Shape And Size

So what actually happens to speakers setup in the open air? Firstly, the direct sound from the speakers is all you hear. There are no reflections coming from walls, ceiling, floor etc. Secondly no sound comes back to you a second time from a rear wall. In open air there is no reverberation time yet every room has a reverberation time. You can work out the reverberation time of your room by using the Reverberation Calculator Page.

the high frequencies are directional, most horns have a spread around 30 - 45 degrees the low frequencies emanate in all directions from a speaker cabinet, that's why you can place a subwoofer anywhere in a room because the ear can't pick direction in low frequencies

Speakers Outside

reflections off the walls add and subtract the waveform causing colourations what happens here is that the low frequencies reflect off the wall and add to the normal waveform causing cancelation and doubling of the sound

Speakers Inside

As you can see the walls really muck up the sound waves because they reflect back off the wall behind the speakers and the reflected waves arrive at the listener in and out of phase. As a result they add and subtract from each other creating confusion in the bottom end. Also the highs and mids are going to reflect off the walls and the room rear wall. 

Standing Waves

Another problem created inside is parallel walls and standing waves. Standing waves are when a sound reflects off walls that are opposite each other and a wave equal to the distance is formed. As you move around a room with standing waves you can hear as you walk in and out of a standing wave. In one spot the bass is booming yet in another there is hardly any bass. Makes it hard to figure out how much bass you have? It works like this:

as you move around a room the waveforms add and subtrack making it hard to figure out correct frequency balance out of phase waveforms cause room colourations in phase waveforms add to each other causing room colourations

Typical Standing Waves

What happens here is that if you stand at the high point of an in phase standing wave you hear double the volume of the frequency yet when you stand at the same point in an out of phase standing wave the waves cancel each other and you hear nothing. It's pretty hard to figure out your sound frequency balance when this happens throughout your control room. It happens at all the octaves of the frequencies as well so if the frequency is 440Hz it also happens at 880Hz , 1760Hz ,3520Hz,etc. This is what creates coloration in the room. As you move around the room the frequency response keeps changing causing room coloration. It's also a problem in the studio if your mic is sitting directly in an out of phase node!! So the first thing you must do is eradicate all the parallel walls in a studio design. I believe that a wall must be at least 12 degrees off parallel to stop parallel wall standing wave interference. That's either one wall at 12 degrees or two walls at 6 degrees each. If you can afford to make the angle bigger, do so. Also you can create angled walls within a rectangular room by adding acoustic treatment. (I'll demonstrate that in the pages on wall treatment). (Note: having non parallel walls doesn't entirely stop standing waves - they still form within a room but along different lines of repetition). The main reason for having angled walls in a control room is because of reflection control of the high frequencies for true imaging from your speakers.

This effect also happens between the floor and the ceiling so to stop the effect you must angle one! well it's not going to be the floor is it. I must state here that putting angles into the ceiling is expensive so I would recommend that for the home studio you use the acoustic treatment to break up the ceiling. (See wall and ceiling treatment pages)

Room Modes 

The formula for determining the fundamental frequency of a standing wave for a particular room dimension is:

f = V / 2d

Other standing waves occur at harmonics of the fundamental frequency - that is 2, 3, and 4 times the fundamental.

Fundamental Frequency Calculation

Enter the value of one of your room dimensions and you measurement system. 

If you are working in: 

Metric insert the speed of sound as 343 m/sec.
Feet and inches
insert the speed of sound as 1130 ft/sec.
Then click on any other field and all fields will be calculated

Speed Of Sound
Metres/Feet/ per sec
Room Length
Meters/Feet 
Fundamental
Hz
1st Harmonic
Hz
2nd Harmonic
Hz
3rd Harmonic
Hz

Thus a room with an 6 metre dimension has standing waves forming at 

yet a room dimension of 3 metres gives 

In other words rooms with dimensions that are multiples of each other create similar room modes - so avoid room shapes with dimensions that are multiples of each other.

The Kick Drum: It is also interesting to play with the calculator in other ways. Try entering the measurement of a 24" - (2 ft) (0.600M) kick drum. A kick drum being a circle is a continuous parallel wall and it will have a fundamental frequency. You will find that the fundamental frequency of 20" - 24" bass drums is around 300Hz - wow ! do you recognise that frequency when equalising kick drums??

So what size should a control room be? These days people are building bigger and bigger control rooms because so much happens in the control room now. Often the bass player and keyboard player actually sit in the control room. I often have the vocalist singing in the control room while the bed tracks go down, so a good size control room is a good idea. 

So many people think that you need a big studio and end up putting a poky little room at the end and call it a control room. I suggest that a control room ideally should be at least 6m x 5m with a minimum ceiling height of 2.4m. This size room means a good sized working area with space for the musos, friends and hangers on. Also because you don't want rear reflections to interfere it is better to start with a longer front to back dimension than the side to side dimension. i.e. 6m x 5m.

The studio, on the other hand, requires a different set of considerations. The first to consider is do you want just one room or more? The trend nowadays is for more than one studio so the you can get isolation between players and different acoustics for each room. The acoustics you want for a live drum sound is totally different than you want for a vocal for example. I recommend at least 2 rooms. You can put drums in one, guitars in the other, keys , vocals and bass (DI) in the control room or maybe the band in one and the vocalist in the other. Getting separation between guitars and drums is usually OK in a good sized room but keeping them out of the vocal track is hard if they are all in the same room. Just a note here - the kitchen is an important room to consider. Gallons of coffee and tea are drunk in studios. An engineer was once asked - how do you normally have your coffee? "Cold with a fly in it !! "- was the reply. Also a kitchen allows for a fridge for the beer and a microwave for the pizza, vital ingredients in every album recording session. Toilets (bathrooms for you in the US) are also a vital service a studio should offer. The coffee and beer has to go somewhere.