Ok, I've looked at explanationsFebruary 7 2010 at 3:13 AM
Vince (Login MoxiFox)
Response to Not false at all.
from numerous sources now and no ... I don't see any doubling of low frequencies (or I should say "halving") from proximity effect but rather, a sharp amplitude increase resulting in the low frequencies, from proximity effect. The lower the sound frequency, the more the amplitude of the output is increased. (The effect of turning up the very low bass slider on an equalizer. )
In simple terms, it's kind of like listening to a speaker playing in open air. It sounds very "thin" because the same frequencies are being vibrated from the front of the cone as are being vibrated from the back of the cone but they're 180 degrees out of phase and cancel each other before they get to your ear. (Same effect from a directional mike; side sounds are canceled because the sound waves strike the front and back of the mike diaphragm equally, IN PHASE and therefore, the diaphragm isn't propelled either way. A mike and a speaker effectively work opposite because the mike is generating an electrical signal to be amplified .... while a speaker is using an electrical signal to generate sound ... and so, an open-air speaker, listened to from the side, will be very thin, whereas a directional mike getting sound waves from the side, will produce a very tiny electrical signal.
If you get directly in front of an open air speaker, it sounds better and ... the CLOSER you put your ear to the speaker, the deeper and stronger the low notes become. It's taking "too long" for the back waves to arrive at the front to allow for cancellation now, because your ear is only an inch or so from the diaphragm and the sound has to go about a foot, in order to get around to the other side.
Now, if you put a speaker into a heavy closed box, you get an "air suspension" speaker box .... with an essentially flat frequency response on the output. If you put a mike diaphragm into a solid closed "drum" so that no sound waves can contact the back side of the diaphragm, you also get a relatively flat frequency response with no proximity effect. This kind of mike will be "omin-directional" ... picking up sound equally from all around itself.
The uni-directional mike then, works mostly for sounds coming directly in front of it ... canceling almost all sounds to the side. The omni-directional will pick up sounds from all sides .. with no cancellation effect, because the back of the diaphragm is sealed off from getting sound waves.
Now, if you put a speaker into a tight heavy box and then have a hole of certain size cut into the box, with a tube or chamber of a certain length behind it going to the BACK of the speaker diaphragm -called a "tuned port"- the sound waves coming from the back of the diaphragm will come out of that port out of phase with the waves coming from the front of the speaker diaphragm so that the back waves will "enforce" or add to ...... the front waves ... in delayed mode. Say the cone is moving backward .... it creates a compression of air inside of the box which then travels through the chamber and port, coming out of the front "late". By the time that compression wave is emanating out of the port, the speaker cone is already moving forward again, creating a compression wave in front of itself. The two compression waves blend, essentially DOUBLING the volume of compressed air. Thus, the tuned port speaker box will ENHANCE low frequencies around the center tuned frequency, falling off on each side of the tuned frequency .... creating a "hump" of artificially amplified sound close to the tuned port frequency. This, of course, is how sub-woofers can blast out "bass" at higher levels than actually appear in the recorded signal.
Well, the same thing is true with a directional mike. Sound waves from a talker or singer's voice -when in close- are "amplified" by an effective tuned-port because their sound waves take a bit longer to arrive at the BACK of the diaphragm than directly from the front. If it takes a wave longer to arrive at the back, it will reinforce the next compression or refraction cycle coming in at the front. It's kind of a "snap the whip" effect of vibration ..... and the lower the frequency, the more pronounced it becomes because of the length of the waves. That, of course, means the response rises toward the "hump" of its tuned port center frequency .... just like the ported speaker box.
So the proximity effect doesn't actually create any false sub-harmonic frequency ... it merely amplifies the low frequencies artificially ... that already weakly exist.
OK, but ...
Then there's the feedback factor from the speakers producing the sound of the bass singer. Without being "live" ... I betcha there wouldn't be NEARLY the same effect. The positive feedback from the speakers in the auditorium enters the singer's mouth, which acts as a cavern reflector and also, from his cupped hand to his ear. They learn just how to do it to get the maximum effect. It wouldn't surprise me a bit, if there's a heterodyning effect between their voice and the feedback which DOES produce a beat frequency which is half the frequency of what they're producing with their own voice.
All of this is my own understanding, coupled with what I've learned about sound production on my own and from what I read tonight. Feel free to disagree or give counter argument.