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Echoes, reflections, reverberation time, a lot of these terms get confused and I thought I’d just illustrate some characteristics of echoes and put some time signatures on them and this will give us some feel or idea and separation. These are complicated things to understand and the definitions behind them are very empirical and if you don’t really have a background in physics it can be a little difficult.

So I’ve taken some of those complex things that our laws of physics tell us define echoes and I’ve broken them up into parts I think that we can understand and get some kind of – I call it conversationally competent feel for what an echo is.

So let’s get back to some basic – direct sound from a source is the first sound you will hear. So it’s the first sound if I’m speaking to you and you’re standing in front of me, that’s the first sound that you’re going to hear. So we have this direct that we need to be aware of. And then we have the reflected energy from surfaces. Most of us communicate in buildings, in rooms that have boundary surfaces.

So we have these two sounds. We have direct and reflected energy. Reflected is always the second sound, direct is always the first. So we have to keep that in mind. Now, the reflected energy has a component to it, called a time delay. Because sound travels at a fixed rate of speed at a certain atmospheric pressure. So anything that bounces off the surface of the room will definitely travel a longer distance than the direct sound which travels straight to you from my voice to your ears. So everything we hear is always a combination of those two.

And the reflected energy always has a time signature to it. The direct energy always has a time signature in it. So if the difference between the direct and the reflected energy, the time gap difference between them is less than 15-20 milliseconds, they combine to form one sound. Now, for those of you that are familiar with time signatures this number should ring some bells. 15-20 milliseconds is the time signature that we assign to reflections in our 2-channel listening system from the sidewalls.

So we always try to shoot for treatment that gets the time signature of the sidewall reflections and the direct energy of our listening position within this time frame range. Okay, so that should ring a bell there. And why? Because we want to form one sound. We don’t want to be sitting in our chair in our listening room, our mixing room or any room for that matter and hear more reflected energy than we do direct.

Okay, so then there’s this thing called the precedence effect. Okay. That is where you identify a source first, okay, the direct sound first and that is a combination of many, many variables. But our goal is always to try to identify the source first and then minimize the impact of the environment or the room we’re in.

So if we exceed this time gap that we talked about here of 20 milliseconds then we get into the separate domains. We get the reflected sound and the direct sound and you can hear it. And this is the beginning of echo and we’re going to discuss that further as we move forward.

Alright, echoes. Two walls separated. We all have walls in our environment that we listen to music and communicate in. So if we have walls we’re going to get a series of regularly spaced echoes, okay? And that’s what we have to look at here. And I want you to get a feel for echoes and the definition of an echo because we’re going to add all those together and then we’re going to get something called reverberation.

But let’s stay with echo here. We’ve all heard this term, flutter echo. Flutter echo is echo, it’s just kind of a sister or a brother of an echo and it’s really confined to smaller spaces at higher frequencies. Flutter echo is a great thing. I heard the other day in my car when I was driving. I rolled down the rear windows. I was going about 80 miles/ hour and I heard the air against the car and the aerodynamics of the car was creating this frequency. And I got out my phone and I shouldn’t have been doing this while I was driving but I realized that it was in the 900-1500 cycle range which is right where flutter echo occurs.

So small space, the car, air moving back and forth in a small space. And we get that in acoustics. The smaller the room, especially in vocal rooms, and the energy level is high we can get a term that’s flutter echo. So remember, it’s a series of reflections that interfere with the direct sound and have a certain time signature. So that’s the conclusion we’re trying to arrive here.

When the time signatures are close in milliseconds and they pass that 20 milliseconds mark that we just discussed we’re going to get a term where you can’t distinguish between direct and reflected energy. And there’s when inaudibility comes into play and there’s where we get the term reverberation. Now, we’re going to go through reverberation in another video. But I want you to see that echoes, flutter echoes, they all lead somewhere. So there’s a starting point of what we call a reverberation.

Now, reverberation and we’ll discover this in our other videos, in order to get down to the point where we have audibility we have to decrease the energy by 1 millionth of its value or 10 to the minus 6th power. We won’t go crazy with the scientific stuff but any RT60 time that’s in the 1-3 or over the 3 second range will produce that kind of inaudibility.

So I hope this helps with echo. I didn’t want to use too scientific of a definition here but I want you to realize that it’s a series of reflections of boundary surfaces that have a certain time signature and it’s always in conflict with the direct sound. How much of a conflict it’s in depends on how much we have to reduce it and the appropriate time signature of it.

This is an unedited transcript from our video series from Acoustic Fields. There will be some errors in grammar and sentence structure that occur during this translation process.

For complete understanding and comprehension, please view the video which is included in this text. For any additional information regarding this topic or others relating to room acoustics, please contact us directly at:

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Dennis Foley

I am an acoustic engineer with over 30 years’ experience in the business. My technology has been used in Electric Lady Land Studios, Sony Music of New York, Cello Music and Films founded by Mark Levinson, and Saltmines Studios in Mesa, Arizona, along with hundreds of others.

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