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Old 26th March 2013, 03:37 PM   #1
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Default FTRCelestion FTR15-4080FD and THAM-15 (Bad Idea? 6mm xmax)

Hi all,

After spending hours with HornResp trying to get a 12" TH to look more like the THAM-15 I gave up and decided to just build a set of THAM-15s. Parts Express recently had a sale on celestions so I took advantage of that and ordered two Celestion FTR15-4080FD drivers which seemed to sim well enough.

Well, to make a long story short, PE had erroneously listed the xmax at 10mm when, in fact, it's 6mm. They've agreed to pay for the return shipping owing to their error, however, before I send them back I want to make sure that I'm not overreacting.

Does anyone have any experience, or lacking that, simply thoughts/opinions with these drivers in a TH? My sims suggest that the xmax difference will result in a dramatic reduction in SPL.

tnx,
gs

Last edited by ghettosynth; 26th March 2013 at 04:07 PM.
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Old 26th March 2013, 04:38 PM   #2
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Quote:
Originally Posted by ghettosynth View Post
Well, to make a long story short, PE had erroneously listed the xmax at 10mm when, in fact, it's 6mm.
Does anyone have any experience, or lacking that, simply thoughts/opinions with these drivers in a TH? My sims suggest that the xmax difference will result in a dramatic reduction in SPL.
Your sims are correct.

A driver like the BC15TBX100 costs more because it can displace nearly as much air as two of the FTR15-4080FD, a 6 dB increase in clean SPL.
If you want small and loud (and known high quality), gotta pay the piper.
That said, you might look at displacement for dollar comparisons, there are some car drivers that have a lot of bang for the buck.
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Old 26th March 2013, 07:28 PM   #3
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Originally Posted by weltersys View Post
Your sims are correct.

A driver like the BC15TBX100 costs more because it can displace nearly as much air as two of the FTR15-4080FD, a 6 dB increase in clean SPL.
If you want small and loud (and known high quality), gotta pay the piper.
That said, you might look at displacement for dollar comparisons, there are some car drivers that have a lot of bang for the buck.

Sure, but the BC15TBX100 only has a 9mm xmax. I purchased these becasue PE said that they had 10. The next step up Celestion has 8mm xmax and (was) $100 less than the B&C. I certainly would have paid the extra $45 per driver to get something that was going to work. I only went with the driver that I did because it seemed well suited based on the data that I had.

In sim I get about 123dB or so from the Celestion and 126dB from either the larger Celestion or the BC15TBX100.
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Old 26th March 2013, 07:56 PM   #4
OMNIFEX is offline OMNIFEX  Jamaica
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The majority of the noise in the THAM 15 is above 55 Hz. XMAX won't play a huge factor under those conditions. Its like a Scoop. Woofers in Scoops rarely use any of its XMAX for the majority of the SPL is above 50 Hz.
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Old 26th March 2013, 08:32 PM   #5
stinems is offline stinems  United States
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I'll note that I don't have real-world experience with either driver, but...

I don't think the Celestion is as much of a dog as you might think. Not to take anything away from the B&C 15TBX, which I agree is a monster in its own right, I'm not so sure its such a huge leap ahead of your Celestion. Celestion seems to use the old school method of xmax rating: (VClength-Gap)/2 (for overhung). B&C, Faital, and others have gone away from that since their advancements in motor design, apparently, have made that too conservative of a thumbrule (at least for the marketers). A look at the Celestion's impedance curve will tell you that is no slouch of motor. Not sure if it has demod ring(s) like the B&C, but its quite flat and non-inductive. A bit more research shows the FTR series having improved cooling mechanisms too.

In a nutshell, my guess is the 6mm of xmax on the Celestion is a fairly conservative rating.

Cheers,
Sam
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Old 26th March 2013, 09:17 PM   #6
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Originally Posted by stinems View Post
I'll note that I don't have real-world experience with either driver, but...

I don't think the Celestion is as much of a dog as you might think. Not to take anything away from the B&C 15TBX, which I agree is a monster in its own right, I'm not so sure its such a huge leap ahead of your Celestion. Celestion seems to use the old school method of xmax rating: (VClength-Gap)/2 (for overhung). B&C, Faital, and others have gone away from that since their advancements in motor design, apparently, have made that too conservative of a thumbrule (at least for the marketers). A look at the Celestion's impedance curve will tell you that is no slouch of motor. Not sure if it has demod ring(s) like the B&C, but its quite flat and non-inductive. A bit more research shows the FTR series having improved cooling mechanisms too.

In a nutshell, my guess is the 6mm of xmax on the Celestion is a fairly conservative rating.

Cheers,
Sam

Ok, so when we measure the 15TBX100 by the same method I get (25-12)/2 = 6.5mm and the 15PS100 has (21-11)/2 = 5mm. What PE must be reporting is what B&C refers to as XVAR which is determined by the driver's ability to reproduce a sine wave under a predetermined distortion level, yes?

So with the B&C drivers this allows for a 60% increase over xmax. Of course, I can't assume that the Celesition's will perform the same way, but even if they can achive half that at say 30% over xmax, then the xvar rating of the celestion will be similar to the lower priced B&C. It seems to me that it's probably reasonable to assume that the lower cost celestion will behave similarly to the 15PS100 and the higher cost celestion will behave similarly to the 15BTX100 with respect to excursion and power handling.

By that, I simply mean that, given that some people have been reasonably happy with the 15PS100 in the THAM-15, that I would probably be reasonably happy with the Celestion. Or, said differently, that any difference between the Celestion and the 15PS100 will be marginal at best?

I'm not looking for absolute answers here, just a seat of the pants comparison to decide whether the Celestion will perform adequately in the THAM-15. I can certainly send them back and get the 15PS100, or, spend twice as much and get the 15TBX100.

At the end of the day, we're talking about $300 and I've blown more than that in a single weekend just having a good time. I don't need the ultimate performance, I was just concerned that the driver I chose was severely inappropriate for the application. It sounds like I should have purchased the higher priced Celestion, and I might still do that, but I'm not sure if it's worth the hassle returning the lower cost drivers. The next model up also sims well and has a stated xmax of 8mm, assuming that xvar is greater, this is probably comparable to the 15BTX100.

I will be using these with a pair of JBL SR4722 tops (not SRX). They are rated at 600 watts but, IIRC, they really only take about 300 or so. I was planning on driving the subs in 4 ohm bridged mono with about 2kw.

tnx,
gs
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Old 26th March 2013, 09:37 PM   #7
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Originally Posted by ghettosynth View Post

I'm not looking for absolute answers here, just a seat of the pants comparison to decide whether the Celestion will perform adequately in the THAM-15. I can certainly send them back and get the 15PS100, or, spend twice as much and get the 15TBX100.

The next model up also sims well and has a stated xmax of 8mm, assuming that xvar is greater, this is probably comparable to the 15BTX100.
You can look through B&Cs specs, you will find that Xvar is not some linear relationship to Xmax, it varies with the magnet structure and coil design. Some have hardly any difference, some quite a bit.

You can't assume Xvar specs on other unmeasured drivers. Well, you can, but they would only be guesses.

I have no experience with Celestion (other than hearing Marshall stacks ) but my experience in comparing Eminence to B&C is the B&C at Xvar have less distortion than the Eminence at Xmax.

If that is true with the Celestion, paying double for nearly double (6 to 11) the usable excursion is worth it, if you want the maximum LF potential from a small cabinet.
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Old 26th March 2013, 09:48 PM   #8
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Originally Posted by weltersys View Post
You can look through B&Cs specs, you will find that Xvar is not some linear relationship to Xmax, it varies with the magnet structure and coil design. Some have hardly any difference, some quite a bit.

You can't assume Xvar specs on other unmeasured drivers. Well, you can, but they would only be guesses.
I get that, which is why I'm asking questions here.

Quote:
I have no experience with Celestion (other than hearing Marshall stacks ) but my experience in comparing Eminence to B&C is the B&C at Xvar have less distortion than the Eminence at Xmax.
Ok, good to know.

Quote:
If that is true with the Celestion, paying double for nearly double (6 to 11) the usable excursion is worth it, if you want the maximum LF potential from a small cabinet.
Right. Are these Celestion pro drivers new? I'm finding very little information about them on the web?
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Old 26th March 2013, 10:20 PM   #9
stinems is offline stinems  United States
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If I had to guess I think PE misread the datasheet for the Celestion and transcribed the 10mm gap depth listed right below the 6mm xmax.

You're right, I think the 4080HDX is intended for subwoofer duty more than the 4080FD which, given its upper end response and lower moving mass, is more of a midbass with improved bass capabilities.

Architecturally, I'd say the 4080FD is at least on par with the 15PS100 though. Doesn't have triple roll surround, but it has a funky double roll ('flexirol'). Both have a double spider, 4" copper voicecoil on a glass former. I'd bet the 4080FD has an edge in motor cooling and inductance linearity over the 15PS100.

If I were you I'd give the Celestion a try. How is PE's return policy on slightly used driver, or one that didn't live up to your expectations? Would they at least refund towards the B&C if the Celestion doesn't work out?

By the way, from B&C's website:
Evolution is a process that affects not only products, but also their technical specifications. Constant advances in research provide more and more precise methods to measure the performance of loudspeakers, and describe their features. Thiele – Small parameters have become the universal language for describing loudspeaker behavior in the small signal domain. Nevertheless, they comment little on the working limits of loudspeakers in the large signal regime.

These limits are customarily indicated by Xmax, the maximum linear excursion. This value is typically measured according to the AES2-1984 standard, corresponding to a maximum of 10% total harmonic distortion (THD) with a sinusoidal signal (though most manufacturers, including B&C, now typically provide data for Linear Mathematical Xmax, not measured Xmax). Recent research shows that this method can yield ambiguous results, and even different numerical values for the same loudspeaker. The main limit of this measurement is that it looks at the output signal instead of the physical features of the driver itself. On the contrary, the most up-to-date instruments for distortion analysis can measure the variations in loudspeaker parameters when they are fed with high-level signals. In this way, an excursion limit can be fixed, beyond which the parameter’s variation becomes excessive.

The “X var” value reported in our data (generally after the traditional “Xmax” value) is measured this way. Beyond this excursion limit, the magnetic field seen by the voice coil, or the total suspension compliance, or both, drops to less than 50% of their small signal value, producing high distortion levels, strong variations from small signal behavior and power compression. The new technique yields different results from the standard measurement based on THD. B&C Speakers believes that this added information gives a more accurate and reliable description on loudspeakers behavior in actual operating conditions.


Cheers,

Sam
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Old 26th March 2013, 11:24 PM   #10
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The “X Var” is only by B&C where everyone else will state xmax and/or xmech. This means using the “X Var” by B&C will not enable you to make a fair comparison amongst other drivers since no other manufacture other than B&C offers “X Var.”

Historically Celestion offered mathematical xmax on there 15 & 18-inch drivers. Mathematical xmax is not popular anymore due to marketing hype. I do recall in the mid 1990’s feeding each Celestion 18-inch driver 600 – 800 watts (and sometimes 1000 watts) which offered a 3.25 mm xmax with no issues of mechanical damage.

I actually still own 6-8 of those old Celestions today, retired, sitting in cardboard boxes.

The choice is really up to you if you know you are going to drive the loudspeaker with no remorse why paying for an extra 3mm is worth it. It is usually better to just use a pair of bins than trying to extract every ounce of SPL (which will guarantee distortion) from one bass bin. Bear in mind the majority lose drivers due to electrical damage (burned up voice coils) than mechanical damage (exceeding the xmechanical limit).
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Last edited by OMNIFEX; 26th March 2013 at 11:31 PM.
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