hi, im supposed to be constructing a pair of ear defenders with some sort of filter to allow the voice frequencies through, for my 3rd year university project. It is supposed to be geared towards the work enviroment such as factory process or construction where the noise floor is high and vocal communication is important.
I have some basic electronics skills and can design a band pass filter. The trouble is im still extremely confused as to what frequencies I need to let through. I have read conflicting information about the human voice frequencies and would be very greatful if someone could point me in the right direction or reassure me of what I understand.
I was originally going to construct it in an analogue way with a simple R-C/op amp circuit. Would there be any benefits of doing it digitally instead?
From them you can deduce that human hearing is nonlinear and has a peak at 3kHz. If you're trying to design something that allows voice frequencies to pass, I'd start there and go downward to as close to 125Hz and see what happens. Even if you only get to 1 kHz before your algorithm gets funky, that may work for you.
Going above 3kHz (incidentally, the response of analog telephones of old was 3kHz, because Bell labs did the math) get's you some intelligibility, but there really isn't a lot of energy in the human voice up there; just shine, air and some sexy breath sounds.
I have been using Hearos (http://www.hearos.com/) for years bot in noisey environments and as a sleep aid. I have compared them to noise canceling headphones and think they do an equivalent job for much less money and less bother.
There are ear plugs with "engineered diaphragms" to flatten (improve) the frequency response, but the rubber (or whatever) and design don't make for comfortable wearing.
The method that most custom-molded earplugs use to reduce overall SPL while still retaining almost-perfect fidelity is by first creating an airtight seal. This allows for the use of a semi-rigid diaphragm in-between the free air (external to your ear canal) and the air mass trapped in the ear canal by the aforementioned airtight seal. The rigidity of the diaphragm determines how many dB of attenuation is incurred; I'm not sure how exactly the frequency response of the filters does not affect the sound quality much, but I'm sure a lot of money has been invested in R&D for it.
Sounds like a fun project...I hope I've helped a little bit. Have fun and please keep us up-to-date on your progress and the end result.