# Narrow band bass trap question



## jimbo_9er (Mar 12, 2007)

First, I want to say "Thank you kind sirs" for all the great information and assistance provided by the folks here. Ethan, you are a gentleman and a scholar. Your explanations of acoustics are so clear, that i feel like I really "get it" after all these years. I have been a recording engineer and musician for 30 years and you have opened up a whole new world of understanding for me. Thanks again! ... and REW software has made experimenting fun instead of tedious.

Now to the questions: If I want to treat a null, do I target the freq of the null? It seems that the null should indicate the freq of the sound waves that caused it, but I wanted to check to be sure. I will be building some membrane resonators and I have collected the formulas and data in order to build them to target the freq of my choice. 

Since my problem freq is 70 Hz (8' ceiling) do i need to put the traps on the ceiling? Are they any more effective there?


Here's where I am so far:


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## terry j (Jul 31, 2006)

hi jimbo

might be an idea to repost the graph, but 1) change the scale to log and only go up to 200 and 2) change the db scale to 45-105.

That will bring it into line with the accepted format for graphs, as all graphs then will have the same 'look'.

Can't pull mine up right now to tell you exactly how to do it, but in the sticky posts in the bfd/rew forum there is an explanation. "getting graphs ready for posting"


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## jimbo_9er (Mar 12, 2007)

Thanks terry, I'll repost the graphs later.


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## Ethan Winer (Jul 21, 2006)

jimbo_9er said:


> Your explanations of acoustics are so clear, that i feel like I really "get it" after all these years.


Thanks very much.



> If I want to treat a null, do I target the freq of the null?


The problem with nulls (peaks too, but especially nulls) is their frequency changes as you move around the room. Even moving your ears (or measuring microphone) a few inches can change the frequency. For this reason I favor broadband absorption in most rooms. That way you reduce the peaks and raise the nulls for all frequencies they might occur.



> Since my problem freq is 70 Hz (8' ceiling) do i need to put the traps on the ceiling? Are they any more effective there?


Perhaps. Ideally the entire room would be treated with bass traps three feet thick. If you have the REW program that lets you experiment and see quickly which bass trap placements help the most. So invite a bunch of tall friends over for beer and chips and have them hold traps in different places while you measure.

--Ethan


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## jimbo_9er (Mar 12, 2007)

Thanks Ethan! I know you've answered such questions a lot. I have extensive broadband trapping (~120 cu ft in corners and rear wall,) 703 panels for first reflections, 2" mineral fiber (8pcf) clouds suspended an inch from the ceiling on approx 50% of ceiling and i feel like I am ready to attack that 70 Hz null. I commonly take readings from 3 listening points in the room, plus a variety of misc and indeterminate spots. That null wanders from 60 - 80 Hz so I am wanting to design for 70. The room is 14 x 23 x 8. In about a year, I plan to vault the ceiling, but I am already doing work in this room and I want it to be as good as I can reasonably get for the meantime.

BTW - I think I got a good deal on the mineral panels - 6 ea 2" x 24" x 48" for $109 inc. shipping. Anyone want the link?


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