Minimizing standing wave

[gmed]

How do you minimize standing wave cancellation in s apeaker enclosure? Does woofer placement and bracing matter? I am already building a Translam oval rounded wall enclosure. From what I know, lateralizing the woofer minimizes this. Any hints?

[joecarrow]

In general, people try to follow the "golden ratio" for cabinet dimensions. This spreads out the frequencies at which standing waves can exist, minimizing their effects. Another approach that is sometimes used is a transmission line. You can read more about that here: http://www.t-linespeakers.org/ Also at http://quarter-wave.com.

Keep in mind that there are two types of unwanted resonance in a loudspeaker. There is cabinet resonance (which a curved wall translam can do a great job of overcoming), and air cavity resonance. The frequencies at which resonance will occur are dictated by wavelength and the internal dimensions of the cabinet. Yes, driver placement will matter, but if you have a spherical cabinet there will be a resonance that driver placement cannot help. The best approach, in my opinion, is to size the enclosure for the operating bandwidth of a driver.

For example, a sonotube subwoofer would have a large resonance correlated to the diameter of the tube- but it would be in the hundreds of hertz, and is safely outside of the operating range of the driver, which generally only operates below 100 hz.

Another example is the rear chamber of a tweeter. At the high frequencies tweeters produce, the size of a resonant chamber is quite small- but fiber stuffing does a great job of absorbing the sound and preventing it from being a problem.

The high bass is the hardest to get absolutely right, due to wavelengths and the reduced effectiveness of most sound absorbing materials.

Fortunately, many of us don't need to tear our hair out too much over imperfect bass enclosures since our listening rooms contribute enough interaction in this region that the response is all over the place anyway.

In my opinion, the only way to get a real-world room (not a dedicated listening room) to get all of the benefits of a wonderfully faithful speaker is to employ some sort of room correction, or to operate as a dipole.

[kingpin]

Isn't this the reason why people use such things as felt, to line the inside of their boxes.

[gmed]

Thanks for the response. My main issue is standing wave cancellation due to driver placement. From what I have learned in the past, whenever the driver is placed in the middle of an enclosure, there will be sound wave cancellations, so to fix this, you must place the driver on one side. That means no symmetry. But I see a lot of speakers that the woofer or the mid-bass driver is placed exactly in the middle of the enclosure. Does bracing compensate for this? What about rounding over the inner edge of the opening of the speaker hole? When is driver placement an issue?

[joecarrow]

Thanks for the response. My main issue is standing wave cancellation due to driver placement. From what I have learned in the past, whenever the driver is placed in the middle of an enclosure, there will be sound wave cancellations, so to fix this, you must place the driver on one side. That means no symmetry.

Actually, what you're seeing with the "no symmetry" rule of thumb is something done to address baffle edge diffraction. This is unrelated to cabinet resonance, and standing waves.

But I see a lot of speakers that the woofer or the mid-bass driver is placed exactly in the middle of the enclosure. Does bracing compensate for this?

No, bracing has nothing to do with this. What's going on here is this- the midbass driver only operates up to a certain frequency, and most of the garbage from diffraction happens to occur at a higher frequency than that. Thus, it doesn't matter so much if the midbass is centered or not.

What about rounding over the inner edge of the opening of the speaker hole? When is driver placement an issue?

Rounding over the inner edge of the speaker hole is purely a practical thing for small midwoofers that don't have a lot of breathing room. It makes a better connection between the back of the cone and the main enclosure volume. If the baffle is thick and there is not a chamfer, then it's like the woofer is partially in a really small enclosure.

Driver placement is an issue when (as I understand it)-
-The internal dimensions of the cabinet are a large fraction or small multiple of a wavelength of some frequency within the range produce by the woofer
-The distance from the driver to the edge is above a small fraction of a wavelength of the sound produced by the driver. This can be simulated with a free program named "The Edge", among others.

I hope that helps, and isn't too wrong.

[collo]

Moving a driver from a central position does help with resonances inside the cabinet.

For example, the driver to side wall resonance is 1/4 wavelength (velocity node to pressure node).

Side to side resonance is 1/2 wavelength (pressure node to pressure node).

The distance between the sides is double the driver-side-wall distance, so the frequencies will be the same.

To avoid constructive re-inforcement, move the driver sideways by 5 or 10mm. The peak splits into three smaller ones.

If you wan't to play around with this idea, grab yourself a copy of boxnotes and move those drivers around!

regards
Collo

[JonMarsh]

The information pointed out by Joe and Collo are all correct. But some additional comments-

It's rare that a driver will be centered both vertically and horizonally. Horizontally for the midwoofer is often unavoidable- vertically clearly is.

The driver positioning is more important IMO because of the diffraction effects, and I go to some trouble to model these effects in BDS or EDGE prior to deciding on the size and shape and driver location on the front baffle. Even for a small two way system, like the Modula MT.

Internal standaing wavees in the enclosure in the midrange are best handled (IMO) by the use of damping materials on the enclosure walls (Whispermat is my preference) and I also use light stuffing with polyester batting behind the midwoofer, but not filling the whole enclosure, and not near the port entrance. Of course, in sealed systems, everything is stuffed, so discrete standing waves in the enclosure have no chance.

Another thing to think about with regards to "optimized" ported enclousure configurations is using Martin King's ML TQWT configuration; the tapering isn't completely necessary, but the arrangement of where you put the driver and port is important. The M8ta uses that configuration loosely, and works quite well.

Martin King's site

There's been a lot of discussion on this topic at HT-Guide- try the search function, looking for BDS, or Diffraction.