An OB design

el`Ol · « **Reply #40 on:** 31 May 2007, 09:24 am »

Quote from: D OB G on 30 May 2007, 10:42 pm

Hi el'Ol,

Yes, looks good, but as you say, can graphs be compared from model to model? It does have a rising peak at 2000Hz, which the Precision Devices seem to avoid?
I'd be interested to know the comparative prices.

David

Hello David!

Prices are not so different, despite the much more impressive basket of the PD.
The Ciare is 250 Euro, the PD186 is 300 Euro.

Oliver

hasselbaink · « **Reply #41 on:** 31 May 2007, 02:33 pm »

Quote from: D OB G on 30 May 2007, 12:21 pm

Hi el'Ol,

I'd never heard of them, but Precision Devices sure do seem to make some magnificent drivers.
I'm not qualified in the slightest to make any judgement on the matter, but maybe I can offer suggestions as to which might work better with my design technique.
I'd rule out any with an Fs higher than 30Hz (especially if you're going to the trouble of 18" drivers)- that seems to knock three off the list.
There are two that have highish Qts- they might be too high to compensate.
That seems to leave the PD184, PD186 and the PD1850.
The response graphs don't seem to be represented in an absolutely equivalent way. e.g.the PD1850 graph stops- what is the high freq. like? Should you take a chance?
That leaves for me the PD184 or the PD186.
PD186 is better made, with better parameters in other respects.
It's probably one of the dearer ones??

I emphasise again that I am totally unqualified to judge a manufacturer's products that I have never seen or heard.
So that's it for what it's worth.

David

.
.
Hello David,

Nice to hear from a person with good knowledge of OB speakers.
I have a question regarding SPL's in the low register. What is the minimum SPL acceptable at, say 40 Hz, in order not to sound dynamically compressed? According to Linkwitz Riley's spread sheet, it seems to be that woofers with a Xmax of less than 6mm are limited to maybe 80 db at 40Hz. What are your thoughts on the subject?

Cheers
Chris
.
.

D OB G · « **Reply #42 on:** 1 Jun 2007, 12:24 am »

Hi Chris,

I think JohninCR, who answered a similar question on the "Xlim versus SPL" thread, is your man here.
I haven't seen Linkwitz's spreadsheet.
The answer clearly depends on the radiating area, as well as the Xmax (a 1 mm driver of Xmax 6mm is obviously not going to give 80 dB at 40 Hz).
So what diameter did Linkwitz use?
Free air, or with floor reinforcement?

David

D OB G · « **Reply #43 on:** 1 Jun 2007, 03:03 am »

MJK

Posts: 101

Re: An OB design
« Reply #8 on: May 23, 2007, 02:44:17 pm » Quote

--------------------------------------------------------------------------------
If you have a driver with a higher Qts, the impedance curve around the resonance is going to be peaking significantly. How are you going to size the inductor for a nonconstant impedance and still produce the constant 6 dB roll-off? The only way I see to accomplish this is by going active where the drivers impedance does not impact the "crossover" filter component sizing. Doing this would also require a highly efficient woofer to be able to give away significant SPL and still be able to transition to a midrange or full range driver.

Hi MJK,

Re the response approaching resonance problem.
I now have Basta.
According to it, the electrical inductance, as opposed to electrical impedance, resistance, or reactance, has a characteristic that I have never seen before.
Inductance is at a maximum at Fs (a random driver showed 5,000 mH !!!), drops to zero, and stays there as frequency rises, and then at some low freq (in this case 100 Hz) rises immediately to the voice coil inductance.
This only seems to apply to open baffle and v = infinity alignments.
So this (although I don't understand why and how- if indeed Basta is right) would explain the empirical results.

David

MJK · « **Reply #44 on:** 1 Jun 2007, 11:13 am »

David,

Quote

So this (although I don't understand why and how- if indeed Basta is right) would explain the empirical results.

Do you believe this result? Does it make sense?

Martin

D OB G · « **Reply #45 on:** 1 Jun 2007, 01:05 pm »

Martin,

I don't believe it.
It doesn't make sense.
But...

According to the writers of Basta, "electrical inductance is... the reactive part of the electrical impedence seen as inductance divided by w."
"(The electrical impedance) curve can be useful if the manufacturer has specified the voice coil inductance at two different frequencies... The voice coil inductance can be modelled as lossy. Measurements of real voice coils show that impedance behaves far from a simple resistor in series with an inductance.
A more appropriate model takes into account "eddy currents" induced in the magnetic pole pieces of the loudspeaker."
The writer(s) go on to provide an equation giving n as the loss factor and pointing out that manufacturers rarely specify the voice coil inductance according to this equation, and which explains why they include a box for their calculation of Le loss in their driver data input from other parameters such as Le (and possibly compliance of the moving mass, as well as others).
So they seem to maintain that the idea of a constant Le is as subtle as considering the nominal impedance as describing the complex impedance graphs we all know.

I have no knowledge of any of this.
Their graphs of single drivers consistantly show the pattern I described earlier (although I must have found an extreme case- another example showed a peak of about 5 mH at resonance, then zero inductance, then Le, as frequency rose).

I might well have misrepresented Basta's contentions.
If they are right, it seems to provide some sort of explanation for the inductor compensation method.
If they (or I) are wrong... the method still works anyway.

David

MJK · « **Reply #46 on:** 1 Jun 2007, 01:19 pm »

David,

The voice coil inductance for a bass driver is essentially zero at the driver resonance (below 100 Hz). The method being described is applicable to frequencies much higher, closer to 1 kHz. Something tells me your simulation is not correct.

Martin

D OB G · « **Reply #47 on:** 2 Jun 2007, 01:55 am »

Martin,

Basta agrees with your voice coil inductance comment below about 100 Hz, except that it makes a sort of asymptotic leap at exactly resonance (below resonance they have it drop slowly).
I have Basta now because of maxro's simulations, which look very nice, and give precise inductance and resistance values, showing the effects of very subtle changes in value, however, they don't quite accord, or really some really don't accord with my measured values using a variable, tapped, multi-inductor, an inductance meter, and with the reponse being measured with "Imp". I would be very wary of questioning a simulation written by knowedgable people about things I know nothing about, but maybe you are right, maybe there is an anolamy somewhere.

In any case, this still doesn't address your fundamental issue.
As we know, if we are to design reliable crossovers in the higher freq range, we need to zobel the speaker for linear impedance.
Why does my technique work (and why does Basta say it works) when I don't (can't?) provide a linear impedance over the relevant freq range? (unless an LCR parallel network was used, as in the case of, say, reducing the resonant freq impedance hump of a tweeter- which I have experimented with, and which seems to make no difference (very large values!).

David

el`Ol · « **Reply #48 on:** 2 Jun 2007, 04:45 am »

Another question:

Why does the Visaton Bobox BB work?
I see just a 12dB x-over, no additional 6dB linearization.

http://www.visaton.com/en/bauvorschlaege/breitband/noboxbb/index.html

D OB G · « **Reply #49 on:** 2 Jun 2007, 06:00 am »

Hi el'Ol,

Firstly, I would say that -10 dB at 32 Hz is a long way from the claimed deep bass.
The conventional way of specifying would be to say that it is -3 dB at approx 50 Hz.
The Visaton BGS 40 bass driver is shown with a falling reponse of 6 dB per octave below 400 Hz.
This surprises me. Normally responses are given on an IEC baffle, and with a Qt of 0.42 you expect a flat freq response rolling off below say about 120 Hz.
In any case, with a 12 dB per octave crossover, 6 dB per octave is "used" just to end up with a flat response below the 250 Hz crossover.The "other" 6 dB per octave compensates for the baffle loss.

David

Rudolf · « **Reply #50 on:** 2 Jun 2007, 01:33 pm »

The Nobox was measured in an anechoic room . Visaton shows the difference between those measurements (a) and a typical room (b) in this diagram:

Visaton drivers are measured in a DIN baffle of 135 cm X 165 cm

scorpion · « **Reply #51 on:** 2 Jun 2007, 05:39 pm »

IEC = 135 x 165 cm and from the element placement in the Visaton picture it seems that IEC and DIN are identical.

/Erling

Rudolf · « **Reply #52 on:** 3 Jun 2007, 07:59 pm »

Thanks Erling,
for stating this more precisely. I should have cross-checked this before posting.

Rudolf

el`Ol · « **Reply #53 on:** 21 Jun 2007, 11:06 am »

Quote from: D OB G on 23 May 2007, 03:10 pm

To answer your question, that's why I go for middle Qt drivers, to avoid the resonant peak, and to give room to fine tune the inductor impedance and dc resistance.

David

What about taking a high-Qts speaker and flattening out the impedance peak with a series notch filter?

JohninCR · « **Reply #54 on:** 21 Jun 2007, 04:32 pm »

Quote from: el`Ol on 21 Jun 2007, 11:06 am

What about taking a high-Qts speaker and flattening out the impedance peak with a series notch filter?

How is that going to get around the high Qts driver's inherent inability to stop itself from flopping around at resonance? Isn't impedance taming just for the amp's benefit?

Daygloworange · « **Reply #55 on:** 21 Jun 2007, 04:41 pm »

Man, that's one serious setup for measuring a tweeter!

Cool!

Cheers

el`Ol · « **Reply #56 on:** 22 Jun 2007, 05:49 am »

Quote from: JohninCR on 21 Jun 2007, 04:32 pm

How is that going to get around the high Qts driver's inherent inability to stop itself from flopping around at resonance? Isn't impedance taming just for the amp's benefit?

I was asking because the new 15" from Hemp looks good from what it does in the higher regions. It stays within a 5db margin and is probably suitable for a 6dB x-over. What`s strange is the hole at 45Hz. Does anybody have an explaination?

D OB G · « **Reply #57 on:** 23 Jun 2007, 05:26 am »

Hi el'Ol,

With my design technique, if I want a system Qts to end up being chosen somewhere between 0.56 and 0.7, and given that I am going to raise the Qts of the driver by the DC resistance of the inductor in series, as well as any extra resistance I need to achieve my target Qts, the Qts of the driver needs to be low enough to cope with this margin (usually around 0.3- 0.35 when two nominally 8 ohm drivers are used in parallel).

High Qts drivers aren't suitable for my technique.

An interesting observation:
High efficiency drivers tend to have (there are exceptions) a low Qts.
High Qts drivers tend to be less efficient. So using a high Qts driver in the conventional OB way ends up with a similar efficiency, on average, as a low or mid Qts high efficiency driver run with a series inductor, and the loss of efficiency that results.

David

el`Ol · « **Reply #58 on:** 23 Jun 2007, 05:52 pm »

I found a high-Qts 15" driver from Selenium that looks even better than the Hemp.
http://www.seleniumloudspeakers.com/site2004/catalogo/pdf/wooferpw3_15pw3_15pw3-slf_new.pdf
Why can`t I find low Qts drivers with equally good top end?

Russell Dawkins · « **Reply #59 on:** 23 Jun 2007, 08:08 pm »

Quote from: el`Ol on 23 Jun 2007, 05:52 pm

I found a high-Qts 15" driver from Selenium that looks even better than the Hemp.
http://www.seleniumloudspeakers.com/site2004/catalogo/pdf/wooferpw3_15pw3_15pw3-slf_new.pdf
Why can`t I find low Qts drivers with equally good top end?

At only $70, that looks to be a worthy contender. I'm not used to seeing a combination of low mms (71gm) and low fs (40Hz) like that. Selenium must be doing something different, like having a very compliant spider and surround. xmax is a little limited, though.
On the surface of it, it would seem to be ideal - now, I wonder how it sounds.

I floated this driver on another forum and someone wrote back to say he had a couple of Selenium speakers and his impression was they offered outstanding value.