|
Inductors in Series with Speakers:
Using an inductor (A.K.A. a coil or choke) in series with a speaker will block higher frequencies while having little effect on the lower (bass) frequencies. The coil should have very low D.C. resistance. This will assure that there is little power loss in the coil and the power is efficiently transferred to the speaker. If 2 coils have the same value (usually stated in millihenries) but one is wound with larger wire, the larger wire will allow more power to be sent to the speaker. If cost is part of determining which one to buy, you must decide whether cost or efficiency is more important.
Making the Connection:
For a simple 6dB/octave low pass filter, the inductor (coil) is simply connected in series with the speaker. It does not matter whether you insert it in the positive side or the negative side. If the coil you're using is relatively large, I'd recommend glueing the coil down to the enclosure (instead of letting it hang from the speaker terminal).
In the following graph the violet line shows how the signal level drops off (rolls off) as the frequency is increased. You can see the voilet line is virtually unaffected (virtually no decrease in the signal level) below ~400hz. At point A, you can see that the signal is down 3dB at 1khz. As I said on the previous page, the 3dB down point is the crossover frequency (for Butterworth alignments). It represents the point where the power reaching the speaker is 1/2 of what it would be without the crossover. If you were driving the speaker directly with a 1khz sine wave at 100 watts of power and then inserted the inductor, only 50 watts of power would get through to the speaker (if the drive level from the amp remained constant). At point B, the signal level is down by 6dB. This represents the 1/4 power point. If the frequency of the sine wave was increased to 1250hz and the level from the amplifier remained constant, the power at 1250hz would be only 25 watts. At point C (3750hz), the signal is down by 12dB. This means that the amount of power reaching the speaker at 3750hz is 1/16 as much as it would be without the inductor.
|
