The job of a loudspeaker is to change the varying electrical current at the amplifier output into a varying sound pressure level, without adding any coloration or distortion to the signal. Compared to a perfect input signal a typical speaker will introduce harmonic, frequency and phase distortion. In practice the latter two will dominate because it will be very difficult to provide phase/frequency accuracy over the range 20-20Khz. Most practical speakers still use the principle developed in the 1920s where a coil of wire in a magnetic field moves a diaphragm.
The choice of a good replacement loudspeaker is always a very difficult task compared to a similar exercise of replacing your amplifier or CD player; the difference in sound between of amplifiers over a range of prices is usually very small and mainly relates to greater power and less distortion. There is however a wide and obvious difference in the characteristic of sound from a range of similarly priced and sized loudspeakers, a visit to any good shop or demo. room will reveal loudspeaker sounds to suite all tastes.
Inspection of speakers in the mid price range will reveal cabinets made from several types of material (wood/plywood/MDF) and filled with similarly varying types of acoustic damping material. Then there are the drive units which will have the moving diaphragms made from metal/paper/plastic/fiber or a range more exotic materials each with their own sound characteristic. Next of course there will be the choice of baffle type, this may be in the Popular form of ducted port reflex or infinite baffle. All this means that the designer has the very difficult job to marry the different types of item and end up with a commercially valid loudspeaker which produces a good sound at a reasonable price.
It is misleading to talk about a loudspeaker having an accurate sound, i.e. if it is tested in an anechoic chamber and found to have an ideal response, it will sound significantly different in a normal listening room where the reflected sound from walls and ceilings will add to the main wave. The important point to remember when choosing a speaker is to take the room into account, most obviously large speakers do not work well in small rooms. A loudspeaker has synergy with a room when it produces a sound stage in keeping with the room, so in a relatively small room say 3 x 4 m. you select a small speaker with a good but limited base response, this will produce a balanced sound without the overbearing base response which would occur if a larger speaker were used. When the correct choice is made you will be able to sit and listen almost anywhere in the room and hear a balanced sound, a speaker which is to large will exit the room resonance's producing to much base in one area and far to little in another.
Whilst talking about small or bookshelf speakers is worth mentioning that they usually employ two drive units, a base/midrange operating up to around 3 kHz and a tweeter covering 3 to 20 kHz. This means that the single crossover point is bang in the middle of the operating range. However on larger more expensive speakers there are likely to be several driver crossovers. In loudspeaker design it is the maintenance of correct amplitude/phase response at crossover that determines the true quality.
You should take advantage of room characteristics to achieve the best balance of sound, if you choose a good quality small speaker its base response will be enhanced or degraded by moving it relative to the walls or corners. The ideal height for speakers is that which puts the speaker at the same level as the head when seated in the listening position.
In truth a good loudspeaker is worth its money, if you have such a loudspeaker you will find that you can listen for hours on end to your favourite music. It will have that foot tapping quality which involves you in the music without causing fatigue. This is a condition which is not easy to establish; it requires trial and error and the help of a good dealer.
Finally you should consider the loudspeaker connecting leads, these should be capable of carrying the current from the amplifier to the speaker. In practice 5-10amp. capacity will be very adequate. You should take care to ensure that all connections are well made and do not offer any resistance to the current flow. Put politely you should ignore all suggestions that very expensive cables offer any advantage. They do not; their main function is to separate you from your money.
Dave McGhee
Ganymede Test & Measurement
email:
web: http://www.wavecor.co.uk
1st August 2001