I am trying to recreate a strange happening I experienced many moons ago. The scene is this. Party at my house. Van parked outside with two box speakers on the roof supplying the music. Now comes the freaky part. When walking by the van there was a dead spot in the music. I think what it is called is noise cancelation. I’ve been trying to duplicate that effect with no success. I tried mono output to get all things equal. But no dead spot. Is it that rare of an occurence or am I missing something. Thanks
Yes, sound from two speakers can cancel each other at a particular location - a location which is 1/2-wavelength farther from one speaker than the other. But this depends on the frequency. Different notes cancel at different locations, so you can’t cancel out all the sound at one postion unless the music consisted of a pure sine wave (i.e. a single pure note).
Still, I can’t explain what happened at your party. Can you describe the situation in more detail? Are you sure the music didn’t just happen to pause when you passed by the van?
If we were talking about a simple audio signal like a sine wave or something, it could be that the dead spot was caused by sound waves bouncing off a wall behind you and coming back to cancel themselves out, but music is a bit more complex; the sound waves coming back would be from the music that was played a fraction of a second earlier and so they wouldn’t exactly match those on the way out.
What is needed is two complemetary waveforms (sound waves aren’t waves in the sense of ‘up and down’, but rather ‘compression and rarefication’).
When you connect a loudspeaker to a plain DC supply, it will make a loud click and the cone can be observed to pop outwards then return to the original position; when you disconnect, it pops inwards.
However, if you connect the terminals the other way around, it will pop inwards first, then outwards on disconnection.
This is where I go off into speculation now; if one of the speakers was wired the opposite way to the other; in theory at least, it would be producing a compression wave when the other one is producing a rarefication wave and vice versa.
With a mono signal, it’s concievably possible that these might cancel each other out at certain points.
Sorry if my science is a bit shaky, but the above seems to make sense to me.
Or do I mean compression and decompression?
or am I just way off target anyway?, dunno
Dead easy this one:
Go to your hi-fi. If your speakers have(basically speaking!) two leads going into the terminals at the back of 'em (one being positive, one negative), one ONE SPEAKER only, flip the pos and the neg lead over. (so pos goes to neg and vice versa)
Then play your fave CD and put your head in the middle of the stereo field. (i.e in the exact centre between your left and right speaker)
Not only will the centre of the image seem dead, but you’ll notice you’ll loose some of the bass frequencies.
Feel your brain get squished and your eyes bug!
Enjoy!
that’s what I said, wasn’t it?
It’s exactly what you said. But it was pitched to the Peanut Gallery (no science, just process). Don’t sweat it, Mangetout, Bimble was backing you up.
OK
It depends a lot on the acoustics of the room or area you’re in. It’s true that a “dead spot”, which is the result of destructive interference, will only be a dead spot for certain frequencies, but given the nonlinearities of our built-in listening devices, the effect can be quite dramatic even if only one small frequency band is dropping out, especially if that particular frequency plays a large role in the music (i.e. a hip-hop song with a simple bass-line which is mostly the same note).
In addition, the dead spots for a given frequency will be distributed about the room in a repeating (but also nonlinear) pattern. The size of the pattern will vary with the frequency. Overlay many of these patterns on one another, and some of the dead spots will coincide. Probably the van setup outside the party resulted in a spot where a large number of the frequencies had dead spots, dramatically increasing the effect.
I tried to represent it visually here: http://www.codsquad.com/~galt/interf/. It’s a gross simplification, addressing 3 distinct frequencies with two sources and no reflections. You can see that the resulting waves are pretty chaotic, and that the spots where all 3 frequencies destructively interfere are pretty rare. But there are patterns, and under odd circumstances, they make themselves very obvious.
Oh, and this is still exactly what the others said. I just get a kick out of coming up with particularly illustrative ways of explaining things. 
I was gonna go into ramblings about possible (but highly unlikely - I just like throwing curve balls into the mix) large-scale comb-filtering and stuff like that… but that got covered as well…
And as for Peanut Gallery - I’m allergic to peanuts; can we go for “Fruit ‘n’ Nut Gallery” instead?
Sure thing. “Fruit ‘n’ Nut Gallery” is probably more accurate anyway. I haven’t seen a Peanut post yet. (Wait…was that mean? Is some one gonna take offense?)
galt - everything you say makes sense, but you’re talking about interference patterns; cancellation of one wave by an identical on at points where they meet and are out of phase…
…but if you connect one of your speakers up the wrong way, then you don’t have two identical waveforms, in fact, you have waves that are exact mirror images of each other (at all frequencies) - wave B has troughs that match each peak of wave A and vice versa - so at a point equidistant from the two speakers, all parts of the wave should cancel out.
Just FYI from an audio engineer. What y’all are talking about is known as ‘phase cancellation’.
Thanks, I understand the concept, but I was trying to give a general explanation of how this can happen. He never said the van was set up with one speaker wired out of phase, so I’m not assuming that was the case. Either way, my diagrams apply, but you get more dead spots if you have the wave sources out of phase.
By the way, it doesn’t sound like you’ve ever actually tried wiring your speakers that way. I have, and I think you’ll find that the effect is not always as dramatic as you would expect. This is because a) you’re still getting indirect waves which are reflected off objects and travel different distances before reaching your ear, thus aren’t properly canceled, and b) even if your two ears hear exactly opposite signals, your brain doesn’t automatically average them and hear nothing.
You’re quite right galt, I’ve never tried it, it was all theory and was based on the hypothesis that one speaker was wired backwards…
Other factors that might foul up the practical realities of this might include:
[list=1]
[li] the fact that you have two ears spaced slightly apart (so you’re not hearing sound at a single point)[/li][li] atmospheric diffusion (or is it diffraction?) of the sound waves.[/li][li] Hi Opal[/li][/list=1]
My apologies if I seemed to be attempting to discredit your very elegant explanation, I intended no insult.
and my point 1 appear to be the same…
…but rather than the listener hearing two complementary signals (one in each ear) and the brain cancelling them out, I’m proposing that each (or a single) ear happens to be in a position where both complementary signals chance to meet in such a way as to cancel each other out due to the physical properties of sound waves in air.
Don’t discount your first point in comparison to my point b. They’re subtly different. The fact that your ears aren’t in the same location is important aside from the issue of mixing the signals together in your head. However, in the case where you’re sitting directly between the speakers, the difference in distance is irrelevant, since the point is that the ears hear their respective signals at the same time, which they will (since the distance from the left speaker to the left ear is the same as the distance from the right speaker to the right ear).
In other words, even if the brain did perfectly mix the signals in your head, there are cases where the ears not being in the same location would still make a difference. So you bring up an interesting point.
A great discussion going here. Being the OP, I’ve been racking my brain for more details. This happened about 20 years ago but the effect has never left my memory. I hope to have some time to try the reverse polarity theory.
OK. IIRC The speakers were about 7 feet off the ground pointed in a hap hazard way but intersecting at some point. The dead zone was at about 5’6" or my ear level and about 9 or 10 feet perpendicular to the van. I think I could turn and face the van and still keep the effect. I don’t think the background was much a factor. There was a run of the mill tree and the house was maybe 18-20 from the van. I also remember the feel of the dead zone. It was like how your ears feel when you go up in an airplane. Sort of that fuzzy silence. Thanks for all your comments
Were there turntables involved in the playing of the music?
Two copies of the same song, played simultaneously through a mixer can cause the same thing.