Phonak Acoustic Phone Program

I think the answer is that the network bandwidth available back in the day for transmitting telephone signals was limited so to achieve maximum capacity for many customers, AT&T figured out that only a frequency range of about 3 kHz between about 400 to 3,400 Hz was needed.

How Telephones Work1

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Thank you, that makes a lot of sense.

I kept the highs lowered like the software set it as and lowered the lows by 12 dB and I can now finally hear well with DuoPhone on public transport. I’m happy!