Basic theory on wave propogation

[Tomorrow]

Hopefully my question on wave cycles will be answered. I've only asked it several times now.

--Sincerely,

And I think it has been answered several times by these knowledgeable people. Please rephrase your specific question.

 

[mtrycrafts]

Hopefully my question on wave cycles will be answered. I've only asked it several times now.

--Sincerely,

You may want to enroll in a college class on this? Then, you can ask the Phd professors to your heart's content.

 

[Savant]

Vaughn, bro, you're killing me here. But I'll give it one last go:

I just want to know, for interests sake, if the sound wave travels from the speaker to the rear wall and back to the front wall (or is it back to the speaker again ? )

Please remember that I know that this is determined by frequencies, in terms of wavelength, but I want to get a general idea of what contitutes a cycle. A cycle is the distance from one compression peak to the next, which is great, but in real world terms, I just need to know how the wave travels in the room itself.

c=fλ

c = ~1130 ft/s
f = frequency, Hz
λ = wavelength, ft

λ is also equal to, as you've pointed out, the distance between compression or rarefaction peaks in the wave. So, if the distance from loudspeaker to wall is 14 ft, then one complete cycle of an (approx.) 81 Hz wave will "fit" in the space between. (f=c/λ=1130/14=~81 Hz)

Whether a complete cycle would "fit" in the distance from loudspeaker-to-wall-to-wall or loudspeaker-to-wall-to-loudspeaker would depend on the the distances and frequencies in question.

As for "how" the wave travels in the room, that will depend on the directionality of the loudspeaker, among many other things. (It should be noted that the above explanation of waves "fitting" makes quite a huge assumption of plane wave behavior. Sound travels in spherical waves. Therefore, plane wave theory can only get you so far...)

N.B.: If this doesn't answer your question, I honestly don't know what you're asking and I am very sorry for the misunderstanding!

Final question, peaks and rarefactions; a peak occurs at full wavelength and rarefaction occurs at 1/2 wavelength ? Am I correct in my assessments ?

In general terms, compression and rarefaction are 1/2-wavelength apart, yes.

 

[Vaughan Odendaa]

I'm sorry if I'm giving you a difficult time. All that I wanted to know is, if the wave cycle travels from speaker to rear wall and back to speaker to constitute a cycle, or, alternatively, if the wave must make a return trip to the opposite wall in order to constitute a cycle.

Sorry. There are no other people who can help me better understand these things. This forum has been a valuable source of info for me and I've learned a lot, but some (I assume) simple things just escape me and I wish to learn a little bit more.

I think I've heard Ethan talk about a sound wave traveling from speaker to wall and back and that constitutes a wave cycle. If Ethan is around here, I'm sure he'll elaborate. Again, sorry for making you go through all the trouble.

I really do appreciate the time you take to help me understand these subjects. So thank you.

At least I was correct on one of my questions. Heh.

--Sincerely,

 

[Ethan Winer]

Vaughan,

> I think I've heard Ethan talk about a sound wave traveling from speaker to wall and back and that constitutes a wave cycle. If Ethan is around here, I'm sure he'll elaborate. <

I'm not an expert with wave propogation. My understanding is that all waves emit from the source and expand outward. At high frequencies the sound travels like rays, but at low frequencies it's more like blowing up a really big balloon until it fills the room.

I know that waves do hit the rear wall behind you and then bounce back. I'm sure this is the case because the reflections create classic comb filtering even at very low frequencies. Though you probably need a room large enough for at least half a cycle to "develop" over the room length. I think the term is "pressure mode" when a wave is too large to fit. Again, this is like a balloon.

Either way, the wave always bounces back to the front wall in front of you. The source may be a loudspeaker out in the room, but the reflections occur between the parallel surfaces. Beyond that, I have to defer to Jeff's expertise.

--Ethan

 

[Tomorrow]

I'm sorry if I'm giving you a difficult time. All that I wanted to know is, if the wave cycle travels from speaker to rear wall and back to speaker to constitute a cycle, or, alternatively, if the wave must make a return trip to the opposite wall in order to constitute a cycle.--Sincerely,

No. A cycle has nothing to do with the room-wall distance or echoes. A cycle in waveguide propagation has a very specific definition. Cycle is a periodic event, and in sound, it is a wave measured from peak to peak (output from the speaker or sound source). (This is by convention. One could also measure it null to null.) In this hobby, the number of these cycles per second are identified as Hertz....it means the same as cycles per second.

Is this what you're trying to find out?

EDIT: The longer the wavelength, the fewer number of cycles may be counted per second. Thus a 20 Hz subwoofer signal has a much longer wavelength than a 500 Hz signal. Longer and shorter wavelength signals behave somewhat differently in different media (room surfaces for example, or water, etc). This is one reason why, when you're driving down the freeway at 55 mph and you hear the subwoofer in that guys tricked Honda three cars in front of you, you only hear the sub and not the other higher (filtered) wavelength sounds that are at that moment deafening him. LOL.

 

[bpape]

Exactly. I think you're confusing the wavelength of a specific frequency with how it interacts within a closed space - 2 very different things.

Bryan

 

[Savant]

budz and Bryan: Thank you for your perceptive contributions! It appears there has been confusion regarding the term cycle.

Vaughn: In my limited capacity to understand English only from the perspective of an acoustics geek, it appears that I have but one definition for the term cycle in my brain - see budz' post above.

As for low frequency wave propogation in small rooms - the actual explanation I think you're looking for - Ethan's captured the essence above. Is that enough detail?

 

[Tomorrow]

budz and Bryan: Thank you for your perceptive contributions! It appears there has been confusion regarding the term cycle.

Vaughn: In my limited capacity to understand English only from the perspective of an acoustics geek, it appears that I have but one definition for the term cycle in my brain - see budz' post above.

As for low frequency wave propogation in small rooms - the actual explanation I think you're looking for - Ethan's captured the essence above. Is that enough detail?

No need for the , Jeff. That detailed and accurate explanation of yours took me down memory lane....college from a long time ago. (Vaughan, there will be a test tomorrow! ) It helps to be a simpleton like me in order to give a simple answer.

 

[Vaughan Odendaa]

Alright. Then perhaps I was confusing cycles with wave propogation, at least how I was describing it. Fair enough.

And what is this talk about a sound wave traveling from the speaker to the wall and back ? I know that resonance occurs when the sound wave folds onto it's own axis and "ramps up" with each successive cycle. More and more energy builds up from each wave cycle.

But what I am confused about, is this whole round trip "cycle" that I've heard about. Perhaps there are two different definitions of cycle, but one definition I've heard is that it is a 360 degree travel in the air.

Peak to trough and then peak to trough. Or something like that. So when I ask what constitutes a cycle, as in when a sound wave propogates from the speaker to the wall and back, I'm sure that this is different then the other definition of cycle that people here are mentioning.

Perhaps someone can help me given this new information I'm injecting. I hope I'm becoming more clear now as to what I want clarified. Thanks.

--Sincerely,

 

[Tomorrow]

Alright. Then perhaps I was confusing cycles with wave propogation, at least how I was describing it. Fair enough.

And what is this talk about a sound wave traveling from the speaker to the wall and back ? I know that resonance occurs when the sound wave folds onto it's own axis and "ramps up" with each successive cycle. More and more energy builds up from each wave cycle.

But what I am confused about, is this whole round trip "cycle" that I've heard about. Perhaps there are two different definitions of cycle, but one definition I've heard is that it is a 360 degree travel in the air.

Peak to trough and then peak to trough. Or something like that. So when I ask what constitutes a cycle, as in when a sound wave propogates from the speaker to the wall and back, I'm sure that this is different then the other definition of cycle that people here are mentioning.

Perhaps someone can help me given this new information I'm injecting. I hope I'm becoming more clear now as to what I want clarified. Thanks.

--Sincerely,

Vaughan,

I don't wish to sound rude at all. But what you have said here causes me to think that you haven't understood your own questions or the answers that we've provided. Perhaps if you re-read a couple of the recent posts the answer will pop out at you.

Other than that, I strongly suggest that you get an entry level book on acoustic fundamentals. It would likely be more enlightening to you than we can be here in this format.

 

[Vaughan Odendaa]

I know what a cycle is and I gave a correct definition for it from the beginning. I may have confused this later on for something else that I wanted clarification on. Sorry for that.

What I am having difficulty with (for the record, I think there are perfectly competent people here to answer what I want sufficiently ! ) is what this wave motion or rather, 360 degree motion that the wave must travel ? Perhaps I was confusing cycles for wavelength.

Hence the wave traveling to the wall and back. And hence my need for clarification if this entails the sound wave traveling from the speaker to the wall and back to the speaker. If someone could just please explain what I'm trying to describe I would be very happy, and I don't mean to be rude either, but I don't think what I'm asking is beyond the reach of the more experienced members here.

I mean, I know what I want answered. It might not be worded correctly, however and I'm sorry for that. If my question was answered on the previous pages I promise you that I would not have bothered to continue asking even more questions.

--Sincerely,

 

[Vaughan Odendaa]

And I have the Master Handbook of Acoustics. The book doesn't describe or explain every last detail.

I'm starting to get frustrated now. I don't appreciate being told that my questions were answered on the proceeding pages when they really weren't otherwise I would not have wasted everyone's time to ask the same questions again. I'm not stupid.

I really don't appreciate that at all.

 

[Tomorrow]

And I have the Master Handbook of Acoustics. The book doesn't describe or explain every last detail.

I'm starting to get frustrated now. I don't appreciate being told that my questions were answered on the proceeding pages when they really weren't otherwise I would not have wasted everyone's time to ask the same questions again. I'm not stupid.

I really don't appreciate that at all.

Yes, trying to phrase a question about something that confuses you, or understand an our answers about it, can be darn frustrating. I wasn't at all suggesting that you are stupid. I was suggesting the same thing that Jeff and MTRY were...that perhaps you would be better suited studying the matter with primers or college courses. You have a good deal of curiosity.

We're not ignoring your questions, as you have suggested in previous posts. We have answered your questions as we understand them. Communication is sometimes difficult in i-forums.

Go easy.

 

[mtrycrafts]

You have a good deal of curiosity.
Communication is sometimes difficult in i-forums.

Go easy.

More reason for him to pursue a professional setting, a formal acoustics college courses. Penn State is a good one. After all, we are not down to the basic fundamentals to explain what he needs to satisfy himself, especially on the inet

 

[gmichael]

I didn't read every post, so please forgive me if this has already been answered. Most of the answers I saw were way more complicated than the question asked required. So let this very simple man give a stab at it.

Imagine something is vibrating. Hopefully it's a speaker. As it moves to the left, it will compress the air molecules on the left side of it. As it moves to the right it will pull the air molecules to the left farther apart. I believe that this results in the low pressure area you are refering to.

The opposite happens on the right side creating a wave that is 180 degees out of phase. These two waves should not be aloud to play together unsupervised.

The end.

 

[gmichael]

Oops, I missed the (Second thing, wavelength, or cycles)

Let's say that the speaker is dead center to start our cycle. It then moves to the left (for example) smashing air molecules together as it goes. It reaches the far left point. This would be the top of the sine wave (or 90 degrees) Then the speaker moves to the right until it goes as far to the right as it's gonna get. Now you are at the bottom of the sine wave (or 270 degrees) When it comes back to the center again you have completed your 360 degree trip (or one full cycle).

Don't sweat the wall. It doesn't factor into this.

 

[Savant]

Vaughan, mon frère:

And what is this talk about a sound wave traveling from the speaker to the wall and back ? I know that resonance occurs when the sound wave folds onto it's own axis and "ramps up" with each successive cycle. More and more energy builds up from each wave cycle.

But what I am confused about, is this whole round trip "cycle" that I've heard about. Perhaps there are two different definitions of cycle, but one definition I've heard is that it is a 360 degree travel in the air.

You mentioned twice above that the concept you are trying to work out is something you "heard" elsewhere. Might you be able to link us to, or otherwise point us in the direction of where you heard it? Something we can expand upon based on its context, perhaps? Just trying another approach. I really do want to help you, bro.

 

[mtrycrafts]

I didn't read every post, so please forgive me if this has already been answered. Most of the answers I saw were way more complicated than the question asked required. So let this very simple man give a stab at it.

Imagine something is vibrating. Hopefully it's a speaker. As it moves to the left, it will compress the air molecules on the left side of it. As it moves to the right it will pull the air molecules to the left farther apart. I believe that this results in the low pressure area you are refering to.

The opposite happens on the right side creating a wave that is 180 degees out of phase. These two waves should not be aloud to play together unsupervised.

The end.

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

 

[Ethan Winer]

Vaughan,

> But what I am confused about, is this whole round trip "cycle" that I've heard about. Perhaps there are two different definitions of cycle, but one definition I've heard is that it is a 360 degree travel in the air. <

One cycle at a given frequency has a wavelength some number of feet or inches. This is directly related to the frequency of the sound, and unrelated to the size of the room the sound is playing in. So lets use 200 Hz as an example, playing in a room that's 20 feet long. Each full cycle of the 200 Hz wave is 5'8" long, and the room resonances fall on 28.25, 56.50, 84.75, 113 Hz, and so forth. As you can see, the room dimension and wave frequency have nothing to do with each other. If you play 200 Hz in a different room with a different length, the room will have a different series of natural resonances, but the 200 Hz tone will still be 200 Hz and will still sound like 200 Hz.

Does this help?

--Ethan