[John Casler]

John,

I was thinking that I when I open the speakers, I would route a thin set of wires into the cabinet.  I would have to run them so that the bass/midrange could then be put over the wires.  I'd then play the normal test tones from ETF.  I'd tape the mic somewhere, near a corner perhaps.  I could use several different locations.  It would be interesting to go front to back (or side to side), but that might take more rigging than what I'd like to do. I could easily perform a test with stuffing/wool and  ...

Come to think of it, (obviously I am no acoustician) is it possible to have any standing waves, of any LF significance in a cabinet with dimensions of less that 6 feet?

Using the dimensions of a Floorstanding cabinet, the wave might be too long to cause a SW problem in such a small space and short distance

Any experts out there?

[ctviggen]

I don't see why you couldn't have standing waves having frequencies on the order of the size (length/width).  True bass, like 20Hz would be too long.  Here's some sites:

http://speakers.lifetips.com/cat/61238/speaker-cabinets/
http://www.silcom.com/~aludwig/Loudspeaker_construction.html

The wavelength in feet is 1137.30 ft/second divided by the frequency (at 25C, and vice versa).  We'd need to get about 6 feet (is that the height of the speakers?), which would be about 190 Hz.  For 1.25 feet in width, it'd be about 910 Hz.  At frequencies near these (and multiples thereof), you could create standing waves.

[skrivis]

I don't see why you couldn't have standing waves having frequencies on the order of the size (length/width).  True bass, like 20Hz would be too long.  Here's some sites:

http://speakers.lifetips.com/cat/61238/speaker-cabinets/
http://www.silcom.com/~aludwig/Loudspeaker_construction.html

The wavelength in feet is 1137.30 ft/second divided by the frequency (at 25C, and vice versa).  We'd need to get about 6 feet (is that the height of the speakers?), which would be about 190 Hz.  For 1.25 feet in width ...

Yep. It's the dimensions of the enclosure (speaker, living room, concert hall...) that govern the standing waves. Carefully applied damping can reduce the amplitude somewhat, but just putting it on the walls isn't always enough. Breaking up the volume with baffles or partitions shifts the frequency of standing waves up (due to the smaller dimensions of each part of the total box), and higher freq has a harder time exciting the walls of the enclosure; which is why it's desirable in a speaker cab. (The baffles also act to stiffen the enclosure walls, further reducing problems.

Another way is to avoid rectilinear shapes. That spreads out the standing waves to different frequencies of lower energy each.

[_scotty_]

John, If you were to put an eye bolt into the top of the empty speaker cabinet and suspend it from from the ceiling you have a good approximation of a large wooden bell. If you then strike the cabinet with a mallet and measure
the resonant frequency and the harmonics that result from the excitation supplied by striking the sides of the box you know where your problem frequencies lie. You could also mount a fairly wide range driver in one of the woofer cutouts facing inward and measure the impact the box has on the response curve that results from playing a sinewave frequency sweep through the driver. An impluse response test played through the driver and a waterfall plot taken from the inside of the box would be even more informative.  If the  software will allow it a waterfall plot that measured the
resonances resulting from striking the sides of the box with a mallet would be the best way see what was happening in the box.
  Then one would apply various braces and sound absorbing materials to the inside of the box and measure repeatedly until your treatement has reduced the magnitude of the resulting resonances. Ideally you don't want to absorb any bass energy at the frequency the vented alignment is tuned to thereby avoiding a loss of low frequency reinforcement that the vent is supposed to supply.
 To state the obvious, this a diametrically opposite design approach to making a speaker with no precision applied interior bracing or resonance dampening inside it.
Scotty

[skrivis]

I can't resist.

Transmission line enclosures tend to be quite stiff because of the baffles making up the line. Since the line is tapered, there are fewer parallel surfaces to encourage standing waves.

I kind of look at the TL as reducing the magnitude and increasing the bandwidth of resonances - including standing waves. You're going to get resonances in a speaker system. Enclosure, drivers, x-over, air in the enclosure - these will all show resonances. The key is to try to control them as best you can.

A properly damped TL reduces resonances, and makes the remainder more resemble a broad shallow upswelling, rather than a sharply peaked curve. They also damp the driver resonance, as evidenced by the impedance curve. (Sealed and vented boxes have more rise in impedance at resonance = more resonance.)

I have found this broad low-level resonance to be less objectionable than that of other speakers. YMMV.

[_scotty_]

Interestly enough some "transmission lines" have impedance curves  
which have the double hump characteristic of a vented alignment centered on the frequency to which the line is tuned. The impedance curve should show a single low hump at the system resonance and a series of smaller peaks
of greatly reduced magnitude at even multiples of the fundamental frequency.  As a sidebar many transmission lines are 3/4 wavelength lines instead of  one quarter wavelength lines because the designer failed to take into account that the speed of sound in the line when stuffed  is slower than in air. This makes the line length necessary for the line to be a quarter wavelength long at the tuning frequency much shorter and the losses of bass energy lower yielding much better efficiency. In other words a transmission line does not have to be inefficient compared to other designs if properly done.
Scotty

[CornellAlum]

Totally off topic at the moment, but you guys have GOT to try this bh5 mod.  The amount of bass I picked up in my 626rs as well as the confluence of drivers (I got that term from TAS ) is amazing!

[skrivis]

Interestly enough some "transmission lines" have impedance curves  
which have the double hump characteristic of a vented alignment centered on the frequency to which the line is tuned. The impedance curve should show a single low hump at the system resonance and a series of smaller peaks
of greatly reduced magnitude at even multiples of the fundamental frequency.  As a sidebar many transmission lines are 3/4 wavelength lines instead of  one quarter wavelength lines because the designer failed to take in ...

Some models show that all enclosures are variants. As such, a TL with an impedance like a vented box isn't a surprise. A proper TL _should_ actually reduce the freq of the resonance as compared to a vented box, and it should also smooth out the impedance curve.

Someone told me that a double hump impedance curve in a vented box is actually the single hump of a sealed box with a notch out of it (due to the vent). Others have said this isn't a valid way to look at it.

Just to be pedantic, a TL of improper length is still a 1/4 wavelength pipe, it's just not the wavelength you wanted it to be. I'm also not sure that the idea of the speed of sound in a stuffed line being radically different is correct. King's models don't really support that view, and I don't feel Augsperger's do either. The shape of the line may be more of a factor...

Some people see the purpose of a line to be to completely lose the rear wave. It's kind of a real world way of doing an infinite baffle. Others feel you should be getting useful output from the end of the line at very low frequencies. (King and Fried fit in here.)

I'm not an expert on TLs or general enclosure design, so I'd suggest taking a look at Martin King's models, as well as Augspurger's work. Martin approaches it from a mechanical viewpoint (tuned pipe), while Augspurger takes an electrical approach, similar to T-S.

As always, YMMV. hehe

I _will_ say that I've found that TLs do a wonderful job, and I haven't heard any other designs that do as well. (I have not heard BC's current designs, so my comments really can't apply to them. I defer judgement until hearing them.) (Ted B, are you listening? hehe)

Along with VMPS, I want to hear NHT's xD speakers. (I also want to hear Green Mountain's products, Dave Ellis', SP Tech, and Karl Schumann's Ultimate Monitors, along with a longer listen to Linkwitz's Orion.) (North Creek and Olson's Arial are interesting too.) All of these represent designers that have impressed me with their ideas. Too bad I live in the audio wasteland of NE Ohio...

[skrivis]

Totally off topic at the moment, but you guys have GOT to try this bh5 mod.  The amount of bass I picked up in my 626rs as well as the confluence of drivers (I got that term from TAS ) is amazing!

By "confluence of drivers," do you mean that it appears like you're listening to more of a single sound source than a collection of drivers?

Interesting. I'd think that the BH5 could reduce the tendency of the cabinet to "speak," and also possibly reduce re-radiation back through the speaker cones from the back wave. They might also reduce box resonances, although I think the walls are not the best place to do that.

I'm glad it's working well for you though!

[skrivis]

John, If you were to put an eye bolt into the top of the empty speaker cabinet and suspend it from from the ceiling you have a good approximation of a large wooden bell. If you then strike the cabinet with a mallet and measure
the resonant frequency and the harmonics that result from the excitation supplied by striking the sides of the box you know where your problem frequencies lie. You could also mount a fairly wide range driver in one of the woofer cutouts facing inward and measure the impact the box h ...

The name escapes me right now, but someone was doing "exploding wire" tests of empty enclosures that would also work well for this kind of thing. You want to rapidly excite the box, with the initial event stopping immediately so it doesn't get in the way of measuring what the box itself is doing. (This may be less relevant now that we have computerized testing that can separate the initial event and show only the later ones. Heyser did a lot of work on this...)

[_scotty_]

I have to agree you skrivis, King's and Augspuger's work should be required reading for anyone thinking about DIYing their own TL loudspeaker.
http://www.quarter-wave.com/
http://www.t-linespeakers.org/

   The reason I suggested using a driver mounted to the enclosure or striking the sides with a mallet was to directly excite the box walls and measure the different resonances that resulted from each wall and upon retesting find out how effective the bracing was in suppressing structural vibration modes.
The exploding wire test is useful in determining how effective the dampening treatments applied to the wall were in suppressing the enclosure's cavity resonance behavior.  
Scotty

[skrivis]

I have to agree you skrivis, King's and Augspuger's work should be required reading for anyone thinking about DIYing their own TL loudspeaker.
http://www.quarter-wave.com/
http://www.t-linespeakers.org/

   The reason I suggested using a driver mounted to the enclosure or striking the sides with a mallet was to directly excite the box walls and measure the different resonances that resulted from each wall and upon retesting find out how effective the bracing was in suppressing structural vibration mod ...

I can just picture Brian throwing his body between one of his cabinets and Scotty with a large mallet.

Brian, do you walk softly and carry a large mallet?

[skrivis]

   The reason I suggested using a driver mounted to the enclosure or striking the sides with a mallet was to directly excite the box walls and measure the different resonances that resulted from each wall and upon retesting find out how effective the bracing was in suppressing structural vibration mod ...

Have you found that gross cabinet resonances are prevalent? I would think that cavity resonances are more common.

BC, how does the BH5 effect these? Has it made a difference in both for you?

[ScottMayo]

Brian, do you walk softly and carry a large mallet?

Somehow, I can picture scenarios involving Brian, a large mallet, and various people on AC.

[skrivis]

Brian, do you walk softly and carry a large mallet?

Somehow, I can picture scenarios involving Brian, a large mallet, and various people on AC.

Resonance test! BAM!

Or else we use the spark test. Tesla coils anyone?

[CornellAlum]

Brian, do you walk softly and carry a large mallet?

Somehow, I can picture scenarios involving Brian, a large mallet, and various people on AC.

    I even know of a couple of candidates

[CornellAlum]

Just thought I would add what I have heard now that I havce the larger done with bh5 as well.  Wow, is all I can say.  I thought the improvement with adding the bh5 to the 626rs was good, but it is of a greater magnitude with the larger.  I thought I had taut, well defined bass before, but the amount of sheer authority the bh5 has brought to the larger is absolutely first rate and incredible.  

The changes to the 626r are less obvious but certainly worth the effort.  They seem to just sound better, with additonal clarity, especially in the lower mids, as well as a better transition at the crossover points with less of the smearing quality I had noticed before.  Presence is up from before as well.  I also think the mids have been brought up several decibels, which is a great step in my opinion.

It was a royal pain to do, but p[robably the best 5 hours or so I have spent.  BTW, Josh's tin snips idea for cutting bh5 was wonderful, thanks!

[skrivis]

Just thought I would add what I have heard now that I havce the larger done with bh5 as well.  Wow, is all I can say.  I thought the improvement with adding the bh5 to the 626rs was good, but it is of a greater magnitude with the larger.  I thought I had taut, well defined bass before, but the amount of sheer authority the bh5 has brought to the larger is absolutely first rate and incredible.  

The changes to the 626r are less obvious but certainly worth the effort.  They seem to just sound better, with  ...

Just curious, but... BC, have you found any measured differences in response that would back this up?

I know, I know, I'm ever the skeptic.

[ctviggen]

How could you measure the response?  Most programs, like ETF only measure steady state response and not any type of instantaneous response.  How does modifying resonance of the cabinet affect the steady state response of the speaker?

[CornellAlum]

I believe Bruna measured the before and after with bh5 in one of his threads I read when he was modifying his 626r's, and he was able to detect a 2db bump in the lower octaves.  I will see if I can find it when I get some time later today.