Sound diffusion systems - Diffusors

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Seeing that each loudspeaker specializes in a certain frequency band, in order to reproduce the entire audible frequency spectrum (20 Hz - 20 KHz) it is necessary to use more than one loudspeaker at a time. However it is also necessary to filter the signal before it reaches the loudspeakers so that each one of them receives only the frequency it is able to reproduce. To do this, we'd use low-pass, pass-band and high-pass filters [Filters ] combined in a single electric circuit called crossover.

9.11.1. The crossover

A crossover circuit consists in filters which sub-divide the entrance signal into many signals which each cover a certain frequency band. For example a 3-way crossover generates three signals: one containing the low frequencies addressed to the woofer, one containing the middle frequencies addressed to the midrange, and one containing high frequencies addressed to the tweeter, as shown in the following diagram:

Sound diffusion systems - 3-way crossover

3-way crossover

The typical response of a 3-way crossover is illustrated in the following figure:

Sound diffusion systems - Frequency response of a 3-way crossover

Frequency response of a 3-way crossover

Let's see what happens to cut frequencies. To guarantee that the frequency bands are correctly distributed to the different loudspeakers, the cut frequencies of the filters are made to overlap. For example the lower cut-off frequency of the pass-band filter corresponds to the cut-off frequency of the low-pass filter. In the example shown on the previous diagram we can see that both the cut frequencies have a value of 80 Hz. The latter, just like all the directly adjacent frequencies, shall be reproduced both by the woofer and the midrange, therefore it shall be reproduced by two loudspeakers simultaneously. This increase gets perfectly compensated by the fact that the cut-off frequency has a gain drop of 3 dB, and so the sum of the two sounds produced by each loudspeaker, returns the original amplitude [Combination of sound sources ] . If we move towards the left or the right, the same frequency shall be reproduced by both loudspeakers. As the reproduced amplitude of one speaker icreases, the other one reduces thereby compensating for the other one, so that the sum always remains constant.

The action of a crossover can take place in two separate points on the amplifying chain, with different results and prices:

  • Active crossover: this type of crossover consists in an active circuit, or in other words, it is equipped with autonomous powering and acts on the signal before it gets amplified. Consequently, at the crossover's output (which we shall suppose to be a 3-way one) we will have three signals, each one carrying a certainn frequency band, which will be amplified separately. This allows us to use amplifiers designed for the reproduction of a specific frequency band and therefore to reach far higher quality standards. This setup is illustrated in the following figure:

    Sound diffusion systems - Scheme of an active crossover

    Scheme of an active crossover

  • Passive crossover: in this case the signal reaches the crossover having already been amplified. Seeing that one single amplifier has been used to amplify the signal, this crossover doesn't need to be powered. This setup is illustrated as follows:

    Sound diffusion systems - Scheme of a passive crossover

    Scheme of a passive crossover

    This solution is far cheaper, but undoubtedly inferior quality wise if compared to the previous crossover, because it only involves one amplifier for the entire audible spectrum and will therefore provide a less accurate amplification of the signal.


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