Porting / SPL / volume vs shape of cabs…

[plumberstrummer]

ok, new information i’d like to run through the brain trust here. I had a 15”x15”x14.7” size in mind for the single 112 (aiming for 54.2 liters), and the engineer sent me this:

“…it’s not recommendable to have low frequency tuned cabinets where two (actually almost all three) dimensions are equal. Such a cab may be prone to the build up of internal resonances, standing waves etc. Basically that cab may resonate at the frequency which corresponds to 15”, i.e. 900Hz (and to all lower frequencies that might generate a 2nd or 3rd harmonic that falls in the 900Hz range.) Your frequency response curve might see a peak in that area, and I don’t think that’s what you want.
Ideally you should try and avoid simple ratios between dimension, like 1:1, 2:1… as much as possible…”

@agedhorse
What relative dimensions are preferrable, then? Is this as big of a concern as i’m being led to believe? i’m happy to build outside of the boxy dimension, but then i want to shift the sizes in an intentional way…

is the engineer trippin over nothing, or… ?

 

[Wasnex]

ok, new information i’d like to run through the brain trust here. I had a 15”x15”x14.7” size in mind for the single 112 (aiming for 54.2 liters), and the engineer sent me this:

“…it’s not recommendable to have low frequency tuned cabinets where two (actually almost all three) dimensions are equal. Such a cab may be prone to the build up of internal resonances, standing waves etc. Basically that cab may resonate at the frequency which corresponds to 15”, i.e. 900Hz (and to all lower frequencies that might generate a 2nd or 3rd harmonic that falls in the 900Hz range.) Your frequency response curve might see a peak in that area, and I don’t think that’s what you want.
Ideally you should try and avoid simple ratios between dimension, like 1:1, 2:1… as much as possible…”

@agedhorse
What relative dimensions are preferrable, then? Is this as big of a concern as i’m being led to believe? i’m happy to build outside of the boxy dimension, but then i want to shift the sizes in an intentional way…

is the engineer trippin over nothing, or… ?

Not sure how critical it is, but this (specifically post #3) may be the rabbit hole you are looking for:

Do Speaker Enclosure proportions matter...Fibonacci numbers

I know some will say that only the 'air space' interacting with a given driver matter. The actual shape of the enclosure is of little consequence. I have always believed that dimensional ratios play into design. (and please don't quote me the inverse square law-- that is really rudimentary)...

www.audioheritage.org

 

[plumberstrummer]

Not sure how critical it is, but this (specifically post #3) may be the rabbit hole you are looking for:

Do Speaker Enclosure proportions matter...Fibonacci numbers

I know some will say that only the 'air space' interacting with a given driver matter. The actual shape of the enclosure is of little consequence. I have always believed that dimensional ratios play into design. (and please don't quote me the inverse square law-- that is really rudimentary)...

www.audioheritage.org

“Acoustic ratio” you say…. Very nice

 

[agedhorse]

ok, new information i’d like to run through the brain trust here. I had a 15”x15”x14.7” size in mind for the single 112 (aiming for 54.2 liters), and the engineer sent me this:

“…it’s not recommendable to have low frequency tuned cabinets where two (actually almost all three) dimensions are equal. Such a cab may be prone to the build up of internal resonances, standing waves etc. Basically that cab may resonate at the frequency which corresponds to 15”, i.e. 900Hz (and to all lower frequencies that might generate a 2nd or 3rd harmonic that falls in the 900Hz range.) Your frequency response curve might see a peak in that area, and I don’t think that’s what you want.
Ideally you should try and avoid simple ratios between dimension, like 1:1, 2:1… as much as possible…”

@agedhorse
What relative dimensions are preferrable, then? Is this as big of a concern as i’m being led to believe? i’m happy to build outside of the boxy dimension, but then i want to shift the sizes in an intentional way…

is the engineer trippin over nothing, or… ?

Yes, it can matter somewhat but like everything with speaker design, it depends.

All 3 identical dimensions could be considered worst case, but this can be mitigated to some degree by bracing and/or materials choice.

 

[JimChjones]

For what little its worth this complete amateur has tended to use the so called "golden rectangle" (1.618:1) for the face, because its supposed to look good (!) and is not a simple ratio, and then set the depth at what gave the right volume. If the depth then seemed too close to one of the others then juggle. There can be other factors though, with my current bass cab I started with the depth that would fit in my music gear cupboard and let that dictate the other two! Picking a width that the amp head won't overhang may also be a consideration.

 

[plumberstrummer]

Thank you all so much for your help! I think we have a plan dialed in now, and are basically ready to print…

Now, I just have one new question (maybe this deserves its own thread… ) ~

As far as high pass and low pass filters for the speaker cabinet/crossovers, do we have any recommendations? We are doing a 412 with the mix of speakers, andI am looking to protect the bottom 125 Hz from two of these speakers. I’m not sure if it is better to get a crossover that can divide across those hertz, or if it is better to have two different filters… I imagine this is not a cheap effort, but I’m not as concerned about that? (Hoping that’s not famous last words…)

If anyone can point me in a direction or two, I’d really appreciate it!

 

[agedhorse]

Mixing speakers? Good luck. You are now setting yourself up for a new disappointment IMO.

Doing a speaker level 125Hz HPF will be more expensive than better drivers that can handle the bandwidth. Not just cost but size and weight too. Then you will need to address the phase shift cancellations that are certain to occur between 100 and 150Hz.

I'm an experienced (professional) speaker designer, I COULD do it successfully but I wouldn't bother because there are MUCH easier and less expensive ways to design a superior speaker. The cure is (much) worse than the disease IME.

 

[plumberstrummer]

Mixing speakers? Good luck. You are now setting yourself up for a new disappointment IMO.

Doing a speaker level 125Hz HPF will be more expensive than better drivers that can handle the bandwidth. Not just cost but size and weight too. Then you will need to address the phase shift cancellations that are certain to occur between 100 and 150Hz.

I'm an experienced (professional) speaker designer, I COULD do it successfully but I wouldn't bother because there are MUCH easier and less expensive ways to design a superior speaker. The cure is (much) worse than the disease IME.

Thank you, I do know and respect your experience… it is enough to go back to the drawing board, but I have reasons for trying to make this work if possible. If I can just share these points, you can tell me I’m wasting my time and I might back out of the whole project if needed:

1) it is not “mixed speakers” exactly, it is basically two different fully separate 2x12 cabs in one - two guitar speakers I know sound correct, and two dedicated bass speakers just to carry the low end

2) the bass speakers are tuned for the frequencies I’m after, and have been tested and tried already. We are only adding a second separate 2x12 guitar speaker cab to cover the higher freqs.

If I were to put a dividing line at 200hz, only letting the guitar speakers see the above 200hz frequencies, could I avoid the possible phasing issues in that zone or would it take special tuning I am not experienced enough to even attempt?

I know this seems dumb, and it may be. It is for a very specific effect I need for this amp, and I am concerned it won’t be accomplished by full range speaker cabs… maybe there is one out there, I am willing to investigate. I am willing to concede it’s it impossible. Even so - is it possible this could work if done right by someone experienced, like yourself?

 

[agedhorse]

Check your messages.

This is a case where biamping would be more appropriate, especially if you are looking to overdrive the signal into the guitar cab. Just beware of the power limits of the guitar speakers.