How precise?

[JerryLove]

I was looking at some of the volume calculators: and I'm wondering how much a small variation makes in sound.

If a speaker wants, say, 9ft^3, and the enclosure ends up with 10ft^3, how large an effect is that likely to have on performance? I'm worrying that the "effective volume" caluclations are going to get pretty complex with the effects of materials and the like if the precision need is exacting.

 

[jliedeka]

A cubic foot will make a difference. Smaller amounts won't be that big of a deal. If you play around with a box modeling program like WinISD, you'll be able to see the difference in response curves.

A larger box may give you a lower f3/f6 but will have reduced power handling because it needs more watts for the lower frequencies. It's hard to say exactly what will happen without modeling, especially for a ported configuration.

For the speakers I'm building, I used estimates and measurements for the volume displacement of the drivers, crossover, bracing and port. I think I'm within a liter of what I was going for so not too bad.

A slightly large box might not be a bad idea for another reason. As the speaker plays, the voice coil heats up. That changes the QES which affects QTS. The result is t hat a slightly larger box will be more suitable for a warm driver. Check out Curt C's page http://speakerdesignworks.com/ for more detail on this.

Jim

 

[annunaki]

With a sealed enclosure, a 5% variance will have little to no audible effect. With vented enclosures, a 5% variance can cause a large tuning misalignment.

It is important to be quite accurate with enclosure volumes sealed or vented in my opinion.

 

[Swerd]

Remember that the woofers you can buy often vary by as much as 10% from the manufacturer's published T/S values.

Its funny how jliedeka and annunaki agree on the facts, that you should aim to hit within ~5% of your calculated target. But one says you must be quite accurate and the other says so much.

 

[highfigh]

I agree about using WinISD- you can select the optimum size for the box and it shows the graph for that driver, with or without vent. You can make changes to the box size, vent diameter or tuning frequency and get instant graphical feedback. It doesn't show power handling and some of the other data available in Bass Box Pro 6, but it's definitely worth downloading (free). It also calculates first and second order crossover component values, based on impedance and F3.

If you want to be more accurate with any drivers you have, the WT3 is sold by Parts Express and will do impedance/phase angle plots, get T-S parameters and find the Vas. It plugs into the USB port on any decent computer and is easy to work with. So far, I haven't seen wild variations when testing multiple units of drivers but it's not quite "cookie cutter", either. The ones I have tested most often were for a home theater I'm helping with and based on the measurements and WinISD, they sound really good.

 

[Alex2507]

one says you must be quite accurate and the other says not so much.

Fixed.

You keep helping me with speakers and I'll proof read for you.

 

[Adam]

Fixed.

You keep helping me with speakers and I'll proof read for you.

The rest of us let it go, but you just had to point it out, didn't you?

[BTW, I don't want to just threadjack. I'm quite interested in this topic. Matt's latest build has me intrigued, and one of my friends could make that cabinet easily.]

 

[JerryLove]

OK. So how does one account for the effect of things like rockwool that increase the effective volume? Is that in the software as well?

Also, I notice that we have several (at least one from Chris) "speaker improvement" mods, like the one for the Infinity Primus bookshelves that seem to reduce the interior volume signifigantly.

Further, does it tend to be percentage or actual size. When sealing off a mid-range in it's own enclosure: should I worry that my hexagon doesn't have the same volume as the circle I modeled?

My real worry in my own DIY, is that I'm not sure how high I can keep the precision once I accidentally make the inner materials (bracing or the like) thicker than I thought.

And you guys already hijacked my "feeler for sale" thread. I'd really like to see what I can learn from this one.

 

[Alex2507]

OK. So how does one account for the effect of things like rockwool that increase the effective volume?

I'll say what I think and let it be corrected for the sake of learning.

Damping makes reflected sound act like it's in a larger enclosure by reducing the amplitude of the reflection as if the the boundary were further away. My recollection of a WmAx post is that 3lb/ft^3 OC703 should be treated as 5% of it's actual volume in an enclosure and 6lb/ft^3 OC705 should be treated as 10% of it's actual volume.

I should probably be reading my book on speaker building instead of making guesses.

'feeler for sale'? ... no dude, for rent ...

 

[Swerd]

Alex - thanks for the proofreading & editing. Your correction is what I meant to say.

JerryLove - I know a guy who drives himself crazy trying to be more precise with his speaker box building than may be possible by typical DIY methods or needed. Its painful for me to watch him work. It is my opinion that cabinet building is not as precise as you might imagine. At least that is true for the way I and the few others that I've seen actually build them.

I emphasize that I have not made a systematic study of this, it is only opinion or wishful thinking (take your pick). If you built 10 cabinets that you wanted to be 0.5 ft³, they certainly would not all be identical. What would the variation actually be among those 10 cabinets? Similarly, if 10 different DIY builders each made 0.5 ft³ cabinets, what would the variation be? And finally, how big a variation would actually make an audible difference? I don't know anyone who cares to do that experiment.

If a calculated box volume should be 0.5 ft³, and yours ends up 0.45 or 0.55 ft³, don't sweat it. There is some wiggle room in woofer bass performance.

With sealed cabinets, you can fine tune the box by adding or removing stuffing after listening. With reflex cabinets, you can tune by adjusting the length of the port tube. When you build the box, leave a way to make those adjustments before you glue everything irreversibly.

 

[highfigh]

OK. So how does one account for the effect of things like rockwool that increase the effective volume? Is that in the software as well?

Also, I notice that we have several (at least one from Chris) "speaker improvement" mods, like the one for the Infinity Primus bookshelves that seem to reduce the interior volume signifigantly.

Further, does it tend to be percentage or actual size. When sealing off a mid-range in it's own enclosure: should I worry that my hexagon doesn't have the same volume as the circle I modeled?

My real worry in my own DIY, is that I'm not sure how high I can keep the precision once I accidentally make the inner materials (bracing or the like) thicker than I thought.

And you guys already hijacked my "feeler for sale" thread. I'd really like to see what I can learn from this one.

I don't know of any software that accounts for the changes from dampening material but some manufacturers (like Tymphany) recommend a certain percentage of space filled, on their application sheets.

As far as the internal volume, you can substitute six triangles for the circle, using the radius of the circle that's the same as the dimension from the center to the center of the inside surface of the sides. This would be the altitude of the triangle and by multiplying that by 1/2 the length of the side, you'll find the area, which would then be multiplied by the height. Multiply that by six and you have the internal volume and the only thing you need to add to that is the displacement of the driver. Using a circle is technically inaccurate, but if the 10% tolerance is used, it may be close enough for Jazz. The volume of the bracing should be easy enough to calculate. What do you mean "accidentally make the inner materials thicker than you want"? Is that anything like "I cut this piece four times and it's still too short"?

 

[JerryLove]

With sealed cabinets, you can fine tune the box by adding or removing stuffing after listening. With reflex cabinets, you can tune by adjusting the length of the port tube. When you build the box, leave a way to make those adjustments before you glue everything irreversibly.

This sounds like a great suggestion. Thanks!

 

[WmAx]

WinISD pro has user settable co-efficients to account for box dampening in the cabinet and it's effect(s) on frequency response. However, you have to know the approximate degree of actual effect of the material used, in order to accurately enter the co-efficient values into the software.

-Chris