Hi Electricman,
Unlike the deaf old geezers who invented lousy-sounding telephones and AM radio, most people can hear up to 20KHz. Most of the intelligence is up there: did you say "s" or "f" and "ick" or "six" and so on. Those deaf old guys didn't care, lousy-sounding 300-3KHz was much better than Morse Code, and they didn't know how to go lower or higher anyway.
They were very worried about crosstalk. Horrors if you could hear your neighbour! Capacitance between their wires coupled high frequencies, so they got rid of the high frequencies. They didn't realise that their balanced wiring cancelled crosstalk. They were also very worried about high distortion, so cut the highs more to reduce it from their terrible-sounding transducers.
I wasn't talking about noise on AM radios, I was talking about the high frequency rolloff they have above only about 2KHz. AM radios must attenuate 10KHz extremely to avoid an annoying 10KHz interference-between-adjacent-stations whistle. When they use a simple cheap low pass filter, it begins its cutoff at about 2KHz.
Modern telephones with electret mics sound much better than the old carbon ones that cutoff above 3KHz. But the telephone network is also bad. When I worked with telephone systems I asked Bell, "Why is the response at only 2.5KHz and above, down 15dB when I call next door?" They said I was far from their equipment and the spec allows -7.5dB one-way. My location was at their limit and I had terrible sound! So I sold an equalizer circuit to many customers with my boardroom telco conferencing system that boosted 3KHz +10dB to make it sound nearly normal, and as soon as they heard it they said, "Wow, we must have that!" Don't tell Bell, it was illegal, but no complaints.
Even television announcers sound bad with their fancy little lavalier mics hidden under their turtle-neck sweaters. Since their mic is close to their chest and throat, their voice booms loudly but no high frequencies come through. Lately they have been using headset mics (due to my complaints) and they sound much better.
I worked with a sophisticated intercom system for many years. Its sound quality (and price) blew away the competition for large systems in life insurance companies (room to room), banks' head offices (hundreds of stock-traders' communication) and airports (announcements over the PA) where quality was important and price didn't matter. Its response was flat up to about 12KHz, it had very low distortion, you could also play background music through it and it sounded great! I added bass-boost to the background music to make it sound even better! I had to tweak it a little to make it pass 15KHz and meet the airport's "broadcast quality" spec. I use its stations' 3" speaker and my bass-boost circuit in my clock radio and it sounds like a hi-fi with a sub-woofer! It weighs a lot more than an ordinary little clock radio but looks the same.
Have you ever heard good wide-band speech through just a tweeter? It sounds odd like whispering, but is crystal clear. It produces only those frequencies you want to get rid of.
So if you want to sound like you are talking through a thick pillow, cutoff frequencies above 4KHz. At first I thought you wanted to reduce loud background noise or people or snakes hissing at you. :twisted: