Crossover 102 - Electronic Crossovers - Page 5

However, by definition, a Linkwitz/Riley filter circuit can only be even order, where a Butterworth can be either even or odd. There is also a very important difference in how the turnover frequency or crossover point is defined. Butterworth filter networks are defined by the half power or -3 dB down points in frequency response. Where as the Linkwitz/Riley filters are determined by the -6 dB down points. I'll repeat this for you, Butterworth measure -3 dB down at the crossover frequency, while Linkwitz/Riley are -6 dB at the turnover frequency. This very important fact is often not understood by the typical sound provider. Even though a Butterworth and a Linkwitz/Riley filter set of the same order may share the same crossover point, they are going to use different value components to arrive at the common frequency.

I believe it is now time to discuss the summation of these two filters at the crossover point. First we must define coincident and non-coincident signals. If two different signals are at the same level and are not coincident (or starting from the same exact moment in time), then the most that they can add when summed together is +3 dB. This is also true if they are the same common frequency but exhibit a significant difference in degree of phase angle. If however you have two absolutely coincident signals in both frequency and level, then they will sum to +6 dB when added or mixed together.

Butterworth filters when combined are said to have a smooth power response through the crossover region. Since the crossover frequency is defined as the point at which the spectrum is attenuated or down -3 dB, the actual summation of the transducers is essentially flat when the power is averaged. There is still a little dip around the crossover frequency because the filters are not in phase with each other at this frequency. Now there are some analog electronic crossovers that introduce some signal delay in one or more outputs. However the steps can be quite broad depending on the chosen crossover frequency.

Okay, how do I set up a variable electronic crossover?
First of all there is no magic crossover frequency point. The raw frequency response of each transducer or driver must first be examined, to ensure that the intended drivers can indeed effectively reproduce the chosen range of frequencies. The second and most important consideration is to know the actual sensitivities of each of the component drivers in the system. The sensitivity of a loudspeaker is the internationally accepted standard of 1 Watt @ 1 Meter. With one Watt of power sent to the driver, a measurement is made on axis at a distance of one Meter to determine how loud is the sound pressure level (SPL). Once you know the sensitivity of the drivers you can then set the gains of the crossover properly.

Let's say that we have a three-way system, and the low frequency device can handle 500 watts continuous and produce an SPL of 100 dB (1W, 1M) with a frequency response of 45 Hz to 2 kHz (+/- 3 dB). The mid frequency driver can also handle 500 Watts, but it has a sensitivity of 103 dB (1W, 1M) from 70 Hz to 2.5 kHz. The compression driver on a constant directivity high frequency horn can handle 80 Watts continuous, and has a mid-band efficiency of 112 dB from 800 Hz to 3.5 kHz, with -6 dB per octave roll off above 3.5 kHz.

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