How "real" are internal standing waves?

[dscline]

Wow, this has been immensely helpful... thank you! Collo, I didn't realize Boxnotes was your software. This update came just in the nick of time... Got the box mostly built this weekend, but I now won't bother with any special attempts to attenuate quarter wave resonances. Just the standard damping that Brian suggests with his kits.

Thanks!

[Collo]

Glad to be of help.

The non existence of those quarter-wave resonances certainly is good news for us sub builders.

Dont be scared by the 20dB level of the measured resonances. The meter was placed exactly at the point where these were strongest. The level outside the box, particularly if its a sealed design, would be much lower. Throw in some damping and you're good to go.

Good luck with the build....

[simon5]

I like your old sub hehe, I'm happy you still keep it !

[Pikefish]

Very interesting and well done!

Perhaps the dreaded quarter-wave resonances don't occur because the drivers free air resonance is too low for it, by this logic they may occur below Fs, not a problem with sanely sized cabinets, except maybe large midrange cabinets, keen for round 3 with a longer tube and high Fs driver?

Do you suppose those superimposed ripples were cone breakup?

[Collo]

I don't see how the fs of the driver would change whether a quarter-wave resonance existed or not.

The ripples are most likely from assorted reflections inside the tube. Cone breakup was evident above 1.5kz, which is why I stopped the graph there.

Applying the 1/3 octave smoothing filter in REW gives a clearer result......



The red line is the same as in the previous graph, ie the internal resonances for a 500mm spacing.

The blue line is whats measured outside the box, 100mm from the cone.

Again, no quarter-wave resonance is seen.

[Pikefish]

Cheers for the outer-box specs, you can really see that the pressure maxima near to the driver really does hold it back/drive it. Lucky we can damp these things at the velocity maxima. Those ripples were probably caused by the diaphragm being a cone rather than a plane eh?

I was thinking that a wave would be reflected by the driver if it's Fs was too low for it to be displaced. By reflecting it, only a half wave resonance is reinforced.
If the length of the tube was such that the quarter wave resonance was Fs, maybe a quarter wave would be reinforced, and leave the cabinet through the diaphragm. Ok in theory?

[Collo]

The only way for a quarter-wave resonance to occur is if one end of the pipe is open.

The tests show that the driver acts as a closed end. Fs doesn't seem to have any role in this process.

[GM]

Are you planning to update sites such as this one? http://www.hometheatershack.com/foru...nclosures.html

GM

[Collo]

I'm planning to do a writeup on my site describing the experiment and its conclusions, including links back to this thread and a parallel discussion at HTS.

I'm fixing the resonance calcs in boxnotes and sonosub.exe first though...

[GM]

OK, I just had remembered seeing the thread and noticed it hadn't been very active, so wondered if it was still on your 'radar'.

The other thread is interesting, but again, a sealed cab is like a room, all reflections, so just like a room it only has 1/2 WL resonances as long as all windows, doors are sealed airtight, otherwise there are Helmholtz resonances to consider, ditto air leaks in a sealed cab.

GM