Originally Posted by JackNC
Geddes recommends a 2nd layer of sheetrock on walls and ceiling with green glue between it and the first layer. Has anyone tried that or know how well that works relative to bass traps?
Originally Posted by Bill Fitzmaurice
A second layer of sheet rock would be used to reduce transmission through the walls and ceiling to adjacent rooms or the environment. Acoustic panels are used to reduce wave reflections inside the listening room.
Originally Posted by Martycool007
Bill is correct. The reasoning behind using two layers of drywall and green glue/clips are for sound proofing.
Well, JackNC is inquiring about bass traps, and how a second layer, or double thickness with GG in between, impacts it's bass damping/bass trapping effectiveness.
It's really important to understand a common acoustic distortion characteristic of small rooms, even treated small rooms/HTs, is an un-even decay times. It's very tough to properly damp the lower octave energy, due to the wavelength size inherent to the low freqs. Most importantly, one needs uniform decay times top to bottom. A room that has too much MF/HF absorption, relative to the LF absorption, just allows the LF reverberation to drone on, resulting in an unbalanced sound bass clarity and detail that could be much better.
The low bass clarity and detail results from the decay times masking of that part of the playback, as the lower level is overly bathed in reverberant noise. The low level details within the material simply can't be heard due to the uneven and slow decay of the more prominent sounds...masking. This results in a scenario whereby the listener can't resolve the reproduced acoustic cues that accompany the imaging. Characteristics like the decay of the recorded space... this impacts imaging spatial characteristics.
If the room's response time exceeds the material's recorded reverberant energy and is of sufficient level, the recorded details are masked.
In many rooms that difficult to achieve, but we try. Ideally, striving for uniform decay times, ... even if they're excessive, still is a step in the right direction. Similar to overly thin absorption EQ'ing a reflection, if the decay times are somewhat uniform, then at least there's no gross changes in reverberant energy.
Originally Posted by JackNC
Actually, we are both/all three correct. Here is a quote form Premium Home Theater by E. Geddes, talking about constrained layer damping with two layers of Gypsum board with a layer of mastic between:
"The first goal is sound isolation. The CLD panel does not transmit sound as effectively by converting it to energy. Second, it adds considerable low frequency sound absorption..... With a fairly hard surface treatment it will basically act as a damper at lower frequencies, one whose damping coefficient will decrease with frequency - which is exactly what we want.!"
Its fairly expensive technique (requires two layers of Gypsum on spring clips) but he claims it works very well and best of all, when done its invisible and takes up little space in the room.
Absorption of sound energy at bass frequencies, can be achieved with a resonant absorber... the drywall.
Just like the drywall example Jack and Geddes are talking about, a mass suspended by a spring will vibrate at it's natural frequency
The mass of the panel, in concert with the combined springy-ness of the damping layer of GreenGlue, and the stud/air cavity, are resonant at some natural frequency. The sound energy is absorbed because of frictional heat losses of the motion.
Damping increases as the velocity increases, and velocity is highest at resonant frequency. You change the resonant frequency with the mass.
Everest states 1/2" sheetrock, on 16" studs, possesses about .29 absorption coefficient at 125hz, and it increases at 63hz.
He states 1/2" sheetrock, 16" studs, with the typical 3.75" airspace, resonates at around 60hz.
Also, double 5/8" lowers the freq to around 38hz.
Adding the mass lowers the resonant freq.
As Jack points out, in the Geddes example, both bass trapping increases, and sound egress control increases.