Precision devices

mike.e said:
OB has certain requirements.
High displacement/multiple units
Moderate Qts

See the SEARCH function for OB projects completed.


The info available is somewhat confusing. I've read that you need a driver with a QTS >0.5, then SL uses a Peerless 12" with a QTS of 0.2 :xeye:

Displacement is an issue but isn't this mainly targeted at those trying to get down to 20Hz and as far as I know Xmax of 7mm isn't bad. The Fs is a bit high but I don't want to get down below 40Hz with these speakers, if needed I can use a low mono sub to fill in below them. What attracts me to these speakers is the low mms with a decent motor, shouldn't this be good for fast response?

The reason for my question is the lack of decent affordable drivers in the UK, the Peerless that SL uses is twice the price of the PD and for 4 units the cost is simply too much.

Thanks for the response
Kev
 
Recently the german magazine Hobby Hifi did a test of all variations of Mono-pole and Di-pole subs varying from I-Pole to W-Pole of which the U-pole (like an open backed cabinet) seemed to give very good results. Depth was kept to no more than 30cm to avoid having standing waves in the usable range. The N-pole and W-pole types also seemed to lower the fs of the driver due to coupling the air mass directly behind the woofer.

The only difference between high Q and low Q drivers was their efficiency in the upper bass and midrange. In the lower bass they were practically the same.

By coincidence I have ordered a pair of Precision Devices drivers and am going to experiment with different di-pole type cabinets.
 
Which drive units have you ordered?
When will you start playing with dipole bass?
I currently have a Goodmans 201 on an open baffle which sounds amazing. This PD driver will be mounted on the same baffle and equalised with a Behringer FBQ2496. If it doesn't quite do what I want I can order another pair and go with the dipole arrangement you suggest, probably a U-pole.

I'll be interested to know how you get on.
Kev
 

derf

Member
2004-04-01 11:04 pm
Herts
Qts isn't much of a factor, unless you want to take the driver to around 200-300hz, where it starts to get more sensitive, thus requiring more eq/bigger resonance peak(higher qts) to keep it flat down to fs.

Displacement is an issue but isn't this mainly targeted at those trying to get down to 20Hz and as far as I know Xmax of 7mm isn't bad. The Fs is a bit high but I don't want to get down below 40Hz with these speakers, if needed I can use a low mono sub to fill in below them. What attracts me to these speakers is the low mms with a decent motor, shouldn't this be good for fast response?

I think what most people either forget/don't know is that SL is quite possibly using 12" drivers for aesthetic reasons, as much as ones pertaining to sound quality(they do have to sell after all). Rather than looking for huge amounts of Xmax, it's preferable to look for larger surface area, which leads to less excursion/less distortion. Hence why I think you'll find 15" and 18" drivers so attractive for this kind of application.

For example, the Delta 15LF(quite a steal at around £50) has 856.3cm2 of surface area, compared to the PD driver at 530.93cm2. Now, most people use 2 drivers per side, so the total surface area for both sides is: 3425.2cm2 surface area with the Delta and 2123.72cm2 with the PD. Factor in an fs of 41hz and a not inconsiderable 4.8mm xmax and it certainly looks tempting.
 
Hi Kevin

If you don't wanto to go that low, but want the best bass, the may I suggest the PD2450 24" driver from Precision Devices. It has an Fs of 30.477 Hz, so it won't do much below that (unless you EQ the hell out of it). :D It should work very well on an open baffle, and will provide you all the bass you'll need. Here is the link:
http://www.precision-devices.co.uk/ShowDetails.asp?id=17

Enjoy,
Deon
 

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KevinTams said:
The info available is somewhat confusing. I've read that you need a driver with a QTS >0.5, then SL uses a Peerless 12" with a QTS of 0.2 :xeye:

You are right it is confusing. I tend to side with Linkwitz, but people with practical approach have done well with high Qts drivers. Here's what Linkwitz says about Qts. I wonder if people have missed this as I'm surprised at the lack of mention of it on dipole threads.

From Linkwitz FAQ
Q34 - What is the optimum Qts for the drivers of a dipole woofer?

A34 - The low frequency roll-off of a woofer and its associated group delay are optimal, from what I have observed, when they follow the response of a 2nd order highpass filter with Q = 0.5. When a driver is mounted in a dipole W-frame or H-frame its mechanical resonance frequency Fs decreases to Fd, due to air mass loading, and Qts increases by a similar percentage to Qtd.
For example, a driver with very strong motor, Fs = 18 Hz and Qts = 0.2 might have Fd = 16 Hz and Qtd = 0.22 as determined from an impedance measurement of the baffle mounted driver. With Qtd < 0.5 the low frequency behavior of the woofer is characterized in the complex s-plane by real axis poles at -69 Hz and -3.7 Hz and by 3 zeros at the origin. One of these zeros is due to the front-to-back dipole cancellation with its 6 dB/oct low frequency roll-off. The frequency response of this 3rd order acoustic highpass filter must be equalized to obtain a flat response. A suitable target response could be Fd = 20 Hz and Qtb = 0.5. It is easily realized with two shelving lowpass filters. The first filter with a pole at 20 Hz and a zero at 69 Hz corrects for the low Qts of the driver. The second filter with a pole at 20 Hz and a zero at 400 Hz compensates the 6 dB/oct roll-off due to dipole cancellation. The 3.7 Hz pole is low enough in frequency so that the response is dominated by the 2nd order roll-off below 20 Hz. An advantage of a low Qts driver is the ease with which it can be equalized for an optimum response with Q = 0.5.
A driver with a smaller motor might give Qtd = 0.7 and Fd = 20 Hz, which leads to a pair of complex poles in the s-plane. This can be readily changed to a 3rd order Bessel highpass response by using a shelving lowpass filter with a pole at 20 Hz for the necessary dipole roll-off compensation.
Likewise, if Qtd = 1 and Fd = 20 Hz, then an additional pole at 20 Hz, from the dipole equalization, leads to a 3rd order Butterworth acoustic highpass response. Third order filters introduce more group delay than 2nd order ones. It is therefore advantageous to use Qts < 0.5 drivers, even when they require driver roll-off equalization in addition to the normal 6 dB/oct dipole correction.
 
Derf, Deon
thanks for the info, Unfortunately smallish room and demanding wife means that 12" drivers are all I can fit on the baffle. :bawling:

Richie00boy
I've read the quote from SL several times now and I think I understand the gist of it :dead:
You're correct about me missing it but I doubt it would have meant that much to me if I had seen it :) If I understand correctly the fact that I'm looking to have a -3dB at 40Hz means in this case that a low QTS driver will allow me to use less eq to achieve my desired SPL assuming I use large enough xmax and sd drivers.
 
KevinTams said:
Which drive units have you ordered?
When will you start playing with dipole bass?
I currently have a Goodmans 201 on an open baffle which sounds amazing. This PD driver will be mounted on the same baffle and equalised with a Behringer FBQ2496. If it doesn't quite do what I want I can order another pair and go with the dipole arrangement you suggest, probably a U-pole.

I'll be interested to know how you get on.
Kev

I have ordered a pair of PD2150's. This will give me a total cone area of 3260cm2 (nearly the same as four 15" drivers) with an Xlin of 12mm and Xmax of 38mm; fs = 22Hz and Qts = 0,19. I want to start out with building a pair of simple U-poles (basic box with the back missing) and experiment with them and then later on build a more visually pleasing design. I want to see what happens when I play the two seperatly or face them firing at each other from a close distance - sort of W-pole like. I will use them up to about 80Hz max. :)
 

derf

Member
2004-04-01 11:04 pm
Herts
Yeah, the 21" and 24" PD's are big ticket items, which is no suprise, as how many people are they really expecting to buy them?

What made you go with 2 x PD21*, rather than multiple 15"/18" drivers, Geenius?. Might of been a bit cheaper, could of gone for a Stereo setup etc.

Just wondering...

:)
 
derf said:
hey, very cool ;)

Didn't fancy a baffle though, do they do ok without one?

Surpisingly well! At the moment they are running with a simple 2nd order passive crossover connected parallel to the main speakers. Bass is powerfull and well detailed, only the lowest octave is a bit down in level. I will be adding a baffle shortly.