Room acoustics and how to treat a
room is a HUGE topic, this page only a scratches the surface of Room
acoustics as there are so many different variables that effect
sound. Only the main topics will be discussed here and should form a
good starting point for learning what you want in this topic. This page
concentrates mainly on improving the sound of a room, NOT sound proofing
a room which is completely different. By
sound proofing a room, your not only keep the sound out, you also keep
sound in the room which can cause problems such as large RT60 and slap
echo's. The following covers these topics and
will help you to correct problems that rooms and sound proofing cause.
This is written for both small and large rooms which may be used for
What makes a room sound good ?
Only one thing makes a room sound good. REVERB. The reverb of a room
needs to be equal right across the frequency spectrum for this to be
true. If the top end dies away really fast, yet the bottom end or the
mids hang around twice as long the reverb may destroy the room sound.
Also too much reverb in a room even if its balanced reverb can destroy
the intelligibility of spoken words or faster paced music. I'm sure
you've starting singing in the bathroom before and realized how much
better your voice sounds in a room with REVERB. A room without reverb
does not sound very nice, most people would agree on this.
What is Reverb and RT60?
Reverb is made up of heaps of echo's/reflections off hard and
reflective surfaces. Each walls reflections combine to make up a smooth
tail to a sound called REVERBERATION. If you are speaking and the sound
carries on after you have stopped speaking, that's reverb at work. A
nasty form of reverb is the ECHO which is not smooth. A particularly
nasty type of echo is the FLUTTER ECHO, which is mainly caused by
parallel reflective walls. RT60 is the term given to the length of
reverb. If the sound takes 3 seconds to die down to 1 millionth of the
original intensity (-60dB) , then the RT60 of the reverb is 3 seconds.
Churches and reverb
Traditional churches were acoustic nightmares (for fast paced music
and spoken word) as the room was designed to amplify and have very long
reverb times. Choirs on stage would sound 3 times as large with larger
reverb times, hence the design of cathedrals. This reverb sounds great
on the choir, but on the spoken word the intelligibility is greatly
effected. The congregations struggles to hear the words and will very
quickly become fatigued. Modern churches with drum kits and full rock
bands that play very fast also suffer from the long reverb as each beat
of the sound carries well into the next or even 6 beats time. Hence the
reason why many church's are using acoustic treatments to control the
sound. The following will help churches with acoustic problems greatly
as large RT60's are normally found in larger rooms. Its much cheaper to
design a good room from the start then try to fix a problematic room
Building a good room
To build a good room for sound there are three main things to take
into account. First is that Bass takes space to correctly form (at least
1/4 of a wavelength is needed), so smaller rooms should have high
ceilings just like early church's. Secondly the dimension of a room
should NOT be a multiple of the same number...eg. 20 foot by 30 foot by
10 foot high. A room with these dimensions would have serious problems
as a common frequency or ROOM MODE would become very prominent and cause
feedback as the room will become RESONATE around this frequency and
harmonics. Lastly the room should not have or as few as possible
parallel walls/roof. 12 degrees offset is normally sufficient that
eliminating these room modes from occurring as the walls are not a set
distance apart. 6 degrees for each wall is not a lot to offset and can
fix a lot of standing waves not to mention the reflections.
How do I fix excessive echo/reverb,
and make a room sound better ?
If a room has a problem with echo's you will find that if you treat
only one wall of a room which you think is a problem, the others walls
that are untreated will become more apparent. So treating all
walls/roof/floor is necessary to combat echo problems with absorption,
which will over suck a room and leave it dead. If you don't want a dead
room then you can fix with two methods through scattering the sound
called DIFFUSION and secondly through ABSORPTION.
Remember how above I stated that smooth and equal reverb sounds good
in a room ? Well most people when treating a room will do one of two
things. The first is to concentrate on absorption and suck all the
reverb/life out of the room. Anechoic chambers are rooms used for
testing sound equipment mainly microphones and speakers. These rooms
have zero reflections through the use of BLACK EYE's and absorption over
all walls. If someone who is not used to working in such an environment
is placed in a room with no reverb they will nearly always throw up and
lose their sense of direction. Hearing is used for balance and sense of
direction and without reverb and reflections the human body feels very
weird. This is taking it to an extreme, but still proves the point that
absorbing too much sound out of a room is going to make people feel
uncomfortable and make the room feel cold.
The second thing that people do wrong is only absorb half the sound.
By this I mean only the top end (treble) is absorbed and the bottom end
(bass) still has a large RT60 making the room appear dead but still have
a BOOM to it. Making the reverb in a room more smooth and equal through
diffusion and some absorption in problematic frequencies will result in
the best acoustic treatment for that room. Going back to what makes a
room sound good, you first need to listen to the room and decide if the
room echo's, or has a smooth reverb tail. Secondly you need to then look
at the reverb time of the top end and the bottom end. If the room is
small and has hollow plaster walls then the room wont/may not have
problems with the bottom end and Diffusion will be the best way to
improve the rooms sound. The hollow walls can create a bass trap and the
small room dimensions mean small RT60. The rooms echo's will be smoothed
out into reverb by the scattering of the sound off diffusion panels. The
second common type of room is the large concrete room that has solid
walls. This type of room will need bottom end absorption to stop the
BOOMiness of the room and diffusion to smooth out the sound. A large
room may also have too much reverb so some full spectrum absorption
should be added to reduce the RT60 of the room. The absorption should be
scattered around the room and not just on one wall. 10 - 40 % coverage
of total wall space is normally enough.
To sum up you need to turn echo's into reverb with diffusion panels,
and secondly you need to ensure that the bottom end had the same RT60 as
the top end. After achieving those two things you can use broadband
absorption to reduce the overall reverb time (RT60) spreading the
absorption over all the walls not just one. This is hard to get right
and why professionals are normally called in for advice who can
calculate how much of each type of treatment is needed in a room.
Voice over rooms and ISO
This type of room is best when heavy use of broadband absorption all
around is used. More than 70% coverage of the rooms walls and roof
should be covered in a absorbing material. The floor doesn't matter that
much as the human ear cant hear as well from below.
Drums sound best in a live/reflective environment A wooden room tends
to bring out the best in a drum kit and for that matter most acoustic
instruments. The room contributes HUGELY to the sound of a drum kit and
using absorbing materials in a drum room will destroy the sound. In this
type of situation if the room is small to mid sized the best treatment
is using reflective diffusion panels to smooth out the rooms sound.