How about this (cough) (cough) radioshack driver .

[bakalorz]

So, I realize that Radio shacks drivers are typically not the best ... but, I had 3 of them from a defunct car stereo system, and also a bandpass box in a home theater.

Just around the same time I started reading about IB, I happened to get a hold of 4 more very cheaply.
1 more, and I would be able to do an 8 driver IB

so is there anything that would make these in particular not very good in an IB setup.

(I realise that the xmax is so-so, but, for the price, I can get 8 drivers, I would only have been able to get 2 of the parts express 15" IB driver)

I believe the power handling will be pretty limited (only about 30 to 40 watts/driver, but hopefully 8 of them will make up for that ...)

the particular driver is part # 40-1019
which is a 12 inch driver
and the specs are:
Frequency Range ... 31-150 Hz
Resonant Frequency ... 31.3 Hz
Power Handling ... 200 Watts RMS/400 Watts Max
Nominal Impedance ... 4 Ohms
Piston Area ... 82.5 In2, .0532 M2
Magnet Weight ... 28 oz
Flux Density ... 11.52 Tm
DC Voice Coil Resistance .. 3.41 Ohms
Voice Coil Inductance ... 2.2 mH
Total Q Factor ... .57 Qts
Electrical Q Factor ... .60 Qes
Mechanical Q Factor ... 13.97 Qms
Equivalent Acoustic Volume ... 3.09 Ft3
Mechanical Suspension Compliance ... .220 mm/N
Mechanical Mass of Cone Assembly and Free Air Load ... 118.0 g
Mechanical Mass of Cone Assembly only ... 110.9 g
Peak to Peak (maximum) Linear Excursion ... .7187 In
Sensitivity . 89.0 dB (1 Watt @ 1 Meter)
Cone 12 inches, Wet Look Treated Fiber
Voice Coil ... 1.5 inches, Kapton MTB Former
Surround (material of construction) ... Polyether Foam

So any gotcha's to using 8 of these in an IB

Martin

[ThomasW]

It's not who makes the driver it's the specifics. With that low Xmax and the fact that they're 12"s, you'll have dead drivers after the first big transient unless you have a couple dozen of them.

[ryans]

Looks like they'll displace a little over a liter each. So 8 of them puts you at about 8.5 liters. That's a little on the low side ... but not by too much. There's been several posts from guys with 2 Dayton IB15's who have been happy. 8 of these would give you a considerable advantage over a 2 IB15 setup. The Fs is actually half an octave higher than the IB15, so I think you'd have some protection against very low frequency transients.

2 dozen of these would give ya 25 liters!!! That's like 4 Avalanche 18's.

[dewardh]

The higher Fs does not give any "protection" at all (that would apply only in a sealed box). In fact it makes things worse . . . the speaker's impedance is at its highest at resonance, which does give some (slight) protection. Below resonance impedance falls again, and the amplifier is able to deliver more power to a less loaded cone. The proposed installation *must* have a properly spec'ed high pass (rumble) filter. I suggest examining the cone excursion plot in WinISP and considering the implication of full amplifier power trying to move the cone 5-10 times Xmax against no resistance.

Which is not to say that those drivers cannot produce satisfying bass in an IB, given the ample volume displacement (which is, ultimately, the thing which determines how much noise they can make), but *they must be protected from overexcursion*. Thomas makes the not unreasonable assumption that not everyone will understand what it takes or how to do that, and recommends overcapacity to be on the safe side. It makes sense . . .

[ryans]

Hmm ... some confusion here, maybe? Higher Fs means that response will start rolling off (on the low end) at a higher frequency. So, holding everything else constant (if you could actually do that), a woofer with a higher Fs will produce less output on the very low end and likewise have lower excursion on the very low end.

Woofer excursion is not directly related to impedence. Any of the simulators I've seen treat the input source as a near-perfect voltage source (not a current source), so the effects of the impedence curve are taken into account in the simulation and I don't see any correlation between impedence and woofer excursion.

I ran the simulation and it shows 8 of these woofers can handle a total of 400 watts all the way down to 11Hz (with no rumble filter). (Edit: should be 250 watts down to 16Hz, no filter).

"I suggest examining the cone excursion plot in WinISP and considering the implication of full amplifier power trying to move the cone 5-10 times Xmax against no resistance."

Huh? We don't know what "full amplifier power" is. For a 400 watt amp (Edit: should be 250 watt amp), it'd be just fine. Not 5-10 times Xmax. And the woofers do have resistance even in IB enclosures. Take the reciprocal of compliance and you get the resistance (mechanical, not electrical). You can easily see this by pushing on the woofer and feeling it push back.

I agree that gross overkill is one way to guarantee success. But its not the only way. 8 of these appear to have as much displacement as 3 Dayton IB15's and you'd want 200 watts to take full advantage of it. Its not the sort of setup that we'd go recommending to people starting from scratch, but this guy's already got 7 of 8 woofers. The additional cost to buy 1 woofer and build a manifold is pretty cheap. Still gotta get an amp, but maybe he can find or borrow an old receiver to try it out.

I don't see anything that makes this a definite "NO", so why the discouragement? Wouldn't you rather see guys who are already this close (in terms of having equipment on hand) give this a try? The risk is low and we stand to learn something from trying something new.

So, bakalorz, I say try it. Worst case is you decide to upgrade the woofers later. You could plan ahead for that and make enough room in your manifold to go to 4 or 8 15-inch woofers later on.

[dewardh]

A couple of things. I'm certainly not trying to discourage anyone . . . the price is right, and the result will certainly be better than not. I only suggest caution regarding overexcursion (and yea, with those drivers 5-10 time was a bit of an exageration).

But the higher Fs *does not* cause lower excursion at lower frequencies. And speaker impedance at resonance (and below) *does* effect the power delivered to the driver. At resonance the speaker impedance rises. So, with a constant voltage amplifier current to the speaker falls, and power falls as as a result. Below resonance impedance falls again, and thus the power actually deilvered to the speaker rises. Since the only thing restraining the cone is the suspension . . . the acoustic load is trivial . . . most of the amplifier power goes to trying to tear the suspension apart. This is where IB differs from a sealed box . . . in a box that power goes to compressing air and heating the voice coil.

When I simulate these drivers with WinISP pro I see it reaching Xmax at 27 Hz (with a 400 watt amp), but probably safe, depending on how they're wired, in all but the worst case ELF scenario with a 200, assuming that the array is wired for 8 ohm (I should note that I do not completely trust this part of the WinISP sim, since they don't, for example, when you specify 2 speakers, ask whether series or parallel . . . and it does matter). That would produce a quite exciting SPL (105-110 dB) at anything above 15-20 Hz, far better than any multi thousand dollar "commercial" subwoofer, but also not a match for some of the more extreme systems discussed here. More than enough from my perspective . . . I'm building mine with only 4 twelves, although with half again more Xmax and a Fs of 17 Hz. But I'm not looking to flex the windows or crack the plaster, and my main speakers won't play over about 105 dB, so subs that could get louder wouldn't anyway in real use.

Sure . . . go ahead and build em. Nothing to lose, eh? Seriously . . . it will sound very good. But humor a perhaps overcautious old geezer . . . put a high-pass on it, just to be safe . . .

[chrisbee]

dewardH

You mention having 105dB speakers and not needing to exceed that level with the subwoofer... but doesn't one need considerably more dynamic headroom from a subwoofer than from the speakers?

With film DVDs in particular there is an increasing tendency to make sub-wrecking LFE tracks. With ever-lower frequencies and ever-higher potential SPLs. Isn't most of the energy in the sound track going into the bass and sub-bass?

[ryans]

Oops, yeah dewardh, you're right. I forgot to increase box size beyond the default Q=0.707 box in my simulation. So 8 woofers can take 250 watts to about 16Hz.

I don't disagree that impedence rise affects the power input to a woofer. But input power is not the sole determiner of cone excursion. If it was, wouldn't you get very poor response at Fs and higher excursion above that?

Most of the amp power turns into heat, regardless of the enclosure. That's why a woofer's efficiency term is so low. A small fraction of the remaining power goes to compressing air in most sealed boxes. You can pretty easily calculate the pressure change for a given box size and woofer from max + excursion to max - excursion and its just not very much in the vast majority of cases. For instance, an old Adire Shiva displaces about +/- 0.8L. But its usually put in a sealed box of 40L or more. PV = nRT. In this case the change in Volume is +/-0.8L out of 40L = 2% change.

I still say that (all else constant) a higher Fs gives you less output at low frequencies and less output means less cone excursion.

WinISD doesn't need to know how the woofers are wired. It does require that they're wired to achieve equal power distribution across all woofers. But if that's the case, then you can figure how much each woofer gets. 200 Watts and 8 woofers - 25 watts each. Then it can figure what the voltage across each woofer is based on the nominal impedence. It doesn't care if you got to that power level by putting all 8 in series and a whole lotta volts, or all in parallel with 1/8th the voltage rail.

[dewardh]

chris:

Pretty much all my source material is digital (CD, DVD, sat), so it is by definition peak limited. I equalize for flat (as flat as possible) in room response. So if I set levels such that 0 dBFS (that's all the bits on and the red led flashing) gives me 105 dB from my mains where do I get anything over 105 dB from the subs? I might want to run the sub a few dB "hot" just for effects, but still . . . (and room gain gives me that, anyway).

ryans:

When WinISP asks for "system input power" under "signal" I assume (perhaps wrongly) that they are calculating voltage into 8 ohm. At least that's what they appear to do when they then calculate "amplifier apparent power load". If that is the case then series (double the load impedance) or parallel (half the load impedance) should impact the actual power that the defined amp can deliver, and selecting for 2 drivers should give you a choice . . .

Very little of the input power is actually converted to sound. At higher frequencies most of the power goes into accelerating the cone, at lower frequency into stretching the suspension (plus heat in both cases). Different failure modes result . . .

[ryans]

"I assume (perhaps wrongly) that they are calculating voltage into 8 ohm."

This is from the WinISD Pro documentation:
"Cone excursion shows how much driver cone moves with sinusoidal excitation at chosen powerlevel. The powerlevel is controlled in "plot"-tab. The power applied can be related to excitation voltage with following relation: Eg=sqrt(P·Re), or P=Eg²/Re where Eg is the RMS voltage applied to driver's terminals, P is the input power in watts and Re is the DC resistance of the voice coil. "

So they're calculating voltage across Re.

If you experiment with 1 woofer vs 2 woofers (with 2 woofers in a box 2X the size), you'll see that cone excursion is the same when the power to the 2 woofer project is 2X the power to the 1 woofer project. So the simulator just divides the power by the number of woofers and assigns that much power to each woofer. The voltage drop across each can be determined by the above equations and it doesn't matter if you get there by large total voltage and woofers in series or smaller total voltage and woofers in parallel.

[dewardh]

> it doesn't matter if you get there by large total voltage and woofers in series or smaller total voltage and woofers in parallel

It doesn't matter in the simulation, but it does matter in the real world. Which is, in part, why I don't completely trust the WinISD simulation.

With real world amolifiers you cannot just arbitrarily increase amplifier voltage . . . it is limited by the power supply rails. So if I specify an amplifier power (into the nominal 8 ohm load) what I am telling you is the amplifier's maximum output voltage. You can get more power if you lower the load impedance (by connecting speakers in parallel), assuming the amplifier can deliver the additional current. You can *not* get more power if you connect the same drivers in series . . . you get less. So a *real* amplifier driven to its maximum output will cause a different excursion in the drivers, depending on how they are wired. That's the problem with a simulation that makes its own sometimes oversimplified assumptions . . . perhaps it can predict what will happen if its assumptions are true, but that doesn't tell you what's really going to happen.

WinISD will also draw a cone excursion curve that shows excursions 2 or 3 or 4 times Xmax. Not only does it seem not to know anything about suspension limits, it also does not seem to know that further excursion (and perhaps the life of the driver) stops when the voice coil is completely out of the gap.

Another way of putting all this is that while the WinISD simulation is useful in the "normal" range of boxes and behavior it fails at the limits . . . and in this case (maximum excursion below resonance in an infinite baffle) predicting behavior at the limit is exactly what we're trying to do.

I believe it is better in such situations to counsel caution. I also think that the subwoofer proposed in the original post of this thread will work, and work better than anything you can buy in a store for anywhere near the price, as long as not too much is expected of it in extreme low frequency extension and a little more than the usual care is taken to not overdrive it.

[ryans]

"but it does matter in the real world"

No, it doesn't. 10V RMS across the VC of a pair of woofers will give you a certain amount of excursion (at a known frequency) and it doesn't matter if they're getting the 10V in parallel (10V total) or in series (20V total) ... If they're each getting 10V RMS across the coil, the output is a given. RMS power will be 2*10^2/Re for the pair.

"With real world amolifiers you cannot just arbitrarily increase amplifier voltage "

I was speaking of using different amplifiers and impedences to achieve a certain power level. WinISD assumes that you've figured out what your final impedence will be and that you've correctly entered the max power you amplifier will output into that load.

"WinISD simulation is useful in the "normal" range of boxes and behavior it fails at the limits "

According to your example, it fails when you go _beyond_ the limit (beyond the suspension limit). When you're _at_ the limit, its still (relatively) accurate. So its still a useful tool for estimating performance at the limit.

Alright, I think I've blabbered on long enough on this topic (and a great cheer is heard from all the spectators )