# Need some help with room acoustics



## Tobias (Feb 20, 2007)

Hi,

I have recently been thinking about buying new main speakers and/or amplifier, but as I read through reviews and articles I see that many recommend trying to do something about the room acoustics before buying new equipment (which sounds reasonable). The problem is that I have not much knowledge about it and how to figure out what to do to improve on it.

I've read about a method where you have someone moving a mirror along the sidewalls and then you sit in the listening position, and at the points where you can see the front speakers you should put up some material. But I don't know what material to use, or how big it should be.. 

I have taken some pictures of my room, the dimensions is approximately 5 meters (16ft) wide and 7 meters (23ft) long, and it's basically concrete, with a carpet covering the floor.

This picture is taken a bit to the left of sweetspot:








This one is the stairs down to the basement:








The left wall of the room:








Behind the listening area there's an area of the basement with not so much in it (can't use it due to this stupid pillar ) (And yeah, I need to fix the cables )








Here is the right wall of the room:








Finally a measurement with Radioshack SPL meter and REW. The curve is smoothed out (1/2 octave setting in REW). I can do some other measurements if needed. I think the area from 15-200 looks badder than it really is due to the scales on the axis  - I can post a graph of this frequency area if needed.

I could also make a sketch of the room with dimensions if needed.

I hope someone can help me with what, if anything, I should do with my room before I go buy new equipment 

Thanks in advance.


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## bpape (Sep 14, 2006)

Actually that's pretty typical and actually better than it really is because you smoothed it. Turn off the smoothing and that's what's really happening.

The material for reflection points on the side wall can be 1" or 2" rigid fiberglass or moderate density mineral wool. Normally, one would put up 2'x4' panels. Also, don't forget about corner trapping for bass frequencies. Lastly, some panels on the front wall help a lot in a multi-channel environment - not only to keep the surround reflections from coming at you from the front, but also to help smooth the bass response issues caused by the distance from speakers to the front wall.

The scale you're using is correct on the frequency axis. Try setting the level axis (vertical) to be more like 50-80db. Lastly, pretty much everything above 400Hz is meaningless in a graph like this. Set the graph to show 10-400Hz - that's where the real room issues are and we want to see as much detail as possible.

Bryan

Bran


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## Tobias (Feb 20, 2007)

Thank you for your answer  I wasn't sure about how far up to measure, I suspected I was taking it too high 
I added the smoothing because the higher frequencies was very jumpy, but here is a 'fresh' measurement with the scales as you said. I'm a little bit in doubt about the target curve (blue), since I was a bit in doubt wether to use "subwoofer check levels" or "main speakers check levels" in REW settings, but the results are only differing by 1 dB or so.

But here is the graph. The sub is by the way equalized with a BFD.


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## Ethan Winer (Jul 21, 2006)

Tobias,

> I added the smoothing because the higher frequencies was very jumpy <

I consider the "crossover point" between bass and everything else to be around 300 Hz. So I use very high resolution up to 300 Hz, and then 1/3 octave above that. Measuring accurately at mid and high frequencies is very difficult, so above the midrange you're not usually measuring what you think.

> The sub is by the way equalized with a BFD. <

The first thing to do when considering bass traps and other treatment is to disable all EQ or other active "correction" devices. You need to know how the room really behaves. Likewise, if even if you do end up using EQ - and chances are good you won't need that band-aid once you have enough bass trapping - the EQ must be adjusted _after_ all the traps are in place.

--Ethan


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## Tobias (Feb 20, 2007)

Can you (or someone else ) get any information out of this graph regarding how to make improvements?

I have tried the method with the mirror, and unfortunately two reflection points is at the stairs (picture 3) and at the radiator (last picture), respectively. I don't know if it's possible to put up some panels anyway?
Maybe above the radiator and right under the stairs.. though there's not much space.. 
I can make some pictures of what I mean if something is unclear 

The other two points should be more accessible... though a pipe could be in the way (last picture)..


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## Tobias (Feb 20, 2007)

ethan: Thanks for your input. I'll try to post a measurement without any EQ asap then


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## bpape (Sep 14, 2006)

Might have to consider reflection panels on stands. Regardless, the decay time in the room will want a decent amount of broadband and 2"

Most likely the peaks and nulls up to 100 (disregarding the 20Hz that only an EQ can fix) are a function of seating position and tweaking the speaker/sub position

Bryan


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## Ethan Winer (Jul 21, 2006)

Tobias,

> Can you (or someone else ) get any information out of this graph regarding how to make improvements? <

Not really. The main thing that graph is telling you is that you need bass traps. Though as Bryan said, speaker and listener placements are a factor too.

--Ethan


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