For a start, speakers need to be matched to an amplifier and a listening room, with the key things to take into account being the speakers
- Power Handling -
- Sensitivity -
- Frequency Response -
- Impedance -

Power Rating

Speakers are delicate things and easily become damaged by amplifiers with too much grunt. A too-powerful amplifier will also lead to sound distortion. For these reasons you need to know the power output of the amplifier you'll be using and whether your speakers will work with it. The power rating of a speaker (measured in watts) indicates how much power it can accept from an amplifier. Peak power ratings are high, but cannot be sustained without threat to the drivers. Manufacturers often give a maximum power figure only as a warning to help prevent speaker damage.
The real information about a speaker's power rating is found in its watts RMS measure, meaning the maximum continuous average power it can handle. For example, a 200-watt amplifier would blow a 30 watt RMS speaker off the planet if cranked up to full power. More speakers are damaged, however, by under-powered amplifiers driven into clipping.

Sensitivity Rating

A speaker's ability to convert electrical signal into acoustic energy is knwon as its sensitivity rating. In other words, it is a measure of how much you get for what you put in. Sensitivity is expressed as sound pressure level (SPL) and is rated in decibels (dB). It is measured by driving 1 watt of power into the speaker and measuring the loudness at a distance of one metre. A high SPL (eg 90 dB/W/M) means a speaker produces a goo, loud sound relative to the amount of power being driven into it.
So, if you have low-powered amplifier and like your music loud, you'll need a speaker with high sensitivity - an SPL of at least 90dB. Heavy furnishings deaden sound, so speakers below 85 dB in such a listening room would need a high-powered amplifier to deliver satisafactory sound levels. (A high-powered amplifier, of course adds to the risk of speaker damage). In general, a speaker with a higher maximum power capacity and greater sensitivity should be capable of being driven to higher volume levels than others.

Impedance

Impedance is the "load" or "resistance" a speaker presents to the electrical signals interpreted by an amplifier. Most domestic speakers are rated at 4 or 8 ohms. Speakers with a load much lower than the amplifier is designed to drive (eg 4 ohm speakers with an 8 ohm amplifier) could damage the amplifier, and while an amplifier driving speakers higher than its specified impedance will not be damaged, the sound level will be lower. If you want to drive more than one pair of speakers from the same amplifier simultaneously, then (unless the amplifier is stable at low impedances) each speaker must present an impedance of at least 8 ohms.

Frequency Response

Frequency response describes a speaker's ability to reproduce frequencies of the audio spectrum at a constant reference level. More plainly, it is a measure of how well a speaker preserves the tonal characteristics of the original music. Good speakers have a bandwidth of 30 Hz - 20 KHz, and do not discriminate between frequency ranges. With bass frequencies expressed as hertz (Hz), and high frequencies as kiloherz (KHz), this rating is seen expressed alongside a decibel measure of variation either side of the speaker's frequency response (42 Hz - 18 KHz ±3dB). A 3dB variation is specified as acceptable. Inside this, and variations will not badly affect sound accuracy. If presented with a graph showing frequency response, know that the flatter and more extended the line, the better a speaker's frequency response.

Driving Force

Speaker design - cabinet style and size, number and placement of drivers - affects the sound a speaker produces, and while some may handle sensitivity and sound quality better, no one type is necessarily better than the other. Most speakers, for example, have front-facing drivers which provide strong direct sound, as well as reflected sound from nearby walls. But bipole, dipole and omnipolar designs radiate sound, in differing proportions, to front, back and side hemispheres. This rpovides a greater sense of spatialness from reflected sound, but does not give precise impressions of front-facing models.
Driver specications will often list the diameters of tweeters and woofers. To deliver bass, a woofer has to move a lot of air, so effective cone area specifications should reflect a large diameter (about 300mm). Tweeters need to be small and light to handle the more rapid high frequencies, so diameters should be about 25mm. Generally, the best sounding systems have several drivers. Two and three-way designs help achieve greater sound accuracy by distributing the bandwidth between the drivers in either two or three frequency ranges. And, while small speakers will produce moderate listening levels in small rooms, only large floorstanding models provide the deep bass and dynamic range to fill a lrge room at moderate levels.

Home Theatre Speakers

Speakers that double as stereo-only and home theatre system channels need special consideration. Front left and right speakers need to be magnetically shielded if they are to be placed within one metre of the television screen. They should also be timbre matched to provide a seamless passage of sound between left and right channels. Likewise, centre channels should be magnetically shielded and timbre matched to the front speakers. Timbre matching is necessary for the rear speakers only if your system is Dolby Digital®.
Speakers generally disperse sound across a wide area, but THX® speakers disperse sound in a narrow beam. This provides more directionally realistic dialogue, which is great for home theatre but unsuitable for two-channel listening. So, THX® speakers need to have a switching function to make them useful for stereo mode.

What to Listen for

Creating accurate sound is what good loudspeakers are all about. If what you hear makes you feel like you are sharing the same air as the performer or orchestra you're listening to - and all the technical specifications in hades can conjure this - then the speakers are doing their job. Listen to vocals and instruments to determine distinct bass, midrange and treble frequencies : Trebles should be clean and crisp, bass should be deep but not boomy. What you hear should be clear, not soft, blurred, coarse or shrill. Listening should not be painful, but enjoyable.
While a hi-fi store will have a fair collection of recordings to show speakers off to their best advantage, it's a good idea to take your own music along. Not only will it give a better indication of what loudspeakers will suit your musical tastes, they may demonstrate whether your music sounds better or worse than it does at home. (Remember to use the same recording on each pair of speakers you audition).
All speakers add coloration - tonal deviations from the original sound - so selecting a pair is just a matter of finding a coloration, be it brittle, mellow or warm that you like. Keep in mind that size, shape and furnishings of your listening room will influence what you hear. Rooms with heavy furniture, carpet and furnishings will be less reverberant than those with lots of exposed windows, wooden floors and few furnishings.