Professionals who dispense hearing instruments know that it is almost inevitable that some adjustments to the recommended first fitting will be required. Follow-up visits can be time consuming and, if managed improperly, frustrating for both the professional and the hearing instrument wearer. As a consequence, the wearer may become dissatisfied and return the device for credit or leave the hearing instrument in a drawer un-used. These reactions are detrimental not only to their communication abilities but to the entire hearing care profession as the negative experiences of dissatisfied customers are shared with potential future users.
Having excellent clinical skills and a thorough understanding of fitting technology is important for the successful dispensing professional. This paper provides some direction for making adjustments on multi-channel compression hearing instruments, the predominant type of emerging technology in todays hearing instrument market.
Multi-Channel Compression Instruments
Multi-channel compression hearing instruments are devices that provide independent non-linear signal processing in many discrete frequency regions. Multi-channel capability allows wearers to understand speech better in certain noisy environments. For example, in pediatric fittings, it helps them to monitor their own voice and assists in their speech production.1 For the dispensing professional, the capabilities of multi-channel instruments allow greater flexibility to fine tune the settings on the hearing instruments to meet the wearers needs.
The advantage of multi-channel compression processing over single-channel linear processing is exemplified in situations with limited-frequency background noise. A kitchen, for example, can have high-frequency noise due to the clanging of dishes, silverware and pans. Individuals wearing a single-channel linear hearing instrument in these environments often turn down the volume control of their hearing aid. This user manipulation reduces the intensity of the high frequency noise, but also decreases the audibility of speech in other frequency regions.
Multi-channel compression hearing instruments process sounds independently in smaller frequency ranges. One way to envision this is to imagine a two-channel linear hearing instrument with two volume control wheels. This instrument would allow the individual in the kitchen to turn down the volume of the higher-frequency noise and leave the volume of the lower-frequency sounds at the same higher gain setting. While this decreases the audibility of the high-frequency speech sounds, the low-frequency speech sounds remain intact. Obviously, it would be difficult to place multiple volume control wheels on a hearing instrument and then train the wearer in their proper manipulation. Thus, multi-channel hearing instruments require non-linear processing.
Fig. 1. Input-output curves between a linear hearing instrument and a non-linear hearing instrument matched at a conversational input. Note the extra output from the non-linear compared to the linear hearing instrument.
Input-output curves are useful in demonstrating the difference between linear and non-linear hearing instruments (Fig. 1), For the linear hearing instrument, gain is constant at all input intensities. This is illustrated by the straight line that forms a 45° angle with the x-axis on the input-output graph. If the professional increases the gain setting on a linear hearing aid, all input signals are increased in output intensity by the same amount. While this may solve the wearers complaint of not hearing soft sounds adequately, it may also result in conversational sounds and loud sounds being too loud or uncomfortable. With a linear hearing instrument, the dispensing professional can only instruct the wearer on the proper volume-control adjustment in different listening environments.
Non-linear hearing instruments often use wide dynamic range compression (WDRC) processing which reduces gain as input levels increase. The WDRC instrument shown in Fig. 1 shows gain reduction above the 20 dB HL input level. This point is known as the compression threshold (CT).
The input-output curve shown in Fig. 1 is controlled by three compression parameters. The compression threshold controls the gain of the hearing instrument at input levels around the compression threshold. Since the CT is typically below 60 dB SPL, it controls gain for very soft sounds. A second parameter is the hearing threshold level (HTL) or Gainmedium parameter. This controls gain for conversational level sounds. The last parameter is the UCL or Gainloud parameter that controls the gain of the hearing aid at high input levels. Together these three parameters determine the resulting input-output curve of the compression hearing instrument. Because of their unique contribution to the I-O curve, a simple but powerful tool to fine tune or troubleshoot complaints for a compression hearing instrument is to determine the input level at which the complaint is based. If the complaint has to do with soft sounds (low input signals), CT is the parameter to adjust. If the complaint is related to conversational level input, the Gainmedium parameter will need adjustment. Likewise, if the complaint has to do with loud sounds, the Gainloud parameter will need to be adjusted.
In order to accurately determine the input level of the wearers complaints, it is necessary that the wearer effectively communicates his/her complaint and that the hearing care professional accurately understands that complaint. In other words, effective communication needs to occur. In the real world, however, not all wearers are trained in effective communication or in the specific meanings of hearing-related words. Dispensing professionals cannot assume that the client knows how to effectively explain the nature of his/her problem with the hearing aid. Therefore, it is helpful to have a plan for effective communication.
The first step in understanding a clients complaint with a hearing instrument is to listen carefully and determine the crux of the problem. This can usually be found in a few key words spoken by the wearer to describe the problem. The professionals task is to consider the various meanings of these key words in order to determine the true nature of the complaint. During this questioning phase, the professional should also pay attention to the wearers non-verbal cues.
Key Words: Key words are descriptors that reflect the wearers perceptions with the hearing aids. For example, words like too loud, too soft, shrill, raspy, tinny and rumbling are often mentioned. Some professionals have a list of these key words available to help their clients describe their perception with the hearing instruments. The wearers look over the list of descriptors and choose the ones that best describe their perception. Widex has collected a list of common descriptors that are frequently used by hearing instrument wearers to describe their perceptions with hearing instruments (Fig. 2).
Figure2. Widex Solution Identification Worksheet.
Clarifying Ambiguities: After determining the key words, the professional must clarify any ambiguity in the wearers description. While there are a number of ways to say the same thing, there can also be several meanings for the same word or group of words. It is important to be precise in the definition of the problem.
An example of a client comment is that the hearing aid is not loud enough. Although the immediate reaction is to increase the gain of the hearing instrument, such action may not be the most effective unless one is certain that not loud enough means sounds are too soft and not sounds are not clear.
If the wearers complaint is a lack of loudness (too soft), a gain increase for low frequency sounds might solve the problem. This is because low frequencies contribute significantly to the overall loudness of speech. However, if the complaint is a lack of clarity, a gain increase for the mid-frequencies may be more appropriate.
Learning about the Listening Environment
Following the understanding of the actual problem, it is important to understand the listening environment in which the problem occurs. This entails obtaining specific details regarding the input intensity levels as well as the types of background noise. For example, the wearer may report that the problem only occurs at the dinner table. What is unknown is whether the situation is quiet or noisy, with only a few people talking, or whether a loud television is present.
Knowing the background can help the professional estimate the intensity of the input signal. For example, speech in a noisy environment is most likely occurring at a high input level because of the Lombard reflex. This can signal the dispensing professional to adjust the gain parameter controlling high input levels (UCL or Gainloud) and not the gain parameter for soft or conversational level sounds.
Where to Start Adjusting?
Adjustment can begin after the professional has obtained accurate information regarding the wearers reactions to the hearing aid. There are a number of tools that can assist the professional in making educated decisions as to what hearing instrument adjustment is necessary. Flow charts organize the potential solutions to a problem based on a logical order. Kuk2 provided a detailed discussion of how to construct a flow chart for hearing instrument troubleshooting. This tool is helpful for problems such as feedback or occlusion.
Another tool for problem solving is the input-output curve.3 As was discussed previously, the I-O curve provides a representation of the output intensity for the range of input intensities. If the output scale is in hearing level (HL), the professional can infer from the audiogram if the amplified output signal may result in certain perceptions by the wearer (e.g., aided output is not audible, aided output exceeds UCL and is reportedly too loud, etc.).
Worksheets as Tools
The Solution Identification Worksheet (Fig. 2) created by Widex is designed to assist wearers in describing their reaction to their hearing instrument. It is also helpful for the professional in making fine-tuning adjustments.
The worksheet is organized to provide guidance in making the adjustments. The first column provides potential changes to the different hearing instrument parameters for the low-, mid- and high-frequency regions. In the center of the worksheet are the descriptors clear, bright, comfortable and natural. If the wearer uses these words, the dispensing professional would not need to make any modifications to the hearing instrument (i.e., the left column for that row indicates No Modification).
Recall that non-linear hearing instruments decrease their gain as the input signal increases. When adjusting hearing instruments of this type, it is important for the professional to determine whether soft, moderate or loud input sounds need to be adjusted in gain. The worksheet divides these adjustments based on the input intensity (soft, moderate or loud). If the wearers description falls on the top portion of the chart, the dispensing professional should increase gain and therefore output for that input intensity; if the wearers description falls on the bottom portion of the chart, the professional should decrease gain for that gain parameter.
The final classification of descriptors in the worksheet is based on the frequency region for adjustment: low-, mid- and high-frequency. The right three columns represent these regions. As an example, if the wearer indicates own voice boomy, the professional should decrease gain of the loud input gain parameter (UCL or Gainloud) in the low frequency channel.
Another example may be a wearer complaint of too loud. It is obvious that a decrease in gain is necessary. The question is which gain parameter (for soft, medium or loud input) and which frequency region? The worksheet directs the professional to consider reducing gain parameters of moderate and loud inputs in the low- and mid-frequency regions. The soft-input range is not included, because it is uncommon that excessive gain is applied for soft sounds to make them too loud.
At this point, the dispensing professional needs to obtain more clarification from the wearer. Based on this clarification, it might be necessary to change a single hearing aid parameter or combination of parameters to improve hearing aid satisfaction. First, the professional should ask the wearer if the loudness problems are related to moderate and/or loud sounds. This will determine what output regions to adjust. For example, if the concern is only for loud input levels, the UCL or Gainloud should be reduced.
As indicated earlier, clarification of the listening environment can also be useful. Multi-talker background noise will have higher intensity low-frequency noise as the input when compared to listening in quiet. In the multi-talker noise, a gain decrease in the low-frequency region for loud input signals would be appropriate.
As another example, a wearer who cannot describe the problem may choose the descriptors listening in a sea shell and endings of words are unclear from the worksheet. The professional finds listening in a sea shell on the worksheet and determines that a decrease in the low-frequency region for soft inputs is recommended. The professional also finds for endings of words are unclear that an increase in gain for high frequency, soft inputs is recommended. It is important to recognize that these are suggestions and starting points. The wearers will need to provide more feedback to verify if the correct adjustment is made. Also, hearing instruments across and within manufacturers vary by the number of channels and compression parameters, as well as the flexibility for adjusting these parameters. The product-specific parameters will direct the exact adjustments.
When fine-tuning hearing instruments in response to client comments, two major areas need to be understood clearly. The first is an understanding of the individuals perception of the hearing instrument. The description of the sound quality is important, but information on the listening environment and input intensity is also valuable. The second area is a clear understanding of the wearers hearing instrument. Todays high-technology hearing instruments have the potential to increase the sound quality and offer more flexibility for adjustment. However, the professional must know how to utilize this flexibility.
Prior to making adjustments to a hearing instrument in response to a clients complaint, the hearing care professional also needs to determine how the problem can be best resolved. With todays advanced hearing instruments, thousands of combinations of parameters can be manipulated and, often, one change to address a specific listening situation can have unexpected consequences for another listening situation. Thus, the dispensing professional needs to keep in mind that an adjustment to fix one complaint may, in turn, cause a problem somewhere else. Particularly with respect to complaints that do not relate to audibility, intelligibility or comfort, some dispensing professionals say they first try to counsel the patient on the problem prior to making adjustments to the hearing instrument.
This article provided a systematic approach to effective communication and adjustment of multi-channel compression hearing instruments. Professionals are encouraged to use multiple fine-tuning plans depending on the clients needs. For example, a hearing instrument wearer who is having difficulty finding the right words would benefit more from the worksheet presented in this article than a custom flow chart. By having a number of resources, the dispensing professional can more effectively perform hearing aid adjustments and have more time for aural rehabilitation and counseling.
John A. Nelson, PhD, is a research audiologist with Widex Hearing Aid Co., Long Island City, NY.
1. Kuk F & Ludvigsen C: Hearing aid design and fitting solutions for persons with severe-to-profound losses. Hear Jour 2000; 53 (8): 29-37.
2. Kuk F: How flow charts can help you troubleshoot hearing aid problems. Hear Jour 1999; 52 (10): 46-52.
3. Kuk F: Using the I/O curve to help solve subjective complaints with WDRC hearing instruments. Hearing Review 1998; 5: 8-16, 62.
Correspondence can be addressed to HR or John A. Nelson, PhD, Widex Hearing Aid Co., 35-53 24th St., Long Island City, NY 11106; email: [email protected].