# Dip at 75hz....need to build a membrane trap?



## jlird808 (Nov 2, 2010)

I was just about to start looking for the parts I need to build this deep bass trap EW posted here:

http://www.ethanwiner.com/BTPlans.gif

Then I noticed it says it goes down to 80hz. Will it still cover some of my 75hz dip? Can I modify it slightly to grab the 75 out of the room?

Thanks!:T


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## nottaway (Jan 19, 2009)

How do you know you have an issue that needs treatment?


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## tonyvdb (Sep 5, 2007)

A 75Hz dip could be caused by a phase issue at the crossover of your mains and sub. Have you tried adjusting your phase on the sub?


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## jlird808 (Nov 2, 2010)

Im one of the project studio guys here so no mains and sub.


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## bpape (Sep 14, 2006)

Looking at the dimensions and mass, I'd say it's pushing to get to 80Hz. Also, the dip you have is more than just one thing. Looks like at least 2-3

What are the dimensions of your room?

Bryan


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## hjones4841 (Jan 21, 2009)

Brian: Looks like he is pretty much near field to the speakers. How about a little parametric EQ just to bring it up a little? Could the 75Hz dip be coming from the floor? Would be interesting to see what happens with the speakers moved around a bit.

jlird808: Where is the mic position for the measurements WRT to the listening position? In the chair?


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## bpape (Sep 14, 2006)

My guess is a cancellation off the wall behind him but no way to know for sure without dimensions and seating position in relation to the length.

Bryan


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## jlird808 (Nov 2, 2010)

Sorry for the short hiatus!

The room dimensions are approx 16' from the wall behind the desk to the wall behind the seating position. The ceiling is about 10.8 feet tall. The width of the room is about 15.2' at its widest point but my room is also L-shaped as u can see below.

Sorry I forgot to post my room layout. Everything is up-to-date except that the BLUE and white corner traps flanking the desk actually go almost to the ceiling now. The one one on the left is an R30 superchunk with a 30" face...reaching about 10'5 feet tall. The left side, howver, is about 3-4" thick 705 but only about 18" across. It doesn't quite reach the ceiling either...about 9' tall.

I actually have EQ filling in the hole for the most part. I'd kinda like to address it without EQ though...

The mic is setup right where my head would be in the chair. It is somewhat "nestled" in the chair when I test. Should I try without the chair?


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## bpape (Sep 14, 2006)

I would definitely look at something on the wall behind you. That distance could easily fall in that range for a cancellation.

Bryan


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## jlird808 (Nov 2, 2010)

bpape said:


> I would definitely look at something on the wall behind you. That distance could easily fall in that range for a cancellation.
> 
> Bryan


Hey thanks...so should I look at just building a fat broadband absorber for back there? Ceiling/back wall corner soffit trap maybe?

THX!


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## bpape (Sep 14, 2006)

Think more centered behind you rather than the corner probably. Use something at least 6-8" thick.


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## jlird808 (Nov 2, 2010)

bpape said:


> Think more centered behind you rather than the corner probably. Use something at least 6-8" thick.


I may have the perfect place! Unlike the room layout image I posted, that "curtain" behind me is actually two windows with blinds. BETWEEN THEM I have a wrapped 705 panel that is only 1.5" thick (but spaced off the wall about 1.5") at about 36"w x 44"h set on a waist-high bookshelf. I could easily add some more slabs of 705 to the back of this and make it 6-8" thick as u suggested. I could also technically make it taller but I couldn't really make it any wider.

I'll try to take a picture of this rear wall with the current 1.5" panel later this evening.

Thank you!!

:T


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## jlird808 (Nov 2, 2010)

Popping back in here...gonna build a 6-8" slab this weekend. 

One concern though, based on this pic of my back wall, is the area just TOO NARROW? 



















That 1.5" panel was just there to stop first reflections, as its about 10' behind my head. I can't make it any wider, but I can definitely make it THICKER and TALLER. The tops of those windows are actually about 10' up. The walls continue another 2' to the ceiling. 

So would it be worth it to fatten up that trap on the back wall, seeing as how its relatively narrow? Make it go to the ceiling maybe? 

Here's my floor plan again...again, the curtain is inaccurate, its not one long complete wall.










THANK YOU!!!!!


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## bpape (Sep 14, 2006)

Can't hurt. Not sure if you'll have enough surface area to be fully effective but it will still help the overall decay time in the room. I would look at a bare minimum of 6" thick. Thicker and/or spacing off the wall will help extend the reach farther.

Bryan


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