This post won't enable you to choose a set of speakers by reading specifications or manufacturer's blurb, but hopefully it will give you an idea what you should be looking for, and why.
So, what are suitable speakers for mastering ? Well for example we have B&W Matrix 801s in Studio 1 at SRT, Genelec S30Ds in Studio 2 (In general I don't rate Genelecs, but the S30s have a ribbon tweeter and are in a class of their own). Other common names include ATC, PMC and Dunlavy. What all these monitors have in common (apart from costing at least as much as a large family car !) is that they have:
- Flat, or neutral frequency response - meaning their response is very even over the whole audible range. So, for example, B&W 801s come with a graph measuring their response - it only varies by +/- 1 dB from 20 Hz to 20 kHz.
- Acurate phase response - meaning that the stereo image is very detailed, giving an absolutely solid mono center image, but also a deep, convincing "three-dimensional" soundstage - at least, on a decent recording. The stereo imaging in our studios is so good that it's common for people to ask if some of the sound is coming out of the center speaker (used for surround work), even though they are listening to pure stereo. And on the occasions where a mix element is accidentally phase-reversed, it leaps out and draws attention to itself.
- Clean, controlled dynamics - with only minimal distortion introduced, even at high listening levels. (Using a suitable amp is important for this, too, of course.)
Of course there are many other aspects of speaker's capabilities I could list, but it's almost more important to realise that these characteristics mean mastering speakers should be:
- Revealing - allowing you to hear details you'll miss on other speakers - faults, like clicks, hiss and distortion; but also the positive sides of the recording - accurate stereo imaging, "depth" (meaning the impression of a three-dimensional sound field), impact and dynamics. Listening on a mastering system should almost be like wearing headphones, in this respect.
- Clinical - Mastering speakers don't make things sound good. Unlike speakers for tracking or mixing, they shouldn't add "vibe", or "feel". Often they might be described as "bland", "boring", or at best "civilized". In fact, they are reliable and acurate. Only the very best recordings will sound good on them ! In fact that leads us neatly to:
- Unforgiving - It's often shocking to hear how bad many recordings sound on a mastering rig - the uncompromising accuracy uncovers limitations masked by lesser reproduction systems. While this won't necessarily make for a very satisfying listening experience, it's essential for the mastering engineer to hear and correct any problems or limitations in the original mix.
Together, all these requirements add up to help the engineer achive translation - when it sounds it's best on the mastering system, it will sound great everywhere else, too.
So, if you've got this far, you're probably thinking "how the hell do I choose speakers which meet all those criteria ?". If you can't rely on printed specifications alone, this is a tough question. Until you're learnt exactly what a recording should sound like, it's hard to recognise a pair of speakers that make it sound that way.
Probably the first piece of advice is to get some recommendations. This post was inspired by a thread on the Gearslutz mastering forum asking about mastering speakers, and places like this or the SOS forums are good places to start - perhaps less than £500 isn't really realistic, though! Several of the speakers I mention above are available second-hand for "reasonable" amounts of money. My personal favourites, B&W, have ~35 staff working constantly on research and development, and the advances they make for their most expensive speakers always filter down to the cheaper models. I have Series 2 601s at home and am frequently amazed by how good they sound. Not that I'm suggesting you use them for mastering, but considering the difference in size and price, there's almost certainly something in the 600 range to suit.
Next - listen ! Find a showroom where you can test a few different speakers, ideally in the same room, and listen to various different tracks you know well. Even better, get some on demo and try them in your room. Remember you're looking for accuracy, not excitement - mastering monitors reveal limitations that others don't. Choose a range of recordings and be objective. "Sledgehammer" by Peter Gabriel is a great-sounding track, but it has very tight, controlled bass, so if the speakers give it a thundering bottom end, they aren't flat. "Dummy" by Portishead is a classic album, but it's almost entirely in mono - that should leap out at you. Maybe even take a CD with a few test-tones on it - for example a sine wave sweeping up from 20Hz to 20kHz will not only give you an idea about the speakers frequency response, but may also reveal a few buzzes and rattles along the way. (Don't play it too loud, though, or you may find yourself paying for some blown tweeters...)
Finding the right speaker in your price-range is a major challenge for a would-be DIY mastering engineer, but once you've worked with truly acurate monitoring for a while, you won't want to go back.