The truth about room gain ...

[paulspencer]

I once read an article claiming room gain doesn't exist in the average Aussie home. The reason given was that the construction isn't solid enough and the boundary flexes too much. My own experience tends to agree with this.

Room gain is often talked about and taken for granted, and many refer others in here to rules of thumb and calculations.

So I'm starting this thread to uncover the truth and give us more of an idea of what is really going on.

What I'm looking for is some EVIDENCE based on measurements. Compare to simulations and calculations if you like, but let's see what measurements tell us.

Anyone who'd like to participate in this little study, please tell us:

A. The room

1. Dimensions (metres and feet)
2. Construction of walls, floor and ceiling (eg. solid brick, hollow concrete block, 150mm concrete, 10mm plasterboard/drywall on studs, etc)
3. Windows and openings (if the room is completely open to another room volume, this should be included as part of the room)

B. Placement

1. Speakers
2. Listening position

C. Speakers used

D. Measurements and description of how you measured

Background info:

My search showed up one result:
A post by Zaph http://www.diyaudio.com/forums/showt...943#post603943

[paulspencer]

Now I'll answer my own post ...

A. The room

1. 3.9m wide x 4.6m deep x 2.8m high (12.8 x 15 x 9 ft)
2. 10mm plasterboard walls/ceiling, 19mm pine floor boards, weatherboards on the exterior walls
3. One internal door and 1.8 x 1.4m window (double hung sash windows)

B. Placement

1. Speakers 1m in from the corner
2. Listening position 1m from rear wall

C. Sub Rythmik Direct Servo 12" sealed

Now my results. I put the sub in the listening position so that I could move the mic around to where I would put the sub. This is the same thing as moving the sub and keeping the mic in the listening position, but easier. The mic has a ruler flat response down to 20 Hz. I hooked up ultracurve and played a test signal (noise) with the RTA display showing 20 - 20k. First testing nearfield with the mic right up close I find the response is ruler flat with -3 dB points at 20 and 80 Hz. Well, pretty close from looking at the display on ultracurve.

Next I started moving the mic around, trying corner placement, then my actual speaker placement 1m away. Very very close. There is no noticeable difference. Also, the only difference between this and nearfield is the level is higher with nearfield. I can't see any room gain, or any room modes at all!

Moving around into other possible sub locations, I find that in some placements I get a dip in the midbass, but no improvement. The corners or where I actually put my mains are ideal, with no room for improvement.

Opening or closing the door has no measurable impact. Probably because there is so much damping in the room by wall flexing.

My conclusions:

1. The Rythmik is well designed and does what it's meant to do, giving you the response it's meant to achieve
2. My room has no gain at all - the boundaries flex too much and this is why modes are fairly well damped
3. Current placement works well

I'd like to see what others have found, and if you find other results on the net, please show.

If there is interest, I can post images of the results later.

[RobWells]

Hi Paul,

I don't think the ultracurve has enough resolution to show room modes well. Try trueRTA or ETF to get better results.

I've posted outdoor and indoor measurements of my labhorns before, and also indoor against winISD sims of my tempests. (search for 'room gain in real life' for the thread)

I'll post links later as am off to work now.

Room is 21' x 10' x 8' high. Ceiling goes to 10' in the middle. Concrete floor, plasterboard with a 'prefabbed' exterior (not brick or concrete block)

Both times I've seen a ~12dB / oct rise from 30Hz down to ~16Hz.

Cheers.
Rob.

[Mikey p]

There is a thread here with graphs showing no room gain. This is with an IB sub.
http://ibsubwoofers.proboards51.com/...8731620&page=1
You will need to register to access the thread. It was stated that room elements were creating either bass traps or causing cancellation losses. This would be specific to that room in question. I've read in other forums where members have stated finding positive room gain results when their subs were measured near field and then at the listening postion. It apears it's a combination of room shape, size and sub interaction with the specific room as to whether or not it produces room gain.

[richie00boy]

Top thread Paul, will be keeping an eye on it.

[Sonusthree]

Typical!
I've braced the inside of my speakers and now I have to brace my house!

As someone without proper measuring equipment I am relying on you guys to set me straight on this subject.

[AndrewT]

Hi,
I have no access to measuring equipment.
I just use my ears.
My new sub has a very droopy low end response.
This should be bass light.
My ears/brain tell me it is producing deep bass that can be strong when asked.
The active Tannoys it is used with claim flat response down to roll-off at 50Hz and the twin 18inch sub goes significantly deeper & stronger than the pair of 6inch bass/mid units in the main speakers.

I think this is telling me that there is some room gain effect and that the room gain is more effective when the Sd to room volume ratio is higher.

The room is very irregular with lots of doors to the outside and/or to adjacent spaces. Plaster board on 600mmm spaced studs (both metal and timber) I guess the volume is about 400cubmetres.

[richie00boy]

Originally posted by AndrewT
Hi,
I have no access to measuring equipment.
I just use my ears.

Have you been able to compare to live unamplified music?

Originally posted by AndrewT
My new sub has a very droopy low end response.
This should be bass light.

Is this a simulated droopy response or a measured one?

Originally posted by AndrewT
The active Tannoys it is used with claim flat response down to roll-off at 50Hz and the twin 18inch sub goes significantly deeper & stronger than the pair of 6inch bass/mid units in the main speakers.

Well that seems logical for an 18 inch driver vs. a 6 inch one...

Originally posted by AndrewT
I think this is telling me that there is some room gain effect and that the room gain is more effective when the Sd to room volume ratio is higher.

Now you've lost me with your logic there.

Originally posted by AndrewT
The room is very irregular with lots of doors to the outside and/or to adjacent spaces. Plaster board on 600mmm spaced studs (both metal and timber) I guess the volume is about 400cubmetres.

For room gain to exist the room must be sealed, else there is no way for the pressure mode (which is what gives 'room gain') to be activated. What difference do you notice when the doors are open to when they are closed?

[AudioFreak]

Originally posted by paulspencer
Now my results. I put the sub in the listening position so that I could move the mic around to where I would put the sub. This is the same thing as moving the sub and keeping the mic in the listening position, but easier.

By my understanding and way of thinking (and I could be wrong here but would appreciate it if someone could verify either way with facts), moving the sub is not the same as moving the mic when the room has boundaries and we are looking at SPL vs. frequency.

[FullThrottleRic]

My room is about 12.5 x 10.5 x 8 ft, all brick walls with a plasterboard ceiling (loft above).

Measured with an ancient Sennheiser mic which is -3db at 1Hz (!) according to the spec sheet I get about 10db boost at 20Hz and 20db at 10Hz. The 28Hz/q 0.5 setting on my Rythmik DS12 is pretty much perfect to match the room gain and I get strong output <10Hz