In fitting hearing aids, be very careful when you amplify the low frequencies

If you are new to audiology, be careful when you read this article. Today I want to tackle an advanced concept, so you need to have considerable experience and well-developed intuition when you attempt to employ these ideas.

Listed below are hearing thresholds for six patients. For the sake of simplicity I’m showing you their hearing thresholds for only ear (their hearing is the same in both ears). Also, for simplicity, I am showing thresholds for the five standard five test frequencies (250, 500, 1000, 2000, and 4000 Hz):

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Let’s assume we have already established a standard listening program for these patients. We’ll call it the “default” program or Program 1. Now, let’s consider what happens if we add substantial low-frequency amplification (go bass heavy) or if we subtract substantial low-frequency amplification (go bass light).

 

SIX PATIENTS, CASE BY CASE

The first patient has good hearing in the lower frequencies, so I see no reason to go bass heavy with her. She is an excellent candidate for an open fit (a naturally bass light fitting).

The exception to this logic is a hard-of-hearing infant. In fitting a baby, we need to give the child the very-low-frequency rhythm and intonation cues inherent in speech in the zone of 100 to 300 Hz to allow for development of normal language abilities. My old professor at the University of Tennessee, Dr. Carl Asp, would say all hard-of-hearing children need extra-wide-band amplification so they not only assimilate the articulated aspects of speech but also its emotional content.

If Patient 2 is an adult who has not previously worn hearing aids, you will probably give him an open fitting. However, bear in mind that no truly open-fitted aid can give a patient significant amplification in the lower frequencies, because low-frequency sound escapes easily through any type of opening. So, an important aspect of this fitting is designing a fitting program with the patient: open fitting the first 1-4 months, followed by the addition of earmolds to modify this into an occluded fitting, which lets you add substantial amplification to the lower frequencies. This approach makes this new hearing aid wearer happy initially, then helps him get greater benefits once he becomes used to amplification.

Patient 3 and 4 both have “reverse curve” hearing loss configurations. Going bass heavy with these patients is an approach I would recommend only to highly experienced practitioners. When you start adding too much gain in the lower frequencies, you quickly get into an “upward spread of masking” situation, which results in deterioration of sound quality and word understanding. To my experienced peers I would say, “Adding low-frequency gain is like adding salt to your vegetable soup; you need some salt, but be very, very careful how much you add.”

Since Patient 5 and 6 both have very poor hearing, a wide-band response will probably work best. Once again, caution and skill are needed and you must pay a lot of attention to the amounts and type of compression you use.

One last thought. We don’t tend to measure and consider hearing at 125 Hz, yet this is a very important frequency for some patients (patients #5 and #6, for example). If you are considering adding significant amplification in the lower frequencies, it pays to measure the threshold in that zone.

 

1 Comment

  1. Hooray — Dr Bob “gets it” on reverse slope SNHL! He writes,

    “Patient 3 and 4 both have “reverse curve” hearing loss configurations. Going bass heavy with these patients is an approach I would recommend only to highly experienced practitioners. When you start adding too much gain in the lower frequencies, you quickly get into an “upward spread of masking” situation, which results in deterioration of sound quality and word understanding…”

    In fact, Upward Spread of Masking is what wrecks 98% of reverse slope SNHL (also called RSHL) fittings; and if you actually *validate* the fitting with speech-in-noise testing, you’ll almost always see aided scores LOWER than aided: The HA professional plugs the audiogram into the fitting software, selects a “prescription,” hits auto-fit… And *automatically* gets it wrong every time, as NONE of the prescriptive methods is designed for RSHL (though Brian CJ Moore’s CAM2 (CAMEQ2-HF) comes the closest). What is espe is when children over the age of about 6 who have fully developed speech production are discovered to have RSHL, and the audiologist not only plugs in DSL5-I/O but also “verifies” the excess low frequency with a probe mic; but *fails* to validate the fitting with speech-in-noise testing, or even with GHABP or COSI.¹

    Patients 3, 4, & also 5 probably have low frequency cochlear dead zones, which can occur with losses as low as 40dBHL; and of which the boundaries need to be mapped out using the Threshold Equalizing Noise (TEN) test, also by Professor Moore. Having a low frequency dead zone is a serious problem² as it sounds distorted and scratchy, like listening through a blown loudspeaker driver. Robyn Cox has done some research indicating that it is acceptable to amplify down to 0.56x of the boundary, i.e. if the cochlea is dead up to 1000 Hz then amplify down to 560 Hz — And I respectfully disagree, as although it might help slightly in quiet, it *completely* blows up in noise, completely destroying speech discrimination.

    BOOTNOTES:
    1) We have several parents of RSHL kiddies age 5-8 in the Facebook RSHL group; and all have the same complaint that their kids have to take off their hearing aids when it’s noisy;

    2) My own cochleas are dead to past 600 Hz, with the loss occurring when I was 19.

    3) There’s a big Reverse Slope Hearing Loss group on Facebook: Check it out.

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