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Hearing aid logic? total sound pressure in my ear

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help me understand how these things work… just out of curiosity

so for example
from my at 4,000 Hz I have a 55dB loss

so I’m guessing when the aids pick up a 20dB sound at 4kHz it’ll pipe it to my ear canal at approx 55dB. That added to the natural 20dB sound coming through the air would be around 75dB total in my ear but I would interpret about 20dB total sound to my brain
is that about right?

Am I correct to assume that the processor ramps that sound amplification down with increases in ambient sound level… to some point to avoid too much sound in the ear?
so maybe as the total of the ambient sound + the sound from the aid approaches a level such as 85dB…or 100dB in my ear canal, the amplified sound from the aid ramps down to zero.

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You’ve sort of got it, but I think you’re overestimating the amount of gain the hearing aid would supply. Very roughly, for sensorineural losses, hearing aids supply about 1/3 of the loss and for conductive losses, about 1/2. It’s usually done via a fitting formula with proprietary ones being the most common, but v5.0 and being the standardized ones. You are correct that soft sounds typically get more gain than loud sounds. It’s called “compression.” Some people, especially those that have worn aids for a long time, hate it and prefer a more “linear” sound, where soft sounds and loud sounds get equal gain. With what I’ve given you and Google, you can find a lot more info. Audiology Online is a great source. You can take classes for free. It only costs if you want CE credit. You do need to register though. Good luck!

With your hearing loss this is typically what a hearing aid would do while using the prescription formula. Modern hearing aids and the NAL formula use compression. That is when loud sounds are amplified less than soft sounds. The reason is that most hearing loss is not linear. We have a much higher loss in soft sounds than we do with loud sounds. You can see on the graph three input sound levels; 50 (soft), 65 dB (normal), and 80 dB (loud). If the hearing aid was applying no gain, you would have just a straight line at those levels right across the frequency range. However, the hearing aid is applying gain based on your loss. This graph is showing what the loudness levels would be in your ear with those three levels input.

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Below is another way to show it. This graph shows the insertion gain only for soft sounds (top lighter curve), normal sounds (middle heavier curve), and loud sounds (bottom lighter curve). This shows more clearly that loud sounds are being amplified less, and soft sounds amplified more.

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This is based on open fittings. You may have a feedback issue at 3 kHz in your left ear where the gain is near the blue shaded area, It may be better to use closed or partially closed fittings.

so it’s not as simple as I was thinking…(2-dimensional and linear)

Sound was never my area…I’m going to have to study those charts Sierra, when I get some time to think…
and I might just look into those classes. Thanks for that tip MDB!

Get some done and see for yourself what is happening at the TM. Theory is great but where the “rubber meets the road” is what concerns us.

In reply to @bradw

Here is another example of a different prescription formula called 1/3 gain. This method does not use any compression. There is only one curve for gain, and that same gain is used for soft, normal and loud sounds. It is what most analog hearing aids did before digital hearing aids came along. I also changed the fitting type in the left ear to closed so you see the potential feedback areas are suppressed. I am showing you this not because I recommend it though. Linear gain with no compression makes loud sounds too loud for most people, and soft sounds are not amplified enough to hear them. It is an option however for those that are long time users of analog aids and can’t make the transition to what compression sounds like.

You can see that while your hearing loss is in the 60 range, the formula is only providing about 20 dB of gain – hence the name of 1/3 gain.

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Have you tried different fitting formulas? Here is what looks like for you, and below that v5. And, if you are wondering what negative gains in the DSL are achieve, I have no idea! I would like to hear an explanation on that one myself.

You can see however that there is a lot more compression used in the NAL-NL2 formula compared to the DSL v5 in your right ear.

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In reply to @haggis

Here is what the graph looks like for the v5 that has the negative gain at lower frequencies. It seems it just sets a sound level in the ear canal that is lower than the natural sound on the outside. I suppose they could try to adjust to that with a real ear measurement, but I would think it may require some closed fittings to get that kind of attenuation.

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For the v5 graph above for @haggis I used open vented sleeves. It does not change much for closed sleeves, as shown below. I would think the difference might be that one would be more likely to meet the targets with closed.

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