What are good ways to manage loudness complaints with ReSound hearing aids?

Question

Answer

One rationale for hearing aids with wide dynamic range compression (WDRC) is reduced need for volume control because the input level dependent compression will automatically reduce gain for higher input levels relative to lower ones. The logical extension of this is that hearing aids with this technology should be able to provide amplified sound that is audible but not too loud. When fitting ReSound hearing aids, there are three fitting rule parameters that help in getting users off to the best start with their hearing aids. These are the experience level, the “percent gain” and the inclusion of measured UCL values in the target calculation:

• Experience level – applies input level and frequency dependent corrections to the targets based on experience with amplification. Applies to the proprietary Audiogram+ and NAL-NL2 fittings.
• Percent gain – calculates % of prescribed gain and applies to the gain settings while the targets remain unchanged. Provides a quick way to reduce or increase overall gain for any fitting prescription. 70% of prescribed gains are automatically incorporated into the “First time user – onboarding” experience profile. This profile will adjust targets for a person inexperienced with amplification and gains will be set to 70% of targets. Applies to all supported fitting rules.
• Use UCL – if UCL values are available and this option is checked in the fitting menu, frequency and input level dependent decrease (or increase if UCLs are higher than predicted) in prescribed gains will be applied when fitting to Audiogram+. MPO settings will be affected if fitting to DSL5.

If a hearing aid user complains that sounds are too loud once the hearing aids are fit, the Hearing Care Professional (HCP) has two potential technical tools for an immediate solution to the complaint: gains and MPO (Maximum Power Output). We recommend gain adjustments first and foremost, as they will affect the hearing aid user’s perception of loudness of everyday sounds. MPOs are less effective in this respect. In fact, hearing aids with multichannel WDRC may never reach the levels that MPOs are set to in daily environments.¹ And reducing MPOs can have unintentional downsides that are harmful to sound quality – more on that below.

A first step in managing loudness complaints is to probe for more specific information. Are all sounds experienced as too loud? An overall gain reduction may be in order. Are there specific sounds that are too loud? If so, the typical levels and frequency weighting of those sounds provide important information in deciding how to adjust gains. A little-known helper in the Smart Fit fitting software can be found in the Sound Player, which can be selected from the Menu icon in the upper right corner of the fitting screen. While it is obvious that playing sounds for the aided hearing aid user to judge might be useful for validating that loudness for different sounds is appropriate, the Sound Player also provides information for each included sound on frequency content (broadband, or weighted in a low, mid or high range) and typical level (soft, moderate or loud). For example, “Cafeteria ambience” is categorized as having broadband frequency content at a moderate level. A hearing aid user who mentions this type of environment as being too loud might benefit from an overall gain reduction for moderate and perhaps also loud sounds.  In contrast, a vacuum cleaner tends to have loud low frequency content, so a complaint related to this type of sound might be helped with a reduction in low-to-mid frequencies for loud inputs.

Depending on the technology level of the hearing aids being fit, other features that control gain and noise reduction for different types of environments and sounds may also be available. Such features can be helpful because they can address environmentally dependent loudness issues. These include the Environmental Optimizer, the Environmental Optimizer II, and Impulse Noise Reduction.

• Environmental Optimizer – the HCP can adjust overall gain offsets in 7 different environments. This provides automatic, environmentally dependent volume control functionality.
• Environmental Optimizer II – in addition to overall gain, the strength of Noise Tracker II per environment can be set.
• Impulse Noise Reduction – reduces peaks for loud transient sounds. It is effective at lower levels than MPO would typically be set to and does not affect overall loudness of amplified sound.

Finally, those wearing hearing aids with wireless connectivity have multiple options in the Smart 3D app for adjusting overall volume and frequency dependent volume to their liking, and to save these settings for easy access later.

But what about the MPO?

MPO settings in ReSound hearing aids are channel specific and will differ depending on the selected fitting rule. The prescription of the MPO settings for Audiogram+ and NAL-NL2 come from the NAL-NL2 fitting rule. This prescription does not incorporate measured UCL values, while other fitting prescriptions such as DSL5 do. Thus, when Audiogram+ or NAL-NL2 are selected in the fitting software, measured UCL data will not affect the prescribed MPO settings. Regardless of the prescription, MPOs can never be higher than the receiver capabilities. In other words, if the MPO prescription at a particular frequency is 110 dB SPL, but the receiver in the hearing aid being fit can only output 105 dB SPL at this frequency, then the MPO setting will be 105 dB SPL.

HCPs who verify MPOs in the real ear or coupler sometimes express concern that the measured values are “under target”. Similarly, some authors have suggested that current hearing aids show too low OSPL90 relative to their MPO settings when programmed to user settings.² It is important to recognize that the MPO prescription and settings are not actually targets to be met, but rather limits not to be exceeded. If non-linear hearing aids are not fit near full-on gain and depending on the compression ratios and time constants of the hearing aid compressor, measured output for a 90 dB SPL signal at the user’s gain settings may be far below the NAL-NL2 MPO prescription, which might surprise HCPs to know is for linear amplification. In discussing OSPL90 for non-linear hearing aids, Dillon¹ states: “In the absence of a procedure specifically designed for non-linear hearing aids, a practical solution is to use the OSPL90 prescription for linear hearing aids” (p. 331).

Does this mean that the MPO setting does not matter? Of course not. It does matter in that it will keep the hearing aid output from exceeding uncomfortable levels regardless of how the HCP might adjust the gains. It is also important to note that MPO settings can be too low, which was identified as a potential sound quality problem in the article mentioned above.² Output limiting uses either a very high amount of compression, peak clipping, or some combination of the two to ensure that sounds above a certain level are not emitted by the receiver. Regardless of whether compression limiting or peak clipping are used for output limiting, this is very different than lowering gain in that it necessarily adds a lot of distortion to the output signal when the MPO is reached. This is almost always undesirable so it is also a goal of the MPO prescription to prevent the hearing aid from operating in saturation to the degree possible. Just as there is an acceptable maximum output level, there is also a minimum acceptable output level that will maintain good sound quality for speech and other everyday sounds.  

In validating the National Acoustic Laboratory’s approach to prescribing MPO, Storey et al³ investigated the range of acceptable settings for MPO and found that it was influenced by the type of sound as well as the type of output limiting. Generally, the upper limits – or the highest acceptable values the MPO could be set to – were not different for peak clipping versus compression limiting, but higher settings were accepted for impulse sounds than speech. Minimum levels were most affected by limiting type, with lower levels being acceptable for compression limiting than peak clipping. Based on this data as well as clinical experience, ReSound recommends setting the channel specific MPO settings no lower than 95 dB SPL and preferably higher if possible. Consequences of too low an MPO setting can be added distortion and degraded sound quality in many everyday environments. It can even lead to complaints of intermittency where it sounds like the hearing aids are shutting down and coming on again.

Key takeaways:

• Using tools that affect gains are the recommended first line of defense in dealing with loudness complaints.
• A lower measured OSPL90 than the prescribed MPOs for NAL-NL2 is an intentional effect of the design of a WDRC system. The important thing is for the OSPL90 not to exceed these levels.
• Ideal MPO settings will be low enough to avoid loudness discomfort, but high enough to ensure that the hearing aids are not operating in saturation in most everyday listening environments.
• For ReSound hearing aids, it is recommended not to set MPOs lower than around 95 dB SPL, and preferably higher.

References

1. Dillon H. Hearing Aids. 2012. Thieme: New York.
2. Mueller G, Stangl E, Wu Y-H. Comparing MPOs from six different hearing aid manufacturers: Headroom considerations. Hearing Review. 2021;28(4):10-16.
3. Storey L, Dillon H, Yeend I, Wigney D. The National Acoustic Laboratory’s Procedure for Selecting the Saturation Sound Pressure Level of Hearing Aids: Experimental Validation. Ear & Hearing. 1998: 19(7); 267-279.