Rhys Meredith, MSc,
is the head of audiology at Swansea NHS Trust at Singleton Hospital, Swansea, United Kingdom.

The proportion of hearing aids fitted with Wide Dynamic Range Compression (WDRC) is increasing,1 and this amplification strategy is now used in a majority of hearing aid designs. Studies comparing WDRC to linear hearing aids have reported improvements in performance2 and preference,3 particularly in cases with moderate sensorineural hearing loss.

WDRC hearing aids aim to keep a wide range of input sound intensities within a comfortable listening level by automatic adjustment of the amplification characteristics. Thus, compared to linear amplification, WDRC hearing aid users should be able to experience a wider range of environmental intensities without having to manually adjust the volume/gain control. This is a particularly useful feature for those with limited manual dexterity and users of completely-in-the-canal (CIC) devices.

Although studies have reported that a significant proportion of WDRC hearing aid users want a manual volume control or rate the volume control as desirable feature,1,4,5,6,7 many digital WDRC hearing aids cannot be adjusted by the user. It is possible that, although some hearing aid users appreciate the convenience of the automatic adjustment of gain, others find that it does not work well in all situations. The lack of a manual control could be a significant cause of dissatisfaction, as well as increasing the number of fine-tuning sessions required by some hearing aid users.

The present study has a cross-over design and appears to be the first to specifically record user preference for a WDRC digital hearing aid with the manual volume control either activated or de-activated. It also compares the preferences of inexperienced hearing aid users to those of previous users of analog linear hearing aids.

Study Methods
Design. A cross-over design was used in which all participants were fitted monaurally with a medium-power, post-aural, digital, WDRC hearing aid. Participants were initially fitted with the volume control either activated or deactivated on a sequential basis. After approximately 4 weeks the condition was reversed.

Subjects. Participants were chosen from a register of patients waiting to be fitted with hearing aids at a National Health Service (NHS) audiology clinic in the United Kingdom. All study subjects had a bilateral sensorineural hearing loss with no significant conductive component. The hearing loss in the aided ear had to be within the fitting range of the hearing aid used in the study, according to the NAL-NL1 formula.8

A total of 20 participants had no previous experience of using a hearing aid and were categorized as “new hearing aid users.” A further 18 were previous users of linear, analog hearing aids and were categorized as “experienced users.” Details of participants’ gender, age, and hearing levels are shown in Table 1. Printed information sheets and consent forms were mailed to each potential participant at the time of the establishment of a hearing aid fitting appointment. Actions taken during the study participants’ visits included:

Visit 1. Real-ear measurements were completed and the hearing aid was fitted according to the NAL-NL1 formula.8 The hearing aids were programmed using Siemens Connexx (v4.4) fitting software. Further adjustments to the overall gain levels were made, as necessary, for loudness comfort based on the responses to a variety of environmental and conversational voice sounds. To avoid possible order effects, the initial manual volume control setting was either activated or deactivated on a sequential basis.

The initial stage of the Client Orientated Scale of Improvement (COSI)9questionnaire  was conducted, and verbal and printed instructions on the fitting and use of the hearing aid was provided.

Visit 2. The COSI and International Outcome Inventory for Hearing Aids (IOI-HA)10 questionnaires were completed, reflecting the experiences with the hearing aid since it was initially fitted. The manual volume control condition was then reversed and any adjustments necessary for loudness comfort were made again based on a variety of environmental and conversational voice sounds. It was decided prior to the study that any participants who refused to have their volume control set-up reversed could keep their initial volume control condition and would be exited from the study.

Visit 3. COSI and IOI-HA questionnaires were completed for the second time to reflect experiences with the hearing aid since the second visit following the volume control set-up being altered. At this stage participants were also asked to decide whether they preferred to have the manual volume control to be activated or deactivated.

Equipment. All real-ear measurements were carried out on a Siemens Unity Probe Microphone system. The hearing aid used in every case was the Prisma 2M+, a medium-power, four-channel, post-aural, digital hearing aid with digital speech processing. Kneepoints for input compression were set to 45 dBSPL in all channels and compression ratios were not allowed to exceed 2.5:1. Output-limiting compression was used when necessary to match the NAL-NL1 targets for high intensity inputs.

All aids were set with two speech comfort programs and a telecoil setting. The first speech comfort program used an omni-directional microphone; the second program was based on the settings in the first program, but with a directional rather than omnidirectional microphone.

Results
Volume control preference. At the end of the study, 28 participants (73.7%) opted to have the manual volume control set to the activated condition and 10 (26.3%) to have it de-activated. Thus, significantly more participants preferred to have a manual volume control than would have been expected by chance (Chisquare observed vs expected = 4.52, df = 1, p<0.05).

A significantly higher proportion of the experienced hearing aid users preferred the manual volume control to be activated than was preferred by the new hearing aid users (88.9% and 60% respectively) and these  differences in proportion were significant (Chisquare = 4.08, df = 1, p<0.5).

For ethical reasons, at the second stage of the schedule, the study procedure allowed participants to keep their initial volume control setting rather than have the setting reversed. The 6 subjects keeping their initial volume control setting (4 new users and 2 experienced users) did not attend the final third stage of the study, but their preferences and questionnaire results obtained up to this point were included in the results. Table 2 shows the volume control set-up that they elected to keep.

Questionnaire results. COSI and IOI-HA questionnaires were used as a measure of user satisfaction. The results for the new and experienced hearing aid user groups are shown in Figures 2 and 3, as well as Table 3 for both volume control conditions. The results for the two volume control settings were compared using the Wilcoxen signed ranks test. The IOI-HA results were significantly higher when the volume control was activated compared to when it was de-activated for both new hearing aid users (Z = -2.2 p<0.03) and for experienced hearing aid users (Z = -2.28 p<0.02). However there were no significant differences observed between the two volume control conditions for the COSI questionnaire results.

In order to exclude any significant order effect, the final volume control preferences were compared to the settings allocated to participants at their first visit. As can be seen from Table IV, no order effect was found.

As the preferences of the experienced hearing aid group was overwhelmingly in favor of a manual volume control, further analysis aimed at identifying predictive factors for volume control preference was restricted to the new hearing aid user group.

Table 5 shows demographic and hearing level statistics separately for the new hearing aid users who opted for the manual volume control to be activated and those who opted for it to be de-activated. A total of 56% of males preferred to have the volume control deactivated compared to 72% of females. It is also apparent that those preferring the volume control to be deactivated were older and had slightly better hearing. But no variables were found to have a statistically significant predictive value.

Discussion
WDRC hearing aids are becoming increasingly popular. The automatic adjustment of WDRC hearing aid amplification characteristics can improve audibility and loudness comfort through a wide range of input intensities. It can also be argued that hearing aids without manual volume controls have fewer mechanical components that could develop faults and that the gain cannot be altered in error. Some manufacturers’ confidence in WDRC is such that many hearing aids have been produced with no manual volume control option.

There are a number of studies1,4,5,6,7where a significant proportion of hearing aid users indicate a preference for a manual volume control. However, there do not appear to be any previous studies where participants were able to directly choose between hearing aids with or without a manual volume control.

The present study used a cross-over design to test preference. The study was limited to participants with mild and moderate levels of sensorineural hearing loss and used a post-aural hearing aid. Therefore the findings may not be relevant to hearing aid users with severe hearing loss or conductive hearing impairments or to users of in-the-ear (ITE) hearing aids.

It was found that a significant majority of participants preferred to have the volume control activated, and a significantly higher satisfaction score (IOI-HA) was reported when the manual volume control was active compared to the de-activated condition.

Dillon6 found no significant factors that could predict volume control preference. However, in the present study, it was demonstrated that the proportion of experienced hearing aid users (88.9%) who prefer a manual volume control was very high. Similar levels of preference have previously been reported. Kochkin1 surveyed a cross section of hearing aid users in the US and found that a manual volume control was rated as a highly desirable feature by 78% of experienced hearing aid users. Surr et al7 found that 77% of their participants fitted with a WDRC hearing aid with no manual volume control stated that they wanted a manual volume control at least sometimes.

A smaller proportion of participants appeared to need a volume control wheel in a study conducted by Valente et al.5 Their report showed that 44% of experienced hearing aid users found the lack of a volume control on a WDRC instrument to be “somewhat or very unappealing.” However in contrast to the present study and that by Surr et al,7 most of the participants in the Valente et al5 study were already users of relatively modern non-linear hearing instruments.

Although some hearing aid users do not appear to need a manual volume control, the results of the present study and the findings of previous reports would appear to provide evidence against the provision of hearing aids without a manual volume control option, particularly for experienced users.

Conclusion
A cross-over design was used for participants to use a medium-powered hearing aid with the manual volume control in an activated and deactivated condition. A majority of participants preferred the volume control to be activated, and a significantly higher proportion of experienced hearing aid users preferred the manual volume control than the new hearing aid users. The IOI-HA satisfaction score was also found to be slightly higher when the aid was used with a manual volume control compared to when it was de-activated.

References
1.   Kochkin S. MarkeTrak VI: Isolating the impact of the volume control on customer satisfaction. The Hearing Review. 2003;10(1): 26-35. Available online at www.hearingreview.com.

2.   Moore BC, Johnson JS, Clark TM, Pluvinage V. Evaluation of a dual-channel full dynamic range compression system for people with sensorineural hearing loss. Ear Hear. 1992:13(5);349-370.

3.   Humes LE, Christensen L, Thomas T, Bess FH, Hedley-Williams A, Bentler R. A comparison of the aided performance and benefit provided by a linear and a two-channel wide dynamic range compression hearing aid. J Speech Lang Hear Res.1999:42(1);65-79.

4.   Kneble SB, Bentler RA. Comparison of two digital hearing aids. Ear Hear. 1998:19(4);280-289.

5.   Valente M, Fabry DA, Potts LG, Sandlin RE. Comparing the performance of the Widex SENSO digital hearing aid with analog hearing aids. J Am Acad Audiol. 1998:9(5);342-360.

6.   Dillon H, Storey L, Grant F, Phillips AM, Skelt L, Mavrais G, Woytowych W, Walsh M. Preferred compression threshold with 2:1 wide dynamic range compression in everyday environments.  Austral J Audiol. 1998:20(1);33-44.

7.   Surr RK, Cord MT, Walden BE. Response of hearing aid wearers to the absence of a user-operated volume control. Hear Jour. 2001:54(4);32-36.

8.   Byrne D, Dillon H, Ching T, Katsch R, Keidser G. NAL-NL1 procedure for fitting nonlinear hearing aids: Characteristics and comparisons with other procedures. J Am Acad Audiol. 2001: 12(1);37-51.

9.   Dillon H, James A, Ginis J. Client Oriented Scale of Improvement (COSI) and its relationship to several other measures of benefit and satisfaction provided by hearing aids. J Am Acad Audiol. 1997: 8(1);27-43.

10. Cox R, Hyde M, Gatehouse S, Noble W, Dillon H, Bentler R, Stephens D, Arlinger S, Beck L, Wilkerson D, Kramer S, Kricos P, Gagne JP, Bess F, Hallberg L. Optimal outcome measures, research priorities, and international cooperation. Ear Hear. 2000:21(Suppl 4);106-115.

Correspondence can be addressed to HR or Rhys Meredith, Audiology Department, Singleton Hospital, Sketty Lane, Sketty, Swansea SA2 8QA; e-mail: [email protected]