Nodal lines can occur when to waves collide and cancel each other out.
This effect can be demonstrated in water. A transparent container of water is placed over an illuminated overhead projector or other light source. Two identical rods (only at certain distances apart from each other) can be placed in the water with the exact same motion causing circular ripples to be emitted. At certain points, intersecting waves from each rod cancel each other out. The pattern seen by a series of cancelations reveals the nodal lines.
Back in the Middle Ages, when my father was a young college student at West Virginia University, a physics professor of his demostrated the nodal lines effect in water as described above and as described on the website.
This professor said something along the lines of…Theoretically, two trumpet players could stand in a flat field (with the proper distance between them) facing the same direction, play exactly the same note, and a third person standing in the path of one of the nodal lines would not hear either trumpet at all because the sound waves cancelled out.
My general question is: Has this effect every been demonstrated successfully?
…possibly with different instruments that would more easily produce the exact same note such as to amplifiers emitting sound from the same keyboard (or guitar etc.).
If this effect has never knowingly been successfully demonstrated, then perhaps I’ll open a thread in GD debating whether or not it could be done.
Isn’t this the same effect that some so-called noise cancellers use? I recall seeing prototype systems years ago which were designed for high-end motor vehicles. Don’t know whether they ever got into production.
You could widen your search by checking for ‘phase’. In your example where two wave ‘humps’ collide, you get reinforcement (the momentary height of the water at that point is higher than either of the two waves colliding). If two wave troughs coincide, you get cancellation.
Peaks = in-phase, troughs=out-of-phase.
Another experiment to try is to wire one of your hi-fi speakers out of phase with the other (i.e. on one speaker only, swap the polarity of the signal coming from the amp). On most material, the sound will be boxy and thin. That’s (some) cancellation going on.
Not a direct answer but IIRC some of the noise cancellation technology utilizes this concept. here is one link. Do a google on noise cancellation technology for lots of links.
The first is some background, but mentions that Honda have such technology in their non-export Accords, and the second shows headphones with built-in external mics, the signals from which are presumably inverted and mixed in with whatever you’re trying to listen to.
The problem of trying to cancel the sound of two trumpet players would be that the null would occur only at one exact point, and the spacing of your ears would mean that only one ear could get the effect at a time (the nulls would be a few inches apart). Another problem would be with the multipath effect - you’d have to assure that the only sound path from each trumpet to your ear was the direct route, without any reflections.
You can notice a similar effect with your car radio. The signal bouncing off buildings, bridges, etc., causes a pattern of peaks and nulls in the signal strength. The nulls are about half a wavelength apart, and the wavelength of FM radio is about 10 feet. Often, when I’m stopped at a red light, my radio reception will go to hell, and I can fix it by moving my car by about five feet.
I believe you are mistaking two very different things. Noise cancellation just samples the sound and generates the inverted wave. This has nothing to do with nodal troughs in standing waves which is a totally different phenomenon.
Standing waves generated by reflections at impedance changes are common in transmission cables like those feeding transmitting antennas and, in fact, in antennas themselves. Yes, you have points in the cable with zero voltage and others with max voltage and the points with zero voltage have max current and the points with max volts have zero amps and they are separated by 1/4 wavelength and I hope I am remembering all this correctly because it was ages ago since I dealt with it.
Yes, it could be easily replicated with sound if the two waves are perfectly in sync either because they are generated perfectly or because one is the reflection of the other. There is no way to have two instruments play in exact pitch or in sync so there is no way to do it that way but you can do it with loudspeakers and a wave generator.
Seeing that a we are talking of wavelengths in the range of a foot or two it should be easily done in the lab on a desk. You would have a node every half wave which would be inches apart.
Standing audio waves are used in the type of refrigeration which does not use a mechanical compressor but converts Audio energy directly into heat pumping and now I forget the name of that interesting method
For two acoustic waves of identical frequency, full constructive interference occurs when they meet with a phase difference of 0º (peak + peak or trough + trough), i.e, in phase. Full destructive interference (cancellation, I guess) occurs when they are 180º out of phase (peak + trough).
ok, you can do the experiment quite easily in fact. Take a piece of pipe which is transparent. Fit loudspeaker at one end and cover the other so the whole thing is closed. Put some filings or powder inside. Turn on amplifier and feed sine signal. Adjust frequency so that wave length is a multiple of the half wave length and you will have stationary waves which the filings should show. If you shift the f it stops happening until you hit anothe multiple of the half wave. You can adjust the length of the tube instead of the frquency (like a trombone)