Hollywood 24CX-4, 24CX-3, 24CX-2 owner manual How Crossovers Work, Filter Frequency Response Curve

result in a greater overall efficiency of the system by making sure a band of frequencies is always reproduced by the speaker components that are most efficient at reproducing it.

How Crossovers Work

To divide a range of frequencies into two or more smaller ranges, a crossover uses filter circuits. A filter is simply a circuit that lowers the levels of unwanted frequencies in a signal. There are two different kinds of filters used in crossovers:

•High Pass Filters: These reduce the levels of low frequencies in the signal.

•Low Pass Filters: These reduce the levels of high frequencies in the signal.

Filters roll off unwanted frequencies. Imagine that you have a high pass filter with a cutoff frequency of 1000 Hz. That means the filter is supposed to keep frequencies above 1000 Hz while discarding frequencies below 1000 Hz. Do you suppose, then, that a signal fed into this filter at 999 Hz would simply not show up at the output? If you answered “No” you were correct! In fact, a signal at 999 Hz would be virtually as loud at the output of this filter as a signal at 1000 Hz. Signals that are above or near the cutoff frequency of a high-pass filter are reproduced at normal levels. As the frequency gets lower, the level gradually decreases. The rate of decrease is called the filter’s rolloff rate. This term comes from the fact that on a graph of a high-pass filter’s frequency vs. its gain, there is a smooth curve below the cutoff frequency. Here is an example of such a graph:

	5
	0
	-5
	-10
Gain (dB)	-15
	-20
	-25
	-30
	-35
	-40
	20	140	260	380	500	620	740	860	980	1100	1220	1340	1460	1580	1700	1820	1940	2060	2180	2300

Frequency (Hz)

Figure 3 - Filter Frequency Response Curve

Figure 3 shows the curve of a 1000 Hz high-pass filter with a rolloff rate of 6 dB per octave. For every for every octave below the cutoff frequency, we subtract 6 dB from the gain of the filter. The cutoff frequency itself is actually at -6 dB. Filters with higher rolloff rates have steeper looking curves. The TDM 24CX series crossovers have a 24 dB per octave rolloff rate. Figure 4 is what the above filter would look like with this higher rate.

© 2003 TDM Audio, Inc. Electronic Crossover Owner’s Manual Page 7