by BOB S. STUART
[Bob Stuart is Technical Director and cofounder of Boothroyd-Stuart, a British firm which manufactures the Meridian line of hi-fi equipment. For several years, the line has included active loudspeakers.]
To the common user, it may not seem too strange to refer to a loudspeaker as "active." After all, in reproducing music, the loudspeaker seems to play a more active role than any other part of the hi-fi chain.
The dictionary recognizes many uses of the word active, but the relevant use for us comes from electro n c theory, where passive networks that respond to externally applied sources are contrasted with active networks that have internal sources of power.
This is the first requirement-an active loud speaker has internal power sources.
However, that is by no means the whole story, especially since there are passive designs which use power for establishing transduction (for example, polarization in passive electrostatic speakers or energizing in ionic designs).
Almost since the beginning of audio engineering, it has been understood that sound reproduction over the frequency and amplitude ranges of music cannot be achieved accurately by a loudspeaker with a single radiating element. The reasons are very simple and apply equally to moving-coil (or dynamic) drive units, ribbon elements, and electrostatic membranes.
Essentially, flat frequency response calls for frequency-invariant sound pressure, and this can be achieved only with a transducer that produces a constant acceleration at all frequencies. Thus, to achieve the same response, a transducer has to displace greater volumes of air at low frequencies than at high frequencies.
In fact, for each octave that the frequency is lowered, the displacement of the radiating element increases fourfold. Consequently, the requirements for a bass radiating system in terms of displacement are very different from those of a tweeter. In addition, the re- [...]
[...] only two 25-watt amplifiers to achieve the same sound. This explains why active systems can be designed to work well with lower powered amplifiers, or to produce surprisingly high levels for the apparent available amplification. If the example is carried on to three 25-watt amplifiers in a triamp system, we might find that in order to get the same volume of sound in a passive loud speaker system, we'd need a mighty 250-watt power amp! It is clear from the above that al though an active system is harder to design, there are efficiency benefits, and it is by no means a foregone conclusion that an active system need be more expensive to build.
Bass Response
So far, I have described how a de signer, by taking a multi-amped approach and by using a low-level electronic crossover, can produce real benefits in the performance of a loud speaker system. Although there are clear differences of operation, we are still discussing the construction of an active loudspeaker so that its performance will emulate that of a passive design. There are, however, huge benefits possible if the loudspeaker is designed as a system from the outset.
By using dedicated electronics, for ex ample, it is possible to change many of the standard rules of loudspeaker sys tem design.
In a passive loudspeaker, there exists a fixed relationship between the low -3 dB cutoff frequency (f), the volume of the box (Vb), and the efficiency (e): e = Vb/f3 x K, (K1 is a constant relating to the system's design). Try as we may, in a passive system this relationship cannot be bro ken, and less bass or lower efficiency are the penalties of smaller cabinets.
The designer of an active system is, however, presented with several new tools which essentially allow him to alter the constant K, in the equation.
Techniques that have been used include making the output impedance of the bass power amplifier negative to increase damping and lower the bass cutoff, or making it frequency- and/or level-dependent to squeeze extra low end or apparent efficiency from the system. The essential point here is that the response and performance of the active system can be synthesized by considering a total system of driver + box + amplifier + crossover + power supply. Other valid techniques to change the constant in the equation are the use of low-frequency boosting, auxiliary filtering, and various types of motional feedback.
Once we admit these possibilities, the design and performance of the active loudspeaker can enter a new do main. Not all the possibilities are beneficial, and some changes may intro duce different problems. For example, negative or variable output impedances will lead to greater variation in manufacturing, and to changes in sound quality at different volume levels, all of which can be remedied. Motional-feedback techniques are conceptually the most intriguing, but this technology is full of pitfalls, not the least of which seems to be the general problem of low feedback factors, deciding which part of the transducer to control, etc. Motional feedback has one major disadvantage for the de signer of cost-effective systems: The feedback process will, in general, pro duce an extension and lowering of distortion in the low-frequency output of a bass system, but only at the expense of reducing the high-frequency output of the driver. The normal analogy here is that feedback will result in a reduction of operating bandwidth of the driver due to practical limitations, and this essentially bars true motional feed back from two-way systems. No serious consideration could be given to a loudspeaker that used feedback over only part of the frequency range of a driver.
I believe all loudspeakers should be designed to be acoustically small, to achieve a near-point source radiation pattern in order that the system will give good, lifelike imaging and be easy to listen to. Not all loudspeakers are designed with this overriding consideration in mind, but when it can be achieved, its very worthwhile benefits include a more realistic sound and a smaller system to house.
The basic principle discussed above, our so-called interactive bass principle, uses a system design of cabinet, driver, port, power amplifier, and filtering in the electronic cross over. Its performance is shown in Fig. 7. The interactive bass system uses auxiliary filtering, and the particular alignment gives a sixth-order roll-off and an extra octave of bass compared with the passive speaker of the same volume and efficiency. Put another way, a passive speaker delivering equivalent bass response would need to have eight times the volume or twice the linear dimensions. The chosen alignment is also the one which minimizes the cone deflection of the bass units for a given output. This point is a very interesting one, for at first sight you might expect that all speaker systems were the same in this respect for a given output. The alignment chosen, in fact, results in cone movements one third as great as those experienced by the same drivers on equal broad-band excitation in a passive sealed box.
To the Future
In the final analysis, the most important thing that any loudspeaker has to do is to convincingly reproduce music in a way that is easy to assimilate and relaxing to hear. My belief is that to reach those goals, the attentive de signer can have a set of really powerful tools at his disposal, if he is prepared to design the loudspeaker as a system from the very beginning, and if he em ploys dedicated, integrated, multi amped electronics. The system can be smaller, clearer, and just easier and nicer to use. Equally important is the fact that ease of adjustment in the factory will result in a more consistent product.
I often draw an example, contrasting the way people buy motor cars with the way they buy hi-fi systems. Clearly, it might be possible to select a car by choosing the body from one source, the chassis from another, the motor from a third, and many who did so might feel that the result was interesting, unique and so on. How many of you really feel that the end result would perform as well as a carefully balanced, integrated design from one manufacturer? Even if it accelerated well, how good would it be on the corners? Active systems are the way forward, and even if it takes direct digital technology to ensure it, we will be seeing more and more loudspeaker designs moving this way.
(adapted from Audio magazine, Sept. 1987)
Also see: Meridian D600B Speaker (Mar. 1991)
= = = =