How important are the internal proportion in a cabinet

[knifeinthesink]

Im assembling a cabinet and I realized that for cosmetic reasons I want to change some proportions. The changes to internal volume will be ok but Im unshure about some other things.

I understand that the internal dimension of a cabinet should ideally correspond to the golden ration and that the worst internal shape is square. How important is this in the bigger scheme of things. How detrimental are standing waves inside the cabinet?

[joecarrow]

It's most noticeable when the size of the square (or even worse, the diameter of a circle) corresponds to a fraction of a wavelength of a frequency within the bandwidth of the speaker. For example, your speaker plays 50 hz to 500 hz. If the inside of the box is a cube with a side the length of a half wave of 100 hz, then you would expect to see some peaks and dips in the frequency response at 100 hz and some multiples of 100 hz due to energy in the resonator formed by the shape of the cabinet.

It's not important to follow "The golden ratio" of 1.618... It's more important not to make your walls something like 12, 12, and 24 inches. Once you get over 500 hz or so, polyfill stuffing starts to get fairly effective at absorbing internal waves.

Basically, you can pick another ratio- just so it's an irrational number. Maybe pi over 2, or the square root of five, just something to keep it from being a really simple ratio like 1/2, 1/3, 2/3, etc.

What's the new internal dimension you're considering? I'm sure someone will be able to tell you if it will be problematic in your desired bandwidth.

[knifeinthesink]

thanks for the interesting reply.

original internal dimensions were 8"h x 5"w x 6"d.

New dimensions would change depth to 5.5".

Might be better then the original.

Speaker is a very small two way with a 4" audax mid woofer tuned to apx 95hz.

What is the effect of standing waves inside the cabinet. To be a problem they must effect the audible waves but how?

[joecarrow]

The cavity within the cabinet creates a resonator tuned to whatever frequencies correlate to large areas bounded by parallel sides. At some frequencies the cone's movement is robbed of energy as it builds up the standing wave. Sorry I don't have a better link to it- but there are definitely some websites out there if you google enough. Quarter-wave.com might have a few tidbits, and I'm sure that linkwitzlab.com has some discussion of it.

So, I just googled for this: http://www.cord.edu/dept/physics/p128/lecture99_35.html

And according to this (and I'm aware that the acoustics may be somwhat more complicated than this physics lecture), your lowest frequency for a standing wave would be roughly (speed of sound)/(length). Taking the speed of sound to be 1100 feet per second, I found your 6" walls to have a standing wave at about 1100 hz. The lowest resonance would be along the 8" dimension, at 825 hz. Fortunately, this frequency is easily killed with some polyfill stuffing.

Unless someone out there can point out some math problems, I don't think you'll have problems with standing waves due to dimensions in such a small cabinet.

[knifeinthesink]

Thank you!

That was exactly what I needed to know.

[collo]

I became interseted in this some time ago and wrote some software to calculate what frequency the standing waves occur at. Boxnotes makes it easy to adjust your dimensions to minimise their effects. It does other stuff too, such as allowing you to print out your cutting list.

[knifeinthesink]

excellent!

Thanks for bringing to my attention.