by victorxray » Sun May 27, 2007 2:54 am
Technically a square wave isn't a 'limited sine' but actually a super set of the fundamental frequency plus the 3rd harmonic at 1/3rd volume, the 5th harmonic at 1/5th volume, 7th harmonic at 1/7th volume, and so on (theoretically to infinity). If you low-pass filter that square wave as was recommended however you'll knock out most of the harmonics and it won't sound so 'bright'. (you could also take a sine and add distortion which will also add harmonics).
The practical problem I have found with a pure sine wave is that it's too linked to different speaker and room responses. I can make a great sub on my studio system (M-Audio BX8s with no sub) but it sounds totally different on my finessed hi-fi speakers I have on my stereo (I know as I always check). Because we are talking about 40Hz, 50Hz, 60Hz and that's right in the bottom end of good speaker's response curves.
Standard concert tuned A is 440Hz (A4). This means that E0 is just above 20Hz (20.6Hz), E1 is 41.2Hz, E2 82.4Hz etc. If you had a sine-wave bass line from E1 to E2, the problem that you might have, is that different speaker systems will have cut-offs lets say in the 40 to 60Hz range which is right in the middle of your bassline.
Also many speaker cabs, to get this low response, tend to resort to tuned port systems which have the side effect of adding resonant spikes where the system is very sensitive.
So lets say the subs roll off at about 50Hz and they have a resonant frequency of 65Hz. This means your C2 note will jump out of all the others and anything below your G#/Ab will rapidly lose volume. But the next week when you play it on a different system in a different club a completely different set of notes may be lost or emphasised. This is also ignoring room effects because the wavelength of such sub-bass frequencies is down in the metre to half-metre range so the size and shape of the room can drastically affect the tuning.
Anyway if you add a bit of upper harmonics to the sub-sine by using a low-pass filtered square wave in addition to the sine, of course you won't overcome this effect but what will happen is the higher overtones will at least suggest the presence of the lower tones and perceptively to the audience, they will be able to discern the sub-bass part.
Also I believe this effect is what is reponsible for the comment that 'sometimes higher notes sound bassier' it's the speaker system response that's doing this.