Effect of receiver size and Max Power Output on sound quality?

#1

Some hearing aids have larger receivers (sound generators) than others. Receiver-in-canal models (like Phonak Marvels) tend to have smaller receivers than Behind-the-Ear receivers with tubes (like Phonak Bolero/Belongs). This article states that smaller receivers typically are limited to lower Max Power Output (MPO), which correlates with compressed sound at louder volumes:
http://www.hearingreview.com/2008/06/mpo-a-forgotten-parameter-in-hearing-aid-fitting/

I imagine that the lower compression afforded by a larger receiver with higher max power translates into a richer more detailed sound, like an audiophile stereo system with a strong amp and big speakers. Does anybody have experience with this?

Phonak offers 4 different RIC receivers (S, M, P, UP), each with increasing power, meant for different levels of hearing loss. I wonder: do the higher-powered receivers provide richer, fuller sound than the lower ones, and if so, why don’t people just choose those all the time? Are they too big for some ears? More feeling of occlusion? Lower battery life? Increased feedback? All of the above?

–rex

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Phonak Marvel vs Bolero
#2

The bigger receivers are going to provide a lot more mid and low range frequency gain capability. Which, when running at lower gain will have a little better sound quality.

However, when the gain requirement climbs, the larger receivers tend to lose sound quality in the high frequencies. A single driver sound system does not sound nearly as good as a multiple driver system for a reason. The heavier, or longer waves created by the speaker diaphragm in the low frequencies essentially mask the high frequencies. It is very difficult for a single diaphragm to create a 5k sound wave at the same time it is creating a 200hz sound wave, and do both effectively.

I have thought for a few years that the next big thing in sound quality for hearing aids would be multiple drivers. It would be a game changer.

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#3

I have not researched this aspect very thoroughly, but my thoughts are that if the manufacturer’s fitting software is used properly it will guide you to the best receiver power for your hearing loss. I suspect what you will find if you look at the detailed specifications for hearing aids that as the power goes up the distortion levels also go up. The maximum bandwidth can also go down. Here is a detailed spec sheet for the KS8. That would suggest that one should select the lowest power receiver that still provides the needed amplification to address your hearing loss. Since just about everyone’s hearing will deteriorate over time, some consideration should be given for future loss.

Here is how my hearing loss fits on a S receiver:

You can see that at the 4k to 8k range it simply does not have enough power for the left ear. I ended up getting a M receiver and this is how it fits. It covers the whole range and provides some margin for future hearing loss.

I think the best one can do is ask to see how your hearing loss fits on the capability graph of the HA’s you are considering. You don’t want one that is underpowered or you will get into clipping issues. And you don’t want one that is too powerful or you could suffer sound quality loss.You could also get into over amplification at frequencies where your hearing is good. I have a little bit of that with my right ear in the 1-1.5k range. But, that is better than being under powered.

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#4

I have a purly conductive loss so require a lot of power. I find the Ultra Power HAs give me better sound that is less compressed then when I tried the Power HAs.

My audiogram fits into the Power HAs and is meant to be too good for the Ultra Power hearing aids.

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#5

Sierra, what’s your highest gain number in the software? I have a 57 db gain receiver (the Phonak P receiver). I don’t quite fit the fitting range for these but we are trying them anyway because the SP requires an encased mold which I wanted to avoid because of convenience.

Anyway, my highest gain number is 54, and that’s a high frequency with DSL (known for high frequency boost) so I’m not maxing out my receiver. I think there is a little bit of wiggle room in the fitting range estimates. The 57 receiver may actually go a little above that anyway, but if I got into that I would probably get the SP receiver then.

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#6

It starts to become complicated because the receiver can produce so much, but it usually results in a higher level in the ear. The way I read it, I have about 10 dB of headroom with a 80 dB input signal. The HA is putting out about 110 dB and is capable of about 120 at the frequencies where I need the most gain. It does beg the question as to what happens when the ambient sound levels go above 90 dB… Does compression kick in and prevent amplifier overload? Perhaps that is when the sound goes to mush!

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#7

There are two ways of limiting loudness…

Output limiting, and compression.

Where compression is concerned, as you exceed the knee point, which is the point where compression kicks in, and is typically very low in modern tech, the signal will be altered by compression. The higher the ratio, the more “trash” you get in the signal, the higher the THD in the output signal. You can see this exhibited by listening to a song on a decent stereo off of a disc, and then listen to the same song off of an ipod or cell phone. The difference is dramatic.

Output limiting basically says to loud sounds;" YOU SHALL NOT PASS!" and knocks the top off of it. Again, as the output signal from the hearing aid approaches it’s limit, the distortion can be up to 10 times higher than when the signal is at a normal input/output level.

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#8

@eric.cobb, thank you for your insightful answer! I never would have guessed that smaller can translate into better sound, but it makes sense. That helps explain why Phonak offers so many receiver options for people with varying degrees of hearing loss and suggests the smallest one that can meet the amplification needs.

I imagine that multi-driver HAs are coming! The Bose hearphones might already have that…

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#9

The task for a HA speaker is much more modest than the ones required for even a home sound system. To produce bass in a large room you need a speaker that can move a lot of air. It requires power and mass. A big speaker like that is not well suited for higher frequencies, although the crossover point to a tweeter can be as high as 4.5k. Compared to a home sound system the frequency range is much more limited. The bass doesn’t go nearly as low and the highs not nearly as high. If there ever was a situation where a single speaker could deliver the full expected range, it would be in a hearing aid.

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#10

Total failure of the engineering needed to achieve completely different outcomes, under different load conditions, life expectancy, and environmental issues.

Like saying, ‘that mouse-trap you bought is rubbish at catching elephants’.

If you’re actually interested in learning anything about, ferrofluid damping, balanced armature, charge inversing, 1.0Volt, conventional broadband receivers; I suggest you have a look at some of the Knowles Technical bulletinson them.

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#11

I am not saying that there is not sophisticated technology in a hearing aid speaker. Obviously there is to build it that small. What I am saying is that the issue of getting a wide frequency response from a single driver is much simpler. That is because the mass of the speaker is so low, and the volume of air that needs to be moved is so low. I don’t think there will be any multi speaker hearing aids in the near or even distant future. They are simply not needed.

One thing to keep in mind is that Bose made their name using very inexpensive 5" paper cone speakers to deliver the full frequency range.

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#12

I’m not an Audiologist nor do I work in speaker tech so please just take this comment as one from a user of a hearing aid ------ A very non technical view

My hearing loss (62dB) needs my aids to have fully open fittings and to work with my residual hearing (low + high), avoiding compression etc. A fairly unusual situation I know, but “Receiver” quality matters lots with my odd type of loss.

I found the best compromise for me was to go one size up in receiver power (MPO) from what was officially needed. Going up a power reduced frequency range a little, but it gave me better quality sound. Richer sounds and less distortion overall. For me it just worked better. Just my personal experience though for what it is worth.

The only down side of that is they are larger of course which could be difficult for some people.

Thanks for all the technical details above. Fascinating stuff.

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#13

Bose made their name with great marketing of a crappy product. But of course that doesn’t contradict what you said. Now they seem to be more serious about fidelity. There are a ton of earbud companies so one would hope that Knowles and their competitors are keeping up with that side of the audio business too. Some of the earbuds have quite large drivers.

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#14

I would have to agree with you. They used cheap 5" speakers in the Bose 901 that delivered quite poor audio quality. That said Dr. Bose did have a new discovery. He figured out that if you reflected sound off the back and side walls you could make the sound much larger due to the physical delays in the longer sound path. It made the Bose name. Later they did the same thing but better electronically, and made a fortune. Not my first choice in audio quality, but they did break new ground.

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