Standing Wave Info

From:  http://pwkdesigns.com/forum/viewtopic.php?f=3&t=377

Travis:

Standing waves are terrible in bandpass/blowthrough setups with parallel baffles. Standing waves have been known to kill woofers and there is still no computing software to determine where and when standing waves will be present unless a micrometer is directly used on the box baffle. Found this video from a fellow DIY user who can explain it better.

http://www.youtube.com/watch?v=-gr7KmTOrx0

Me:

Just out of interest, how exactly are you determining that standing waves are terrible in bandpass / blow-through setups with parallel baffles, or are the cause of woofer failures? And how do you use a micrometer to determine the presence or the position of standing waves?

Travis:

Yes, had it wrong. It is a reasonance-meter, I got stuck working on some equipment currently with me and forgot the name.

As for how I determined standing waves are terrible… How would I not be able to? I have tried multiple designs that you have done for my blow-through that within over a year the speaker closest to the port mouth would out of nowhere just die. I’ve tried many different scenarios in my head and nothing is leading to logical answers. Talking with world record holders, they offered simple advice. Angle baffles to kill standing waves. It’s been proven time and time again.

Me:

You still haven’t explained how standing waves affect bandpass enclosures. All you’ve done is state that you’ve had speakers closest to the port fail. What can you tell us about the behavior of standing waves near the port and how you identified them, of all things, as the cause of the failure?

First, let’s say that a pair of parallel baffles of some arbitrary chamber in a bandpass enclosure are spaced as far as 30 inches apart. Already, this equates to a fundamental frequency of 226.2Hz, to say nothing of its harmonic components. Even if your enclosure plays up to 80Hz, that’s still a round trip distance of 169.6″ that a wave would have to travel before the phase of its reflection could interfere at the origin of the subsequent cycle.

How then are you applying the math to make low frequency standing waves in any conventional size chamber not only possible, but also the cause of woofer failures? On top of that, what makes bandpass enclosures more prone to this effect than any other design where a woofer’s piston fires directly against a parallel baffle?

Travis:

Standing waves = turbulance = out of phase = non symetrical cone movement = coil rocking

At the port the standing waves were causing an unsteady movement of air. See above discription^

Math for the standing waves was pretty simple for me. Acoustic wave formula is all over the web. 344/(hz)x3.3 = wave length in feet

344 / frequency (lowest fundamental wanting to play)
x 3.3/4 = xx (in feet as far as an object has to be to take full advantage of that frequency)

Made perfect sense to me as how we were able to figure out what frequency it was that caused the port mouth to start having turbulance. From there it was simple math.

Applies in the same scenarios where a regular “bass reflex” aka vented enclosure are in a trunk. Trunks/suvs with vented boxes act the same. I’m sure you’ve played with woofer placement from hatch and flat areas in trunks and suvs. To close to a surface and it will sound different as opposed further away. Acoustical wave is a funny thing but pretty easy to account for when knowing the formula and the speed of sound.

Me:

*sigh*

I would probably feel a little better if at the end of your post, you told us that you were joking. Unfortunately, I have a feeling that you weren’t, and this is the wrong place to spread misinformation.

“Standing waves = turbulance = out of phase = non symetrical cone movement = coil rocking”

Where are you getting this nonsense from? Standing waves describe a phase grouping relationship as it applies to resonance, not to the flow profile of air. These are two COMPLETELY DIFFERENT frames of motion. Then, you equate this comparison to “out of phase.” What’s out of phase? If there’s no definitive phase relationship then there isn’t a standing wave, by the very definition of the phenomenon. And what does any of this have to do with the symmetry of the cone movement?

“At the port the standing waves were causing an unsteady movement of air. See above discription^”

Again, standing waves don’t affect the flow profile of air anymore than a ripple in a pond affects it’s current (or lack thereof.) Not only could the geometry of any conventional size port not sustain standing waves corresponding to any bass frequency, but your above reference to the contrary is completely wrong as well.

“Math for the standing waves was pretty simple for me. Acoustic wave formula is all over the web. 344/(hz)x3.3 = wave length in feet

344 / frequency (lowest fundamental wanting to play)
x 3.3/4 = xx (in feet as far as an object has to be to take full advantage of that frequency)”

There’s no such thing as the lowest (or, for that matter, the highest) fundamental frequency, nor is it selectable from any given range of frequencies. It is a single definitive value corresponding to a single definitive distance and, as I’ve already pointed out, even a value of 80Hz corresponds to a distance of 169.6″. Any frequency below that would have an even longer span.

“Made perfect sense to me as how we were able to figure out what frequency it was that caused the port mouth to start having turbulance.”

Did it? Walk us through it.

“From there it was simple math.”

So it wouldn’t be too much trouble for you to offer us a simple mathematical example.

“Applies in the same scenarios where a regular “bass reflex” aka vented enclosure are in a trunk. Trunks/suvs with vented boxes act the same.”

Doesn’t that contradict the specificity with which you approached bandpass enclosures at the beginning of this post?

“I’m sure you’ve played with woofer placement from hatch and flat areas in trunks and suvs. To close to a surface and it will sound different as opposed further away.”

Yes, this is due to compression, acoustic loading, and the tapping effect, not standing waves.

“Acoustical wave is a funny thing but pretty easy to account for when knowing the formula and the speed of sound.”

So you say, but it looks to me like you have a LOT of back-peddling to do before you can attempt to qualify that assertion.

Travis:

WOW!!! Glad I have referred to other people for work.

So Pete is clearly stating sound waves move no air at all.
#2 Pete says acoustical loading isnt affected by standing waves.. lol I’m sure Frankie Rio would have a hay day with this.

I dont think I should further press-on.

But, for the record, if you cant hear sound waves. Than I guess air doesnt vibrate and standing waves dont move. For air to not vibrate then moving object with frequencies is impossible because air doesnt move/vibrate.

By the way, its nice to know Pete does something other than take months to “design” “bass reflex” enclosures. Its nice to know I am the discussion of the ol’ hillbilly hangout.

Me:

“WOW!!! Glad I have referred to other people for work.”

Great, so what are you doing here other than instigating petty drama?

“So Pete is clearly stating sound waves move no air at all.
#2 Pete says acoustical loading isnt affected by standing waves.. lol I’m sure Frankie Rio would have a hay day with this.”

Can anything you say stand on the merit of its own scientific validity, or do you operate based purely on how you think others might react to something that you clearly do not understand?

I’m going to take one last stab at trying to explain this to you, this time using the video at the beginning of this thread as an example. Think of the string as a volume of air in a two-dimensional time-frequency domain. It represents a fluid medium by which waves travel along the horizontal axis, and the amplitude of which is depicted by the vertical axis. So, a pressure wave is created at the left end of the string, it then travels right across the string, is reflected at the right end, and travels back toward the left. This back-and-forth travel represents one frame of motion, however the string (our volume of air) in and of itself, represents another frame of motion, hence, it does not move left or right with the wave. In fact, it could move completely independently of what the wave is doing if, for instance, the demo was set up on a rolling cart.

So – no – waves (standing or otherwise) don’t move air. And if you STILL don’t get it, at least that gives everyone else here some insight into the level of understanding on which you base your assertions.

“I dont think I should further press-on.”

Yes, it’s probably best that you don’t…

“But, for the record, if you cant hear sound waves. Than I guess air doesnt vibrate and standing waves dont move. For air to not vibrate then moving object with frequencies is impossible because air doesnt move/vibrate.”

No, standing waves don’t move, they just occur at static intervals based on frequency and the distance to the reflection point. If hearing depended on standing waves (moving or otherwise,) you wouldn’t be able to hear a sound source in an anechoic chamber.

“By the way, its nice to know Pete does something other than take months to “design” “bass reflex” enclosures. Its nice to know I am the discussion of the ol’ hillbilly hangout.”

Thanks for caring.



5 Responses to Standing Wave Info

  1. Pingback:Does Anyone Understand Cabin Gain - DIYMA.com - Scientific Car Audio - Truth in Sound Quality

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  3. Avatar sameoldme
    sameoldme says:

    Was he using subwolfers to do these standing and sitting waves test? 😉

  4. Well, nobody told this guy that the speed of sound isn’t set in stone.

    During the design of PMC’s transmission lines, they discovered that the speed of sound in an enclosed space is quite a bit faster than the mathematical predicitons, so when they tested everything they found the line could be shortened considerably.

    In fact, the variations in sound pressure resulting in distortions of the speed of wave propagation, this non-linearity of wave propagation speed and direction of force, this is actually what results in diffraction.

  5. Avatar da bishop
    da bishop says:

    here’s the interview where he mentions some of the varying speed of sound issues: https://vimeo.com/24010655