Single 18" LF Box vs Double

K

Kip Conner

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Mar 13, 2011
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Athens, GA
I'm getting ready to purchase new subs and wanted to get your real world opinion on an idea. The sub cabinets that were being used with these mid-hi boxes were Apogee AE12 Double 18's. They were getting about 1800 watts parallel in the box, but as you know- they are heavy for one person to move around. The height is great because of the ground stack, but that's about the best factor.

What I am considering doing is replacing the double 18 boxes with single 18 boxes (OAP LF118) with an updated driver and amp that make put them in the 3000w range (vs. the double 18 1800w box).

Removing the ground stack height from the equation, do you think it's a good move?

I've consulted the rep for the drivers and he thinks that the driver is a good fit for the VAS of the existing closure, but I'm just curios if anyone has made this move. The idea would be that the single boxes are easier to move and if on the off chance I blew a driver I wouldn't end up with a passive radiator in a double 18 cabinet.

I'm certain there are some advantages through coupling...
 
Re: Single 18" LF Box vs Double

  1. Watts don't mean anything. If you're specifying multiple boxes to get to a target SPL, you should be looking at that, not watts.
  2. Two single 18s vs a double 18 will have exactly the same output if the double box is exactly double - double the volume, double the port area, and double the watts (read: the same input voltage).
  3. From speaking with people that know more than me, like Phil Graham, one of the most important aspects of subwoofer design is the port area. Closely consider this in your calculation; it can be far more detrimental than the specs on paper.
  4. There is no 'coupling' either. When two identical speakers are used together and their outputs sum 100% constructively, there is a 6dB increase in output. This comes from 3dB as a sensitivity increase (twice as much cone) and 3dB from the power handling increase (double the power is now extracted per an input voltage). I think most people think that there is magic coupling when subs are used together because this guarantees the outputs to be 100% combined constructively. Using subs separate will yield the exact same output, but only where the outputs of both sub stacks are combining 100% constructively - which only happens in the 'power ally'.
 
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Re: Single 18" LF Box vs Double

Thanks Silas, I think that I am making the right decision with the singles. While I don't have a target SPL, I just need it to live up to the SPL of the Apogee's while giving me the option to create an end line fire system. In this case, the volume (as used by one driver) will be decreased a little, but the output voltage will be increased by double. I don't have EASE to calculate so I'm having to go on my gut and the forum experience...
 
Re: Single 18" LF Box vs Double

I dont care for single 18 reflex speakers (at least small to mid sized cabs) because of stacking and moving between truck/warehouse and dancefloor. it is much easier to use leverage to tilt up a double 18 than stack another on top of it. Not into stacking, how about loading a ramp or liftless truck? the box has to be lifted much less to get over the lip.

Between the truck and the dancefloor, it is much easier to control the pushing of double 18s than singles. Sure, you can strap singles together, and straps are necessary for so many gigs, anyway, but there are enough things which already take up time...
 
Re: Single 18" LF Box vs Double

Randy Pence said:
I dont care for single 18 reflex speakers (at least small to mid sized cabs) because of stacking and moving between truck/warehouse and dancefloor. it is much easier to use leverage to tilt up a double 18 than stack another on top of it. Not into stacking, how about loading a ramp or liftless truck? the box has to be lifted much less to get over the lip.
Click to expand...

This was going to be my comment - a cube is about the worst kind of box to wrassle by yourself.
 
Re: Single 18" LF Box vs Double

You could alway goes for a newer "lite" 2x18 with neo speakers. I have some that are just over 100 lbs. with the new Faitals the handle plenty of power and sound good too. Of course I also have a few single 18's for smaller gigs that only need one sub.
 
Re: Single 18" LF Box vs Double

Evan Kirkendall said:
I like single 18's for my personal rig. Easy to move around, and easy to scale up and down. Sounds as good as a dual 18. They can be a little cumbersome to move around, but single 18s aren't really heavy, right?


Evan
Click to expand...


It's not so much the weight, as it is the shape. A cube means you have to bend over and use your back muscles in a bad lifting position. Much easier to screw up a back, or just pull a muscle this way. 218's have the benefit of being long and big enough that you would never lift them, and you don't have too. You just slide that dolly under and walk to the truck.
 
Re: Single 18" LF Box vs Double

Silas Pradetto said:
There is no 'coupling' either. When two identical speakers are used together and their outputs sum 100% constructively, there is a 6dB increase in output. This comes from 3dB as a sensitivity increase (twice as much cone) and 3dB from the power handling increase (double the power is now extracted per an input voltage). I think most people think that there is magic coupling when subs are used together because this guarantees the outputs to be 100% combined constructively. Using subs separate will yield the exact same output, but only where the outputs of both sub stacks are combining 100% constructively - which only happens in the 'power ally'.
Click to expand...

The 3dB increase in efficiency is what people generally mean by "coupling", but it does depend on the positions of the sources. If you add a second identical source separated from the first in open space and drive it identically, you're putting an extra 3dB into the system and you get an extra 3dB out. As you say, this isn't spread evenly over space but is +6dB at some places and -lots dB at others, and this varies with frequency.

Now do nothing other than push the two sources together (relative to wavelength) and you get (ideally) +6dB everywhere. That's a net 3dB gain from the increase in radiation resistance caused by the sources being close together, aka "coupling". A single source against a solid boundary does the same thing. It's thus not really about cone area (same cone area in the two cases) but about radiation resistance.

This doesn't really affect the choice between single or double 18" boxes, however, as the same phenomenon occurs with both.

Nick
 
Re: Single 18" LF Box vs Double

Nick Hickman said:
The 3dB increase in efficiency is what people generally mean by "coupling", but it does depend on the positions of the sources. If you add a second identical source separated from the first in open space and drive it identically, you're putting an extra 3dB into the system and you get an extra 3dB out. As you say, this isn't spread evenly over space but is +6dB at some places and -lots dB at others, and this varies with frequency.

Now do nothing other than push the two sources together (relative to wavelength) and you get (ideally) +6dB everywhere. That's a net 3dB gain from the increase in radiation resistance caused by the sources being close together, aka "coupling". A single source against a solid boundary does the same thing. It's thus not really about cone area (same cone area in the two cases) but about radiation resistance.

This doesn't really affect the choice between single or double 18" boxes, however, as the same phenomenon occurs with both.

Nick
Click to expand...

What could be added, the 3dB increase works only, if the sources are small compared to the wave length.
Mr Danley explained this some years ago on the other place,
but it should be clear from Nicks excellent explanation, think self interference

Uwe
 
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Re: Single 18" LF Box vs Double

Hi Uwe,

Uwe Riemer said:
What could be added, the 3dB increase works only, if the sources are small compared to the wave length.
Click to expand...

Yep. With all things acoustic, it's a good idea to view the world in terms of wavelength. Something that's a major obstacle at 20kHz, for example, can be invisible to 20Hz. And what's "close" at 20Hz is miles away at 20kHz.

One corollary of this is that the efficiency gain for a particular pair of sources is greatest at low frequency and reduces as frequency increases. As frequency gets higher, not only does the gain decrease, but changing directivity patterns develop. FWIW, some time ago I tried putting numbers to this using a simple model of point sources intended to represent varying sizes of a grid of 18" drivers. (It's an inadequate model but, I think, instructive nonetheless.) I wrote:

The radiation pattern is three dimensional but, for simplicity, I'll just consider measurements in a ring in the horizontal plane around the subs. With two drivers together, we pick up almost 6dB everywhere at 50Hz but, by 100Hz, the gain to the sides (i.e. 90 degrees off axis) is down to under 5dB. Averaged around the subs, it's about 5.5dB. You could say the efficiency gain is already tapering off.

Now try an 8x4 array of drivers. On axis, we gain 30dB (five doublings from one driver). At 90 degrees, we gain about 23dB at 50Hz and 16dB at 100Hz. Averaged around the subs, we're up around 24dB at 100Hz, and we've introduced significant directivity.

As the array grows, the near field extends further, so you need to get further away before the directivity pattern settles down. Within the near field, of course, you'll get different results at different distances from the source. As the array gets bigger, if you're too close, the 6dB per doubling on axis (power alley) stops working because the path length difference to the centre and edges of the array becomes significant, i.e. the varying directions of arrival don't add coherently. With a 16x16 array at 100Hz, for example, the gain at 10m on axis (in the model) is about 35dB rather than the expected 48dB.
Click to expand...

Nick
 
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