Group delay from vented boxes

[rabbitz]

Damo

Now try a vented box so you are familiar with both types.

[Ron E]

Originally posted by Peter Marcks
Extension in a ported subwoofer is a function of internal volume and port length.

Actually Vas/Vb, Fb/Fs, Qts, Le and box losses are the box sizing factors. Tuning frequency is defined by port area and length for ported boxes and and Vap/Vb and mass for PR's. It is not impossible to tune a 20 liter box to 20Hz, but possibly impractical depending on required output.

Ported subwoofers also tend to roll off very quickly below the tuning point, so that even in-room extension falls off relatively sharply below tuning point.

As I said , it depends on how you define extension

With, say, a subwoofer enclosure that is 12" cubed, very low extension is not a very realistic goal for one who wants to design a ported subwoofer. It would be much more realistic to try to achieve that in-room extension in a sealed box subwoofer or a subwoofer with PR(s).

And what is the tradeoff?? Sensitivity. Small boxes (sealed or PR or ported) also can have a nonlinear compliance issue.

Some people consider ported to be just a special case of PR boxes - all are in the reflex family - one has a compliance where in the other the compliance is short-circuited.

In general, a reflex box can use a woofer that will be more sensitive and/or have deeper extension (defined as F3) than a woofer selected for an equivalently sized sealed box.

[Peter Marcks]

Originally posted by Ron E
[B]Tuning frequency is defined by port area and length for ported boxes and and Vap/Vb and mass for PR's.

The point being that port tuning is directly affected by the enclosure size and port length, per a given port area, of course.

It is not impossible to tune a 20 liter box to 20Hz, but possibly impractical depending on required output.

Practically is important. Sure, one could design a very small and very long port, and one could achieve a very low port tuning using a very small enclosure. It would not make much sense though to design a ported subwoofer in, say, a 12" cube subwoofer, especially if very low extension (20Hz in-room, or 16Hz in-room etc) was the goal. The porting would eat up some of the precious little internal volume remaining in the cube, and the subwoofer would suffer from severe port turbulence-related noise in the low frequencies because it would not be possible to achieve a very low port tuning and a very large port area at the same time in such a small box.

And what is the tradeoff?? Sensitivity. Small boxes (sealed or PR or ported) also can have a nonlinear compliance issue.

Absolutely. Small size does involve some serious tradeoffs, irrespective of the design.

In general, a reflex box can use a woofer that will be more sensitive and/or have deeper extension (defined as F3) than a woofer selected for an equivalently sized sealed box.

Yes, but especially when looking at relatively small enclosure sizes, it is much easier to practically achieve low extension (20Hz in-room, or 16Hz in-room etc) using a sealed box subwoofer than a ported subwoofer.

Sincerely,

[dscline]

Originally posted by paulspencer
There was a comparison of a number of commercial subwoofers, including a Velodyne 18" serfvo subwoofer, an Adire Sadhara, Servodrive Contrabass and a number of others. The sub which was rated as the best was a Genelec which had four 12" drivers in vented configuration. A number of the subwoofers were sealed and I don't recall any of them being rated as superior in any area of performance.

It sounds like you are referring to the Ultimate AV 12-way sub comparison. If so, I don't think you've completely accurately represented the results. They listed it as "state of the art", and I would hope so: it's a $10k sub. They listed the (sealed) Velodyne as the "Best Real-World Solution", and went on to say "It hangs in there with the best of the bunch in every category but distortion, where it is the best of the bunch" and "Without help from a mike, a tone generator, and a high-quality outboard parametric equalizer, no sub in the survey can match the DD18 for bass quality in a typical home theater setting." It sounds to me like they liked them both, but they were completely different animals (and completely different sizes and costs).

[paulspencer]

If so, I don't think you've completely accurately represented the results.

Yes that is what I was referring to, but you will note that they said a lot of things. They list the Genelec first and the "state of the art." If they have picked any as a winner then it's this sub. I think the statement was accurate enough to make the point I was making, although going from memory I'd forgotten that statement about the velodyne.

I'm not sure I agree about their statement on distortion. The Genelec is lower overall. Look above 40 Hz where distortion is going to be most important - the Genelec is below 1% even at 115db above 40 Hz, whereas the Velodyne is 4% at that level. It makes me wonder what would happen if the Genelec also used a servo ...

Of course what is lacking is identifying the types of distortion, because THD alone doesn't say very much according to some recent research.

I also think that they have not fairly rated the Adire Sadhara. I emailed Adire about the results and they mentioned that an old unit was used which had some problems with the box with rattles. But even according to their own results they seem to have poorly rated it. Last I checked it was cheaper than they mentioned and it looks far better than subs reviewed that cost a lot more. Amongst those subs I'm not expecting it to be the best, but I'd expect a fairer rating should count it the value for money winner.

[damo21]

If by the OP you mean the one who started this thread, that would be me!
Actually, I was weighing up my options and ended up building a 30L sealed sub, mainly because it was easier to build the box and not have to worry about fancy ports.
I haven't got a comparison yet, but I certainly love the clean acoustic bass guitar plucks in some of Brubecks jazz!

I think it's time to start looking at some diy TLs This is great!
If I'm still keen as mustard after all that, I might try the vented sub design I mentioned earlier to compare

It's surprising how sexy a cube of mdf can look with a 12" driver in the front of it!