Stupid WinISD question

[netgeek]

Please forgive my ignorance, but I'm really wondering about this. I have been just starting to use Win ISD and I'm wondering if I'm missing some fundamental point. Here's an example:

Using a Tangband W8-740c (8" woofer), which has the following T/S parameters:

Fs28 Hz, Vas 23 Liters, Qts .30, Xmax 12mm

For a sealed enclosure, the program shows an optimum Vb of 0.178 cu. ft. with an F3 which is approximately 65 Hz.

For a vented enclosure, the program shows an optimum Vb of 0.306 cu. ft. which yields an F3 of approximately 38 Hz.

Here's the problem - Using the vented enclosure, it specifies a 4.02" diameter vent - with a length of 6.109 Ft. !!!!! Surely, this can't be for real. Are we supposed to adjust this for 1/4 wavelength or less?

Clearly, I'm doing something basically stupid, don't understand the program, or I'm missing the point altogether and should go buy some Bose speakers to use as firewood while I have a beer and think about it.

Can someone here point me in the right direction?

Thanks!!!

[cm961]

I'm simulating it and getting the same results. In fact thats why I haven't ordered that driver yet. I'm a little iffy about the Tang Band specifications. Most of their drivers graph really, really well for what they cost. So they either know what they're doing or they stretch it to make sales.

As for the 6 ft pipe, I'm getting that too. You could try using a smaller diameter port. Just remember to make sure the mach speed is less than whatever so it doesn't make noise. Also remember you can move the port to the outside and also put bends in it to fit it in. But with such a small volume you won't get very far. That's why I question these TB drivers. Says right in the data sheet that they're meant for ported enclosures though.

Pete

[Volenti]

Some drivers just don't model well and lead to enclosures that are physically impossible to build (or work for that matter)

The specs could be incorrect but I've personally tested drivers that then model to impossible enclosures.

[kelticwizard]

Netgeek:

A) You can use larger than optimum boxes. In this case I would suggest 0.5 Ft³.

B) Drone Cones, (Passive Radiators), were invented for cases where vents seem impossible.

C) I can't model one yet, but if you want to start a new thread, someone is likely to help you to see what a Transmission Line looks like with this woofer.

[Stephen D]

Originally posted by Volenti
Some drivers just don't model well and lead to enclosures that are physically impossible to build (or work for that matter)

The specs could be incorrect but I've personally tested drivers that then model to impossible enclosures.

That's true but not the case here. This driver is actually modeling quite well & as would be expected of such a low Q driver of it's size tuned as low.

Here's the low down.

For sure physics are pushing against you when trying to achieve low tuning & small box symultaneous in a ported enclosure because as box volume goes down port length must go up for same tuning & port cross section. Also as for port alone, as port cross section goes up length goes up to maintain same tuning.
Usually things work out though for a given speaker diameter because smaller speakers can generally use smaller cross section port, & that is where the main error is in this case.

netgeek;
The 4" diameter port is way more than need be. Running a 100 watt input test sim in WinISD Pro & viewing the port air velosity graph shows that a 2.5" port will do. Modeling for QB3 alignment in WinISD Pro I get similar (insignificantly diferent) numbers than you with a 4" pipe but the 4" is just a default setting WinISD is throwing out at you. It's up to you to tailor the port cross section & length as small as you can get it keeping an eye on port air velocity.

keep in mind a QB3 is not the only alignment choice or even necessarily an optimum alignment (as often stated) for this or any other case. The range is infinite,.. but anyway for a QB3 alignment I come up with a 0.29 cubic ft box volume & tuning of 36.8 Hz. With that & a 2.5" diameter port I get a port length of 31.66". Yes, still quite long but manageable with some tricky design. There is also plenty leeway to raise the box volume some & still get a very good freq response curve & power output allowing port length to be shortened further. Experiment with little larger box & watch what it does to shrink port length & how well response graph looks. Do this until you reach a satisfactory compromise, though it will still be fairly long port length to box size ratio in this case. Remember port volume displacement is to be added to the alignment volume to equal total volume of box (also along with speaker displacement volume).

For this case I might use rectangular port, fashioned as integral part of cabinet walls. Front baffle 9"x9" then form a maze of 9" high walls to form cabinet exiting as port. Would all & all end up a very small woofer with good SQ. This speaker models well for ported but walks on the edge & challenges the designer.

[Stephen D]

Also port can be totally external of woofer box, perhaps integrated as part of podium for MH speakers or something like that.

[wintermute]

Try 20L box 8cm ID port Fb 31.72Hz (that results in approx 65cm port and the freq response when port resonance is taken into effect is much smoother! Sorry I'm a metric person, and didn't take the time to work out the box volume to be the same, just went with what unibox came up with (12.6L) and played a bit and came up with 20L.......

I modeled it with .5 ohm series resistance in Unibox.

Tony.

[netgeek]

Thanks for the responses - a relief to know that I'm not really losing my mind (well, at least not on this issue )...

I'd be interested in any thoughts on using this same driver in a sealed enclosure with the goal of reproducing anything from about 100 Hz and below (to as low as possible).

The project is to take something like the Parts Express BR-1 configuration, roll it off at around 100 Hz, and then graft it on to a "subwoofer" that would give some additional punch to the lower frequencies. What started this was a friend's desire to put together some inexpensive speakers for an HT setup that would meet some pretty rigorous requirements in terms of space and "SAF" factor. In other words - relatively cheap, relatively small footprint, etc. Discrete subwoofers will not be allowed !! So, the goal is to design some small "towers" (which tentatively would make the "SAF-approved" list) and which are sort of "full-range". SP levels are modest, particularly since there will be four of these things in a small room (with the option to expand to six later). My friend's interest is primarily in music content, while his wife is after the Home Theater stuff.

Originally, I wanted to build a quasi-dipole thingy, but cost and space won't allow it.

The current plan is for a 3-way config using active crossovers and EQ (if need be) - all of it powered by embedded "gainclone"-type amps. So, the subwoofer section can be "tweaked" (ala Elliot or Linkwitz) to extend the F3.

Any comments/suggestions are most welcome!

Thanks,
Bill

[kelticwizard]

Originally posted by netgeek
So, the subwoofer section can be "tweaked" (ala Elliot or Linkwitz) to extend the F3.

Any comments/suggestions are most welcome!

Thanks,
Bill

Since the sub is only 84 dB to begin with, I sure hope that none of your active tweaking involve lowering the output level to achieve a smooth frequency response. You have a low SPL per watt to start with.

[wintermute]

If you want low bass I think you are going to need to go with vented with this driver. doesn't seem to model very well in a sealed enclosure. Although I admit that even a 65cm port in a 20L vented enclosure would be a challenge

Tony.