Variovent placement...

[arcturus]

I've just got assembled a "prototype-enclosure" for my subwoofer project... The volume of the sealed enclosure is about 82 liters (about 2,9 cu.ft). It hasn't got anykind of stuffing inside, yet.

The 12" woofer I'm currently using is the 8ohm version of Dynaudio's 30W100 XL.

Inspired by this article,

http://mywebpages.comcast.net/kidder...Subwoofer1.htm

i might be interested in adding a home-made "variovent" to the enclosure.
But does it matter where in the enclosure it is placed? Would be nice if it could be placed at the bottom, together with the woofer...

And how about the tuning of the Variovent? Is there some advices, or should I just use the trying - listening -method?

I'm using a cylindrical-shaped enclosure, made from very strong plastic sewer pipe. It's inner diameter is 59 cm, and inner height 30 cm. At this time, it has end-caps from 22mm veneer. The final version propably has two 25mm MDF sheets glued together, in both ends.

[GM]

It depends on the app. For a sub you want it as far away as you can get it. WRT tuning, if you have the driver's specs you can optimize the design as a ML-TQWT using MJK's MathCad worksheet. http://www.quarter-wave.com/

GM

[kelticwizard]

I am not aware that you can vary a Variovent's tuning. It is not a conventional vent as for a ported box, just a muffled hole to relieve the pressure inside the box.

My understanding is that it's purpose is to take down the final Qtc of a sealed box.

According to the specs published in WinISD, an 82 liter sealed box gives you a final Qtc of .8. A lot of people would take that and leave it as it is. For a long time, that was considered the optimum box tuning.

However, the modern trend is a Qtc of 0.5. I cannot tell you how to vary the tuning of the Variovent, if indeed that is possible. But I can tell you when your box is acting like it has a Qtc of 0.5.

At Qtc=0.8, your response at resonance, (44 Hz), will be 2 dB down from the midpoint.

At Qtc=0.5, your response at resonance, (44 Hz), will be 6 dB down from the midpoint.

I have a suggestion, but it is speculation. I have not used a variovent. So this measurement system is just an idea.

My suggestion is that you download a freeware tone generator from the net:
http://www.satsignal.net/ - Audio Tools-SweepGen

Either run the 44 Hz signal through your speaker, either directly from your sound card if it has speaker level outputs, or from the sound card line level outputs to an audio amp, (any one will do, even a boom box). Then take any SPL meter and measure the level with the Variovent well plugged or covered. Then, uncover the Variovent. The enclosure will be acting like it has Qtc=0.5 when the output from the cone is 4 dB down, at 44 Hz, from what the SPL was with the Variovent covered.

In other words, when you cover the port, the response at 44 Hz should jump up 4 dB, when you uncover it, it should go down 4 dB.

The SPL meter does NOT have to be calibrated, since you are just measuring the difference, at 44 Hz, of the SPL when the Variovent is covered and when it is not. You do not need smooth frequency response to measure that.

If the Variovent does not take the SPL at resonance down far enough, then add another one. Or be satisfied with the lowering of Qtc that one Variovent affords.

Incidentally, only the output at resonance is affected by the Variovent. The speaker's midpoint SPL will not be affected.

[GM]

Sure you can, by varying the density of the vent. Take all the resistance away and it's a BR. Conversely, make it resistive enough and you can completely damp Fs, so you can either view it as lowering the final Qtb (vented) or Qtc (sealed) depending on how resistive it is.

The rest of your musings assumes a golden or acoustic ratio cab, i.e. it has a ~uniform particle density in the cab's controlling BW, so with an increasing aspect ratio, there's potential for more gain over a wider BW.

GM

[kelticwizard]

Originally posted by GM
Sure you can, by varying the density of the vent. Take all the resistance away and it's a BR.

I used to think that, too. However, after checking out the diameter and length of the port in commercial Variovents, if the resistance was removed the box would be tuned much, much higher than any tuning you would consider for a Bass Reflex.

There are people who make a box with a port tuned to a more conventional frequency, then stuff the port. It seems to work well.

But the systems I have seen on the web, if you buy one or two variovents as they recommend, and remove the stuffing, you have box tuned very, very much higher than any configuration imaginable.

To the extent that any box with a hole is a Bass Reflex, yes. But in the sense that it is a conventional Bass Reflex with stuffing in the port, no.

[GM]

used to think that, too. However, after checking out the diameter and length of the port in commercial Variovents, if the resistance was removed the box would be tuned much, much higher than any tuning you would consider for a Bass Reflex.

True, but it's still a BR, i.e. 4th order.

There are people who make a box with a port tuned to a more conventional frequency, then stuff the port. It seems to work well.

I resemble that remark! Again, it's a variovent also.

But the systems I have seen on the web, if you buy one or two variovents as they recommend, and remove the stuffing, you have box tuned very, very much higher than any configuration imaginable.

Again, you prove my point.

To the extent that any box with a hole is a Bass Reflex, yes. But in the sense that it is a conventional Bass Reflex with stuffing in the port, no.

What's 'conventional' have to do with what I said? you can take a 'conventional' alignment and make it aperiodic to any varying degree dependent on stuffing resistance.

GM

[arcturus]

I did some impedance measuring with Speaker Workshop, because at least in the WinISD, applying some losses (Ql parameter) to a sealed box changes its impedance curve.

As you can see, decrease of the peak is from almost 30 ohms (sealed, red curve), to bit over 21 ohms with the stuffed port. It's about 30% reduction, and in WinISD this translates into Qtc of ~0,76. With some stuffing inside the enclosure, the final Qtc might well be in the 0,7 region...

I made the resistive port from two pieces of 6,8 cm diameter plastic pipe. Then I glued the pieces together with epoxy glue, and put plastic net between the pieces (to prevent the stuffing material falling into the enclosure).

Because the port is only 6 cm long, unstuffing it causes booming to the sound. The reflex tuning frequency occurs at 35 Hz with that short of a port, causing a 4dB peak at 53Hz.

[kelticwizard]

Well, it lowered it a little bit, but not much.

I remember seeing some older aperiodic boxes, and instead of a relatively small variovent, they had rather sizable square holes or slots cut into them. The slots seemed to have an area considerably larger than that of a variovent clone that you have made. Inside the slot was stretched some stuffing.

I read an article from Weems way back, and he said the same thing. The slot was pretty large, and the inside had speaker stuffing stretched across it. As usual with aperiodic enclosures, he advised to stuff it to taste. this was in the years before Speaker Workshop, in fact before the home PC.

Moreover, it seems to me that most aperiodics, (what the Variovent box is) have a port without a tube of any appreciable length, and if you removed the stuffing, the box would be tuned well above resonance.

Having never built one, I feel somewhat unsteady giving advice on this. But if I were building it, I would try increasing the area of the vent by at least 2 or 3 times, then measuring. I would also try cutting a square slot in the enclosure, instead of adding circular ports, but that is up to you.

I guess it depends on what you want. A closed box sub with a Qtc of 0.7 is considered well tuned by many people already. I don't know if you want to make the leap to 0.5 Qtc tuning.

[kelticwizard]

Rob Wells built a subwoofer with Qtc of 0.75. With room gain it was almost flat down to 20 and below.

Although he wishes he could make it Qtc = 0.5, the Qtc = 0.75 driver performed quite impressively as far as bass extension, when placed in a room.

Read this post and the following post by Rob Wells:
http://www.diyaudio.com/forums/showt...440#post366440

In fact, the whole thread is worth looking at, I think.

[morbo]

I have built the ACI Spirit (now called Opal). The design called for an aperiodic design, and the instructions said to drill 4 holes about the size of a penny in a vertical row at the bottom right rear corner of the enclosure (far from the woofer) and over top of this glue a large piece of foam from the inside, if thats useful to anyone.