Copyright Michael Karbo, Denmark, Europe.
Chapter 3. Traditional loudspeakers
We have seen, that sound is blast waves, which, ”vibrate” in the air, and we have seen that music consists of tones that can range from circa 16 Hz to 20.000 Hz.
To be able to reproduce music we have to have a unit, that can reproduce these airwaves. In practice loudspeaker units do this. Whatever the media (digital or analog) the sound recording has originated from, it will have to be reproduced through a loudspeaker.
What is a loudspeaker?
Loudspeakers are found in all sorts of sizes because they are used in telephones, in headphones, computer loudspeakers, in radios, television and stereo equipment. But the principle of a loudspeaker is largely the same irrespective of its design.
Figure 9. A typical loudspeaker unit with a large moveable membrane.
It is the membrane, which makes the air move, and the membrane is controlled with the help of an electromagnetic signal (just like in the Figure 4 on page 3). Look at the figure below. The big membrane moves with the help of the coil, which is suspended in a powerful magnetic field. When we apply alternating current to it, the membrane will vibrate in time to the pattern of the current.
Choice of loudspeaker
The ideal loudspeaker would have to reproduce the complete range of tones from 20 to 20.000 Hz. And it should be neutral – which means, that it must not ”add to or take away” from the sound. It must reproduce the recording exactly. In practice, there aren’t any loudspeakers than can live up to these requirements.
Luckily our hearing is very individual. There is a lot of psychology in how we interpret sound – some love, for example, opera or music from Thailand, while for others they really are a pain to listen to. So when it comes to evaluating the quality of a set of loudspeakers, there will be a lot of difference from person to person.
When choosing loudspeakers, it is best to hear them in practice. The problem here is that a locality has an enormous importance for the sound’s character. Some hi-fi shops have an arrangement where you can try between perhaps 12 different loudspeaker models. Give yourself time to listen to the differences. Take your favourite music with you and listen to it. There is a lot of difference between loudspeakers and there are a lot of bad loudspeakers, no doubt about that.
Figure 10. Curve of a middle tone unit. You can see that the curve is flat between circa 100 and 6000 Hz, so that area is covered well by the unit.
Sound experts can use curves, like the one in Figure 10, to assess the quality of a loudspeaker. The problem being that the curve doesn’t tell us anything about the loudspeaker’s sound. It tells us how deep a bass and how high a treble that can be reproduced. And it tells us whether the reproduction of the whole range between bass and treble is smooth. But, even so, two loudspeakers with almost the same curves can sound very different.
Several units share frequency ranges
The simplest loudspeaker is a so-called full tone unit. It is intended for the reproduction of the full sound spectrum from bass to treble. But if the sound quality is to be good, it requires a loudspeaker unit, which is advanced and expensive. Which is why so-called multipath systems are used in practice, where there are several loudspeaker units assembled together, which can cover the whole of the required frequency range.
Try to remember, that the low tones consist of very long blast waves, which contain the most energy. It is logical, that if a loudspeaker is to reproduce very low tones, then it has to be big. Just as, on the other hand, as piccolo flutes are very small, then a treble unit isn’t very big either. Because it is not necessary to supply the air with large amounts of energy.
Here below you can see a so-called two-way loudspeaker. It is a wooden cabinet fitted with two units, a bass unit of 6” and a little treble unit (a metal dome). These two units divide the frequency range between them:
Figure 11. A typical two-way loudspeaker, where two units cover the complete frequency range (B&W model DM303).
If you read the specifications for this excellent loudspeaker (I am, myself, a big fan of the English B&W loudspeakers), then the tone range covered is 72Hz - 20 kHz ± 3dB with the comment on reference axis. This means in practice, that there is a great decrease in the volume when the bass tones fall below 72 Hz. The rest of the tone spectrum is reproduced really well.
The value ± 3dB (decibel) indicates, that all tones are reproduced with almost the same volume – and that is what is required if the reproduction is to be High Fidelity. The comment ”on reference axis” means, that the measurement is taken at right angles with the loudspeaker units. The treble sound loses volume, you see, if the unit doesn’t point directly at the listener.
This loudspeaker has a very little and, therefore, shelf-friendly cabinet of about 16 litre; which is why it cannot reproduce the really deep bass tones.
If you want a deeper bass, than can be given by the loudspeaker in Figure 11, then there are two possibilities:
In both cases, the loudspeaker system will usually be expanded to a three-way division of the tone range. The advantage of a 3-way loudspeaker is that the three units can be more specialised. You can use, for example, a big bass unit, which can cover the lowest tones.
Figure 12. Example of a 3-way system split up.
With a subwoofer
The problem with a deep bass is that it requires really large loudspeaker cabinets. During the last few years, therefore, there has been a greater tendency to use a subwoofer for bass reproduction. This is a specialised bass unit, which is individually fitted into a cabinet.
Figure 13. A subwoofer gives deep bass to both stereo channels.
A subwoofer means that you can utilise the fact that you cannot determine the direction of the lowest bass tones. Recordings in stereo usually consist of sound that comes from the right and the left side respectively, which gives an important feeling of space in the music. But bass tones are different; you just cannot hear whether they come from the right or the left. This can be utilised when making a common bass unit. At the same time it gives a more ”compact” solution, because you only need one bass cabinet.
Furthermore, the system has the advantage of the fact that a subwoofer can be placed anywhere in a room. It can be hidden in the corner, stand under the table or wherever it is most convenient. Because the sound is not direction determined, its position is immaterial. You just need to know that if you put a bass loudspeaker in a corner, then the bass will be amplified. This applies to all loudspeakers.
Figure 13 shows an arrangement where two compact loudspeakers from B&W (model DM303) are supported by a subwoofer. This system can very well be used for normal stereo equipment but the best effect is achieved with an active system. These types are especially used for computers and DVD/surround-sound equipment.