JBL 4642A Dual (18 in) Subwoofer System (How many?)

[Thomas Baham]

Hi, I'm going to be a little vague. I have a 40'X70" Theater auditorium. I have one JBL 462A Dual 18" subs. I was told I should have two of these for the size of the room. Is this true, I have great base, but more might be better? Do I put it up front of rear of the auditorium? Thanks for the help

[Marcel Birgelen]

I guess you're referring to the JBL 4642A?

Purely judging on the size of the room, I'd say that a single one of those would be on the low end. But if the installer correctly measured and calibrated your theater, he should be able to tell you if the performance of a single unit is sufficient.

Keep in mind that for a second unit, you almost certainly will need extra amplifiers too.

In general, I like to put subwoofers as closely together as possible, at least as long as they're all fed from the same source. Spreading them across the room will often cause a lot of destructive interference.

[Thomas Baham]

Yes the JBL 4642A. I will purchase a second Amp to power it (No problem) . Just wanted to make sure its the right choice. I feel that it is adequate, but needs a little bit more ump. I was told before there should be a second sub, but wanted to make sure where to put it. I have room in front on the other side of the center speakers. (one on the left, new on will go to the right). I live in a small town, so having someone come out is very expensive, and not many people know about sound in auditoriums.

[Carsten Kurz]

In a large room like that (and I assume it is an older build?), you may actually get away with less LFEs than in smaller, modern auditoriums. Low frequency usually builds up nicely there because the length of the room fits the wavelength of the LFE. However, adding another one may still be beneficial. I think a very solid LFE section is a good way to impress even sensitive older audiences, because it sounds big, without yelling at peoples ears with an otherwise underperforming setup.

In general, there are two preferred ways to place dual LFE speakers - immediately adjacent to each other slightly off center and tightly to the back wall, or tight into the left and right front corner, if there is enough space there. You may try both ways and decide yourself. Usually, not much work to try both placements.

- Carsten

[Jarod Reddig]

As the saying goes, there is no replacement for displacement.

[Marco Giustini]

If my math is correct you require 4572W with a single sub placed on the floor which is clearly not delivered by a single 1200W unit. With two units installed side by side the requirement drops to 1143W which is basically ok but you'd be running those units at full power during loud sequences, which is never great for sound and extension.
Three units would give you peace of mind as the requirement is going to be 508W only - hence the subs would run at half their capacity with little power compression and great sound.

The above is just math and it's based on fader 7. If you never run fader 7, then two units are going to be ok I suppose. At 6.0 you end up with the same 573W requirement but using just two units.

[Steve Guttag]

In order to sustain the maximum potential level that could be recorded on the sound track with the fader at 7.0 (0dB), you'd need about 2550-watts and your dual 18" only has 1200-watts to deliver...and that presumes your sub is in a baffle wall (sensitivity drops as it moves from 2π to 4π space). One should protect for this on subwoofers as the subwoofer track can indeed receive sustained (more than a split second) of high level. The sub channel is almost alway a generated channel to create an effect rather than merely the bottom octave of the main channels (bass management). As such, "continuous pink noise" is the power rating to pay attention to...the other channels can normally work with continuous program...which is just a 3dB level over continuous pink noise).

So, a properly designed system, for a room of that size would have the subwoofers in a baffle wall, off center-line, tight together, and two units (minimum, but not need to go beyond that). You should have about 1300-watts total continuous power available to them (evenly distributed). So two QSC DCA1222 bridged or a single DCA2422, stereo would work. As you move away from 2π space, you'd need to double that amount of power. Note, the baffle wall also will help your low-end response and tighten up your overall speaker performance.

[Marco Giustini]

Hi Steve,

Just for conversation's sake, could you elaborate how you ended up with the 2550W figure? I'm considering a 21m long auditorium, 12m wide. According to the spreadsheet I made the subwoofer ends up being 14m from the listening position. For a single 4642A that makes 3964W to reach 115dB peak at 7.0 - this is considering the sub sitting on the floor and no baffle wall. Again, not questioning you, just checking!

[Steve Guttag]

Your mistake was the 115dB value. The subwoofer is not a wideband speaker. It has 10dB of in-band gain referenced to the center speaker at 105dB but that does NOT equal 115dB. You would have to sum the frequencies that the subwoofer will actually be asked to play, in the case of the 4642A, that would be from about 20Hz to about 180Hz (it depends on the movie, I suppose and some would argue that subwoofers top out at 120Hz...which further reduces potential power/SPL levels). If you do that, you'll find that the actual SPL as measured on a C-weighted meter will be closer to 112dB, not 115dB. It is a mistake I see a LOT of people make (including at some prominent sound companies).

[Marco Giustini]

I see - good point. So as a flat subwoofer roughly equals to 91dBC at reference level, that's the number I'm adding 20dB of headroom to - hence 91+20 = 111dB which is consistent with what you say. I guess it's better to advertise 115dB as nobody would ever install enough subs

[Thomas Baham]

Thank you all, let me start with this, I appreciate all your smartnesses....LOL