Relation between Vas and Enclosure volume

[kelticwizard]

Aaron:

What was the volume of your enclosure and what did you tune it to? Oh, and the vent dimensions?

One more thing. Did you break in your drivers before measuring the Fs? Breaking in drivers tends to lower Fs and increase Vas, I believe.

[aaroncgi]

The volume of the enclosure - that's a bit tricky. Do you know how to calculate the area of a D shape cross section? It is a 12" cylinder (11.25" internally) but with the 'front' flattened for a baffle approximately 8.5" wide. It's 62" long, but then subtract 3/4" for the top cap insert. So, if we were to assume it was just an 11" cylinder, that would come out to about 3.36 cubic feet or 95 liters. I figure subtract at least 0.3 cubic feet for the drivers, crossover, port, and extra wood behind the baffle, so let's call it 3 cubic feet or 85 liters. I suppose it could be a bit less, so if anyone knows how to calculate area of a D shape, we could get more exact.

In any case, this is the volume (85 liters) I used to tune the enclosure. I initially started with a 3" x 17" port (flared), which WinISD tells me was about 18Hz (I was still thinking the 850122 had an Fs of 37Hz). That sounded a bit too thin, so I cut the center section in half, giving me 11" of port length. This in theory is somewhere around 22Hz. I could probably go down to the minimum length of 5", and expect the sound might not change all that much because the Fb would still be way below Fs. F3 would go down a little and output above 30 Hz would be greater. But, that would push the group delay peak farther up into the audible range.

The drivers were played pretty hard for a couple weeks before I measured Fs, so they should have been broken in. The fact that they measured between 49-52Hz seems to point in that direction, as most notes I have seen put the new batch at around 55Hz Fs. I also noted that the Fs was a few Hz lower at higher signal levels (like 4-6V+).

Of course now, I don't use them full range, because I have my subwoofer, so all that bass is lost. Truth be told though, my new room doesn't have nearly as much gain as my old apartment did, which is bad for my ET's full range performance, but good for my giant subwoofer.

[Pjotr]

Hi,

Don’t understand the relation Vb needs to be somewhere below 2x Vas. With most speakers I end up with a much smaller BR box than Vas, to over 4 times smaller and rarely over 1x time Vas. Note that Qt also plays a big role here.

Be careful with Vas. If you want to measure Vas yourself then be sure the speaker is run-in fully. Vas reflects the compliance of the speaker and this can vary a lot after running in.

[tda]

Hi MJK,
The constructed enclosure was just a test enclosure. I will build a new enclosure with a very large dimension comparing the other two dimensions, so the enclosure will act as a TL. In you reply you mention the word “Classic TL”. All your discussion is referring only to classic TL (TL offset), or also to all your sheets (Ported Box and ML TQWT)? Do you think that in a classic ported box(modeled with WinISD for example) the pressure is equal everywhere inside the box and in a ML TQWT the pressure is not equal?

[sreten]

Originally posted by Pjotr
Hi,

Don’t understand the relation Vb needs to be somewhere below 2x Vas. With most speakers I end up with a much smaller BR box than Vas, to over 4 times smaller and rarely over 1x time Vas. Note that Qt also plays a big role here.

Vb does not need to be 2 x Vas at all. Its that there's not much
point going beyond 2 x Vas in trying to extend bass because
the speaker compliance begins to dominate.

At Vb = Vas the compliances are equal and I'd call this a big reflex
box, as you say Qt is usually arranged such that Vb is typically
1/2 Vas but this does depend on the Qt of the driver.

Vb >/= ~ 1.5xVas generally needs very low port tuning to
avoid bass peaking if it is possible at all with the driver.

I'd also say if Vb > Vas then its a rule of thumb an alternative
driver would give better performance with the box volume,
especially in bass SPL capability, as the implication is a bigger
driver could have been used.

sreten.

[Pjotr]

Ok, thanks Sreten,

That makes it clearer. For BR boxes Vas is not very useful as a selection criterion IMHO. Qt is more suitable. To high a Qt and you end up with a large box indeed and sometimes it is impossible to tune to a decent alignment at all. It is not for nothing Qt of units intended for BR’s is somewhere between 0.2 and 0.3.

For a TL, a Qt of around 0.5 is a more suitable choice.

Cheers

[sreten]

Originally posted by Pjotr
Ok, thanks Sreten,

That makes it clearer. For BR boxes Vas is not very useful as a selection criterion IMHO. Qt is more suitable. To high a Qt and you end up with a large box indeed and sometimes it is impossible to tune to a decent alignment at all. It is not for nothing Qt of units intended for BR’s is somewhere between 0.2 and 0.3.

For a TL, a Qt of around 0.5 is a more suitable choice.

Cheers

IMO you have to consider Qt, Vas and Fs because its
simple to make a low Qt and Fs driver by having a high Vas.
What you need is low Fs combined with moderate Vas
and a Qt around 0.3 to 0.35 for good extension.

For a 88dB/W 6.5" driver something like :
30Hz Fs, 25 litre Vas and Qt = 0.35.

I regard an innappropriate Vas as a rejection criteria.

sreten.

[Pjotr]

Hi Sreten,

Low Fs and a moderate/small Vas are contrary to each other in some way. If a speaker has both its efficiency is unavoidable low then.

Cheers

[kelticwizard]

Originally posted by Pjotr
Hi,

Don’t understand the relation Vb needs to be somewhere below 2x Vas. With most speakers I end up with a much smaller BR box than Vas, to over 4 times smaller and rarely over 1x time Vas. Note that Qt also plays a big role here.

I am a little surprised to hear that. A speaker has to have a Qt well over .4 and be tuned below Fs for a box much larger than Vas to make sense.

Thomas W took a HE 15 and put it into an enclosure about twice it's Vas. It was the only way he could get it to have output below the Fs of 25 Hz. He used an equalizer to take care of the dip above the tuning frequency of 18 Hz. That was not the ideal situation, but it was what he was forced to rely on to use a driver with a 1.5" one way excursion to get low bass output out of it.

In most cases, there is a wide range of choices for drivers with Qts of .4 or below. Not that many take drivers with Qts over .5 to use for reflex encloures. As Small has shown here, the transient response gets worse the greater the Vb is compared to the Vas.
http://www.diyaudio.com/forums/showt...547#post303547

If anything, on a few threads here I found myself the only person arguing that a driver with a Qts of .5 can be put into an enclosure larger than it's Vas and be tuned to below it's Fs and produce a perfectly usable box, as long as Vb is not that much larger than Vas. Making a reflex box with the Vb four times the Vas is cetrtainly beyond normal practice-very few would be inclined to do so, considering that there is no Thiele-Small alignment which calls for it.

[kelticwizard]

Originally posted by aaroncgi
... so let's call it 3 cubic feet or 85 liters. I suppose it could be a bit less, so if anyone knows how to calculate area of a D shape, we could get more exact.

In any case, this is the volume (85 liters) I used to tune the enclosure. This in theory is somewhere around 22Hz.


....The drivers were played pretty hard for a couple weeks before I measured Fs, so they should have been broken in. The fact that they measured between 49-52Hz seems to point in that direction, as most notes I have seen put the new batch at around 55Hz Fs.

I did some figuring. Assuming similar Vas/Fs ratios, if your drivers Fs was 50 Hz, the Vas would shrink to about 17 liters. Which would make your enclosure about six times the Vas.

You tuned the enclosure to 22 Hz. Just as a quick experiment, have you considered running a tone of say, 25 Hz or so through your speakers, and see if there is any difference when you cover the port and when you leave it clear? There are freeware online tone generators, such as this one provided by the Taylor family on their home page:
http://www.satsignal.net "Audio Tools"