How to choose a Qtc for a diy box

Cpyder

So here's the plan. Take a pair of SR6500 woofers and a pair of Vifa XT25bg60 tweeters along with a pair of SR6500 crossovers and put them into a custom made speaker box made out of Birch plywood. My question has to do with choosing the right volume for the box. I'm looking for very tight and snappy midbass. I've done some reading and it sounds like a smaller box rather than larger will give me this sound. I've also read that I want a small Qtc but when using an online calculator, it looks like a smaller box yields a larger Qtc. I must be understanding this incorrectly. Do I want a higher Qtc then? Is there an optimum value for the result I want? I've read that a Qtc of 0.707 is a good trade off between snappiness and low bass. But I'd rather err on the side of snappy bass than low bass as I will probably add a sub at a later date. Please help.

Thanks!

FTGV

Qtc is reduced as box volume increases.Assuming the system is capable of <40 hz extension I personally shoot for a Qtc. in .7 range,finding lower Q's too overdamped for my liking.For smaller 2way' s or minimonitors I would likely target a Q in the .8-.9 range witch would give them a little extra oomph to help make up for their lack of extension.


There are some good free box model sim programs such as WinISD,Unibox etc. that you simply input the T/S parameters and it will do the rest.

dudeinaroom

All depends on what your looking for myself I like a Qtc of .55-.6 but that's me. With a smaller driver you should still have a decent amount of snappyness in a larger box. Higher Qtc levels start to give you a mid bass hump and can roll off rather sharp, but if your using a sub the roll off is a moot point. The lower the Qtc the deeper it will dig and the more sterile or accurate it will sound the higher the Qtc the less low end extension you will get, and will have an increasing hump of up to around 4db's or so and will start to sound muddier in my experience. You have to have the Theil/Small(T/S) parameters to build the right box as well to use winisd. Use winisd as a guide as speaker do vary a little from published specs. It will still give you a graph that is pretty close, and you can play around with box volume, port lengths, size and number of(if you decide to go ported). Good luck YMMV.

Cpyder

Thanks guys! I've been experimenting with WinISD a little bit. It looks like as the box volume increases from 1L to 10L, the Qtc goes from 2.28 to about 1. From 10L to 20L, the Qtc goes from about 1 to 0.8. Any thoughts on how a Qtc of 0.95 would sound or is that too high? At a Qtc of 0.95, it looks like there's a 1dB gain around 150Hz and a 3dB drop point (terminology???) at about 72Hz. (See pic)Would this yield the "punchiness" I am after or would I lose too much of the lows. As the box volume goes up and up, the frequency response looks smoother with a 3dB drop point at around 64Hz.

FTGV - Does a larger or smaller box yield more damping? I thought smaller would be more damped?

Dudeinaroom - You say the lower the Qtc, the more sterile and accurate it will sound. I was under the impression that a smaller box (and hence higher Qtc) would be less "boomy" and more accurate. Am I mistaken?

Thanks for your help.

Bonus question: If I doubled the wall thickness, would there be any significant benefits? I already have the wood, so cost is no issue.

dudeinaroom

No a larger box yields lower extension to a point and has a more natural sound at least, like I said that has been my esp. A driver can move faster in a bigger box than it can a smaller one because there will be less pressure/vacuum working against the speaker.

If you double your wall thickness you will reduce cabinet resonance which is a good thing. I recommend using a good wood adhesive as well as screws to do that. Most of the better manufactures will use a thicker baffle board(the front of the enclosure where the speakers attach to). Some will just use nice thick stuff all the way around. Another trick to use is internal bracing from front to back and side to side at various points in the cabinet generally on horizontal centers. You can google some DIY speaker sites to get an Idea of what I am talking about as well as pick up some construction tips(wish there was an internet when I started building). A lot of people confuse snappiness and punchiness. From the stuff I have built a lower Qtc will give you more snappines(fast speed) while a higher Qtc(smaller box) will give you more punchiness(the stuff you feel). If you are using a sub the results might end up just fine with the crossover set between 80-90 hz. I am not a Phd. in any of this, this has just been my experience. I think we have a few speaker builder around here that are more knowledgeable than.. we might get lucky and they may chime in. If I am wrong someone please correct me as I don't want to be misleading.

mhardy6647

in the interest of a balanced forum :-) some folks do like Qtc slightly higher (even as high as 1) to get a little "warmth" in the midbass without so much of a hump as to turn the system into a boom box.

I'd personally vote for "optimum damping" (Qtc = 0.707 a/k/a [(SQRT(2))/2])... but... as they say, YMMV.

FTGV

Cpyder wrote: »

. (See pic)Would this yield the "punchiness" I am after or would I lose too much of the lows.

IMO with a smaller system with a higher f3 such as this I think it's benificial to have higher Qtc (such as .95) which will give the impression of greater bass weight and extension and sound better balanced over all.Small speakers with lower Q's tend to sound lean and possibly foward and bright because of the apparent lack of bass.(assuming ofcourse not augmented by with a subwoofer).If you want more extension out of that driver you can might look at using a port ,though box volume requirements will increase.

Does a larger or smaller box yield more damping? I thought smaller would be more damped?

The box/driver resonance is better controlled or damped as box volume increases, efficiency also increases.However with the smaller volume the tighter air spring in the box will exert greater control over the drivers suspension.

You say the lower the Qtc, the more sterile and accurate it will sound. I was under the impression that a smaller box (and hence higher Qtc) would be less "boomy" and more accurate. Am I mistaken?

The higher the Qtc the more the system will have a tendancy towards boomy one note bass.More so with systems that reach into lowest octave <40hz or so.

Bonus question: If I doubled the wall thickness, would there be any significant benefits? I already have the wood, so cost is no issue.

I would look at proper bracing as a better way to reduce cabinet talk but if you can laminate the ply's properly then sure.

dudeinaroom

If you feel like having some fun you could also look into transmission line enclosures.

Cpyder

Got a reply from Polk. Hmm... wasn't expecting this answer.

Me:
Currently I'm using a pair of SR6500s in my car but I'm looking into putting them into custom made enclosures and relocating them to my house. Is there an optimum volume of the box for the mids? I would prefer tighter, snappier bass. Would a smaller box or larger box be better for this application? Is there an optimum Qtc to shoot for?

Polk:
The speakers were designed for best performance in an "infinite baffle" type of installation. This means there isn't a specific enclosure size that will suit that speaker. The best recommendation would be to make the enclosure as large as possible to not load down the speakers. Regards, Ken, Polk Audio

[Deleted User]

Hello,
I checked with the folks in the engineering department and my response was correct. The speaker was designed to operate without the need of a specific enclosure. The only requirement is that the front sound wave and the rear be kept isolated from each other to prevent cancellation. This is done to allow the speaker to be used in door pannels, kick pannels and rear decks where the internal volume can vary considerably. The speaker's suspension is suficiently stiff to not need the damping action of a specific sealed enclosure. In fact, if the internal volume is too small the bass output will be reduced.
I hope this additional information is helpful.
Regards, Ken

Cpyder

There's a reason I keep coming back to these forums. And this is it. Thank you so much Ken!

Cpyder

Sorry, I didn't mean to leave everyone else hanging. Thanks for all your input guys. Definitely learned a lot!