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Author Topic: Peavey EU218 Subs  (Read 14006 times)

Phil Lewandowski

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Re: Peavey EU218 Subs
« Reply #20 on: December 23, 2010, 12:25:38 am »

Duncan McLennan wrote on Thu, 23 December 2010 00:00

Bennett Prescott wrote on Wed, 22 December 2010 23:09

I am very surprised that nobody has yet said anything in this thread about the method Peavey used to load those drivers in the cabinet.


I was thinking the same thing.


So my question is, what in the world is a "vented, isobaric chamber"?
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Bob Josjor

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Re: Peavey EU218 Subs
« Reply #21 on: December 23, 2010, 07:07:47 am »

Duncan McLennan wrote on Wed, 22 December 2010 22:00

Bennett Prescott wrote on Wed, 22 December 2010 23:09

I am very surprised that nobody has yet said anything in this thread about the method Peavey used to load those drivers in the cabinet.


I was thinking the same thing.


From Peavey's site: "a pair of 18" neodymium-loaded loudspeakers arranged in a vented, isobaric chamber."

I haven't dug deep into the cabs, but I did take the grill off of one of them.  A side view of the cabinet would look something like this:
[V]
The speakers are loaded into the cabinet on each side of the "v", facing each other.

Bad description and bad illustration. Very Happy

Someone more versed in sub design than myself can tell me the pro's and con's of such an arrangement.
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Duncan McLennan

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Re: Peavey EU218 Subs
« Reply #22 on: December 23, 2010, 01:11:35 pm »

That doesn't sound like an isobaric arrangement to me.
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Bob Josjor

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Re: Peavey EU218 Subs
« Reply #23 on: December 23, 2010, 02:46:02 pm »

I actually took another look and the arrangement is more like this the attachment below.

Again, poor illustration.  Think of the box seperated into three chambers.  The speakers are mounted to the center walls of the chamber, facing each other.
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John Roberts {JR}

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Re: Peavey EU218 Subs
« Reply #24 on: December 23, 2010, 03:14:55 pm »

Your crude image is too crude to inform me.

From the description "vented, isobaric chamber" sounds like a combination of both technologies. Isobaric uses a second active driver to modulate the rear chamber pressure (changing apparent volume) and thus the tuning of the box. The port likewise has an effect on the apparent volume and tuning of the back chamber.  I suspect it is dominated by each in different low frequency ranges.

It seems like an all things to all people design, more bass from a smaller lighter box. I notice this one uses Jon Risch's biased air flow direction port cooling (US pat # 6,549,637), so by definition two ports, one biased to push out warm air, the other to suck in cool air. A fully closed box would not exchange much air, so I don't know how important the ports are for box tuning, or just to assist power handling. Hopefully they are complementary and work together to improve both.  

JR

 
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Jay Barracato

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Re: Peavey EU218 Subs
« Reply #25 on: December 24, 2010, 03:18:02 pm »

Isobaric literally means "same pressure". Of course, I only know the term from my degree in meterology and have no idea of its importance to sub design.
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Jay Barracato
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John Roberts {JR}

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Re: Peavey EU218 Subs
« Reply #26 on: December 24, 2010, 03:36:14 pm »

Jay Barracato wrote on Fri, 24 December 2010 14:18

Isobaric literally means "same pressure". Of course, I only know the term from my degree in meterology and have no idea of its importance to sub design.



http://upload.wikimedia.org/wikipedia/en/thumb/a/ac/Isobaric_spk.PNG/210px-Isobaric_spk.PNG

My description doesn't agree well with wiki and in this case I would take their word for it.

http://en.wikipedia.org/wiki/Isobaric_speakers

JR
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Jay Barracato

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Re: Peavey EU218 Subs
« Reply #27 on: December 24, 2010, 03:43:56 pm »

From the drawing, it looks to be a constant volume situation, where the forward motion of one cone is matched by the second cone keeping the volume constant. Work could be described as delta(PV), which in this case should be close to zero. In other words I would guess that the rear cone allows the work done by the forward cone to be changed into sound in the forward direction, rather than being lost in changing pressure in the rear chamber.
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Jay Barracato
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Rory Buszka

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Re: Peavey EU218 Subs
« Reply #28 on: December 24, 2010, 10:56:30 pm »

Selling an Isobaric sub as a "dual-18 loaded enclosure" sounds like a classic case of the marketing cart pulling the engineering horse, and this could come back to bite Peavey in an unexpected way, because the way the EU-218 operates is quite different from an ordinary dual-18", with a different set of priorities from most compact subs like this one.

Isobaric loading of two drivers is done to couple the two cones together via a 'constant-pressure' volume of air. The volume of air between the drivers is small enough that it acts almost like a rigid mechanical coupling between the two drivers. This puts the motor structures of both drivers to work pushing and pulling the Vd of a single driver. The maximum amount of air that can be displaced is still the same as the amount that can be displaced by a single driver before exceeding its mechanical limits, but now the amount of force the motors working in tandem can bring to bear on the cone is roughly doubled. Both cones can either face forward, and the coupling chamber can be a separate partition built into the subwoofer's enclosure, or the coupling chamber can be formed by the air trapped between two woofer cones turned face-to-face with the frames bolted together at the gasket, with one of the woofers wired out of phase so both cones still move in the same direction, and the effect is about the same for each configuration. And I think I know exactly why this is being done in a pro audio sub. It has to do with how driver manufacturers operate, and the expectations that pro audio clientele have for subwoofer performance based (often incorrectly) on driver diameter.

A single 18" driver has slightly less than double the piston area of a pair of 15" drivers, which have slightly less than double the piston area of four 12" drivers, which have slightly less than double the piston area of eight 10" drivers (and so on for eights, sixes, fives...). It's common for driver manufacturers to build several cone diameters using the same motor structure for each of them. Eminence has a few standard 4" VC motors, a few standard 3" VC motors, a few standard 2.5" VC motors, and so on, and this reduces the number of parts they need to stock and the development effort for each custom driver they do. Using a custom motor for a particular driver pushes up the price of the driver significantly. Looking at B&C and Beyma's range, those manufacturers also build standard models having several different cone diameters on the same motor structure. B&C offers a 12TBX100, a 15TBX100, and an 18TBX100, all with 4" VCs on the same motor structure. However, as one steps up through the standard driver diameters (12", 15", 18"), the piston area that those motors need to act upon increases exponentially, which means that the same motor can generate higher pressures inside the enclosure when it is coupled to a smaller diameter cone. Driving a resonant system at lower frequencies to achieve the same magnitude as higher frequencies requires increased force, and reducing the force that must be applied means increasing the compliance (in this case, reducing the needed box pressure means making the enclosure bigger for a given tuning frequency). As you try to shrink the box for a given tuning frequency, you need a driver with a more and more powerful motor. That explains why the JBL MRX518S (an example with which I have some experience) begins to slump off with decreasing frequency starting at 60 Hz even though the tuning frequency and -3dB point are both right around 40 Hz for that particular box. The frequency response starts falling off prematurely because the driver's motor can't keep up the necessary amount of force needed to drive the small box at its tuning frequency. This is a tradeoff that's made in many, many of the pro-market subwoofer designs using eighteens, because most pro audio professionals (and many of the ones who may be purchasing 'prosumer' products) typically expect that the 'proper' diameter for a subwoofer driver is 18", and that smaller drivers (for whatever reason) just don't produce bass as well as an 18" driver does (even though the speaker designers among us know this isn't necessarily true), but at the same time, the Guitar Center prosumer market demands smaller and smaller subwoofers, but increasing the motor strength means using more ferrite (or neo) and more steel, and the motor is already the most expensive component of almost any pro audio driver.

To prevent the frequency response of a compact 18"-loaded subwoofer enclosure from dropping off with frequency, in light of the above, the designer needs to increase the motor strength. This can either be done by designing, tooling, and manufacturing a purpose-built motor to drive a large cone in a small box, or by using the isobaric principle to couple two standard drivers together to the same air load, effectively doubling the motor strength without the additional expense of designing, tooling, or building a beefier motor to produce in smaller volume. Both approaches result in a more expensive sub for the end user, but the isobaric approach may make more sense to the manufacturer.

Where I see this potentially biting Peavey in the behind is that the EU-218 is marketed as a dual-18" sub. Yes, there are two voice coils, and two motors (and thus the thermal dissipation capacity is doubled), but both motors are still acting on the piston area of a single 18" driver, so the sensitivity of the compound-loaded pair of drivers is the same as a single 18" driver, and the mechanical limits of the compound-loaded pair are also the same as a single 18" driver. A single EU-218's compound-loaded driver pair will need to go through twice the excursion that a typical dual-18's drivers would, in order to displace an equivalent air volume (and making bass is all about displacement). I'd expect the mechanical failure modes of these subs to be torn cones, surrounds, and spiders, instead of burned voice coils. The upshot to all this is that it looks like the isobaric technique is effective in extending the response of this compact sub into the 40-Hz range without the low-end 'slump' I've noticed in other compact subs. While this 'slump' can help the sub sound "tighter", the deeper, flatter response of the EU218 will sound fuller and more satisfying when reproducing kick drums and low-bass fundamentals. There have been a couple of subs to apply this technique (including the massive McCauley quad-21", the EONA 618 sub, and a couple models from EM Acoustics), but the Peavey sub just adds another option geared toward 'prosumers'. Also, when the two drivers are coupled together face-to-face (Isobaric Push-Pull), some mechanical and electromagnetic nonlinearities are canceled out, diminishing even-order harmonic distortion, but there is no effect on odd-order distortion, and the reduction in even-order distortion is of dubious benefit, with there being some debate about the audibility of reductions in this type of distortion.

Also, I should note that the 'isobaric' design principle is different from the 'clamshell' principle used on subs like the EAW SB1000 and Community TLF218. In the case of an 'isobaric' design, the drivers have a closed cavity between them. Early on, a high-profile product in the home market to use this principle bore the moniker "Isobarik", which has led to many people unknowingly misspelling the term in written conversation. The correct spelling is 'Isobaric', with a C at the end. The two are not interchangeable; one is a brand of Linn Products, Ltd., and the other is a generic term to describe the loading principle.
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Rory Buszka
(The Gearmonger)

If it works, but you don't know why it works, then you haven't done any engineering.
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