Recruitment

Hearing Loss
#5

One of the reasons I wanted to post the recruitment thread was due to my high frequency hearing loss. Others may have similar situations.

Ed, I also believe you are correct about the audio gram results. With false interpretations of hearing levels in the higher frequencies it is a set up for failure concerning the initial tuning of the aids.

I have found that when the aids are pushed, gain wise, in the upper frequencies they can quickly become very loud and not understandable. This I believe is recruitment in my case. The Phonak program does not see recruitment and tunes for this high frequency loss. This in turn will blow some users out when in noisy areas. So, I have de-tuned or lowered the gain, on my aids in these upper frequencies with fantastic results.

Ed, does this sound correct?

Thanks

#6

I am only vaguely familiar with the Phonak Ipeg software. I would think that the automatic software probably inserts a modicum of compression when presented with losses above 50 db or so. If more compression and/or lowering of the knee point is required to handle recruitment, it is up to the fitter to program it in.

Yes, just sticking linear high volume hf into a big loss ear is surely going to be counterproductive. Ed

#7

Inputting ucl #s (uncomfortable levels) on the audiogram is where recruitment is addressed.
The mpo can be manually adjusted down, leaving the compression ratio lower, helping up to some level.

#8

Thanks shan, these ucl’s will do the trick for loud environments but will still allow for max controlled gain in quieter situations. Does this sound correct?

The recruitment is only controlled when needed. An upper control limit on gain. Much like tuning controls in the petrochemical field.

I was hoping this thread would help others that have similar situations. If anything, it would help them to better explain the problem to their audiologist.

Thanks

#9

This is where digital hearing aids “can” excel by being able to control the UCL and the MPO.

My sister, who has a very profound loss, has an extreme case of recruitment where there is approx. 10db difference between comfort and pain - it’s been a real challenge to get an aid to help her. Thus an experienced audie and a high end power aid is the solution to getting the settings right.

This tread has been extremely helpful to understanding the recruitment “challenge”.

Thanks Rick for starting it and all those who contributed.

#10

With regard to MPO’s (Maximum Power Output) and other output stage limiters. Most if not all of these will cause audible distortion if driven too hard too long. It all revolves around the timing of the MPO circuit. If the timing (the time it takes to activate gain reduction) is fast it will distort the signal envelope. Output gain devices are really kinda a last resort to keep short transient peak signals from reaching the UCL point.

IMO better, less distortion prone, UCL and recruitment control is usually accomplished earlier in the chain with WDRC (Wide Dynamic Range Compression). Ed

#11

The mpo can be manually adjusted down, leaving the compression ratio lower

… I think you mean higher …

My sister, who has a very profound loss, has an extreme case of recruitment where there is approx. 10db difference between comfort and pain - it’s been a real challenge to get an aid to help her. Thus an experienced audie and a high end power aid is the solution to getting the settings right.

This may be hyperacusis … which is VERY rare and may be due to nerve network damage or a brain issue, not recruitment. It is VERY difficult to fit hyperacusis cases … and even when set up properly, the resulting 10dB dynamic range won’t deliver much data to the brain. Power aids aren’t usually needed in such cases as the MPO/UCL comes DOWN at the same time as the threshold goes UP.

When the compression is adjusted properly for our ears, these hearing aids can do a remarkable job of compensating for our recruitment problems.

I regard compression as a tool to correct for the loss of outer hair cells, rather than as a method of dealing with the more general issue of recruitment.

More generally, recruitment won’t take place until the loss is around 55dBHL - 65dBHL i.e. when the inner hair cells start exhibiting damage. In reality the majority of hard of hearing have losses of 55dBHL or below - except possibly at 3KHz or above. In other words recruitment doesn’t have a major impact for most hearing aid users.

Recruitment is indeed a REAL problem … BUT … I suspect that people worry about it too much. It’s not going to be an issue for most HOH.

#12

I don’t disagree with that, inputting ucl’s on the audiogram would be doing just that. At the starting point.
English disp. I am sure you are right also. MPO being a kneepoint of compression and lowering it would raise the ratio. The relationship between G40-60-80 could be closer to normal though. I have put MPO’s, to close, to output and it is distorting.
My real life situation is that every day I will run into situations where sounds are too loud. Maybe it is only a 80-90 dbl. Short term, normal hearing can take this, I have to cover my ears or leave. Smoke alarms are 80 dB +/- at about 2500 Hz, (I know , you can get 500 Hz…) Un aided this should be a mild 25-30 db. Not the case.
I have gone the opposite route than the (everyone should be open fit) craze.
1.5 mm vent is enough. Controlling all of the sounds seems helpful to me.
What is the least amount of amplification/output required to get the most benefit… More bandwidth??? I read an article/interview a few years ago with Harvey Dillon, he suggested there was a loudness quotient , more is not better.
Ed you have really always preached this. I ramble

#13

This is a thread from a long time ago.
It may help some who have issues with pain from their hearing aids. The information is still good today as was back in 2009.

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#14

The issue I have with this recruitment concept, which is believe is more technically described as a non linear response to increasing gain, is that it is not measured. It is simply assumed that if you have hearing loss you have a certain level of non linear gain. The correction for non linear gain gets baked into the prescription formula for compression. I don’t think it is a one size fits all thing, but that is the way it gets treated. You walk in the door, get an audiogram, and then a prescription formula gets picked, probably primarily on the basis or what the fitter usually does, a proprietary one, or one of the industry standards.

The starting point of the problem is that an audiogram is based on threshold hearing, but we live in an environment way louder that our threshold level of hearing. The whole range of gain corrections is based on one single point of threshold hearing. It seems to me that there must be a better way of testing hearing for purposes of prescribing correction than the threshold method. Perhaps the future of hearing correction, is not more bells and whistles in the hearing aids, but more sophistication in the measurement method during diagnosis.

#15

Isn’t that UCL or MCL? Whatever it’s called. Entirely subjective to what you perceive as opposed to real threshold detection but it’s a part of tests that I’ve had.

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#16

That is my understanding too.

Upper comfort level is different with everyone. My wife for example will double over in pain when UCL is reached. We have members here talking about it all the time but don’t understand it as recruitment. At least that’s my understanding. The pros might correct me on this.

Very much a part in fitting aids properly.

#17

I think the synonym is hyperacusis. I believe I have a loudness sensitivity. Quick loud sharp sounds really make me jump. I see other “normal” people where they don’t jump with the same sound. For them it’s just …meh…yeah that was loud…carry on.

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#18

That may be what my wife has. Recruitment and hyperacusis have similarities. Here is a copy and paste from a quick google search.

Difference between hyperacusis and recruitment

Hyperacusis and recruitment are not related. The significant difference between the two is explained physiologically: recruitment is a problem of the peripheral auditory system, meaning the ear and the hair cells, whereas hyperacusis is a problem of the central auditory system.Jun 15, 2007

#19

I think there are three lines of defense against really loud sounds. On is just the louder sounds compression slope. Another is setting the MPO of the aids. And last some manufacturers include a transient noise reduction system. Siemens, now Signia/Rexton came out with it in 2006, and should be on your KS7’s. They call it SoundSmoothing. It has a low, medium, and high, setting, for each program. I have mine all set at high.

#20

@John_Green This article made me think of you.
Do you think this is your issue?

#21

Thanks for that Rick.
I do think recruitment is part of my problem, but probably not all. I liked your explanation. Very few people I have mentioned it to to had any idea what it was. The way I explained it was like a radio tuned to one frequency, but with the volume turned all the way up trying to hear a signal that was not on its frequency. When even a small signal on its frequency is detected, because the volume was at maximum, that signal is overpowering. Not nearly as complete, but simple enough that most people get the idea.

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#22

Yes; and this is “wrong”; but historically the Threshold is the only measurement. (My audi picked an MOL from thin air.)

We “should” ask the client “Now turn this dial to a medium and comfortable volume” (verify it seems reasonable) “and while turning to other pitches you turn dial to make it seem a similar medium comfortable level”.

If I do that to me, below 1kHz my loudness curve is “flat” (‘normal hearing’) but above 3kHz about 42dB is scrunched into 12dB. But there is no standard test to quantify this.

I believe the big HA labs get statistical trends from mass observations, and their first-fit math includes compression based on HL levels and slopes.

The name is bad but let that go. Actually a ‘normal’ ear is NON-linear. You can’t get 120dB through a simple linear device. The inner hairs and the auditory nerves pass a bit more than 60dB. The outer hairs are hyper-sensitive to sound, shiver when they hear their pitch, which “amplifies” the sound to the inner hairs. (If you know “regenerative radio”, same thing.) Together a 120dB range of sound fits in the 60dB range of the auditory nerves.

So there is a natural 2:1 compression from sound to brain.

UNTIL! Those outer hairs, which must be very highly tuned and high-strung, finally give-out. Now we only hear the upper 60dB of ‘normal’ sound. This seems exactly true for me with 55-60dB threshold 1.5kHz-up, and at least that much “recruitment” up there (but there is also something else, probably weak inner hairs).

I can run an equal-loudness curve on myself in a minute, but I kinda despair of trying to do this on people without years of audio experience.

#23

I wouldn’t put much stock in this description of recruitment. This guy’s PhDs are in ancient astronomy (classics) and theology. As far as I can tell, no current hearing researcher thinks that the auditory periphery works this way–I don’t even think that the guys who originally coined this term thought it worked that way about two seconds after they coined the term, which is why it’s confusing because it’s not actually a good word for it. I hate the way audiologists throw around the word “recruitment” when confronted with a patient who is sensitive to loud noise as if it is at all meaningful. All it means is that people with (sensorinerual) hearing loss have steeper growth of loudness than people without. It is present for all people with sensorineural hearing loss, not just some.

Hyperacusis is a better model for abnormally low tolerance to loud noise.

[Also, it gets my back up when someone is like, “I have a PhD, let me explain to you how time REALLY works” and then their PhD is in French.]

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#24

It sounds better if you say it in French.

I thought Killion had a good clarification of “recruitment” but I can’t find it now.

As said, there are all sorts of New Meanings tacked onto the bad name which are not what was originally intended. Sensitivity to loud sounds is related but NOT the same. I recruit highs like McNamara yet very loud does not make me cringe. (I dislike extended loud sounds; they don’t hurt.)

https://www.etymotic.com/media/publications/erl-0133-1974.pdf is typical Villchur: thoughtful, complete, probably correct in its assertions, but not an easy read. He is making “fake recruitment” for “normal listeners” (backward from what we want). In 1974 this was a large rack of expensive equipment. The audio tracks are here: https://www.etymotic.com/media/publications/Villchur1974Soundsheet.mp3

https://www.etymotic.com/media/publications/erl-0025-1993.pdf has some nice curves. The WDRC compression he used to compensate is very micro-1990; digi-aids could fit better.

https://www.etymotic.com/media/publications/erl-0109-2008.pdf is another solid meal. And shows different people have different loudness curves (“recruitment”). erl-0109-694
Note that 2 of 3 “types” are “normal” at “medium loud” and higher; this is not hypersensitivity to loud sound. (The “delayed” type may get upset when sounds come out of nothing for small increase of power.)

Further reading from Killion and peers: Etymotic Research | Publications & Reference

The current Wikipedia article on recruitment is IMHO barely half right.