Playground (not suitable for children under 30 year).

The Problem With Sampled Instruments and Reverberation

One problem with adding reverberation to instruments samples is that said samples normally already contain reverberation to some degree. Wet sampled instruments (at least historically including many orchestra libraries) contain a high degree of reverberation, even if many of them have been edited to contain an optional release tail. The same is true for instruments sampled dry, although they are sampled in rooms with very short reverberation times (a low RT60 value) so that the recording only will contain first and early reflections whose desirable levels are figured out during instrument and microphone placement.

But adding reverb to a sample already containing reverberation will cause comb filtering to some degree depending on sound source and reverb characteristics. One solution would be to sample the instruments in an anechoic chamber (e.g. Musical Instrument Samples from the Electronic Music Studios, University of Iowa) or at least in a hemi-anechoic or free-field chamber (see David A. Nelson, Acoustic Test Chambers and Environments, National Instruments, November 15, 2006).

The Problem With Acoustic Instruments and Radiation

But anechoic recording has its own set of limitations. For instance microphone placement in relation to the radiation of sound by acoustic instruments (Scott Wilkinson, “Resonance and Radiation”, Electronic Musician, September, 1994) need to be taken into account. In an anechoic chamber sound radiated as non-direct sound will not be reflected back and recorded by the microphone like in a studio or auditorium. This is especially troublesome with bowed strings having shifting radiation patterns depending on the tone being played.

Principal sound radiation patterns for the cello.

Conclusion

An omni-directional source, like a starter pistol or carbon arc, is regularly used as impulse during the recording of room impulse responses. After convolution with a sample the output will be analog to the sample being played back as an omi-directional sound source in the room. To be precise: the impulse response will contain all characteristics of the impulse source, room, microphone and signal chain. An instrument that naturally focus its sound forward (as principal radiation pattern), and being mic’ed accordingly when sampled, will then radiate the same principal sound level and frequencies in all directions in the reverberation model. This is however countered under normal circumstances by subjectively balance the dry signal, representing the direct sound, with the wet signal, produced by the reverb, in the mix.

With synthesized impulse responses it would be possible to take the orientation of the sound source and its radiation pattern into account (Perry R. Cook and Dan Trueman, A Database of Measured Musical Instrument Body Radiation Impulse Responses, and Computer Applications for Exploring and Utilizing the Measured Filter Functions, Princeton University, 1998) and filter the dry input to simulate the non-direct sound being radiated and reflected back to the microphone.

There are two extremes to studio building: The situation were you are stuck with a room and have to do the best of the situation, and the situation were you have total control and can do the best you can afford. In here I try to examen the first and most common situation and how you deal with it in three easy steps. While I’m going to approach a room that isn’t necessarily allocated as studio all the basics will still be the same when you slide across the scale to the other extreme.

Just let us clear up one thing right away: Acoustic treatment and sound isolation are two completely different concepts. When you design a studio from scratch you will take both into account from the start of the design process. But with that said it’s worth to know that an acoustically pleasing space will let you play back sound at lower volume with better definition and do not exaggerate lower frequencies and make the room sound less loud. So even without any special sound isolation just treating it acoustically will probably make your neighbors happy.

So let’s get cracking!

The Right Room: Shape and Size

This is the step where you find out how bad the room you’re stuck with are, or if you are lucky, help you choose a better room.

To start with the room need to have a rectangular shape. There are nothing wrong with angeled walls and ceilings, as long as they deflect the sound instead of focusing it, but their behavior is harder to calculate and predict. On the other hand the worst shape would be a perfect cube. Ideally the ratio between the height, width and length of the room should not be multiples of each other. So a rectangular room with 1.0:2.0:4.0 as ratio is likewise hopeless.

Unless you can build a new room from scratch, knowing about the perfect room ratios such as the preferred 1.00:1.14:1.39 ratio (in addition to 1.00:1.28:1.54 and 1.00:1.60:2.33) developed by L.W. Sepmeyer in 1965 or the other preferred 1:1.4:1.9 ratio (in addition to 1:1.3:1.9, 1:1.5:2.1 and 122 other ranked ratios) developed by M.M. Louden in 1971 is a bit overkill. But if you are really into it the paper Determining Optimum Room Dimensions for Critical Listening Environments: A New Methodology (Cox et al., 2001) will probably be of interest.

OK, back to our more or less ideal room. Despite what anyone might tell you: size do matter. A smaller room will be more difficult to treat and sound worse than a large. It’s hard to have a too big room, and if you do, that’s a pleasant problem. A good room should at least have a minimum volume of 72 m³.

So why do the room size and ratio matter?

The problem is called standing waves. When sound reflects off walls opposite each other a wave of equal distance is formed, basically a copy of the sound wave will come traveling from the other direction, these twin waves are called a standing wave. If you stand at the height point of an in phase standing wave the volume of the waves frequency will double. But if you stand at the same point in an out of phase standing wave the twin waves will cancel each other out and you won’t hear any of them at all.

All rooms have standing waves, the question is how to deal with it? This is where room size and ratio enters the picture. The frequency of the standing waves depend on the distance between opposing walls, not to forget floor and ceiling, in a room. The formula for finding the fundamental frequency of a room dimension is quite simple:

f = V/2d

Where f is the fundamental frequency, V the velocity of sound (344 m/s) and d the dimension. A 8 m long room will have a fundamental frequency of 22 Hz in that dimension, but the same phenomenon will also occur in all the octaves of the frequency affecting the harmonics of 44 Hz, 66Hz and 88 Hz and so on as well. The further apart your walls are the lower the fundamental frequency will be and the harmonics closer together making the average room response fairly flat. That’s why size matters!

So if your room was square that would mean you have the same fundamental frequency in the length dimension and the width dimension as well. The resulting standing waves would be in phase and out of phase with each other making the cancellation effect even noticeable. Since we have to factor in the harmonics just adjusting the room ratio to multiples of each other won’t help either, in a 1.0:2.0 room the first harmonic of longitudinal standing wave will have the same frequency as the lateral fundamental frequency.

In most cases the room you are stuck with will probably not fall into the worst case scenario category, so this is nothing to lay awake worrying about in the middle of the night. Most rooms will need acoustic treatment for the low-end anyway. But if you live in a tiny cube then you might want start planning a move.

The Right Place: Listening Position and Monitor Placement

Hopefully, you will now have found out how well your chosen studio room are in terms of dimension. Now it’s time to place the two most important components in the room: your ears and a pair of near field monitors.

The first rule of placement in a studio is symmetry. As you probably already gathered from the discussion about ratios the shorter wall define the “width” of the studio and you will face one of the short end walls. Exactly which one is decided by the position of entries and doors into the room. Ideally you should have the entry door at the rear of the studio either on the back or side wall.

The placement of the monitors are inherently coupled to the listening position (”the sweet spot”). It means we first need to select the optimal listening position and then place the monitors relative to it.

So where is the optimal position?

Yet again the frequencies of the room, the room mode, play a big role because the least optimal position will be in a null. That is were waves of the rooms fundamental frequency or its harmonics crosses the null.

Plotted above are the fundamental frequency of a room dimension (in red) and its first three harmonics. The longitudinal nulls are located where the waves cross the dotted line.

This makes it obvious that sitting spot on center is the least desirable position and once you factor in both the lateral as well as longitudinal nulls the picture becomes even more complicated. So there need to be compromises. One obvious compromise made in almost every studio is that you will be sitting straight in the middle between the side walls to avoid unbalanced stereo imaging. In the same spirit it’s easy to rely on Wes Lachot’s so called “38 percent rule” that states that the best listening position is 38% into the length of the room.

The logic behind the rule is simple: if you plot the first five or six harmonics you will notice that the position least affected by nulls will be 38% into the room. What you might already have figured out is that the actual distance between the 38% length of the room and the disastrous 50% spot will increase with the length of the room giving more leeway between nulls as the room size increases. So yet again, size matters!

Now that we have found our listening position we can start to place the monitors. For stereo imaging the ideal monitor position will place the listener and monitors at points on an equilateral triangle. The listeners point should be behind the head, so that the ears are touching the sides, and the monitors should be placed somewhere along the sides between the listener and the front wall. Exact placement require some slight experimentation, just remember to avoid the obvious nulls. You can also make use of the Wall Bounce Calculator (Excel) to figure how sound waves reflected by the front wall will affect the sound.

Make sure the monitors isn’t exactly midway between floor and ceiling in a null. If that’s the case it better to rise them slightly higher and tilt them down, as long as the distance between your ears and tweeter and woofer is being equal.

It should also be pointed out that monitors should not sit on the console or desk but behind it placed on heavy decoupled stands. This will save you from unwanted early reflection problems caused by the console or desktop and stop vibrations from one monitor affect the other. Securely mounted monitors will give you a better sound definition. Consider the tiny distances that the woofer cone and tweeter dome move in normal use, if the monitor vibrates, either by its own or outside disturbance, the movement will subtract from the bass output and may cancel the tweeter altogether. Place a small piece of Tack-It under each monitor corner to prevent them from moving.

The Right Treatment: Rearrange and Place Furniture

Unless you’re lucky enough to have an allocated room for your studio it’s likely that it already will contain a set of furniture unrelated to music production that need to be re-arranged and turned into ad-hoc acoustic treatments. But keep in mind that any control room with self respect should at least have a sofa at the back of the room that’s not only comfy but also doubles as an acoustic treatment.

First of all, try to arrange everything in the room symmetrically. E.g. if you have curtains on the right wall then hang some, or another material with the same absorption/reflection properties, on the opposing left wall.

Start at the back of the room. Do the room contain any bookcases? Then put them all against the back wall and possibly fill the space behind the books with cloth. They will work as an improvised bass trap because bass absorption = mass and the uneven front surface will do some minor diffusion in the higher frequencies. Next put a bed or sofa in front of them that will help absorb both high and low frequencies.

After having the back of the room sorted out it’s time to treat the front of the room to create a reflection free zone. The idea is to prevent so called early reflections from interfering with the stereo image. This happen when sound from the monitors arrives at your ears through two different paths: one directly and the other after being reflected by nearby surfaces such as desktop, floor, ceiling as well as side walls. All the early reflection points are somewhere between the listening position and the monitors, so any ad-hoc treatment doesn’t need to be very exact, just drape heavy cloth a decimeter or so distanced from the wall and put a carpet on the floor. This will not be a perfect solution but will serve its purpose until you can replace it with proper wall hung traps and clouds in the ceiling.

Conclusion

It’s easy to fool yourself by thinking that draping you room in cloth and putting up foam and egg cartoon on the walls will solve all your problems. Quite the opposite actually. Such treatments is really bad and will only take care of the high frequencies efficiently and none of the lower ones making the room sound boomy. The room size and shape, placement of monitors and listening position are equally important as any other treatment.

As a final touch clap your hands at the listening position and listen to the reverberation and possible flutter echo or ringing to determine how well the treatments have worked and find additional problem spots.

In the next installment I will discuss some simple purpose built broadband traps that anyone can make and the theory behind their placement.

Acknowledgment

Thanks to Ethan Winer and John Sayers and everyone at the Recording Studio Design Forum and their selfless efforts to spread proper information about studio acoustics.

Here’s a type of sign I can’t remember seeing anywhere else like London, Paris, Milano, L.A. or the few cities with subways I’ve visited (realize that NYC and Berlin is missing and that I need to book a vacation). Is this symptomatically Swedish or am I just prejudiced?

Sign found at the end of platforms in Stockholm’s subway system.

Of course Germans have jokes. They’re just not funny.

YouTube

This was made possible by ctrl-c.org YouTube API. I put together the bookmarklet because Adobe’s 32 bit Flash plugin is usually not very happy in a 64 bit environment on GNU/Linux. Now I can watch everything fullscreen in MPlayer.