Removing Echo

[Onlooker]

Could any of the more experienced here help me with a tip on how to remove echo from a recording? Like from a staircase or hallway.

[reggind]

Echo is *really* hard to get rid of, but here are two tips.

First, a lot of echo is caused by sound bouncing off opposing walls. If you clap in a stairwell for example you can here it echo or ring. The good thing is that the walls are a consistent distance apart. While you normally think of sound in terms of frequency, you can also think of sound in terms of wavelength. And there is one frequency who's wavelength corresponds to the distance between the walls.

Now, if you can measure the distance you can actually figure that out. You need to know the approximate speed of sound. You can look up on line wavelengths for different frequencies. Another thing you can do is use the analyze feature in audacity to show you where the spectral peaks are. Generally the worst resonance (which is that standing wave) will be the worst peak in the entire recording as it gets tickled by any broadband sound. You can make a note of the frequency, generally in the midrange area, where the big peak(s) are and than..

The took that works best to get rid of them is a parametric equalizer. A graphic equalizer, and even a third octive graphic equalizer will not do, the filters are way to broad. A parametric equalizer has three controls, and often times only has a couple of sections. One control is the center frequency, or the frequency the filter will work on. Dial this into the frequency where you noticed the big peak above. The next knob of interest is the amount of boost or cut. You will probably want to set that to the maximum amount of cut, typically -12 or -15db. The last knob, and this one is the semi magic one, controls the Q or the sharpness of the filter. You want the filter to be pretty sharp. The game you will play is tweaking the frequency of the filter and the Q of the filter to get rid of as much of the offending sound as you can.

If you are using plug in's for a computer program (ie you have a software parametric eq) this can require a lot of going back and forth and making very subtle changes. You will get better at it the more you do it. If you can find a plug in that lets you change the parameters on the fly, that is much easier, but beware that often times because of the way the filers work they can take up to 10 seconds to fully represent changes. You need to change things very very slowly. If you can get our hands on an old analog parametric eq you will find it orders of magnitude to learn on.

That is pretty much the free solution to getting rid of echos, and truthfully, it leaves a lot to be desired, but it can help.

There is a program called Izotope RX5 that has an echo remover among other things. That seems to be based on frequency selective compression and or gating. It has a few controls, and it does work to a somewhat better extent than just a parametric eq, however if you turn it up too much, the sound starts to get weird pumpy artifacts. That is you can hear the filter cutting in really hard after a sound. If you are using Izotope RX5 and it's echo remover to clean up the sound in a voice over or something with other ambient sound or music, the pumping effects are much less detectable and much less annoying. Sadly for our hobby, that is not the case, we are not mixing in other sounds so there is no masking of the artifacts of the filter.

So, for free you have a parametric EQ you can play with, and for $$ you can play with Izotope .

[thomas]

I am impressed, reggind, your knowledge is really unlimited... Do/did you do this kind of things professionally?

[Onlooker]

Thanks for that reggind. Much appreciated assistance.
So glad I found this place...

[reggind]

I did pro audio for a bit, and audio electronics for a bit. Still a hobby.

To some extent playing with the files here is fun because it is a challenge. You have one take, you did not record it yourself, the recordings are typically very very sub optimal (the best folks can do, I don't think knocking on a door and asking them to do a dry run so you can set your levels would fly). So we are trying to pick the needles out of the haystacks.

On the plus side, the array and quality of free tools we have at our disposal is just amazing.

[psymedia]

will include this in my tutorial for audio cleaning - my tool kit has specific tools to do this i will explain all via the tutorial in the coming days