Geoff Emerick de Fake wrote on Thu, 06 January 2011 05:19 |
Bogic Petrovic wrote on Thu, 06 January 2011 00:43 |
Geoff Emerick de Fake wrote on Sat, 18 December 2010 09:59 | Yes, it looks very much like one of the walls is absorbing too much. Basically, all your walls are bass absorbers. .....
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Room modes appears because reflections, and if walls are absolutely absorptive there wouldn't be a notches because room behavior, in other words, this will be a good thing...
| I haven't said absolutely absorptive. What I mean is due to the flexible nature of the construction, the walls may act as selective absorbers. The description of the problem rules out room modes as the principal culprit since some frequencies would be dominant, which does not seem to be the case. Room modes produce peaks and dips. Tuned absorbers produce dips only.
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I have been in a studio last year that had similar issues, a sharp loss of energy starting gradually under 500Hz and then a sharp tilt down from ~250Hz. A good -4dB trend all over the lower spectrum.
It was a badly semi-treated room with a not so easy geometry we were asked to upgrade.
The problem for the drop in LF was mainly 2 folds:
- side and back walls construction was such that they became rather transparent in the LF (light but very rigid structure). They hence caused a good level of "absorption by transmission" to the adjacent rooms, while the higher frequencies were diffracted back in the room which had little if any treatment at the listening position, creating a clear shift.
- Modal behvaviour was hence weird (not very pro-eminent) and pretty complex (the room had a special shape) and from calculations we discovered that they were spaced in such a way that they likely increased the perception of that shift.
The solution came from adding a certain amount density to the walls, and basically reverse engineer the structure's behaviour in the LF so we would have a good knowledge of the estimated lower coincident frequency behaviour of the walls, which is a good indicator of LF "transparency" roll-in point of a wall, and then treating the room accordingly, knowing the modal response would evolve a certain way in the process.
Doing so allowed to eliminate the shift and raised the room's response to a much higher standard.