A system that allows dispensing professionals to fit larger vent sizes than was previously possible has been developed. Called OpenEar Acoustics, which is available in Oticons Premium hearing aids. Experience with OpenEar Acoustics since its introduction in October 2001 has uncovered a range of interesting issues. By using independent research findings, as well as Oticons own clinical tests, this article addresses a number of questions relative to open fitting technology, including the underlying basis of the occlusion effect, issues with directional microphones, hearing aid venting, digital processing times, feedback cancellation, upward spread of masking, perception of own voice, and the principles of this system in combination with other advanced digital technologies.
1. What is the occlusion effect?
For many users of modern hearing instruments, one of the unexpected disappointments is the distortion of their own voice (ie, the occlusion effect).1 There has been much discussion about origins and effects of occlusion, but the reality remains that there are only two ways to remove the occlusion effect.2-4
- Allow the low frequency sounds to escape via the ear canal through increasing the vent size (ie, open fittings acoustics);
- Create an earmold that completely fills the cartilaginous portion of the external auditory meatusa solution which is impractical as it often leads to physical discomfort.5
Other methods, including fine-tuning tools, generally provide only partial solutions.
While hearing instruments offer many benefits, occlusion can be a source of immediate and even lasting annoyance.1,6 This can range from the perceptual, such as the sound of ones own voice, bodily functions (eg, chewing) or an uncomfortable sensation of pressure blockage in the ear, to the physical, such as itching and skin irritation. For some patients, these perceptions can verge on the unbearable and result in dissatisfaction and rejection of a hearing instrument.
Figure 1. Occlusion increase of own voice with different earmold vents.
Figure 1 demonstrates that, when the ear canal is occluded with an unvented earmold, low frequency sounds can increase by up to 30 dB. When the earmold is opened by providing increasingly larger vents, we reduce the low frequency amplification of the clients voice and reduce the occlusion effect dramatically. To overcome problems with noise occlusion, dispensing professionals have begun to use the an open fitting strategy which attempts to minimize ear canal blockage and results in improved quality of fitting for their clients.7,8
The traditional task of an earmold is to keep the hearing instrument in place and to seal the amplified signal into the ear so that it does not cause feedback. With OpenEar Acoustics and a BTE instrument for users who have a hearing loss of 65 dB-70 dB in the high frequencies, the earmold can be relegated to the task of the former only. While the client may appreciate open fittings, in practice open fittings can be problematic. One of the main difficulties encountered by dispensing professionals is feedback. The vented fitting is generally believed to be the primary cause, but this is not always the case. The real culprit may actually be the instrument itself. Due to internal construction, some hearing aids function close to their threshold of feedback. Even the slightest alteration to their electroacoustical construction can cause internal or external feedback. If hearing aids have this tendency, then the earmold can be used to plug up the ear to eliminate this additional risk.
Whether the open vent or the hearing instrument itself causes feedback, an increase in vent size requires a complex method of digital feedback control that can work without decreasing the overall gain of the hearing aid. Unfortunately, the issue is complex, and will be discussed in more detail below.
2. What are the prerequisites for an open ear fitting?
One cannot just open up the vent size without creating a feedback problem for the user. In order to solve the occlusion effect via larger vents, a hearing aid is required that is capable of implementing an open ear fitting. There are three prerequisites for increasing the vent size of hearing instruments:
A fast digital processing time. It takes time for the DSP chip of a hearing aid to process the signal. If there is a time delay greater than 10 ms between the processed and the direct sound, users may report unpleasant sensations such as an echo.9
Dynamic Feedback Cancellation. While there are a number of solutions to feedback (eg, notch filtering and feedback management), these strategies all change the acoustic characteristics of the hearing aid in an attempt to reduce the gain at the problematic frequency. These solutions do not allow the implementation of larger vent sizes. Therefore, the feedback cancellation system for open fittings acoustics has to allow for rapid identification and elimination of feedback without increasing the processing time or resulting in any distortion of the speech signal. Once this situation is achieved, there is the potential to increase the size of the vent and open the ear canal, thus reducing the occlusion effect.
Low-frequency gain compensation. With a short time delay and digital feedback control, it is possible to provide the client with an open fitting. Opening the ear canal using a larger vent has consequences in terms of low frequency amplification provided to the client. If the earmold is opened, the low frequency energy (<1000 Hz) tends to dissipate, resulting in a loss of low frequency amplification. In this situation the fitting rationale must compensate for the loss of low frequency energy, or the hearing aid will sound excessively tinny due to the increased high frequency emphasis.
3. How do vents actually work, and how do collection vents function?
A common misconception is that the diameter (in mm) of the vent is the important factor in venting a hearing aid. This, however, is an over simplification. The important concept is not the diameter of the vent but the acoustic mass of air in the vent system. Imagine the vent as a column of air surrounded by the walls of a tube. The air inside this tube has an acoustic mass (not the same as physical mass). It takes energy to overcome the mass in the tube, and overcoming inertia is much easier for low frequencies than high frequencies. Hence, this is why vents have most benefit for low frequency sounds. Following this logic, the acoustic mass in a narrow vent is far greater than the acoustic mass in a wider vent. Hence, the wider vent is more effective in transmitting sound, not because it is wider per se but because the mass of air is less. Additionally, at low frequencies, the dimensions of the vent are small compared to the wavelength of the sound.
Acoustic mass is derived by the equation:
Acoustic Mass = constant x (effective vent length ÷ vent cross sectional area)
This formula shows that the acoustic mass depends on the area and not on the shape of the cross section. (Authors note: Adding a length correction to the physical vent length includes acoustic radiation effects from the vent and results in an effective vent length.) So we can see that long vents transmit less sound than short vents, and narrow vents transmit less sound than wide vents.
Figure 2. Example of collection vents in a CIC and an ITC hearing aid. Note that the vent opening is not flush with the tip end; it is flared to make the vent shorter and more effective.
With custom instruments there is, of course, less space for a vent. While the components and digital chips of hearing instruments are becoming smaller, the required space in the ear canal is still limited. One innovation in custom instrument design is to build a collection vent. There are two critical issues when describing collection vents. First, the cross sectional area is not the same all the way through the vent. In a collection vent, the ear canal opening is larger than the canal tip opening. By dividing the vent into sections, the total acoustic mass can be calculated as the sum of the mass for all parts, not the cross sectional shape of the part (Figure 2). When the total sum of masses equals the acoustical mass of a straight vent, the acoustical effect of the straight vent is similar to the collection vent. Second, the ear canal opening of a collection vent is not only larger, it is also flared. This flaring has the effect of reducing the length of the vent, which also makes the vent more effective. Hence, collection vents benefit both from a smaller air mass and also a shorter vent length compared to traditional vents for custom instruments.
For custom instruments, the size of a straight vent is often limited by the available space in the faceplate of the instrumentand not necessarily by the risk of feedback. For example, a CIC instrument often has space for a 1.4 mm vent opening in the faceplate. However, a 1.4 mm straight vent usually does not reduce the occlusion effect significantly, whereas a collection vent with a 1.4 mm faceplate opening does help reduce occlusion effect due to the increased vent diameter at the canal tip opening. For the client, a collection vent reduces the occlusion effect by an additional 3-4 dB on average compared to an equivalent straight vent.
4. How is vent size prescribed?
The vent prescription in fitting software aims to prescribe the largest vent possible without introducing feedback. The prescription must first take into account the amount of insertion gain, and second, the presence of a dynamic feedback cancellation system.
In determining the amount of desired insertion gain, the fitting software will make a calculation based on the degree and type of hearing loss, as well as the gain prescribed by the different rationale. The more severe the hearing loss, the greater the gain prescribed. Additionally, the presence of a conductive overlay will also increase the amount of prescribed gain. Different hearing aid rationales (eg, Adapto Fast, SKI, NAL-NL1, DSL[i/o]) will prescribe different amounts of insertion gain depending on the rationales underlying gain, slope, and compression rules. Therefore, for the same hearing loss, different vent sizes could be prescribed depending on the prescription targets in the 2000 Hz-4000 Hz region. For example, a more severe hearing loss coupled with a hearing aid rationale that recommends more gain will yield greater prescribed insertion gain and a smaller vent size.
The second determinant of vent size is the presence of a dynamic feedback cancellation system (DFC). The DFC, by eliminating feedback, increases the feedback margin of the hearing instrument and therefore allows larger vents to be prescribed depending on the insertion gain targets of the selected rationale. The patient with DFC active will always have a larger vent size than if the DFC was not present in the hearing instrument.
5. What about open fittings and directional microphones?
Some concern has been raised recently about the impact of increasing the vent size and benefit of directionality. So much so that some researchers have suggested that a larger vent will compromise the benefits of directional microphones.10,11 In a directional microphone, sounds from the side and the rear are attenuated in preference to sound from the front. The concern is that the vent will allow low frequency sound from the rear to pass through the vent without attenuation, thus reducing the directionality.
Recent research by Ricketts12 has found that the effect of venting on directionality has no effect on directionality of sounds over 1000 Hz. Hence, even with a 4 mm vent size, the loss of directivity is restricted to below 1000 Hz. With a 4 mm vent and a relatively flat AI-DI, the average AI-DI results for a directional microphone were still approximately 4 dB greater than for the omnidirectional setting.
Fortunately, not all directional microphones are equal. As the independent research by Ricketts12 determined, some directional microphones have more emphasis in the low frequencies than others. Consequently, when increasing the vent size, care should be taken to use a directional microphone where the emphasis is in the high frequencies. In this way, directionality is maintained. The lack of compatibility between the emphasis of the directional microphone and the effects of an open fitting may be why some people have recommended against the fitting of larger vents with directional microphones.11 It is critical when implementing this system that the directional microphone does not have the emphasis in the low frequencies.
Figure 3. Directivity Index (DI) for the Adapto ITE implementing OpenEar Acoustics, demonstrating the high frequency emphasis of the directional microphone.
Due to the low frequency loss of a larger vent size, the Oticon directional system is optimized specifically in the mid and high frequencies (Figure 3). There is 4 dB or less directional improvement in the frequency region from 500 Hz to 1000 Hz, but greater than 6 dB improvement at 3000 Hz through 4000 Hz. Consequently, when implementing Open Ear Acoustics in an advanced digital hearing aid, the directivity of the directional microphone must complement the open earmold so that the consumer will not be disadvantaged by having both a directional microphone and a larger vent. Additionally, the focus of directionality on the high frequency regions will further enhance speech understanding in noise due to the importance of speech information in the high frequencies. With this design, the signal-to-noise ratio (SNR) is enhanced in the frequency region that most affects the overall speech understanding performance.
The effects of the fitting prescription on directivity and open fittings should also be considered. First, we must ensure that approximately 50% gain compensation is used (which was not implemented in the study by Ricketts12). Second, if the client only has a mild hearing loss in the low frequencies, then less low frequency gain is prescribed. Therefore, whether you have a vent or not, the DI in the low frequencies will be lower, so the effects of opening the vent will be reduced. Third, if the client has a significant low frequency hearing loss, the amplified sound will be louder than any additional sound that might reach the eardrum by the direct pathway, even if it can be heard.
The larger vent size does reduce low frequency gain and provides a passageway for sounds to pass through the ear canal. Vent compensation strategies increase the gain, and for many users, the direct low frequency sound may not actually be audible. Crucially, while low frequency directivity is decreased with larger vents, the DI above 1000 Hz is not significantly affected, so that the system allows for the comfort of increased vent size along with the benefits of directionality.
6. What about open fittings and the upward spread of masking?
It has been questioned whether the large venting schemes used in this open ear fitting system can result in upward spread of masking problems. In order to address this concern, we need to accept the following facts:
- A large vent lets sounds (especially low frequency) in through the ventilation without any amplification at all (corresponds to 0 dB insertion gain).
- Whenever there is a positive amount of insertion gain in an instrument, the sound through the vent will contribute only minimally to the users perceived sound (ie, less sound than through the hearing instrument).
Modern hearing instruments are designed to provide amplification of speech sounds (or letting loud speech sounds through with only limited or no amplification). Hearing loss compensation only rarely calls for the device to attenuate incoming sounds. Therefore, the amount of insertion gain in the instrument will typically be greater than 0 dB. Whenever this is the case, the sounds through the vent will always be softer than the sound processed in the instrument.
The situations where the insertion gain will approach 0 dB will be where hearing is close to normal. Also, large vents are associated with smaller hearing losses and thus limited compensation. If these conditions are fulfilled, the sound through the vent might be comparable to the sound through the instrument. However, in situations with close-to-normal hearing, upward spread of masking is not a concern (ie, the risk of upward spread of masking increases with greater degrees of hearing loss and decreasing frequency resolution). Therefore, when discussing OpenEar Acoustics, we should be mindful that the direct sound will usually be less intense than the amplified sound, and contributes neglibly to upward spread of masking.
7. How does the system use DSP?
The advanced digital processing in modern hearing aids takes time to act. If there is a direct route that bypasses the DSP of the hearing aid, then the two sounds may reach the eardrum at slightly different time periods. A concern with open fittings has been that, if the direct and processed sound are significantly out of time (termed group delay), then the user will object to the sound and, in some instances, this group delay will cause auditory confusion (eg, an echo). Of course this sensation is to be avoided, and a slow signal processing time would preclude the implementation of a larger vent, or any vent at all. Agnew & Thornton9 demonstrated that processing delays of greater than 10 ms were objectionable to 90% of users. Therefore, as a guideline, any device that has a vent (ie, especially an open ear fitting) must have a digital processing time of less than 10 ms. The Adapto hearing instrument, for example, implements an active Digital Feedback Cancellation system in addition to Voicefinder. Despite this advanced functionality, the total sound processing time is approximately 3.4 mswell within the guidelines for ensuring no noticeable group delay. Consequently, OpenEar Acoustics is compatible with other forms of advanced DSP aids, as long as the group delay between the aids does not become greater than 10 ms.
Figure 4. The average occlusion decrease when going from no vent to a 3 mm vent for the vowel /u/. 14
8. How does opening ears help reduce occlusion?
With OpenEar Acoustics, the client should have an improved perception of their own voice when compared to a more occluding earmold.13 This sensation is depicted in Figure 4 where the reduction in own voice level for the vowel /u/ is shown. This objectively demonstrates the improved rating for instruments with open ear fittings over previous hearing aids with occluding earmolds. It is important to remember, however, that due to the amplification provided, the client will report differences between his/her unamplified voice and the amplified sensation. The important issue here is that the open earmold will reduce the occlusion effect for the user (especially in comparison to a more occluding earmold) and should lead to greater immediate acceptance of the fitting.
Table 1. Matrix of previous ear vent sizes and the open ear vent sizes implemented in Adapto.
9. What are the clinical results of this system?
As part of the testing of the OpenEar Acoustic system, 52 clients who wore well-fitted advanced digital hearing instruments were provided with the Adapto.13 Table 1 shows that 47 of the 52 users obtained an increased vent size. It should be noted that 2 participants were upgraded from a BTE to an ITE instrument and the larger vent size could not be accommodated. OpenEar Acoustics led to an increase in vent size of 85%, or the vent increased on average from 1.7 mm to 3.2 mm. Importantly, once the vent size was greater than or equal to 3.2 mm, the user was ensured of a virtually occlusion-free hearing experience and more satisfaction/acceptance with the instrument.
To assess the benefit of the system, the dual issues of occlusion and feedback were examined. In terms of occlusion, the physical feeling of wearing the hearing instrument was researched. For this assessment, 65% of clients found that with the open ear system they felt less occluded than with own aid (p<.01). Thirteen clients found no difference, and only 4 clients found the sensation with their previous digital hearing instrument better.
The results showed a significantly higher rating of naturalness of the clients own voice, and substantially more observations as good, natural, or do not think of it than with their previous digital hearing instruments. Physically, more subjects felt their ears were less plugged up with a more open earmold. Regarding feedback, both occurrence and annoyance were rated significantly better (p<.01) with the open ear system than with an instrument with greater occlusion of the ear canal.
Interestingly, Schum & Pogash7 found that experienced users rated the performance of a hearing instrument with the open ear system more positively than new users. It was concluded that experienced users had greater experience with the problems of occlusion and greatly appreciated the benefits of an open fitting. Conversely, new users had no comparison with traditional technologies that provided the unwanted and often-reported side-effects associated with occlusion. In a follow-up study with three different levels of technology, Schum & Pogash15 found that, when new users had experiences of an occluding earmold versus the open ear fitting, they rated the latter just as highly as experienced users on the dimensions of physical comfort and sound quality. Therefore, a users previous hearing instrument experience affects their positive ratings of open fittings, but this does not affect the overall satisfaction with the open ear fittings for new and experiences users.
Modern digital hearing instruments offer the advantages of focusing on issues that hearing-impaired consumers have found problematic in traditional hearing instruments. One of those issues is the occlusion effect. With advanced digital instruments, it is now possible to increase the vent size and reduce the occlusion effect through OpenEar Acoustics. Through an advanced digital feedback system and a larger vent size, it is possible to reduce or eliminate occlusion rather than just manage its effects.
|This article was submitted to HR by Mark C. Flynn, PhD, research audiologist at Oticon A/S, Hellerup, Denmark. Correspondence can be addressed to HR or Mark C. Flynn, Oticon A/S, Strandvejen 58, Hellerup, DK 2900, Denmark; email: [email protected].|
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