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Tim's Limp Mass Bass Absorbers
Old 19th June 2020
Here for the gear

Originally Posted by Tim Farrant View Post
Over the years I have had great success fixing smallish room bass problems with limp mass absorbers. As there are so many threads here about this very problem, I have posted my design here for you to use. It's not that flash, but they do work if you follow the instructions. The most common question is going to be, "how many do I need?". In most smallish rooms (like this one Time to give up on the 80Hz null? ) sometimes only 4 absorbers per problem frequency mounted at the corners will be effective and you can always add more if needed.


Edit; I have added a Excel frequency calculator, enter your mass and depth into the red areas and it will spit out approx frequency.

Edit; I get asked a lot of questions about these so here are a few answers to the commonly asked questions and some points to consider.

1] These traps are intended to tame room modes, so measure your room dimensions and calculate the modes. Measuring with microphones does not necessarily identify problematic modes.

2] These traps may not work in small rooms where the largest dimension is less than 4 meters. A lumpy low end frequency response might be caused by other factors.

3] If you are only going to make just 2 traps, make the outside dimensions (area) larger than shown. Ideally, making a lot of smaller traps with different depths is preferred and 2 small traps might do very little. One trap in all 4 ceiling/wall/wall junctions might be a better way to go.

4] The direction the membrane is facing is in the room does not appear to be critical.

5] In small rooms, some experimentation in placement would be required. Try on the floor in the corner behind your monitors.

6] The idea of mounting them into corners in a cluster as I have shown is that the corner is effectively eliminated, it does not exist anymore, and has been replaced by a low frequency diffusive element (the boxes) as well as the membrane absorber.

7] The membrane across the front should remain limp, do not tension it. Any tension will shift the target frequency and raise the Q.
Hi Tim! Thank you for you contribution! However I wonder if I can change the fibergrall to mineral wool, cause I read that the fiberglass is dangerous?
Old 21st June 2020
Gear Head

I dont think anyone bother to reply again. No one cares!
Old 22nd June 2020
Here for the gear

How loose/limp should the membrane be?
Old 10th August 2020
Here for the gear

Another thread to look at

Secondary Mix Room Treatment Project

Picks up on this one
Old 12th August 2020
Here for the gear

These limp mass bass absorbers are sealed. Prime Acoustics are open. I've read this thread, and perhaps missed if this issue was discussed. Are both equally effective? What is the difference? TIA
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