Room Gain : What can I expect ?

[316a]

Hi ,
I'm about to build some new speakers for the workshop . This is 10ft x 10ft , made of wood , and strongly resembles a garden shed , albeit with double glazing and double skinned thermally insulated walls . I'll be using some Richard Allan CG12S for bass duties , which according to WinISD , will be 6dB down at 50Hz . Say for instance the listening position is central and 2 ft from the back wall , with the speakers corner mounted , is there a 'rule of thumb' as to how much room gain to expect ? After searching the archives for well over an hour and loading subsim.exe (which I could not get to work properly) , I'm still at a loss of what to expect . Much appreciated any advice or links to (easily useable) simulations

cheers

316a

[kelticwizard]

Here is a thread with a curve that is typically used by Calsod.

Here, Rob Wells shows predicted versus measured response for his room. the measured response was much greater than the prediction.

[simon5]

Expect around 2-3 dB gain per octave when going lower, starting at ~100 Hz.

[kneadle]

How does this relate to BS loss?

Dave

[forr]

Hi Simon5
---Expect around 2-3 dB gain per octave when going lower, starting at ~100 Hz.---

I think we should say : 100 Hz (+2 to +3 dB).

Benjamin and Fielder specified an increase to +12 dB/o towards the down direction of the room gain at 25 Hz. I join an Open Office spreadsheet which may help to calculate the rendition of a closed box in an average room. It is a theorical work which tends to show that, if the room gain was the only intervening factor, one could get a response flat (+/- 3 dB) down to 12 Hz when using a closed box having a resonant frequency lower than 40 Hz.

~~~~ Forr

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[316a]

[i] I join an Open Office spreadsheet which may help to calculate the rendition of a closed box in an average room

Hi ,
Interesting stuff , I now have a fairly good idea of what I may expect Does this also apply to a vented box ?

cheers

316a

[simon5]

Let me state it more clearly then.

160 Hz = 0 dB
80 Hz = +3 dB
40 Hz = +6 dB
20 Hz = +9 dB
10 Hz = +12 dB

This will be true most of the time in most rooms made with plasterboard and such...

If sealed boxes were that good, almost no one would be building ported boxes, especially with obvious problems with ported boxes. Room gain cannot cancel the sealed boxes rolloff.

[forr]

HI SIMON5
---Room gain cannot cancel the sealed boxes rolloff.---

Ultimate roll-off of vented boxes is 18 dB/o or 24 dB/o.
The sealed box roll-off is 12 dB/o under the resonance.
According to Benjamin and Fielder, average domestic listening locations have a room gain of +12 dB/o from 25 Hz down to 12 Hz. I have some data which confirm this fact. If the resonance of the sealed box is 25 Hz, its roll-off will be compensated.

The difficulty is to have a closed box with such a low resonance : it means electronic intervention, using some kinds of servos or transforms, and requires a drive unit able of large excursions.

Note that my spreadsheet is a simple theorical work and is intended to be modified according to the user's ideas.

~~~~~ Forr

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[Peter M.]

Sealed box rolloff below resonance, doesn`t that depend on the qtc?
I thought the 12db/octave rolloff was only correct with a qtc of 0.7

[forr]

PETER M

---Sealed box rolloff below resonance, doesn`t that depend on the qtc?---

I said "utimate roll-off". The Qtc is significant at resonance and above.
Martin Colloms's recommandations for closed boxes are : a Qtc of 0.6 for a resonance at 50 Hz and a Qtc of 0.52 for resonance at 40 Hz. I found similar values using my spreasheet.
Note that page 121, in Colloms's book, 5th edition, there is an error in the figure : it is stated "room gain : approx. 5 dB/oct". The graphic scale is set at 50dB, and it shoud be indicated : "approx 2.5 dB/oct".

~~~~~ Forr

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