Time 1. At Time 1 (approximately 0.05 secs) the hearing aid is turned on. Three onset beeps occur at approximately 0.05, 0.20 and 0.35 secs. At Time 2 (approximately 0.45 sec) the circuit becomes active in tandem with acoustic feedback visible at 3260 Hz. By Time 3, (approximately 0.80 sec) the acoustic feedback was detected and cancelled. The time required for this circuit to detect and cancel acoustic feedback was approximately one third of a second (0.80 secs minus 0.45 secs = 0.35 secs). Figure 4 provides the spectrum of the output of the circuit while feedback was present (top panel) and after it was cancelled (lower panel).

Processing Requirements
Throughput Time:

To maximally appreciate a DSP-based open ear hearing aid fitting, it is essential that the "direct sound" (i.e.; unamplified sound which enters the ear canal through the vent) and the "indirect sound" (i.e.; processed, or amplified sound which enters the ear canal through the hearing aid) are fused and arrive at the destination within 5 milliseconds of each other. If arrival times are extended beyond this minimal digital delay (5 msecs) an echo sensation secondary to fusion of direct and indirect sound delivery will occur. Stone & Moore (2003) recently reported on the duration of delays which are acceptable to hearing impaired listeners. As processing delays extend beyond 5 msecs, speech understanding and sound
quality ratings are negatively impacted.

Throughput times of DSP circuits are based on the core structure of the circuitry, which varies across manufacturers. Oticon DSP throughput times are less than 5 msecs and are constant across frequency. These time factors allow an acceptable mixture of direct and indirect sounds without perceptible interference. Thus, significant venting can be accomplished without concern for sound quality problems associated with throughput and fusion issues.

Additional Gain via OpenEar Acoustics:

Williams & Wynne (2003) demonstrated that Oticon's proprietary feedback
cancellation circuit can increase the useable gain up to 15 dB in frequency regions where feedback is most likely. Nonetheless, Williams & Wynne noted a small percentage of people will experience acoustic feedback despite the feedback cancellation circuit, and even after significant gain reductions in one or more of the high frequency bands. Our experience suggests that in these same cases, feedback can often be eliminated by making small changes in the depth or orientation of the receiver in the ear canal.

Vent Size, Shape and OpenEar Acoustics:

Figure 5: Diagrams of an ITC with a collection vent (left panel) and with a straight vent (right panel).

OpenEar AcousticsT essentially enables larger vent sizes while
reducing the negative consequences of occlusion. In particular, horn shaped "collection vents" maximize vent effectiveness. Vents need to be at least 2.4 mm to be fully effective in reducing occlusion for most people. Therefore, we typically construct "collection vents" in our custom products, meaning the vent becomes wider as it traverses from the
faceplate into the ear canal. The vent usually ends before the main canal portion of the instrument. Nonetheless, the size of the vent is named according to the faceplate opening. Therefore, a 2.4 mm collection vent has a faceplate opening of 2.4 mm and increases medially. Acoustically, collection vents have a greater effective venting impact than straight vents of the same faceplate diameter (see Flynn, 2003, and Flynn, 2004).

Larger vents, less occlusion
Perceived Occlusion:

Williams & Wynne (2003) also evaluated the effect of different collection vent sizes on occlusion measured in patients using ITCs. They reported little reduction in the loudness of the patient's own voice when moving from "no vent" to a 1.0 mm collection vent. However, when moving to a 1.4 mm collection vent, a significant decrease in own voice loudness occurs. Although the release from occlusion was not complete using a 1.4 mm vent, it was enough to provide some occlusion relief. Further own voice loudness reductions were observed for the 2.4 and 3.0 mm vent sizes.

For some patients, collection vents in custom products and straight vents in BTE earmolds will still create/allow residual occlusion. However, in those cases, we find that offering a BTE with a thin tube and open dome, or a receiver-in-the-ear (RITE) with an open dome, will solve residual occlusion issues.

Larger vents and their impact on the prescribed hearing aid response:

Figure 6: The prescribed insertion gain of a Syncro ITC with a small vent (blue line), the effect on the insertion response of a large vent and no compensation (dotted black line) and the prescribed insertion gain with 50% vent compensation (red line).

Previously, as an industry, we typically used small vents for patients with significant low frequency hearing loss. In those instances, the size of the acoustic vents were so small that we typically did not see significant impact related to prescribed insertion gain. However, with larger vent sizes, the impact is significant. Therefore, based on measured patients' preferences, we've found prescribed insertion gain (historically determined solely by the patient's audiometric characteristics) should be adjusted in accordance with vent size.

Consider - one reason we open vents is to minimize the trapped, low frequency energy generated within the head by the patient's own voice. That same vent reduces the perception of the low frequency gain of the hearing instrument. Theoretically, it seems that we might want to build that prescribed low frequency gain back into the hearing aid fitting to meet the low frequency insertion gain requirement. However, our earlier studies indicated patients preferred about half the lost gain to be restored. Therefore, as shown in Figure 6, our proprietary Genie fitting software uses approximately a 50% gain compensation strategy when prescribing gain for these fittings.

Adjusting Prescriptions
Fusion Fitting:

Traditionally, as a profession, we've fit hearing aids using a substitution approach. In other words, we've closed off the ear canal and had the hearing aid process all the sounds delivered to the ear. Using the substitution approach, all speech and noise sounds were previously delivered through the hearing aid. With the movement towards open fittings over the past five years, patients can now hear a mixture of direct and indirect sound. With vents sizes often ranging from 3 to 5 mm, the direct sound entering the ear is primarily low frequency energy. Recently, with the significantly larger venting of thin tube BTEs and most recently RITEs, the amount of attenuation of direct high-frequency sound has been reduced such that these sounds are often audible for the patient.

As a profession, we are re-evaluating and modifying our thinking about how sound should be processed when direct and indirect sounds are fused.

Figure 7: The prescribed insertion gain of a Syncro ITC with a 1.4 mm collection vent and of the Delta (RITE with an open dome).

At Oticon, we've been following new principles when prescribing gain for open dome fittings for our thin tube BTE and RITE hearing aids. In essence, we prescribe less mid-and-high-frequency gain because a significant amount of direct sound is now audible to the patient, across the bandwidth of the device. This direct sound adds significantly to the total loudness of the signal reaching the patient's ear. The sound processed by the hearing aid, especially in the case of our RITE devices (Delta 8000 and 6000) is viewed as a supplement to this direct sound, not as a substitution (see Figure 7).

Conclusion:

There are certain features in hearing aids that offer such significant and
unquestioned benefits that professionals often believe they have an obligation to discuss and review these features with almost every patient. For example, binaural fittings, multi-channel non-linear amplification and directional microphones are three examples of features that are almost always advantageous for the patient.

Given that non-occluding open hearing aid fittings offer so many positive
benefits for the majority of patients (those with mild-to-moderate sensorineural hearing loss) we believe this option should be explored with most patients.

As noted above, the early rationale behind closing off the ear canal was based on avoiding acoustic feedback. Now that effective feedback cancellation algorithms are available, every effort should be made to maintain the non-occluded ear canal whenever possible.

References:

Dillon, H. (2001): Hearing Aids. Thieme, NewYork & Stuttgart. Boomerang Press, Sydney. Chapter Five "Hearing Aid Earmolds, Earshells and Coupling Systems."

Flynn, M. (2003): Opening Ears: The Scientific Basis for an Open Ear Acoustic System. TechTopic, in The Hearing Review, May, 2003.

Flynn, M (2004): Opening Ear Fittings: Nine Questions and Answers. TechTopic, in the Hearing Review, March, 2004.

Kemp, R.J, and Bankaitis, A.E. (2000): Infection Control in Audiology. Audiology Online (June 4, 2000)
www.audiologyonline.com/articles/article_detail.asp?article_id=214

Levitt, H. (1993). Digital Hearing Aids. In G. Studebaker & I. Hochberg (Eds.), Acoustical Factors Affecting Hearing Aid Performance. Allyn and Bacon, Needham Heights, MA. Pp 317-336.

Stone, M. & Moore, B. (2003). Tolerable hearing aid delays. III. Effects on speech production and perception of across-frequency variation in delay. Ear & Hearing, 24, 175-183.

Williams, C. & Wynne, M. (2003). Occlusion Reduction Through Vent Variation . Paper presented at eh Annual Convention of the American Academy of Audiology, April, San Antonio, Texas.