Soffitt Mounting JBL LSR28p's

  • Thread starter Thread starter bwindsor
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bwindsor

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Anyone here tried this with these (or any other monitors that have a rear-port design) and, if so what were the results/problems/solutions?
Thanks in advance,
BW
Pinederosa Studio
 
I have no idea :D
but since we're on the topic ... uhhh ... I wanted to ask ... does it make any sense to consider soffit mounting a smaller speaker, say an Event 20/20? Or are near-fields (i'd consider these like mid-fields because they're larger than normal, imo) better off left on stands?
 
bwindsor,

The main reason for soffit mounting is to increase the size of the baffle. The baffle basically acts as a 180 degree horn. The low frequency cutoff of this horn is governed by the narrowest dimension of the baffle. For most front ported speakers the bigger the baffle the better in my opinion.

[side note: The longer dimension has only a slight effect on the rate of low frequency rolloff. A square baffle will have a marginally steeper rolloff than a long rectangular baffle of the same width. The only reason to avoid a square is that the equal widths reinforce each other's edge diffraction effects]

Now, you definitely do not want to mount a rear ported speaker in a closed cavity. This will wreck the response. You may however be able to mount them in an open back baffle without drastically altering the bass response, as long as the baffle is not too large. Something like a 1m (3ft) wide, very rigid and well damped panel would probably be a good compromise. The benefits of the larger baffle would extend down to around 150Hz, but then it's a high enough frequency to where the port output is still low. The system could be suspended from the ceiling. But, don't mount it too close to the rear wall, since you'll start altering the port loading. You might have to experiment a little with placement.

BigKahuna,

The 20/20's would certainly benefit from soffit mounting. Just keep them within the recommended listening distance.

Good Luck!
barefoot
 
bwindsor -I agree with barefoot on this one. You have to be careful what speakers you soffit mount as some speakers like your JBLs are designed to be free standing and have a bass lift to compensate for it. If you flush mount them you will end up with an uneven increase in low end. I had this problem once before when I flush mounted JBLs similar to yours.

I'm sure sjoko will have something to say here as his speakers appear to be mounted in an open backed baffle.

BigKahuna... before we got the Genelecs I flush mounted the 20/20s and they sounded fantastic. Amirel couldn't believe they were the same speakers that he had been working on for the past year. :) I recommend you try it.

cheers
john
 
bwindsor - I just remembered that I emailed JBL regarding the LRS32s and soffit mounting. This was his reply:

Hi John,

Studio designers such as Waterland Design Group, Chris Polonis,
Walters/Storyk and Studio 440 have all used LSR32s in soffit mount
configurations. Several locations including Warner Bros., Eastside
Audio/Video, Rondor Music and Savage Sound, among others.

Attached is a file of the calculated boost in LF due to making it a 2pi space
by soffit mounting. These are theoretically correct, but will be dependant
on the construction techniques, materials and other factors.

Regards,

JBL Professional
David Kimm
Director, Recording and Broadcast

You will see in the file that the boost is around 4db @ 100hz. The Genelecs also recommend a 4db roll off for soffit mount.

ooops - can't attach an xls file. Take my word for it that you will require a 4db rolloff at 100hz :D

cheers
John
 
well bwinsor - you can look at some soffits build for the JBL's at "sjoko's studio" on John's site.

I did a lot of trials to arrive at the final design. But, I have to say that I was very much aided by the shape of the room, with its 22 degree angled rear and front walls, which increased the space behind the monitors considerably.

The ports of the L, C, R front monitors "blow" into a very dead, heavily insulated space. Apart from this there are calculated spaces surrounding the soffits - which are really more baffles than soffits, and at the point of most rear sound transmission there are straw panels. Also the rear of the baffles are insulated.
To complicate matters a bit, my soffit boxes are fixed onto their base by means of 2 sharp points at the front, and spring-loaded bolts at the rear, so they do not transmit any vibration.
A bit OTT perhaps? But according to the peeps from JBL proaudio, they have never heard their monitors sound like this (the room has also something to do with that).

Don't forget the cooling on 28Ps!!! My soffit boxes are airconditioned.

I went to listen at a couple of soffit mounted LSR installations around the LA area. Everyone was over the moon with them, all had done a proper job with large spaces behind the soffits, some with vent returns to the room, some without.

In my setup a -2dB low cut was sufficient. Others (mainly with more ridget structures) required 4 to 6 dB.

Another solution. I did one design last year with 28P's which was cheap and proved to be very effective.
This involved fitting a baffle around the fronts of the monitors, and suspending them from the ceiling. We used Owens-Corning sheets against the walls and at the rear of the baffles and a -4dB adjustment.

Jeez I write some discombobulated stuff!!! Hope it makes sence, but the conclusion is that the LSR's sound brilliant soffit mounted, providing you consider the porting and airflow requirements carefully.

Oh yeah, as you can see from the pics, I put some 20/20's in them when I finished - just to see. You're right John, it made them sound good (well... better then they do). It also amplified their build-in hum :)
 
Thanks for the info sjoko2,

I'm a few months away from addressing this (hole is being dug for new construction in four days) so I will file this away for reference!

Thanks again,

BW
 
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