Low Qts woofer

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I have my eye on a nice but cheap woofer driver with a Qts of 0.27, and a VAS of around 120L, and an FS of 26Hz. What type of enclosure would it be best for? I'm thinking a TL because of the low Qts, but I'm not an expert on these things. Any suggestions? (Note: I plan to make stereo woofers, if that's helpful)
 
The downside of a low Qts driver in a basreflex is that it, in order to get low, will have quite a big groupdelay. On the other hand that doesn't seem to bother a lot of people out there. It's also the easiest solution I guess.

Low Qts drivers are suited for hornloaded but you'll need a huge enclosure to get low.

Personally I use a low Qts driver in a TL (0,297), with an smaller mouth than throat. It's not tuned at the Fs but a little bit higher to get a flatter response. It was simulated with AJ-horn.

Mvg Johan
 
Yeah, you would think so. Too bad that deep bass and efficiency are about the opposite of each other.

If you want deep bass, Xmax, Sd and Fs are the more important factors. Xmax x Sd = Vd (good for comparing drivers). But you still have to use the right Qts driver for the right enclosure.

A Qts below 0.3 is generally more suited for hornloaded, between about 0.3 and 0.45 for basreflex, higher for closed and dipole. It isn't a strict border, they overlap.

A low Qts will generally go together with a high BL and thus efficiency.

For a basreflex subbas enclosure a Qts of about 0.34-0.38 would be pretty nice. As told lower Qts is possible but groupdelay will be skyrocketed. In Europe it's almost fashion lately to make low Qts drivers.

Btw: Cheaper drivers will most of times have an low Qts and a smaller Xmax, or a big Xmax and a high Qts. That makes it easy to see what kinda driver you're dealing with.

Personally I use a PA-driver, so a low Qts/high BL is welcome in that case. It makes the f3: 30 Hz with getting a continues SPL of 126 dB (at 100 Hz), but it takes a 500 ltr enclosure to do so. Hornloaded would turn out much bigger (for this driver) to achieve 30 Hz efficiently, but SPL would be even higher.

The TL only sounds good with HT and slow music in my opinion.

Mvg Johan
 
Why wont a t-line tuned to about 40Hz give me deep and fast bass with this thing?

The model in question is an MCM 55-1480.



Another option I'm considering is the MCM 55-1245. This is even cheaper, and has an "easier" Qts to work with. If I got these I would probably use them in a sealed cabinet since WinISD is telling me that it'll make a pretty good sub. Which driver should I get out of the MCM 55-1480 or MCM-1245?
 
There are few loudspeaker designers out there, like John Cockroft, who contend that low Qts woofers have a more natural sound characteristic when used in a TL enclosure.

There are also those who say that you should only use a high Qts woofer in a TL enclosure.

I personally feel that almost any woofer can be used in a transmission line ... if the system is properly designed.

George Augspurger wrote an article in Speaker Builder magazine a few years ago, where he compared two TL (open ended pipe) enclosures using the same woofer.

One pipe was 6 feet long and lightly stuffed with Dacron.

The other pipe was 2 feet long and heavily stuffed with Dacron.

The two systems measured almost exactly the same response curve. This lead Mr. Augspurger to conclude that any TL line length could be properly tuned for any woofer, provided you took the time to correctly adjust the damping fiber density.

Sometimes, we forget the real reason we got into this hobby in the first place ... to learn, experiment, and build our own great sounding speaker systems. Too many people on this forum are expecting instant gratification.
 
I look forward to experimenting!

I'm building an active crossover at the moment, is 100Hz too high to cross over for stereo subs? My gut feeling is that it'll be ok, but many people have said this is too high (the crossover will be either 12dB/oct or 24, I havn't decided)
 
bigwill said:
I have my eye on a nice but cheap woofer driver with a Qts of 0.27, and a VAS of around 120L, and an FS of 26Hz. What type of enclosure would it be best for? I'm thinking a TL because of the low Qts, but I'm not an expert on these things. Any suggestions? (Note: I plan to make stereo woofers, if that's helpful)

How big an enclosure can you tolerate? Can you consider active EQ?
 
diyAudio Moderator Emeritus
Joined 2001
BigWill:

You have two perfectly reasonalbe options, if all yu want is decent bass down to 40 Hz-or even 26 Hz.

First, I must ask-if these are woofers, as opposed to subwoofers, will you be using a choke in series with the woofer? That is to say, using a passive crossover? If you are, the resistance of the choke will raise the Qts, so an enclosure can be constructed which will give flat response lower.

I went to MCM's page, but was unable to find the dc resistance of the woofer. Just for the heck of it, I am going to assusme it is 5.7 ohms-many woofers have that.

There is a formula to calculate the raise in Qts by adding resistance. David Weems gave it in his Great Sound Stereo Speaker Manual.

http://www.diyaudio.com/forums/attachment.php?postid=93570&stamp=1039479451

So, if you don't mind building a 120 liter box-4 cubic feet-you can just add the following resistor:

R = ((0.38 / 0.27) * 5.7) - 5.7 = 2.3 ohms. A adding 2.3 ohm of resistance will produce a woofer most suited for 120 liter box tuned to 26 Hz. It can be pointed out that if you use a choke n series, it's resistance counts. So if you want to save money on an inductor and get one with a resistance of 1.5 ohms, you would only need a resistor of 0.8 ohms to add in addition to bring your resistance up to 2.3 ohms.

If you want a smaller box, and a Qts of 0.32, that will call for a 2 cubic foot box, (56 liters), tuned to 33 Hz with an F3 of 35 Hz. To achieve this, the resistance, (both choke and resistor), should come to:

R = ((0.32 / 0.27) * 5.7 ohms) - 5.7 ohms = 1 ohm. That 1 ohm can easily be taken up by a crossover choke alone, assuming you are using one.

So adding a resistor and/or crossover choke is one option.

PS: Oops. I just reread your post and see that you were going active in the crossover department. But you can still add the resistor on your own.

PPS: The reisistors do cut down on your efficiency somewhat. However, a woofer with a Vas of 120 liters and an Fs of 26 is likely to be fairly efficient to begin with.

A 1 ohm reistor should cut the efficiency of the woofer about 1.4 dB.
 
diyAudio Moderator Emeritus
Joined 2001
Another alternative is to build a Daline enclosure. They seem to work well with low Qts woofers.

A Daline is an enclosure which is sort of a combo Transmission Line and Vented box.

They can be modelled by Martin J. King's software program, available at www.quarter-wave.com.

The problem with this is that I have yet to master Dr. King's program, so I cannnot give you a specific recommendation. However, I might be able to contact some people on the board who are able to use it, if you are interested in seeing what kind of enclosure would be produced.
 
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