Copyright Michael Karbo, Denmark, Europe.


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    Chapter 4. Active sound units

    We have seen that there is a division of labour in a loudspeaker, so that a big and heavy sound unit looks after the bass signal while a little, light unit deals with the treble signal.  In 3-way systems another middle tone unit is included. But with several loudspeaker units, the sound signal has to be sorted out – the highest frequencies to the treble unit, etc. This sorting can be done either actively or passively.

    Amplification and passive pitch

    A loud speaker requires a lot of power to be able to play. The bigger the loudspeaker, the more the power it requires and this power usage is measured in watts. Big and powerful stereo equipment can, for example, deliver 2 x 100 watts. This means, that there are two amplifiers (right and left channels), which can each deliver a sound signal with a 100 watts effect.

    If the loudspeaker consists of several units, then it will have a crossover network. This is a little circuit board with a couple of electric components (first and foremost a condenser and a coil). The two wires from the amplifier are connected to the crossover network, which then divide the signal into two or three departments, which are sent to the respective units. All loudspeaker cabinets, which contain two or more units, also have crossover networks.

    The active system

    A crossover network is a passive component – it filters the amplifier’s signals so that the treble unit does not receive signals under, for example, 4000 Hz, etc. But, as mentioned earlier, more and more active loudspeaker sets are being used. This means that the amplifier is built into the loudspeaker cabinet itself. The most simple and widespread form for active loudspeakers is the so-called computer loudspeaker:

    Figure 14. Little set of active loudspeakers with an extraordinary good sound. With a common bass and two satellites.

    The set shown above consists of a subwoofer, which contains both filters and amplifiers. There are three amplifiers built together in the subwoofer, because it supplies amplified signals both to the bass unit and to the two small satellites. Satellites often consist of 2-way systems, which have a passive crossover circuit.

    Figure 15. A typical set of computer loudspeakers consisting of two ”satellites” and a subwoofer with all the electronics built in.

    Here below you can see the back of a subwoofer. It is connected from the left to the right: power supply, DIN switch to the two satellite cabinets, ports (from the computer’s sound card, or from a headphone port on an mp3 player, a discman or something similar. At the extreme right there is a volume control, which is used to regulate the level of the bass.

    Figure 16. The bass cabinet contains all the electronics.

    Choose the right set

    Computer loudspeakers are found in innumerable variants and widely different qualities. Many ”experts” look down on these active loudspeaker sets, which they don’t think count for much. My own experience is, however, that if you choose a good brand and maybe search the Internet for reviews of the different models, then you can find really good loudspeakers.

    Figure 17. Mini loudspeaker set for about $40. Really fine sound, but no deep bass. Suitable for computer use.

    I have, myself, been very satisfied with loudspeaker sets from Philips, Monsoon and Altec Lansing (see Figure 20), from whom, I have had really good sets.

    Finally the brand Klipsch has had really good reviews, just like certain Logitech models, but I haven’t had any experience with them personally.

    Figure 18. A Sony cassette/radio/cd/mp3 player. Connected to a set of active loudspeakers of good quality, then it is a complete little hi-fi set in itself.

    The Monsoon set seen in Figure 14, doesn’t take much room. It is, in fact, designed to stand on a desk around a computer.

    The sound is extremely good, there are so-called electrostatic treble units, which is rather advanced; the loudspeakers, however, cannot play very loudly. The set has landed in the bedroom, where they are connected to a ”ghetto blaster”, which can play both normal audio CDs and burnt CDs with mp3 files. In this way we have a little hi-fi set with a really good sound quality (see Figure 18).

    Figure 19. The back of a subwoofer with a bass relex port.

    All the loudspeakers named here are analog in the sense that they require an analog signal (either from a sound card or from a headphone port). At some time or another we had a set of digital Philips loudspeakers; they just had to be connected to a USB port. So the computer sent a purely digital signal to the loudspeaker, which contained a D/A converter of a very high quality. The result was a really fantastic sound with an unbelievable depth. For one or another reason, we don’t hear very much about these USB loudspeakers anymore – they were, in fact, a very fine product.

    The American Altec Lansing firm was founded in 1941 and has since then made professional sound systems for cinemas, etc. all over the world.

    Figure  20. Altec Lansing supplies today a succession of loudspeakers that are suitable both for stereo systems and for all the sorts of computers/games/mp3 gadgets, which can be found.

    We use ourselves a set of Altec Lansing 621 for the living room’s DVD player. They are impressive loudspeakers with a very convincing finish. And they can play loudly; altogether there are 143 watts (RMS) of amplifier built in.

    Multichannels

    Active loudspeakers are especially used for computers but also more and more for the different types of surround sound that are found in DVD sound data. The simplest sound reproduction consists of one loudspeaker, which is, for example, put in front of us. This could be a transistor radio or it could be a loudspeaker in a telephone. This mono sound can be good enough, for instance, for listening or understanding speech.

    If we want a better sound image with a feeling of space and depth, then we have to have more than one loudspeaker. Hi-fi reproduction of music has to have at least two channels – a right one and a left one. This can give a really good sound image if the loudspeakers and the rest of the equipment are of good quality and if the loudspeakers are well placed according to where the listener is.

    But when it comes to reproduction of the feeling of space, then you can get a much more convincing sound if the sound system is extended to having both front and back channels. Film companies discovered this in the 1960’s when film sound went through a minor revolution.

    Figure  21. Surround sound typically consists of six loudspeakers placed around the listener’s favourite chair or sofa.

    Today, this surround sound has moved into our homes, because the sound from DVD, computer games and stereo systems can be surround sound.

    Surround sound is a complicated technology. Firms like Dolby Laboratories have researched in the subject for decades. In 1982 Dolby Surround arrived and in 1987, the more active Dolby Pro Logic. Both were analog sound systems, which via  intelligent techniques with, among other things, phase shift created an artificial spatial effect in the sound. These systems were installed in thousands of American homes, where the first home cinemas were established. Dolby Pro Logic is still used in, for example, VHS tape’s sound track.

     

    System (analog)

    Channels

    Used in

    Mono

    1

    Simple radio/TV

    Stereo

    2

    Hi-fi

    Quadrophoni

    4

    No longer

    Dolby Surround
    Dolby Pro Logic

    4: right, left, centre,
    surround

    Technology that gives spatial effect in sound. Still used with VHS and some DVDs.

    Figure 22. Analog sound systems.

    Digital sound gives an immediate enjoyment of surround sound. Read more about this later on.


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