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Hi there,

I was looking at the drivers in my IB last night, and I noticed that they actually don't use much of their lowly available 12mm XMax, even at a kind-of loud volume. As some of you know, I'm using four of the Mach5 MJ-18 drivers. I started me thinking -- is more always better? That is, is there a sweet spot at which one might want to run their drivers? I'll make a couple bad analogies here -- my car doesn't really have any performance at 100 RPM. It needs to get up to 5,000 or 6,000 RPM. And my all-tube guitar amps don't sound their best at low volumes; I want to get it about half way up to get that sound. (Remember, shooting down my admittedly bad analogies isn't the point of the thread -- I'm just thinking that some things have different performance characteristics when operated at different levels)

Here's another goofy example -- what if I had 100 drivers in an IB, enough power to run them, and EQ'd to flat or whatever house curve was desired. Would the increased number of drivers give me some benefit over the four that I currently use. Currently, the four that I use provide more than enough bass, so I'm not looking for more, just looking for an angle to get better. And, actually, I'm not complaining about my bass in general, just thinking. I do see that 100 drivers would give me more overhead. (And yeah, I know I could buy better drivers, I could EQ differently, etc.).

Now, I don't want to run my drivers to such a point that I have increased distortion, or to the point of some other non-linear problem. I'm just wondering if there's a sweet spot at which a driver should operate...
 

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I don't know about 100, but yeah, more drivers is better.

Since there's more drivers, each individual driver doesn't have to work as hard to produce the same overall sound.
Since they don't have to work as hard, there should be less distortion.

One rule of thumb I've heard is that you can go down an octave by quadrupling the number of drivers. For example, if 1 driver can go down to 160hz, 4 drivers can go down to 80hz, 16drivers can go down to 40hz, 64 drivers can go down to 20hz, 256 drivers can go down to 10hz, etc...

And of course, it's gonna be LOUD. I forget how many dB's you get for the coupling effect.

JCD
 

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One rule of thumb I've heard is that you can go down an octave by quadrupling the number of drivers. For example, if 1 driver can go down to 160hz, 4 drivers can go down to 80hz, 16drivers can go down to 40hz, 64 drivers can go down to 20hz, 256 drivers can go down to 10hz, etc...
I'm having a bit of trouble getting my head around that. I can't see how increasing the number drivers would change the frequency response, eg if one driver is 100db at 20hz and 80db at 10hz wouldn't 2 drivers give 103db at 20hz and 83db at 80hz, the 10hz is louder but its still relative to the 20hz level. I suppose you cuold then use EQ to bring down the 20hz level as there is now extra headroom available.

I use 2 symetrically placed identical subs in my system and I have used REW to graph 1 sub vs 2 subs and the extension does not change, only the headroom.

Harry.
 

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yeah, I'm kinda with Hakka on that point. The analogy I use is that one tenor can sing a certain range, but twenty tenors singing doesn't suddenly mean they can now sing the bass repetoire!!( or however it's spelt).

I did read a thread on the cult which covered this point, and from memory it was something like this. By having multiple drivers, able to play with increased SPL's, it means that a frequency ( say 15 hz) that is always reproduced by a single driver but at a reduced SPL will be 'raised out of the noise floor' so to speak by the sheer fact of their being multiple drivers. So, by default, the frequency response has 'been extended'.

I get the question Otto, and it's an interesting one. My gut feel is that less is better, yet I have a personal example that may indicate otherwise.

My system at has PHL drivers both in the bass and mid. The heritage of these drivers is professional, and as a result they like to be worked hard. At least that is what I was told by the guy who designed them, namely that they need a bit of stick to get them singing. I , from personal listening, tend to agree which is kinda lucky cause I'm a bit of a loud music type of guy. Would have been dissappointed if I listened real quiet like!!

So, from that angle maybe you do have a point??
 

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Discussion Starter #5
Hi guys,

I think you guys are right about the concept of extension -- more drivers means you're going to have more output at all levels the driver is capable of. If you only get a small output level at 10 Hz, but you pile on a bunch of drivers, you can EQ the higher frequencies down to be compatable with that level you desire at 10 Hz.

It's an interesting topic, though, and I intend to disable two or three of my drivers and see what happens. It probably won't happen tonight, but within the next week or two perhaps. Yeah, I know, I still need to do a double-blind amp test! Don't worry, I didn't forget...
 

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More drivers means you can go lower because dB at a given frequency is proportional to the volume of air you can displace. If you don't use an EQ, you still get the same roll off as before which would still mean the same F3 regardless of number of drivers. But once you're boosting the low frequencies or cutting the high frequencies, you can change the F3. That's the point of a Linkwitz-Transform. I'm using ~2400W at 5Hz, but only ~1W at 100Hz.

So, JCD's statement that it'll be loud isn't entirely true. If you have 1000 drivers powered by a gajillion watts of power, theoretically you can reach say 200dB at 100Hz. But you'd never actually ever run it that loud. However, if you tried to calibrate your level at 5Hz to 115dB with an F3 of somewhere around 30Hz, then yes, suddenly 100Hz would come out at 200dB and kill you.

Of course, there's a point at which you aren't getting any benefit because you simply don't need the dB level that could theoretically be provided by the drivers, and the drivers are no longer being pushed to the point where distortion is introduced. And thus, you're just spending money on drivers and power that you aren't going to be utilizing as well as you could.

If your four 18" drivers aren't being pushed close to their xmax, even at 115dB at your listening position, then there's a chance you could put in a Linkwitz-Transform and increase your extension. You'd have to model it to see what you could do without destroying your drivers though.
 

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Discussion Starter #7
Thanks Josuah. I may be interested in learning more about that... First, though, how would I "put in" a L-T transform?

I did disconnect two of my drivers last night, and bumped the gain up a bit on the amp to compensate (about 3dB). I didn't get to play it at any real volume, but my medium-level listening of five or seven songs indicated similar results to what I'm used to. At those levels and with that material, I'm sure even two of the drivers are just barely moving.

I'll keep playing with it...
 

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The Marchand BASSIS is a high-quality adjustable L-T transform, but it is designed to work with multiples of your existing roll off, and might not end up giving you what you want. At (I can't remember)'s suggestion I am using a Behringer DCX2496 (I believe the FBQ2496 works as well) to create a "fake" L-T transform by using a combination of shelf filters to boost <20Hz and PEQ filters to adjust the curve. It's not as good as a real L-T transform because of the 20Hz center frequency limit of the Behringer units. But it's cheaper than the BASSIS and possibly more flexible.

You can also wire a L-T transform circuit up yourself to achieve the specific effect you need in your room, but you'd have to wire up something else if you moved the sub or changed rooms or changed subs.

Stick in some ear plugs and see if you can run 115dB at your listening position of some frequency that is above your roll off. That'd be reference level. But you have to be careful. If your drivers or the amp aren't capable of this, you're going to be causing damage. Especially if you are running a sustained 115dB sine wave. So you'd have to start low and move up. You may not care about reaching 115dB anyway, if you are usually listening to movies at -10dB or more below reference, which many people are.

Even with ear plugs if you do this too long and you'll damage your ears.

Uncompressed music is a little different, but assuming you're not running the quiet passages at 85dB, and instead more like around 55dB or 65dB, you should be fine with that music if you're fine at reference movie levels.
 

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Just wanted to jump in and state that I don't know how the whole "quadruple drivers/1 lower octave" thing works other than sort of what Joshua was saying -- someone I know, who's good at this stuff, made this comment when were discussing single driver line arrays. I may have missed the specifics, but I'm pretty sure I got the "rule of thumb" part.

I tried to do a quick web search and didn't find anything on point -- If I do, I'll forward it.

JCD
 

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The rule is that it takes 4x the power to go 1 octave lower at the same SPL as the reference frequency. In other words, if you can do 100dB at 40Hz with 100 watts, it will take 400 watts to do 100dB at 20Hz.
 
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