# Identifying Frequency of Reflections...Waterfall, Energy-Time, RT60?



## tqnguyen (Sep 2, 2006)

Hi all, this is my first post. Great group of people we have here!

I have a question about which tools in REW to use for dealing with reverberation/reflection. I read through a bunch of posts and my understanding is that for frequencies < 200 Hz, we use the waterfalls plots. For > 200 Hz, we look at the Energy-Time Curve (ETC).

However, in the ETC how can we identify the frequencies of the spikes? For all those spikes that are after 0ms, how can I know which spikes correspond to frequencies <200Hz, which I don't care about in ETC since I have waterfall for that. Further, how can I know whether a spike corresponds to a large reflection at (say) 500 Hz or 3 kHz? This would help me to identify the exact type of room treatment needed.

Would ETC in hand with RT60 be effective here?


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## brucek (Apr 11, 2006)

> However, in the ETC how can we identify the frequencies of the spikes?


Just take the measurement over your bandwidth of interest. You'll find subwoofer ETC plots not to revealing. Use waterfall for that bandwidth. You'll find that when a reflection is present, that it will usually show with different widths with respect to frequency, so it's not a determining factor. 

RT60 is not useful in the typical home environment. The rooms are too small - think gym or church halls.

brucek


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## tqnguyen (Sep 2, 2006)

brucek said:


> Just take the measurement over your bandwidth of interest. You'll find subwoofer ETC plots not to revealing. Use waterfall for that bandwidth. You'll find that when a reflection is present, that it will usually show with different widths with respect to frequency, so it's not a determining factor.
> 
> RT60 is not useful in the typical home environment. The rooms are too small - think gym or church halls.
> 
> brucek


Ahh, that works too. I was hoping for an easy way to see all frequencies on one graph via one measurement, so as to minimize the manual work of taking multiple measurements.


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## DrWho (Sep 27, 2006)

What do you mean by the frequency of the reflections? Or rather, what would you hope to do with such information?

For the most natural sounding room, you're gonna want to maintain a flat power response while keeping the direct sound flat as well. You have to be careful that you don't start creating an unbalanced indirect sound as that will sound quite bad. Usually all speakers have tighter polars at the higher frequencies and wider polars at the lower frequencies....sadly, most absorption is more effective at the highs than at the lows....so if you dial in your speaker to be flat on-axis, then you're quickly going to end up with a bottom heavy indirect sound, which is often going to sound congested/muddy.

Btw, RT60 doesn't mean anything in a "small room". The reflected sound isn't dense enough to achieve the perception of true reverberation, so any tweaks to "improve the RT60" will often lead to mediocre results.


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## tqnguyen (Sep 2, 2006)

With information on the freq. of the reflections, I am looking to add the correct absorption. For example if the bass has too much reflection, then I would add thicker panels. If there is too much midrange or treble room energy, I would go with thinner panels (e.g. 2"). If the thin panels absorb too much treble relative to the midrange, perhaps I could cover the panels with a somewhat reflective fabric to balance out the absorption.

That was the informal thought process in my mind on how to treat room reflections...I was looking for an easy way to identify the frequency bands that had the most reflection so I could target room treatments to that band.

I'm playing with an active crossover on my 2-way vertical MTM floorstanders, so I have a lot of flexibility (and confusion) on trying to achieve a balanced direct on-axis sound and indirect sound, as you mention.


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## DrWho (Sep 27, 2006)

The only time a reflection will contain some frequencies, but not others is when the polar response of the speaker isn't flat - which will be the case with any 2-way MTM. Your low frequencies will be nearly omnidirectional, but your highs will be much narrower. So already off the bat I can tell you that you'll have more low frequency content at *all* of your reflection points (assuming you've got things dialed in for a flat on-axis sound).

Generally speaking, any amount of absorption you add is going to be more absorptive at higher frequencies than it will at lower frequencies. In an ideal world, your speaker would have a flat on-axis and flat polar response, which would mean all reflection points and all of your room's "reverb" would have a flat tonal balance in addition to being flat on-axis too.

Since we're not talking ideal speakers, the problem gets kinda nasty.

I think your best bet would be to aim your speakers to keep as much of the mids/highs off the side walls (and ideally the floor and ceiling too), and then the only place I would put absorption would be just outside the coverage window of your mids/highs (which would be the corners behind your speakers). Come as far forward as is needed to effectively flatten out the off-axis energy launched into the room. And then everwhere else I would concentrate on using diffusion.

Page 2 of this PDF shows a before and after ETC, with some descriptions of key areas:
http://www.rpginc.com/news/library/HT_AcD.pdf
One of the goals is to have an ITD at least as long as the Haas Window, and then have a semi-reverberant sound field that is as dense as possible and decays naturally.

Btw, do you have the ability to measure your speakers outdoors away from any boundaries? Having an active xover is a very powerful tool, but keep in mind that you can only tweak for the polars over the range where the xover passbands overlap. It would be ideal to pick a xover frequency where the polars of your tweeter and your array of mids are very similar (that way the off-axis response has a natural curve to it too). You could do off-axis measurements outdoors to get a feel for where this point occurs and find out if it's within the operation band of your drivers. Distortion is another thing you want to line up too so there's often some compromise required there. The polar response will also tell you how your speakers will spray sound into the room.

Low frequencies are "slower" in the time domain than higher frequencies, so on the ETC plot the low frequency reflections look like smooth rolling hills and the higher frequency reflections look like poles in the ground. Try doing a measurement that only goes up to say 200Hz (like you would do with a subwoofer)....you should notice no "spikes" on the ETC.


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## tqnguyen (Sep 2, 2006)

Thanks for all the setup advice, Mike. It's very informative. I'm definitely fighting the active crossover battle as much as the absorption/diffusion battle. I will try to borrow a laptop and lug the speaker outside before the weather gets too cold. Probably due to my very basic equipment (DCX2496 and ECM8000) and not measuring outside, I currently seem to have a treble-heavy sound in my room.

Also, unfortunately due to room constrains I'll have to push the listening position up against the back wall. I was thinking of deadening this back wall with a lot of absorptive material. At least I don't have any side wall reflections to worry about in this configuration.


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