Frequency lowering comes in two main flavours, frequency compression and frequency transposition. Both of these are strategies to bring very high pitched (frequency) sounds - which those with severe hearing loss can no longer hear - into the range where we can hear them.
Excuse the graph, I know they are a turn-off, but I'm thinking if you're reading this you will be familiar with this one, it's an audiogram/hearing test report. Left to right is frequency, so low pitched sounds are on the left, moving through mid-tones and finally the shriekiest high pitches on the right.
The circles and crosses are this person's (me) threshold of hearing, and it basically drops out of sight after 1.5kHz. That's where a kicking K sound is, or an F#6 if you're musical.
Unfortunately for me and a lot of other people, lots of speech sounds, bird sounds, microwave beeps or confirmation tones are beyond my range of hearing. Not only that, but amplification cannot help me, because a hearing loss of 80dBHL and beyond tends to mean that bit of the cochlea (inner ear) is dead. I have previously explained the hair cells in the cochlea, now I ask you to visualise a field of wheat that has had a helicopter land in it. That's my hair cells. I digress.
So, given that we cannot hear OR amplify this range of sound, how do we receive the information? We move it.
Frequency Compression
This is the easiest one to understand.
We take a range of frequencies coming into a hearing aid in their natural form and we squash them closer together so they fit in a smaller space.
In the above diagram of what a hearing aid does, low pitch is yellow and purple red is high pitch. The sound coming in to the aid has a large bandwidth, it takes up a lot of space, because it runs right from the lowest pitch you can hear to the highest one.
The sound going out takes up less space, it has been compressed. If you overlaid this on the audiogram above, you would see that more of that spectrum falls into my better hearing range.
Essentially this means that a Sh sound is now a different pitch than it usually is, which sounds a little weird but at least I can hear it. And that, fundamentally is the point. Sorry it sounds weird, yes I do understand but you'll get used to it. This is not a platitude, you really do get used to it. Our brains are plastic and they are crying out for input. When they receive input they haven't had for a long time, it takes time to form new connections and make sense of it. But you will, and it will come to sound normal to you.
Frequency Transposition
So far so good. Compression squashes the frequency bandwidth so it fits into your range of useable hearing.
Transposition is better thought of as copy and paste. Whereas compression affects many frequencies, and keeps them in the same place relative to each other (F is always before G in musical terms, 1500 is always before 2000Hz in frequency terms), transposition picks up the part that you can't hear and dumps it into the range that you can hear. Crucially, it leaves the other frequencies that occupy the dump spot as they were. Imagine someone is playing you an F#6 and an F#7 at the same time at the same volume, you will hear them both as F#6.
Shouldn't work then should it? All I can say is that it does, mainly because true sound doesn't work like music, and also because your brain is amazing. It is quite capable of receving multiple signals in that region and picking them apart, so although the C and the F of 'coffee' might be coming in at roughly the same place, you will still hear coffee.
Which One Is Best?
They both have advantages and disadvantages. I use transposition but that's not to say it's right for you. To my mind, the best thing to do with a patient who cannot get on with one is try them with the other. Be sure you pick an audiologist who has the time and patience, and the range of products, to do this. If you go to a place that's covered in Phonak posters, you're coming home with compression whether you like it or not, basically. Go independent if you want independence!
Summary
If there is one fact about audiology, it is that your brain is relentlessly awesome when it comes to making sense of sound. No matter how you slice it and dice it, as long as you give the brain access to it, it will eventually interpret it in a helpful way.
Like a lot of people, I am unhappy that my cochlea is half dead, but I am grateful for the perseverance of my early audiologists in making me persevere with frequency lowering.
As with many things in life, the sooner you start the better you can adapt. Brains are more adaptable at a younger age, and it's always better to start as soon as you notice hearing loss having an impact. So get yourself along for a hearing test, and if you can't hear the microwave beep, ask your audiologist about frequency lowering and whether it's right for you. Tell them you are willing to persevere, as I hope you are.