' s ' and ' f ' sound the same...

So I was in my instrumentation class today (a class I've come to enjoy although I don't usually like the material) and we were talking about low-pass and high-pass filters.  A low-pass filter only lets low frequency signals through whatever it is filtering while a high-pass filter only let high frequencies through.  We then listened to a song that had a series of low-pass filtering done to it.  It sounded really weird by the time we listened to the last one.  Anyhow, now I will tie the title and this all together...So telephone lines have a low-pass filter on them that only lets frequencies of 4 kHz and lower through.  When you are talking to someone, many of the sounds we hear (such as the difference between the sounds ' s ' and ' f ') are of a frequency higher than 4 kHz.  This is also why women and especially children are hard to understand on the telephone.  Their voices are simply made up of high frequencies.  So there you go...a short random blob on filtering.