Breaking up standing waves

[Hosejockey61]

I've read many a post here about offsetting any internal bracing to break up standing waves. Then when I look at build threads (some by the same people) it appears that all of the internal bracing is symetrical.

How much of an offset is truly needed to break up standing waves?

[DjDisturbed]

Re: Breaking up standing waves

I don't know of any way that bracing can effecting standing waves. But I know it will effect panel resonances , if the panels flex at all. If you want to read more about this you can on Linkwitz's page 2 at the bottom.

http://www.linkwitzlab.com/frontiers_2.htm

This will definitely explain it better than myself. But basically a stiffer and heavier panel will lower the resonance of the panel out of the audible range for the most part. Standing waves happen for a different reason. Do a search for quarter waves , if that's what you are looking to suppress.

I hope this helps.

[Pete Schumacher]

Re: Breaking up standing waves

This guy knows how to break up standing waves.

[Hosejockey61]

Re: Breaking up standing waves

I thought I might be looking into this way too much. Thanks for your input.

Pete, 13000 posts....maybe some medication might help :D:p

[billfitzmaurice]

Re: Breaking up standing waves

I've read many a post here about offsetting any internal bracing to break up standing waves. Then when I look at build threads (some by the same people) it appears that all of the internal bracing is symetrical.

How much of an offset is truly needed to break up standing waves?

Bracing can't break up standing waves as wavelengths long enough to create them would simply diffract around the braces. The good news is that most cabs aren't large enough to support standing wave formation at frequencies that aren't adequately damped out by lining the cab with damping.

[r-carpenter]

Re: Breaking up standing waves

I think it's debatable of how important is suppression of standing waves. Large standing waves are not absorbed by the padding and staffing. Smaller standing waves, say equal to a internal width of the cabinet, can transfer through the port. Padding and stuffing will become progressively more effective with higher frequencies but an over stuffed ported speaker will have lower magnitude of the port output. I also remember reading D'Appolito's book where he mentioned that the driver diaphragm is somewhat transparent to the reflected back-wave.
Sooooo. good question me may say.
but yes, bracing itself will not brake up standing waves. It stiffens panels is all.

[edlafontaine]

Re: Breaking up standing waves

You can choose combinations of panel dimensions which will produce or reinforce certain frequencies in a cabinet. Using panel dimension which are 1:2:4 in relation to each other will do this.

A prescribed method to avoid resonances is to use the "golden ratio" to set panel dimensions. Use the ratio 1:1.618

Search the term to learn more. Here's a calculator link.

http://www.calculatorsoup.com/calcul...oldenratio.php

Using Martin King's worksheets allow designing a cabinet where internal reflections mutually cancel, reduce the amount of stuffing needed to suppress the reflections and increase port output slightly.

[timw]

Re: Breaking up standing waves

Some have mentioned the presence of small blimps in the impedance curves of floor standing speakers signifying standing waves present in the vertical axis of the enclosure. They noted that this could be damped with 4" to 6" thick damping material, such as 3 to 4 lb/cu. ft. semi-rigid fiberglass.

[Paul Carmody]

Re: Breaking up standing waves

Yeah, I think you're confusing "standing waves" with "panel resonances." As Bill said, most standing waves are too large to resonate inside most cabinets (except large floorstanding speakers, especially those specifically designed as transmission lines)

Anyway, you can test panel resonances for yourself. If you have an "empty" cabinet, try jamming a brace in there (it could be as simple as a 3" wide strip of wood). Jam it in different locations along the panel walls and knock on the panel with your knuckles. The higher in pitch the knocking, the better.

[NagysAudio]

Re: Breaking up standing waves

The very best way is by rolling some bubble wrap and sticking this roll right in the middle of the cabinet.

[Wolf]

Re: Breaking up standing waves

The very best way is by rolling some bubble wrap and sticking this roll right in the middle of the cabinet.

If you're rolling it up, anything rolled up will work with this premise in mind.
This is not the case.
Later,
Wolf

[jmb]

Re: Breaking up standing waves

Standing waves will develop at the lengths of the reflecting surfaces as they provide the nodes for the reflections. The longest standing wave you could have is the longest internal dimension of the enclosure and then by the formula c=freq*wavelength you can figure out the lowest frequency of concern. For example, a 1 meter internal dimension would result in a standing wave of roughly 343 Hz.

Standing waves are of concern as they can transmit through the enclosure. They can go through the walls of the enclosure or through the diaphragm of the driver. They do not generally get transmitted through a port, however, as this is a high q (i.e. a narrow bandpass) filter.

The resonant frequency of an enclosure wall is dictated by several factors but one that is fairly readily modified is the length between supports. The shorter the length, the higher the frequency of resonance. At resonance, sounds will more easily be transmitted. All materials have a profile of absorption of sound that generally varies with frequency. Shorter wavelengths are more easily absorbed or dampened so raising the resonance of the panel with bracing brings up the frequency of resonance to a point at which dampening materials can more easily dampen the sound. This is why a combination of bracing and wall dampening is most likely to be the most effective method of minimizing enclosure contributions to the sound.

There is not that much that one can do to mitigate transmission through the diaphragm though the use of sound absorptive materials inside the box may help. The use of fibonacci ratio (.618:1.618:2.618) or prime number ratio proportions of internal dimensions and nonparallel sides help to spread out the energy in the frequency spectrum thereby minimizing the amount of energy at any one given frequency making excitement of a resonance less likely.

Jay

[weinstro]

Re: Breaking up standing waves

I've read many a post here about offsetting any internal bracing to break up standing waves. Then when I look at build threads (some by the same people) it appears that all of the internal bracing is symetrical.

How much of an offset is truly needed to break up standing waves?

I doubt bracing would do much. But, you might want to read up on the Hegeman subwoofer. Morrison Audio makes a commercial version.

Regards,

Rob

[[email protected]]

Re: Breaking up standing waves

i remember this thread. someone have several inch wide braces behind their drivers at a 45 or so and someone mentioned that it might help prevent direct reflection from the rear panel back to the speaker. i suppose this is true but they would still need absorbed at some point. it surley could not hurt.

[r-carpenter]

Re: Breaking up standing waves

i remember this thread. someone have several inch wide braces behind their drivers at a 45 or so and someone mentioned that it might help prevent direct reflection from the rear panel back to the speaker. i suppose this is true but they would still need absorbed at some point. it surley could not hurt.

unfortunately it's not a guessing game. Reflections and the effects need to be measured. The magnitude of the standing waves and how effectively they are absorbed is important IMO.
Are we only talking about standing waves or driver back radiation and effects?