Reverberation Definition – Large And Small Room Acoustic Applications – www.AcousticFields.com
Hi everyone, Dennis Foley from acoustic fields.
Today we are going to talk about reverberation. It is a widely used term, but I believe kind
of misunderstood a little bit. It is applied to small rooms when really it is a large room
phenomenon, but that said, we can use it in small rooms for guideline. Today we are going
to talk about reverberation and large room applications. Why it is really important.
We are going to talk about the defining, the rat 60 time, how you arrive at the rat 60
time and what it is all about, what it actually involves, and then the measurement procedure
itself, there’s some dos and don’ts in measuring and we see a lot of errors in this, so we
will talk about those. And then in small rooms what its application is and some guidelines
that can help us out a little bit. Back to our discussion on reverberation. Let
us talk about reverberation as a large room phenomena. What do we really have occurring
in reverberation. We have a series of reflections off the wall surfaces our stage and our seating
area is a broad based area, there’s a lot of surface area here, so we have multiple
reflections off the wall surfaces. So our goal with reverberation is to try to get an
equal distribution of all the energy within a big room. Uniformly spread across the seating
area. So reverberation is mainly a large room phenomena, and it is frequency depending.
At all frequencies we want all sound to sound the same. So you know when you’ve been to
large room auditoriums you have certain seats that are good visually and certain seats that
are good acoustically, the goal is trying to find a balance there. As engineers, what
large room acoustic people try to do is get a uniform chambray or reverberation coverage
in all these seated areas in a large room situation. So let us look into definition
of reverberation. Really it is pressure decay rate. It is a way of figuring once you interject
pressure into the room, how long does it take in a time domain for that energy to drop 60
db. And that’ll give you a rate, an rat rate at frequencies and octave bands. So it is
a decay rate. It is a measure if you will in more common language of how live or how
hot the room is. So it is a time interval for the pressure
to drop at least 60 db. And then we measure it in each frequency and each octave band.
So we introduce pressure into the room, shut the energy source off, and do our measurements.
Let us talk about the measurement of reverberation, this is an issue that’s tricky in small rooms.
Because we usually have a left and a right channel, a 2 channel system producing energy
into the room. What does that do, it causes lots of reflections off the boundary surfaces.
So are we really getting an accurate reverberation time in the room with this kind of setup.
A better approach would be to energize the room in the so called center part of the room,
spread the energy out more evenly in the room, and then you get a more accurate decay rate
if you will at certain frequencies and certain octave bands. So this is I think where some
of the confusion lies because reverberation is really a large room phenomena, but we do
use it in small room measurements. Some people say if it can be measured it has to be important.
Well, maybe but it is really a large room phenomena and it is applied to small rooms
and I thin that’s where people need to have the understanding. In order to take an accurate
reverberation time reading, we need to energize the room equally so if we put our sound source
or energy source in the middle of the room, and radiate out, we get a more even energy
distribution in the room, and thus a more even rat 60 time.
Let us talk about a little bit about the applicably of rat 60 times in small rooms. Well, we have
all head the expression, too dead, too live as it pertains to reverberation times in room.
So reverberation time does have some applicability in small rooms even though it is a large room
phenomena. When we use the term too dead, that really means that our reverberation times
in the room are less than 2 tenths of a second, that’s a general guideline that we use in
small room acoustics. If it is greater than .5 seconds, then it is a little too lively,
or too hot sometimes is the expression, so the normal range is you know, somewhere between
0.2 and 0.5 seconds of reverb time in your small room environment for music. And that’s
usually the objective we’re after here. We did some large room measurements in a cathedral
and it had 100 foot high ceilings and 200 foot walls and we had a 6 second reverberation
time in some of the lower frequencies so you could see it is a wide range of delays here
that we are discussing but in most of our listening rooms that we use for music we try
to keep it between 0.2 and 0.5 seconds. A lot of variability between those 2 numbers
and everybody’s tastes are a little different. Some people like the fact that it is a little
more lively, in critical listening rooms it is a little more dead, thus where the term
live and dead concept came out of. Depends on usage and your personal choices. What reverberation
times that you prefer but in small room acoustics we like the 0.2 to 0.5 second range.