I see this word now and then and did not understand what it meant. Here is a copy and paste of an article that I found to be very helpful and may be to you too.

Recruitment from Hearing Loss Explained

© June 2001 by Neil Bauman, Ph.D.
Question: Some people can’t wear hearing aids because of severe recruitment. Recruitment seems to be such a weird word for this problem. The word means “to enlist.” The dictionary doesn’t describe anything to do with hearing loss and hearing aids. So what really is recruitment and how did it get this strange name?—C. H.
Answer: Good questions. No wonder you are confused! Even many of the hearing health care professionals don’t understand this condition that goes by the strange name of recruitment. There is a lot of misunderstanding about recruitment. Actually, recruitment really is a good word to describe this phenomenon—once we understand what goes on in our inner ears.
What is Recruitment?

Very simply, recruitment is when we perceive sounds as getting too loud too fast.
Before we look at how recruitment got its name, there are two things we need to know about recruitment.
First, recruitment is always a by-product of a sensorineural hearing loss. If you do not have a sensorineural hearing loss, you cannot have recruitment.
Second, there are two other phenomena that often get confused with recruitment. These are hyperacusis (super-sensitivity to normal sounds) and phonophobia (fear of normal sounds resulting in super-sensitivity to them). Both hyperacusis and phonophobia can occur whether you have normal hearing or are hard of hearing. In fact, if you have a sensorineural hearing loss, you could suffer from all three conditions at once!
How Recruitment Got Its Name

Now let’s look at how recruitment “works” and how it got its name. Perhaps the easiest way to understand recruitment is to make an analogy between the keys on a piano and the hair cells in a cochlea.
The piano keyboard contains a number of white keys while our inner ears contain thousands of “hair cells.” Think of each hair cell as being analogous to a white key on the piano.
The piano keyboard is divided into several octaves. Each octave contains 8 white keys. Similarly, the hair cells in our inner ears are thought to be divided into a number of “critical bands” with each critical band having a given number of hair cells. Each critical band is thus analogous to an octave on the piano.
Just as every key on the piano belongs to one octave or another, so also, each hair cell belongs to a critical band.
When you play a chord on the piano—you press two or more keys together but they send one sound signal to your brain. Similarly—but yet different—when any hair cell in a given critical band is stimulated, that entire critical band sends a signal to our brains which we “hear” as one unit of sound at the frequency that critical band is sensitive to. This is the situation when a person has normal hearing.
However, when we have a sensorineural hearing loss, some of the hair cells die or cease to function. When this happens, each “critical band” no longer has a full complement of hair cells. This would be analogous to a piano with some of the white keys yanked out. The result would be that some octaves wouldn’t have 8 keys any more.
Our brains don’t like this condition at all. They require each critical band to have a full complement of hair cells. Therefore, just as our government, when it runs short of military personnel, puts on a recruitment drive, so too, our brains do the same thing. However, since all the hair cells are already in service, there are no spares to recruit.
What our brains do is rather ingenious. They simply recruit some hair cells from adjacent critical bands. (Here is that word recruit or recruitment.) These hair cells now have to do double duty or worse. They are still members of their original critical band and now are also members of one or more additional critical bands.
If only relatively few hair cells die, then adjacent hair cells may just do double duty. However, if many/most hair cells have died, then in order to have a full complement of hair cells in each critical band, any given hair cell may be recruited into several different critical bands.
The Result of Recruitment

The result of this recruitment causes us two basic problems.
First, the sounds reaching our brains appear to be much louder that normal. This is because the recruited hair cells still function in their original critical bands and also in the adjacent one(s) they have been recruited into.
Remember that when any hair cell in a critical band is stimulated, the whole critical band sends a signal to our brains. So the original critical band sends one unit of sound to our brains, and at the same time, since the same hair cell is now recruited to an adjacent critical band, it stimulates that critical band also. Thus, another unit of sound is sent to our brains. Hence, we perceive the sound as twice as loud as normal.
If our hearing loss is severe, a given hair cell may be recruited into several critical bands at the same time. Thus our ears could be sending, for example, eight units of sound to our brains and we now perceive that sound as eight times louder than normal. You can readily see how sounds can get painfully loud very fast! This is when we complain of our recruitment.
In fact, if you have severe recruitment, when a sound becomes loud enough for you to hear, it is already too loud for you to stand.
The second result of recruitment is “fuzzy” hearing. Since each critical band sends one signal at the frequency of that critical band, when hair cells get recruited into adjacent bands, they stimulate each critical band they are a member of to send their signals also. Consequently, instead of hearing just one frequency for a given syllable of sound, for example, perhaps our brains now receive eight signals at the same time—each one at a different frequency.
The result is that we now often cannot distinguish similar sounding words from each other. They all sound about the same to us. We are not sure if the person said the word “run” or was it “dumb,” or “thumb,” or “done,” or “sun,” or? In other words, we have problems with discrimination as well as with volume. If our recruitment is bad, our discrimination scores likely will go way down.
When this happens, basically all we hear is either silence or loud noise with little intelligence in it. Speech, when it is loud enough for us to even hear it, becomes just so much meaningless noise.
This is why many people with severe recruitment cannot successfully wear hearing aids. Their hearing aids make all sounds too loud—so that they hurt. Also, hearing aids cannot correct the results of our poor discrimination. We still “hear” meaningless gibberish.
However, people with lesser recruitment problems will find much help from properly adjusted hearing aids. Most modern hearing aids have some sort of “compression” circuits in them. When the compression is adjusted properly for our ears, these hearing aids can do a remarkable job of compensating for our recruitment problems.


Raudrive: I like your explanation up to a certain point. I was under the impression that the older view of unnatural loudness was as you explained…that is; other inner cells were Recruited.

But I thought the newer explanation was that the Outer hair cells also deteriorated allowing the full sound pressure to get to the inner ones which then circuitously connect to the auditory cortex. I think the newer view is that the outer cells modulate and mitigate excessive sound pressure.

Also in play to a minor extent is the muscle that tightens the ossicles when loud sounds are heard. Ed

Please correct me if I am mistaken. Ed


You are the reason I went looking for a definition of recruitment. You use the term now and then and I did not know what it meant.

The cut and paste description I inserted sounded like a good explanation of recruitment. If it is not correct I would not know.

The reason I brought this to the forum is if I didn’t understand it, maybe others didn’t either. The article helped me a bunch and explained issues I deal with concerning aids.

Thanks for getting me to thinking.

Yes, understood.

Incidentally: If you drive the fluid filled organ of Corti’s inner hair cells, which is located in the cochlea, hard enough, then working adjacent inner cells will be excited and respond but not at the actual input frequency. Because the patient is hearing only a faint sound which may be distorted, the professional may score this as a valid threshold db loss. This is why pure tone threshold testing is so unreliable for the typical severe/profound loss. Ed

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?


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

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.

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.


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.

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

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.

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

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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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.

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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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.

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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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

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.

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