General recommendations on how to treat frequencies below 60Hz

[pask74Participant]

Hi John,

Thanks for doing this!

My control room is currently being re-built due to a water damage 1 year ago.
I've asked a German company to design the acoustics (I'm in Switzerland) and have it done by a local studio builder.
The concept is mainly an angled reflective front construction + acoustic panels on the walls (including bass traps on the back wall). This design is supposed to be efficient down to about 60Hz, so I'm curious to know what your general approach to frequencies below that is.
I've made a few rooms myself, using what I could find on the John Slayer website/forum - with relatively good results, so I understand the basic principles of acoustics.

Thanks again.

Pascal

[John BrandtParticipant]

So the design is RFZ?

Use caution with reflective front ends when using nearfields because the SBIR will be more pronounced.

Broadband treatment CAN be effective down to 40 Hz, but it needs to be about 16" deep. To really cover the LF range below 100 Hz, you need to use a combination of Broadband and membrane trapping. Placement is also critical to the trapping efficiency.

Cheers,
John

[pask74Participant]

Not sure if the design is totally RFZ as the floor will be solid concrete, but yes, I'll have broadband absorbers on the sides, back, and ceiling.
The speakers will be wall-mounted.

Membrane trapping I found are pretty tricky to implement as they should be installed in specific places, right?

[John BrandtParticipant]

RFZ floors are concrete all the time. 😉

Membrane trapping I found are pretty tricky to implement as they should be installed in specific places, right?

Yes, it is very important HOW they are built and where they are positioned. Membranes work in the pressure zone. You're familiar with the Crown PZM? PZM = Pressure Zone Microphone. You can make any omni, small-diaphragm microphone into a PZM by placing it about 1/8" (3 mm) from a boundary surface. Pointing directly at the surface.

Right at the surface of a floor, wall, or ceiling particle velocity ceases and pressure begins. So... a membrane trap must be as close as possible to the pressure regions of your room.

Cheers,
John

[pask74Participant]

RFZ+concrete=OK 😉

So you would recommend to install the membrane as close as possible to either floor, wall or ceiling?
I'd be super interested to know more about that but I guess it could be the topic of a full book?

[John BrandtParticipant]

Book! indeed.

I'm about a week away from finishing mine...(grumble, grumble.. if only people on forums would quit bothering me!..) ...I'm just kidding. I love to do this. 🙂

If you go to my Resources page and download the room mode calculators, you can determine placement by the listed modes.
Simplified:
Axial modes bounce back and forth between TWO surfaces...
Tangential modes hit 4 surfaces...
Oblique modes HIT EM ALL!

The Axial modes are the most powerful and they will be the most pronounced in the CENTER of the surfaces struck.

Put yer traps there! 😀

Cheers,
John

[Author]

Posts