# How to make broadband panels reflective for higher frequencies



## pablovillarealz (Dec 18, 2011)

Hello, I live in Mexico and Im building a HT for my family, it is a dedicated room 100% movies. Room dimensions are 11m long 7m wide 2.41m heigth. All surfaces are poured concrete. I will build 1 side wall of drywall to avoid some columns as well as a back wall same drywall. Final dimensions are 7.15m long 5.27m wide 2.41m heigth. Im trying to make it RT60= 0.3 so all calcs give me I need around 540 sabins. I can only get generic mineral wool 2" thick 4x2 6pcf my calcs using 0.049V/Sa gives me I need 40 of this panels. I will make the typical solution building wood frames for this 4x2 stacking 2 panels to make it 4" thick and wraping in cloth I will ad spacers behind to give 4" of air behind panels also I will use stradle panels 6" ( stacking 3 panels ) for corners floor to ceiling all 4 corners .The rest I will distribute among room covering first reflections first. My question is how can I make some of these 40 panels reflective since my calcs show higher frecuencies reverb time 0.23 or so. Also how many should I make reflective?

I tried to look for kraft paper absorption coefficient but had no luck

I appreciate any help I can get. BTW I cant get OC 705FRK here

Thanks


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## bpape (Sep 14, 2006)

Just use some 5 mil or so plastic on the faces.

Also understand that you don't really want 0.3 across the board. Your lows will be a bit longer and highs a bit shorter in general -more like a curve. From the calculations:

±50ms from 200Hz to 3.5kHz = 247 to 347ms
- ±100ms above 3.5kHz = 197 to 397ms
- <+300ms at 63hz = 597ms
- 300<RT60<600ms

Also remember that RT60 is useful for large space acoustics, not so much for smaller (relative) spaces like home rooms. That said, you can somewhat extrapolate from the above to be slightly shorter.

Bryan


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## pablovillarealz (Dec 18, 2011)

Bryan thanks for your comments, they are very helpful!

How do you recommend me to modify coeficient of absortion for high frecuencies for my calcs once I place the plastic cover to the front face of the panel? Above 2khz should I aasume absorption coeficient of 0.45 instead of 0.95-1.0 ? Should i assume similar to 705 FRK? 

For the corners I m planning to add a wood riser to the back top of the 6" panel and hang a MLV membrane suspended wihtout touching the mineral wool , actually if you turn panel around you will see the membrane, to improve low freq absortion, what is your experience with this? 

Lastly since Im new at this forum would it be ok to post here my calcs? Room layout? Pictures of room?

Thanks


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## bpape (Sep 14, 2006)

For general purposes, yes, you can use 705 FRK to address approx how it will react. Don't get too hung up past the 1st decimal point.

Membrane on the back won't do much honestly. 

Bryan


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