The Nightingale

By Lars Mytting
02 Nov 1996

Crossover

The resonance peak
When using any of the Seas magnesium woofers, be prepared to spend some brainwork and labour to get rid of the resonance peak caused by the cone material. The essence in this removal is a circuit which in its basic form looks like this:

-(filter)---|-----
IND
|
CAP
|
(RES)
------------|-----

The function of the RLC circuit is to make a very steep and narrow "valley" in the filter response, and the clue is to make this to correspond exactly with the peak of the driver. Crucial for success is to hit the peak exactly. The size of the inductor and the capacitor determines the centre frequency and the deepness of the "valley". An optional resistor is used if you need to change the shape of the "valley" so that it corresponds to the shape of the peak.

Using a large resistor, the "valley" will be wide but not deep. With a smaller resistor (or no resistor), the "valley" will be narrower and deeper. Practical resistor sizes for this use range from approx. 3 ohm to no resistor at all.

All components in the filter will act together, so the shape will change if you alter the size of the filter components before the RLC-circuit. This is definitely a job for simulation software like Calsod or Leap, but you will also get a long way if you use the method described by Joachim Gerhard in the interview with him on this subject.

Underneath is shown the effects of LC circuits of 3,3uf/0,25mh and 22uf/0.05mh. All filters use a 1 mh series inductor and no resistor in the LC-circuit. You will note that both responses are centred at 5000 Hz. The 22uf/0.05mh, however, is far wider than the 3,3/0.25, and is also more damped in the 5-20k range. This is generally an advantage, but it may also make it more difficult to achieve a exact filter slope, like the 4th order Linkwitz-Riley, for instance Fig. 7. LC circuits.

100-300 Hz sensitivity
Sensitivity in the lower midrange is a major problem when building speakers. Many systems using standard crossovers have too low sound level in the 100-300 Hz region. This is partly caused by the diffraction step caused by the baffle, partly that many mid/woofers have higher sensitivity in the upper midrange region than in the lower midrange.

Contrary to many others, I enjoy crossing a third woofer a bit high to compensate for this; as I feel that it adds more muscle and impact to the music. I am satisfied using the 10" ScanSpeak for this, but a safer solution with respect to midrange quality is to use two identical mid/woofers, and to roll one of them off at approx. 200 Hz. The other woofer is crossed to the tweeter as usual. (Be aware to solve some complications with phase and impedance).

The alternative is to use a larger series inductor (often around 2 mh) to "eat" the higher sensitivity in the upper mids, but this technique requires that you lower the total sensitivity. Since you cannot easily add gain in a passive system, you have to bring the sensitivity of all drivers down to the "weakest point" of all drivers. Usually this is the mid/woofer sensitivity at 100-200 Hz, where it may be 84-85 dB, resulting in a total system sensitivity of this level.

Tweeter impedance correction
As mentioned, the Revelator use no oil damping, and therefore have a high impedance peak at the resonance frequency. A RLC circuit to compensate for this is shown underneath. The use of the circuit is optional. With a standard crossover frequency at 2500 Hz, the effect on frequency response is small. It causes the rolloff to start about 1/5 octave higher. The circuit must be placed "after" the regular tweeter crossover Fig. 8. Tweeter impedance correction.

--------------- (crossover)
|
1,7 mh
|
64 uf        ScanSpeak 2905/9900
|
6,5 ohm
|
--------------

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