Basic theory on wave propogation

[gmichael]

Hopefully he will see and read your post but would have been best under one of his.

Is there a way for me to relocate it?

 

[Vaughan Odendaa]

I'm sorry. I lost my cool a little bit earlier and for that I do apologize. Perhaps I should try to find the statement in the thread that I was refering to.

That might make things a bit easier. But I really would like to thank everybody who participated in trying to help me better understand these acoustical concepts. I think that it's great to have a forum full of intelligent, knowledgable and helpful people willing to help others out.

I mean, I have been here for awhile now and I still haven't grasped some concepts which are, for all intents and purposes, simple. Funnily enough, the more difficult questions I seem to understand far easier. Have no idea why.

Thanks again.

--Sincerely,

 

[Tomorrow]

I'm sorry. I lost my cool a little bit earlier and for that I do apologize. Perhaps I should try to find the statement in the thread that I was refering to.

That might make things a bit easier. But I really would like to thank everybody who participated in trying to help me better understand these acoustical concepts. I think that it's great to have a forum full of intelligent, knowledgable and helpful people willing to help others out.

I mean, I have been here for awhile now and I still haven't grasped some concepts which are, for all intents and purposes, simple. Funnily enough, the more difficult questions I seem to understand far easier. Have no idea why.

Thanks again.

--Sincerely,

We're really trying hard to understand your question, Vaughan. Can you please restate it, in really simple terms and as clearly as possible? Pretend you're talking to a bunch of 3rd graders (which some of us may be, lol). We'll eventually catch on.

I kind of get the sense you're asking what actually is wave motion, OR how does a wave move from point A to point B. Am I getting close?

 

[Vaughan Odendaa]

Yes. I think you are getting there. Wave transformation, in other words, is something that I would like more information on. In other words, when a sound wave travels from the speaker and hits the rear wall, can you specify what transformation the wave undergoes ?

I think another member did answer this earlier, but I think the wave starts as a compression wave and then hits the wall and becomes a rarefaction, minimum pressure, then reverts back to maximum pressure, then minimum. . .or something like that. Perhaps I'm completely wrong.

So although I'm not as clear as I would like to be, I think you sort of understand where I'm going with this. I need to know what it means when the wave travels and hits the wall and comes "home". What is that ? One cycle ? No ? A 360 degree path back to the place of origin ?

What is that ?

I'm not sure. One wavelength comprises of many cycles. 50 hz is 50 cycles and has a corresponding wavelength. I know that. So the wave has, what, 50 compressions and 50 rarefactions ? Or is it 25 compressions and 25 rarefactions ?

I'm so sorry if I'm making this more difficult than it has to be. I'm just trying to ask the right questions. So the wave travels and hits the wall, is reflected back towards the source, and this whole time I thought that this was a cycle.

According to the knowledgeable people I'm completely wrong on this. I think. I think you all have a better idea as to what I'm trying to figure out, I hope.

Thanks.

--Sincerely,

 

[gmichael]

Does this link help you at all?

http://hyperphysics.phy-astr.gsu.edu/Hbase/sound/reflec.html

 

[Vaughan Odendaa]

Okay, so the article is saying that a high pressure wave reflected at a wall will remain high pressure. Alright.

Cool. But the act of the wave continuing back "home" to it's place of origin is what exactly again ? And a sound wave, for example, of 50 hz, comprises of how many compressions and how many rarefactions ?

I think once those two questions have been answered I will be very happy. Thanks.

--Sincerely,

 

[Vaughan Odendaa]

We're really trying hard to understand your question, Vaughan.

I know that and I do appreciate all the time taken to help me out.

--Sincerely,

 

[Vaughan Odendaa]

According to one site I just visited :

Cycle: One complete interval of a sinusoidal waveform from zero to positive through zero through negative and back to zero.

So I guess this answers my one question. The wave starts from zero amplitude to compression to rarefaction and back to zero, which is back to the speaker ?

 

[Vaughan Odendaa]

And another site :

"A sound wave goes through a cycle of 360 degrees. As the speaker moves forward it goes from rest (0 degrees) through one quarter of its cycle to 90 degrees. As it starts to move back to rest it travels another 90 degrees to the 180 degree mark before moving completely rearward to 270 degrees. As it moves back to the rest position it travels the final 90 degrees to the 360 degree mark. The cycle then repeats itself and we hear the result as music. To summarize, as the speaker moves up and down it travels one cycle which equals 360 degrees."

Which is kind of what the other member posted. Now my contention is what this entails in a room, hence my whole big issue of whether this means that the wave reaches the wall and hits the rear wall or the speaker in order for it to become 360 full cycle. At what point does it become 360 degree within a room.

I guess that has been my question all along, probably the easiest question but the most misunderstood because of miscommunication problems on my side.

--Sincerely,

 

[Vaughan Odendaa]

And again :

"To summarize, as the speaker moves up and down it travels one cycle which equals 360 degrees."

Excellent. What does this mean in real world terms ? Sound wave propogates from speaker to wall, hits wall, changes direction and meets speaker, or ?

--Sincerely,

 

[mtrycrafts]

And another site :

"A sound wave goes through a cycle of 360 degrees. As the speaker moves forward it goes from rest (0 degrees) through one quarter of its cycle to 90 degrees. As it starts to move back to rest it travels another 90 degrees to the 180 degree mark before moving completely rearward to 270 degrees. As it moves back to the rest position it travels the final 90 degrees to the 360 degree mark. The cycle then repeats itself and we hear the result as music. To summarize, as the speaker moves up and down it travels one cycle which equals 360 degrees."

Which is kind of what the other member posted. Now my contention is what this entails in a room, hence my whole big issue of whether this means that the wave reaches the wall and hits the rear wall or the speaker in order for it to become 360 full cycle. At what point does it become 360 degree within a room.

I guess that has been my question all along, probably the easiest question but the most misunderstood because of miscommunication problems on my side.

--Sincerely,

I think I may see one of your misunderstandings. You are equating the reference to 360 degrees as the sound being sent out and returned back, a full circle. That is not what that 360 degrees means. That is just a mathematical expression of a cycle, a wave, etc. Geometry, sine, cosine, tang, etc. Not a physical circle in space. The sound waves will bounce off walls as do light waves, at angles of equal size as it hits the surface. And, in time, it will decay. It has to.

 

[Ethan Winer]

Vaughan,

> The wave starts from zero amplitude to compression to rarefaction and back to zero, which is back to the speaker ? <

No. Using my earlier example, a completed cycle at 200 Hz will be 5'8" away from where it started. The wave doesn't "go back" to anywhere unless it happens to hit a reflecting boundary. Outdoors waves continue forever, and 200 Hz repeats every 5'8". Here's a classic experiment you can try:

Take a long (ten feet) piece of string and tie one end to a door knob. Stand not quite ten feet away and hold the other end of the string in your hand. Then swing your end up and down. If you don't pull it too tightly you'll see waves form along the length of the string.

--Ethan

 

[Vaughan Odendaa]

Ethan, then can you explain what the statements I referenced mean in connection with wave cycles and the change in pressure as they propogate ( ie, from zero amplitude to compression to rarefaction and back to compression ) A number of sites have explained this but I'm not sure.

Do you agree with the statements I referenced in connection with this ?

The wave doesn't "go back" to anywhere unless it happens to hit a reflecting boundary.

I'm talking strictly within the room.

--Sincerely,

 

[Vaughan Odendaa]

I think I may see one of your misunderstandings. You are equating the reference to 360 degrees as the sound being sent out and returned back, a full circle. That is not what that 360 degrees means.

And what about 360 degree phase cycle ? A wave with one compression peak meets up with another wave with a rarefaction valley and you get a cancellation.

Sound waves are longitudinal but can and do travel (often) 360 degrees within a room, right ?

--Sincerely,

 

[Buckle-meister]

Vaughan, I'm jumping in here without having read all the previous posts - never a good idea - but here's my take:

As far as 360 degrees for a cycle of a wave form, I'll explain it as it was once to me. Take the sinusoidal wave, a good wave to use as an example since it's relevant to the topic being discussed:

Say you were to hold one end of a (short) piece of string in your left hand and the other end of the string - connected to a pencil - in your right hand and, keeping your left hand stationary, rotate your right hand, you'd be drawing circles over and over, one on top of the other. Now assume there's a long piece of paper underneath the pencil and someone else is slowly pulling it past you as you continually draw your circles, you'd find that the resulting curve would sinusoidal. If memory serves, this is called a phasor.

...what about 360 degree phase cycle ? A wave with one compression peak meets up with another wave with a rarefaction valley and you get a cancellation.

If a curve is phase-shifted - i.e. bodily moved - 360 degrees either forwards or backwards, there won't be any cancellation because the peaks will line up with the peaks and the troughs will line up with the troughs (though you would get a doubling of amplitude ). What you are taking about is when a curve is phase-shifted 180 degrees. Draw a sine wave and mark off 90, 180, 270 and 360 degrees and you'll see what I mean.

Sound waves are longitudinal but can and do travel (often) 360 degrees within a room, right?

I don't think it's strictly right to say that sound waves are longitudinal as this tends to imply that the sound moves parallel with a longitudinal axis in a cordinate system (where the longitudinal axis is the longest of the three), and this may not be true in practice, for example if someone faces the wide wall of their rectangular shaped listening room. But that aside (just me being pedantic), yes, depending on a number of factors such as frequency, SPL surface absorption and room size, sound will bounce all around a room before eventually dying away.

Does any of this help?

 

[Vaughan Odendaa]

What you are taking about is when a curve is phase-shifted 180 degrees. Draw a sine wave and mark off 90, 180, 270 and 360 degrees and you'll see what I mean.

Yes, but if one compression wave meets with one rarefaction valley, it is 180 degrees out of phase. The net result would be zero or minimum amplitude (as if the air molecules had not been disturbed).

Cancellation.

--Sincerely,

 

[Buckle-meister]

Yes, but if one compression wave meets with one rarefaction valley, it is 180 degrees out of phase. The net result would be zero or minimum amplitude (as if the air molecules had not been disturbed). Cancellation.

I think we're both in agreement here. Yes, two sinusoidal curves 180 degrees phase-shifted would cancel exactly.

And remember, it's phasor not phaser. The latter's used by Captain Kirk.

 

[Vaughan Odendaa]

Heh. I'm going to order some more acoustic books. Some are already on the way. Can you recommend some really great books on acoustics which don't delve too much into mathematics ?

Thanks.

--Sincerely,

 

[Buckle-meister]

Can you recommend some really great books on acoustics which don't delve too much into mathematics?

Sorry mate, I only have The master handbook of acoustics and I believe you have that yourself already.

 

[Savant]

Vaughan,

It's good to read that progress is being made. Re your request for more books to read, I suddenly had a thought just now of a book I read a long time ago and have never recommended to anyone. The main reason being I'd forgotten until just now that I had read it.

It's called Practical Acoustics by Stephen Kamichik. While it is not "light" on the math - there are plenty of equations and such (this is physics, remember! ) - there is also plenty of practical, down-to-earth explanation. You might add that one to your list. As an example, I found that Figure 2-2 on page 13 would probably have answered the question you were trying to pose to us in this thread...

Links to Practical Acoustics:
Amazon (Be sure to check out the "used" offerings.)
eCampus
Powell's
eBay

I would also recommend Acoustics and Psychoacoustics by David Howard and James Angus. Plenty of useful stuff in this one and a little lighter on the math than Kamichik's book.

Links to Acoustics and Psychoacoustics:
Amazon
Music Books Plus
Focal Press
(The three links above are for the latest edition fo Howard and Angus, which is not the same as the edition I have. Even searching some of the used book sites, I couldn't seem to find the earlier edition. Must be quite popular!)