A Physics question

[Dan Schultz]

I've been learning about the physics of sound waves in my physics class, and learned something new. If waves of sound are exactly the opposite of each other than they almost completely cancel each other out. I was wondering if anyone had ever thought of (if it's possible) making two almost identical instruments with their waves just being the opposite of each other?

Good luck with this one guys!

Rick Denney will have an answer

[Chuck(G)]

If you've ever witnessed a demonstraton of cancellation done electronically, you may recall a number of variables.

The first is getting the phase relationships between the two sounds; they must be precisely 180 degrees out of phase at the point of the listener to cancel. With wind instruments and their inability to hold frequencies precisely, this would be extremely difficult.

Now, you can vary the phase relationship between two pure sound waves by varying the distance from the listener of each source, but that brings us to the next point--the sounds must be equal in amplitude at the point of the listener to cancel out.

Note that I said "pure sound" in the previous paragraph. That's because any harmonic content is not likely to share the same phase relationship as the fundamental.

Also observe that other factors, such as reflections can have a strong effect on what you hear.

So, can you do this with two tubas? Not practically. But you can get an idea of what happens by moving around a fairly live room with a single tuba player honking away. You'll discover, for a given note, that there are "dead spots" where the sound is attenuated by the reflected sound interfering with the sound reaching you via the direct path. Adding sound-absorbing material to the room can actually make the sound heard at the dead spots louder.

If you want to goof around, get a nice sinewave generator and an amplifier and a couple of speakers in an anechoic room and you can have a blast.

[Rick Denney]

I've been learning about the physics of sound waves in my physics class, and learned something new. If waves of sound are exactly the opposite of each other than they almost completely cancel each other out. I was wondering if anyone had ever thought of (if it's possible) making two almost identical instruments with their waves just being the opposite of each other?

It's not a question of one sound being the opposite of another sound. It's a question of one sound being exactly out of phase with another sound. If you start the sound exactly one-half wave later on one tuba than on another tuba, they will be out of phase. In practice, it's hard to do even by accident, and for two reasons.

One is that one sound will catch up to the other sound in a hurry. If both are playing a low Bb, the waveform only has to catch up 1/116th of a second to be in phase. A slight intonation difference for a very brief period will do that. In fact, unless the players are very good indeed, they will wander in and out of phase as they attempt to stay precisely in tune with each other. That's what makes the beats.

The second and more important reason is that the tuba waveform is not symmetrical top to bottom. The two sounds in your physics class were pure sine waves, where the top lobe is identical to the bottom lobe, except flipped. When out of phase, they sum to zero and cancel each other out. When tubas are out of phase, they do not sum to zero and don't cancel each other, though the quality of sound will be affected.

If you want to play with the effect, record yourself playing with a stereo microphone, and play it back. Wire one speaker on your sound system backwards from the other. That speaker will be out of phase. It will pull power out of the sound, but it will still be a tuba sound.

Rick "eschewing oversimplification" Denney

[SplatterTone]

Sort of like what has already been written, if you amble on in to your local Hi-Fi Shoppe (if it's properly snooty, that's how it's spelled), and persuade them to let you take speaker systems like this:

and wire one woofer in each system backwards, you will see the bass response just about disappear.

Many many moons ago when I sold Hi-Fi, we got a set of speakers similar to the picture (they were Jennings Research Elan model) where some dufus at the factory wired them wrong like that. We heard it right away, and knew what it was ... although it was a little difficult to believe something this stupid had been done on a speaker system that was rather expensive.

A characteristic of good speaker design is to make the speaker walls as stiff and vibration proof as practical (concrete speaker enclosures have been made, but they are not all that practical) because the vibration from the speaker walls is from the backside of the woofer which is out of phase with the front side.

[tubeast]

This cancelation of sounds can be observed quite well using recorders. (No not electronic devices, but the wooden flutes)
Recorders have a very uniform sound that´s very close to a sine wave, and this makes it easier to mess around with.
Two of them playing in "unison" will almost invariably result in what we call "Schwebung", a kind of vibrato that will decrease in frequency the better in tune the recorders are.

[Chuck(G)]

This cancelation of sounds can be observed quite well using recorders. (No not electronic devices, but the wooden flutes)
Recorders have a very uniform sound that´s very close to a sine wave, and this makes it easier to mess around with.
Two of them playing in "unison" will almost invariably result in what we call "Schwebung", a kind of vibrato that will decrease in frequency the better in tune the recorders are.

I don't know if I'd call that cancellation necessarily. Heterodyning might be a better term, where one can hear the sum and/or difference frequencies. I think of cancellation as occuring only when two sources are of exactly the same frequency and amplitude and of opposite phase.

[Anterux]

An other thing:

We have two ears. Even if it is a pure sound, electronically generated, in fase oposition, wont we may have some difficulty to hear the fenomenum?

Is it possible due to the distance between the ears tha the wave doesnt reach us in a perfect fase opposition?

Kind Regards from Azores.

Antero.

[P.J.]

Is it possible due to the distance between the ears tha the wave doesnt reach us in a perfect fase opposition?

I've taught physics and even with electronic sound generators set up identical amplitudes and frequencies it gets softer, but never totally cancels for what I believe to be the above reason.

[Steve Inman]

It's actually quite a bit cheaper to use foam earplugs ....

[iiipopes]

Re: speakers wired backwards -
About 25-30 years ago when I was in high school I was in a rather large church auditorium for a area youth meeting. As I listened to the person talking at the microphone, I thought I heard the phasing and volume drop, as I was near the center of the room. As I walked a couple of feet in any direction around the pews, it went from completely inaudible to twice as loud. Of course, being a guest, I was not able nor at liberty to find a church administrative official to tell him or her, but I found it quite interesting, being a large church, with a large budget (they had just installed a new pipe organ as well as upgrading the sound system) having this situation. Of course, since it was only one spot in the center of the room, only three or four people might have noticed otherwise, and only when the room was full to capacity, eliminating most reflections.

[windshieldbug]

We have two ears. Even if it is a pure sound, electronically generated, in fase oposition, wont we may have some difficulty to hear the fenomenum?

Back in the "dark" days of music synthesis, we used to play around with Moog synthesizers using exactly this phemomenae;
we'd hook up sine wave generators to a set of head phones and tune the waves until it sounded like the tone was moving from side-to-side, or that there was no tone at all.

As Rick notes, however, you could only make such things happen with sine waves. And headphones. Putting it through our studio VotT's you could only get a very rough approximation.

It wasn't iPods that caused my hearing loss!

[iiipopes]

One of the seminal tomes on this subject is Hemholz, On the Sensation of Tone, which can probably be consulted at any medium to large college/university library. To give you an idea of the size of the volume, phasing/cancellation is just one "small" chapter, among philosophical discussions of the nature of sound itself and the physiology as well as the psychology of hearing, discussion of different tunings and how they affect chordal/ensemble sound, how difference and additive tones work, etc.

[DonShirer]

I often performed the sinewave cancelling demo in physics class. The speakers will not tend to exactly cancel unless they are in the same physical location (one on top of the other will usually do it). If placed apart, they will set up a field of alternate positive and negative interference fringes depending on the phase difference because of the different distances of the ear from the sources.

The effect is put to use in noice reducing (not cancelling!) microphones and earphones, but as others have pointed out, you can't rely on producing it with two actual musical instruments.

Don Shirer
Westbrook, CT

[punk_tuba]

a good read in the subject of theoretical physics (string theory, relativity) is joseph eglar's "Einstein's Violin".