Among all the audio components, the loudspeaker system is where there are the greater differences. We can find an extensive assortment of baffles at affordable price that deliver a great sound but, on the other hand, some models of famed brands cost in excess of US$ 50,000 a pair. We can also appreciate big differences in size: from small type baffle, that fits on a shelf, to enclosures of more than 2 meters in height, which despite hundreds of kilograms. Is there any technological justification for this diversity or is simply whim of design? A trivial answer is that to get real bass a large speaker is needed. That is effective but it seems to be more like a intuitive response than a justified one. The fact is that the principles that govern the behavior of the baffle and the electro acoustic transducers that compose it, is quite unknown to the common to the music enthusiasts, which tend to attribute ideal technical properties comparable to the remaining parts of the sound system, such as the amplifiers, CD players, equalizers, etc.
We must take into account that only a few decades ago it was unaware the behavior that would have a driver, or electro acoustic transducer, within a baffle. Magnitudes such as frequency response, cone excursion, impulse response, group delay, phase delay, were not possible to predict for the driver-baffle system and the final product was the result of long tests and experiments with different volume enclosures, ports, dampening, etc. This because until recently it had not been deducted a mathematical model for the behavior of the electro acoustic transducer within the baffle.
The dynamic loud speaker has remained virtually unchanged since its invention in 1925 by Rice and Kellogg, two engineers at General Electric Research Laboratory. The technological advances has improved its manufacturing process, the materials and the design of the cone, the power of the magnet, but the physical principle has remained unchanged: a diaphragm coupled with a coil which is within the magnetic field of a permanent magnet, oscillates due to the alternating current applied to the voice coil. This simple principle is that allow to produce sound in almost all of the baffles in the market. The electrostatic and plasma speakers, among others, are an alternative way to produce sound but, due to physical and technical limitations, they have had little market penetration.
The advent of sound films was a great impulse for this industry, but the baffle-transducer system remained a mystery. It was not until 1972 that Richard Small, based on previous work of Neville Thiele, publishes, in the "Journal of the Audio Engineering Society", a series of articles covering the analysis of baffle systems, in closed and vented boxes, using a equivalent circuit. The relevance of this is that enable to model the baffle-transducer system as an equivalent electrical circuit of an RLC type, already known and with predictable effects. Now with the help of the Thiele-Small parameters, particular to each transducer, it is possible to predict their behavior in the baffle without building the baffle.
One of the Thiele-Small equations gives us a clue on the size of the baffle, its frequency response and its efficiency, which can be described as: the efficiency (n0) is proportional to the cube of the resonant frequency (Fs) multiplied by the equivalent volume (Vas).This means that, for example, if we want to achieve an extra octave of bass by reducing Fs in a factor of two, while, retaining the efficiency (n0), we need to increase the volume (Vas) in a factor of eight. This explains why it is so difficult to reach frequencies as 25Hz and why it is necessary a big baffle for delivering real bass.
It is therefore those who opt for small speakers, either by easy or cost, are invariably sacrificing a significant range of bass. On the contrary, those who have chosen the large baffle at the expense of the aesthetic, without a doubt the will enjoy a more vivid realistic and pleasant musical experience.
