I have been recently asked some questions about acoustics. Sound reinforcement is one of my personal passions, so I have collected here some tips about it…
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 completly 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 echos. 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 recording.
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 absorbtion, which will oversuck a room and leave it dead. If you dont 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. Anecoic 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 feel very strange.
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 unwarm. 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 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 absorbtion 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 absorbtion 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 booths
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.