I'll try and answer the questions but first I should point out that acoustic design is not my main line of work. I became interested in it years back when control rooms that I worked in sounded so bad. I have done my own research and come up with a few ideas, I am not a qualified acoustics engineer and simply offer you what I have found to work in the rooms I have designed.
Using the limp mass seems to work well and takes up a lot less space than deep traps. I remember installing an array of these into the rear of a control room at Marmalade Studios. The builders were still hammering in the room and you could easily hear the bass reverb reduce as the boxes were covered with the membrane.
To be honest, I have never measured the response of a room in my life, waste of time IMO, too many variables. My method is to calculate the room modes (as best you can if the room is not symmetrical) and build the absorbers to target the dominant axial modes. This generally works. If you can do a tone sweep as has been suggested, this may be another way to find the target frequencies, but at the end of the day, the Q of the limp mass absorber is going to be fairly broad so an absorber tuned to 80Hz is still going to have an effect at other frequencies, so it's not that critical.
The beauty of limp mass is being able to absorb 50Hz with a unit only 1 ft deep.
To answer Rod's question, the membrane is not tensioned at all. I guess if you did, this would raise the Q of the unit rendering it more like a drum.
As to the quantity of absorbers required, I would probably decide this based on the room proportions and construction. If the room had favorable dimensions and smooth modes then it would require less absorption. If the room had very solid walls (ie for sound isolation) then more absorbers would be required as any diaphramatic action from the wall would be reduced. There is no easy answer to this question unfortunately. What is important to try and install them at all wall/wall/ceiling corners of the room to keep it even.
I also have a theory that whilst Helmholtz resonators are useful in the mid band, their use at low frequencies is questionable. I have read that 13th century Swedes used to use "jars" and "sound pots" to prolong the reverbration of dead theatres. These are basically high Q Helmholtz resonators. To my mind, having something that is singing in tune with what you are trying to eliminate does not make sense and something inert like the limp mass is preferred.
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