# How high in freq to adjust for room nodes?

Discussion in 'Archived Threads 2001-2004' started by Kevin C Brown, Oct 3, 2002.

1. ### Kevin C Brown Producer

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Couldn't think of a more succinct but clear way of asking...

EQ'ing a sub is easy, just do the freqs below and maybe a half octave above the crossover.

But if I got a high quality analog parametric eq to try out with my mains, how high should I look at? 500 Hz? Lower, higher?

2. ### Bob McElfresh Producer

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Several of the book and calculators I have seen seem to stop calculating reflections/room-modes at about 300 hz. This follows the theory that you are trying to tame wall-reflections and likely you dont have many walls that match the wavelengths above 300.

3. ### Kevin C Brown Producer

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Bob- Cool.
80 Hz is 13.75 ft.
300 Hz is 3.66 ft. Ok, I thought that would be more meaningful...
Maybe if I looked at all my room dimensions, and just divided by 2 and 4.
Ceiling is 7.83 ft / 2 => 3.91 ft. Close.
Length is 17 ft so that would be 8.5 ft, and 4.25 ft, also close. (No width, family room opens into the kitchen.)

4. ### Isaac C Stunt Coordinator

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Kevin, you might be interested in the Room Reflection Calculator by Yavuz Aksan. It's really not as simple as dividing room dimensions and wavelengths

5. ### Kevin C Brown Producer

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Cool, thanks.

6. ### Michael R Price Screenwriter

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What happens is at higher frequencies the room peaks/dips get narrower and narrower (so they're harder to recognize); our ears also start to realize what part of the sound comes from the speakers and what part comes from the walls.

EQ is a great idea, and you may want to try EQing room response up to 100-150Hz then EQing the close miked speaker response above that.

7. ### Grant B Producer

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There is also the fact most people have some high frequency loss in their hearing.I compensate for mine (hey since I take the time, it should sound good to me!)

8. ### Kevin C Brown Producer

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If I used an eq to compensate for my hearing (loss), that would open up a while new can of worms...

9. ### Chris Tsutsui Screenwriter

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I made a room mode calculator HERE, you'll need excel. Keep in mind that given a single axial length, there is not just one resonance but at least 3 others that can be just as important.
Generally I pay attention to the first 4 room resonances for a given axial length because they are the big factors and are easy to calculate their positions in the room and move speakers around to tune with them.
So given a room length of 8 feet. You will not only get one resonance at 70.6hz, but 3 more that are almost as important as the one preceding it, here they are:
First order: 70.6
Second order: 141.3
Third order: 211.9
Fourth order: 282.5
The Fifth order is 353.1hz and is where most calculators would stop calculating them.
As for equalizing higher frequencies, I could think of a few reasons why you wouldn't stop at 300hz. One as mentioned above is hearing preference. Two could be due to a speakers real anechoic response that dips at a certain upper band. Three, is there could be walls and objects that absorb a lot of high frequencies but reflect mids. It may be beneficial to reduce the volume of the mid band range or increase the highs so the response is flat. (Of course a better solution would be room treatments but we are talking about electronic EQ)
So Kevin, lets say your room is 17 feet long, I'll try and explain the room modes for that length (don't forget height and width though)
Length = 17 feet
First order = 33.2 hz
2nd = 66.5
3rd = 99.7
4th = 132.9
So when you are playing a 33hz test tone and listening between the 17ft wall lengths (smack in the middle). You will be at the "null" of the first order standing wave and experience a loss in volume at 33hz. However, as you approach the walls the volume will increase.
For the 2nd order, the dips in the response will be 4.25 feet away from the side walls or 1/4 distance away. The spikes in response will be in the middle and at the walls.
3rd order adds another null 1/6th distance from side walls with one null in center, and the 4rth order has 4 nulls with one 1/8th distance from each side wall and one 3/8th distance from each side wall. (Remember that between each null is the spike)
The white paper article at http://www.harman.com/ explains this much better with pictures.
To be short: The advantage of knowing about these standing waves and their locations gives you the option of selective mode cancellation (Placing speakers to cancel out these resonances). You can then flatten the overall response without using electronic equalization.

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