If your monitoring isn't up to scratch, your whole production will suffer, so here are some tips on setting up your room so that your mixes always sound their best.
While professional studios have the benefit of elaborate acoustic treatment and design expertise, the project studio operator usually has to make do with an existing room and off-the-shelf monitors. It's not always easy to get accurate results under these circumstances, but, by applying a few basic principles, you should be able to produce good mixes that sound reasonably accurate when played back on other audio systems.
Professional studio designs tend to be based around soffit-mounted monitors (monitors set into the wall), but the majority of project studio monitors are designed to be used free-standing. Nevertheless, 'free-standing' doesn't mean that you can simply put the speakers anywhere in the room and they will work properly. Not only do you need to set up the speakers symmetrically, at roughly head height (relative to the seated mixing position), you also need to consider the distances between the speakers and the room boundaries.
One reason for this is the existence of room modes, which are resonances that occur when a room dimension is an exact multiple of the half wavelength of a sound being played back. A typical rectangular room has three main room modes corresponding to the principle dimensions — width, length and height — which means that not only will there be three frequencies at which you will expect to experience 'hot spots', but there will also be a whole series of modes at frequencies above this corresponding to multiples of the main mode frequencies. To further complicate matters, there are also oblique and tangential modes related to the corner-to-corner dimensions of the room, though these tend to be of lesser intensity than the main transverse modes.
All rooms exhibit modal behaviour, but these modes only become problematic when they are unevenly distributed, and as luck (or lack of it in our case), would have it, the smaller the room, the more uneven the modal distribution tends to be, because there are fewer modal frequencies that fit into the room dimensions. The worst-case scenario is the small room in which two or more dimensions are either equal or exact multiples of each other, as that results in modes overlaying each other, resulting in very significant peaks occurring at the corresponding frequencies. The worst of all the worst-case scenarios is the small, cube-shaped room where all three principle dimensions are the same. As you might imagine, this is a recipe for modal pile-up on an epic scale, which in practical terms means that some frequencies will sound much louder than others, even though the levels measured at the loudspeaker may be the same.
Modal problems of the kind described tend to be responsible for problems occurring below 300Hz or thereabouts. At higher frequencies, where wavelengths are shorter, the modes tend to be more closely spaced, which produces a more even energy distribution. At frequencies above 300Hz, sound reflections from hard surfaces are the biggest problem and, the smaller the room, the closer the surfaces, so reflections get more troublesome. But you don't just want to hear about problems, you want to hear about solutions.
There are computer programs that can analyse your room dimensions and tell you where the most troublesome modes are likely to occur, but a more pragmatic approach is to put your speakers on a pair of decent stands, play some music through them and find out exactly what you're dealing with. The first thing that becomes apparent is that the effect of room modes depends both on the position of the speakers and the position of the listener within the room. Although your speakers should be set up symmetrically and angled in towards the listener at approximately 30 degrees to the centre line, you have a certain amount of positional leeway in all three dimensions.
When you stand a loudspeaker some distance from a solid wall or surface, some of the low-frequency energy will travel backwards from the cabinet and reflect from the wall. Depending on the distance from the wall, some frequencies will bounce back in phase, resulting in peaks in the frequency response, and others will cancel, resulting in frequency-response troughs. Ideally, you'll want to randomise these peaks and troughs as much as possible, and one way to do that is to ensure that the distance from speaker to back wall is different from the distance to the side wall, and different again from the distance to the floor or ceiling. Also, ensure the speaker isn't exactly midway between floor and ceiling for the same reason. It's better to move it below or above the midway point than to have the tweeter exactly at head height, and you can always angle the cabinets slightly to aim the tweeters at the monitoring position if you need to. Modal behaviour also tells us that exact multiples are to be avoided, so if the speakers are half a metre away from the back wall, don't place them exactly one or one point five meters from the side walls.
What applies to the speakers also applies to the listening position, and if you're sitting close to a wall, you'll notice that the bass end sounds louder than it does if you're away from the walls. This is due to the boundary effect, but you may also notice that hot spots occur if your head is exactly midway between the floor and ceiling, which is one reason studios tend to have ceilings over eight feet high. Of course you need to sit midway between the side walls to get a symmetrical stereo image, but you can avoid sitting midway between the front and rear walls.
Your room size and studio layout will tend to dictate your sitting position to some degree, but it's generally accepted that working with the speakers set up along the longest wall is preferable, especially in smaller rooms, as this minimises problems caused by side-wall reflections. Once your sitting position has been decided, then try moving the speakers forwards or backwards slightly while playing music, because some positions will probably produce a noticeably more even sound than others. A piece of music that sequences a wide range of equal-intensity bass notes is useful for checking this (knock something up on your sequencer using semitone step progressions over, say, three octaves starting at 40Hz) because if you suffer from hot spots, those notes corresponding to the frequencies affected will tend to sound louder than the others. Also listen out for dead notes caused by frequency cancellation and try adjusting both the speaker positions and the monitoring position to compensate. Try to get the smoothest response in the bass register before worrying about the rest of the audio spectrum. In professional studios, bass trapping is often used to even out the bass response, but in a small home studio this simply isn't practical, so most of the time we have to rely on speaker placement and the choice of appropriate speakers.
Every room needs some form of acoustic treatment (have you ever listened to a room that's been emptied, ready for decorating?), but in my experience there's little to be gained from trying to second guess the requirements until you've carpeted the room and moved in your equipment and furniture. Fitting a carpet is probably the most significant thing you can do to a room, but the presence of reflective racks and furniture will also have such a profound influence that designing acoustic treatment based on an analysis of the empty room is pretty pointless.
Professional studio designs aim for a reverb time that is more or less consistent across the frequency spectrum, with the low-frequency reverb time being no more than around 1.25 times as great as the high-frequency reverb time. Actually, 'reverb time' isn't the best term to use when describing small-room behaviour, as most studios won't really allow a true reverberant field to build up. For this reason, the reverb time (or RT60) figure has limited significance, and what is more important is that the room response is nominally flat.
How easy this is to achieve depends on how the room is constructed, as plasterboard walls, glass windows and wooden doors tend to allow some bass energy to pass through while solid brick or concrete walls reflect most of the bass energy back into the room. Large bass traps are not an option in most home studios, so a more pragmatic way to tame bass problems is to choose smaller monitors with a less extended bass response. Absorbent heavy furniture, such as a sofa across the back of the room, will also help.
While a carpeted floor will combat the worst tendencies of a room to behave as a reverb chamber, it will only absorb relatively high frequencies, so carpeting the walls in an attempt to achieve acoustic Nirvana is largely counterproductive and more likely to leave you with a honky, boxy-sounding room. It's invariably better to use as little wall treatment as you can, and always keep in mind how reasonable a typical domestic lounge sounds for listening to music, even though it probably has no dedicated acoustic treatment at all — all it relies on are carpets, curtains and furniture.
In most cases you can fine-tune your listening environment by putting about one square metre of acoustic foam on the side walls either side of your listening position. This will kill annoying flutter echoes and clean up the stereo imaging. Reflections from the rear wall can also mess up your stereo imaging, and in larger studios diffusion is often used to scatter these reflections evenly throughout the room. These diffusers are simply irregular surfaces designed according to mathematical principles, but shelves full of tapes, books or studio junk often serve just as well. However, in smaller rooms, there isn't the space for the scattering to be fully effective, so again the stereo imaging suffers.
A quick and simple solution is to hang a heavy curtain or rug across the centre of the rear wall, in front of your shelves if need be, to add some absorption. It can also be useful to hang absorbent material or to place acoustic foam behind the monitors in a small room, and hanging material a couple of inches from the wall will be much more effective than putting it on the wall itself. A further application of acoustic foam above the mixer may also help tame ceiling reflections in a room with a low ceiling. Again you shouldn't need more than square metre to do the trick.
Once you get to this stage, it's a good idea to get used to the sound of commercial recordings played through your system. Live with it for a while and try to establish whether there are still problems you need to address. Sometimes fine-tuning the speaker positions or angles again will bring about further improvements and you may also be able to move your listening position forwards or backwards a little.
Perhaps the most effective means of minimising acoustic problems is simply to increase the ratio of direct to reflected sound, which means sitting closer to your monitors. Nearfield monitoring means you hear more of the speakers and less of the room reflections, so in an imperfect room you hear less of the imperfections. In a small studio, you can work as close as one metre or less from the speakers.
Having suggested nearfield monitoring, I'd also like to suggest that you use such speakers on solid stands, not on the bridge of your console, as the strong reflections from the console surface can significantly change the sound, making it less accurate. If you have the types of speaker stands that can be filled with sand to give them more mass, so much the better.
Unless you're extremely lucky, you won't end up with a perfect monitoring environment, but by paying attention to the points mentioned in this article, you should be able to end up with something workable. Fortunately, the human hearing system is very good at adapting to different environments, so providing you have some way of playing commercial CDs through your system for reference, you should be able to acclimatise yourself to the room before and during mixing sessions.
It may also be useful to mix a few test songs using no EQ at all and then check the result on a variety of other systems to see if the overall result sounds acceptably well balanced. If it sounds bass heavy on other systems, then your monitoring environment is probably bass light, and vice versa. The last thing you want is to be adding too much or too little bass to your mixes because your room is giving you a false impression of what is actually happening, which is another reason to keep referring back to good commercial recordings to gain you a sense of perspective. Ultimately, the main part of the monitoring chain is your hearing.
Your final recourse when taming bass hot spots is equalisation. Now, I'm always very sceptical about the use of EQ to address acoustic problems, as most acoustic problems exist in the time domain, whereas equalisation attempts to apply a cure in the frequency domain. Having said that, by applying controlled amounts of parametric EQ cut at obvious problem frequencies, it is possible to produce a more even-sounding response, but please be aware that this equalisation should be set up from your normal listening position and will only be effective at your normal listening position. The sound elsewhere in the room may actually get worse!
Also, avoid using EQ boost, as it is unlikely to help cure dead spots and will almost certainly make the overall sound a lot worse. A prominent bass end can often be tamed by switching to a different bass roll-off position on the backs of active speakers, and because this isn't attempting to EQ specific frequencies, the effect is likely to be less position critical.