Multiple drivers vs. single driver sub

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AJ34

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Joined 2012
I’d really appreciate your views on multiple drivers vs. single driver subs.

Is it true to say that a multiple driver sub (say a dozen 12” drivers) will give a higher volume at the listening position that a single driver sub assuming:

The same total volume of air is being displaced
All drivers are identical
All drivers are working within their linear displacement range
No appreciable driver voice coil heating
Sealed box cabinets

I’m thinking that a single 12” driver may move 2” peak-to-peak – that’s going to create a lot of air turbulence at the driver’s cone. The sound signal will be of the form High Pressure/High Velocity (HP/HV) and needs to propagate out into the listening space and to the listener’s ears where it will be (and needs to be) Low Pressure/Low Velocity (LP/LV), relatively speaking.

Multiple drivers will generate a fairly LP/LV signal to start with so the necessary transition is less severe. There’s an analogy with horn loading, where the horn ‘manages’ the transition from HP/HV at the driver’s cone, to relatively LP/LV at the open mouth of the horn. Properly designed horns don’t add extra energy to the sound signal - they increase efficiency by effectively managing the HP/HV to LP/LV transition.

Anyone know of any theoretical papers or reports on this subject?
 
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AJ34

Member
Joined 2012
Interesting comments. What you say sounds sensible to me - a larger cone area moves more air which means there's more air to impede the cone's movement, i.e. higher impedence.

I was looking at this from a different angle, I think.

I can't see how large cone/diaphram excursions is an efficient way of producing low bass frequencies simply because there's massive air turbulence in the vicinity of the cone which, I imagine, would result in all sorts of eddies with subsequent energy wastage. Ideally, one would wish to minimise these eddies by smaller, slower excursions which could be achieved with greater radiating area, i.e multiple drive units or one enormous driver!
 
richie00boy

richie00boy

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Joined 2003
readresearch.co.uk
AJ34 said:
a larger cone area moves more air which means there's more air to impede the cone's movement, i.e. higher impedence.
Click to expand...

That's not quite the right way of explaining it. Because there is more cone area with the same force behind it, there is less of a mismatch of acoustic impedance. Think about how a horn tries to gradually expand from a small cone to the open air. This is the same kind of principle of using a big driver.
 
weltersys

weltersys

Member
Joined 2008
AJ34 said:
Is it true to say that a multiple driver sub (say a dozen 12” drivers) will give a higher volume at the listening position that a single driver sub assuming:

The same total volume of air is being displaced
All drivers are identical
All drivers are working within their linear displacement range
No appreciable driver voice coil heating
Sealed box cabinets
Click to expand...
A dozed 12" drivers would have an Sd of about 936 square inches.
A single driver of 34.5" (about 37" nominally) effective cone area would also have an Sd of 936 square inches.

Using your assumptions, the output would be identical if the 12 12" were arranged in a tight square format.

The 12 12" could be arranged in a vertical or horizontal line, which would reduce vertical or horizontal dispersion, which would result in more forward gain than the single 37" driver.

In reality, getting enough Bl and cone strength for a driver of 37" is impractical, which is why the upper size limit for high power long Xmax drivers is around 21-24 inches.

Art
 
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AJ34

Member
Joined 2012
Thanks for your replies.

richie00boy said:
That's not quite the right way of explaining it. Because there is more cone area with the same force behind it, there is less of a mismatch of acoustic impedance. Think about how a horn tries to gradually expand from a small cone to the open air. This is the same kind of principle of using a big driver.
Click to expand...

Understood, thanks.

I always try to use at least 2 woofers and mount them push-push to gain the advantage of active reaction cancelation
Click to expand...

Exactly what I'm thinking of doing. I could even mount each driver compliantly on the cabinet and brace the drivers securely back-to-back to further minimise transfer of driver vibration into the cabinet.

In reality, getting enough Bl and cone strength for a driver of 37" is impractical, which is why the upper size limit for high power long Xmax drivers is around 21-24 inches.
Click to expand...

Quite agree but my intended comparison wasn't between one massive driver vs. multiple smaller ones (with the same total radiating area). I've re-read my initial post and can see that I've not been at all clear on explaining my thoughts, apologies for that - it's something I'm working on! Please allow me to try again...

Take thirteen high excursion 12" drivers. Use one of these drivers to consruct one (single driver) sub and the remaining twelve to construct a multiple array sub.

My theoretical comparison is between these two subs. Now, set the subs up so that the same volume of air is being moved by both. So, imagine the single 12" driver sub is reproducing a 40Hz sine wave and the cone is moving 2" peak-to-peak. Now, compare the output of this with the multiple array of twelve, 12" drivers each cone moving 1/6" (one-sixth of an inch) peak-to-peak - thus moving the same total volume of air as the single driver. Which system, if any, would sound louder at the listening position assuming all drivers are working within linear range and no voice coil heating?

I have a hunch (that's all it is) that the multiple unit sub would sound louder because less of the drivers energy would be wasted in disturbing the surrounding air (eddies, vortices etc). I liken this effect to that of a horn loaded driver where the horn controls the transition from a small area of air with relatively large displacement (i.e. at the driver's cone) to the open mouth of the horn where a large area of air moves with small displacement and is thus efficiently 'matched' to the air in the room.

We may not like horn subs for other reasons but there's no doubting the efficiency of a well-desigend horn. By designing a sub that has massive radiating area but keeping cone excursions low (rather than going for a smaller area with high cone excursions), we're approximating the open mouth of a horn. In fact, practical considerations aside, the more cone area, the better. Do you think?
 
weltersys

weltersys

Member
Joined 2008
AJ34 said:
Take thirteen high excursion 12" drivers. Use one of these drivers to consruct one (single driver) sub and the remaining twelve to construct a multiple array sub.

My theoretical comparison is between these two subs. Now, set the subs up so that the same volume of air is being moved by both. So, imagine the single 12" driver sub is reproducing a 40Hz sine wave and the cone is moving 2" peak-to-peak. Now, compare the output of this with the multiple array of twelve, 12" drivers each cone moving 1/6" (one-sixth of an inch) peak-to-peak - thus moving the same total volume of air as the single driver. Which system, if any, would sound louder at the listening position assuming all drivers are working within linear range and no voice coil heating?
I have a hunch (that's all it is) that the multiple unit sub would sound louder because less of the drivers energy would be wasted in disturbing the surrounding air (eddies, vortices etc).

In fact, practical considerations aside, the more cone area, the better. Do you think?
Click to expand...
I have done the type of comparison you propose. Even though a driver may have a 2" peak to peak linear travel (about 25mm Xmax) it will be at about 10% distortion at that excursion, while the dozen drivers will be at some fraction of 1% THD.

Distortion makes a speaker sound louder, the single speaker will sound louder than the dozen.

As far as eddies, vortices etc., they are not a problem in a direct radiator, to make low frequency sound you have to "move air". Turbulence is a problem with ported speakers, though large ports with flared ends minimize port compression to the point where other problems are generally more in the forefront.

I would agree that practical considerations aside, the more cone area, the better. Every time cone area is doubled, you gain 3 dB, with a dozen cones only 1/10th the power would be needed divided over the dozen cones compared to the single.

In other words, if the single took 1000 watts to reach Xmax and put out "X" dB SPL, the dozen would use only 100 watts to put out the same SPL, at 8.33 watts apiece distortion and thermal compression are not an issue at all.

If you have an attic or crawl space to use, the other issue (a box with 12 times the volume) is not an issue either.

That said, I find a pair of moderate excursion 12" in a 4 cubic foot sealed box adequate for my living room. A single ported 10" with a series resistor covers the kitchen/dining room, and a pair of 4mm Xmax 10" in a 11.5 cubic foot tapped horn give the same SPL mounted in the center of the wall in my backyard. All the subs are driven off the sub output of a Technics receiver.

Art
 
planet10

planet10

frugal-phile™
Joined 2001
planet10-hifi.com
Paid Member
AJ34 said:
Exactly what I'm thinking of doing. I could even mount each driver compliantly on the cabinet and brace the drivers securely back-to-back to further minimise transfer of driver vibration into the cabinet.
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I would not compliantly mount the woofers. It would reduce the push-push coupling (in most cases) leaving energy in the basket reducing the system's DDR.

dave
 
E

Etocynned

Member
Joined 2011
There is a review of the TRW17 rotary woofer here:

Home Page

It speaks at length on the very concept of subwoofer output limitations due to impedance matching of the driver to the air.

I've read the review over and over again and each time learned something new.

I'm a big fan of Tapped Horns and their cousins Tapped Tuned Quarter Wave Tapers(correct name?) Because of the concept of impedance matching.

When i can i will buy and install 2 TRW-17's in my listening space to augment whatever system is running.
 
weltersys

weltersys

Member
Joined 2008
planet10 said:
The cone moves. The basket has an equal and opposite reaction. This is all loaded into the box. If you have 2 tightly coupled push-push woofers most of the reactionary energy is canceled and you dramatically reduce the energy input into the box.

dave
Click to expand...
The dramatic reduction of energy input to the "box" is especially helpful when the box is part of an infinite baffle installed the floor or ceiling, where the mechanical transmission can show up as rattling parts of the house.
 
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