REAL subwoofers!

By Stig Erik Tangen
20 Mar 1996

Filters

The selected box volume and tuning gives a frequency response that falls off smooth down to cut-off (about 16 Hz). This aligment type is named 'Extended Bass Shelf' in the LEAP computer program. The selected volume and tuning was determined by first choosing a cut-off frequency, and place the box tuning frequency around that frequency. Next was to play around with the box volume to get a reasonable frequency response that neither fell off too much, nor created at big bump in the response just above cut-off . The latter type will detoriate the transient- and frequency-response.

The Extended Bass Shelf alignment relies on some EQ to obtain a reasonably flat response. Since active filtering is the only practical solution for crossover networks at low frequencies, it is simply to add another filter circuit in the active crossover to do the necessary EQ. A 2nd order high-pass filter with a Q higher than 1 will EQ the low end, and at the same time make an effective subsonic filter (absolutely necessary when playing LP's!). I recommend a 2nd order filter at 17 Hz with Q=1.25 (see the curves). The crossover should be at least 4th order, or more (I use 6th order). A steep crossover will make it a lot easier to integrate the sound of the subwoofers with the main system. Everyone I know who have experimented with active crossovers, concluded that the steeper slope, the better sound. A 4th order filter is easily made with two stages of 'op-amp' active 2nd order filters (which of course could be any kind of amp, not necessarily an op-amp chip). I use the LM837 quadruple op-amp chip for my own filters. See some kind of 'active filters cookbook' for details about designing active filters.

I resist giving specific filter plans, as those will be dependent on what main system you will use. Crossover frequency, level, phase etc. should absolutely be matched to the main system. There is unfortunately no 'quick-fix' to use that will give 100% satisfying results.

One solution which I use myself, is to let the subwoofer system continue from the point where the main system has it's natural cut-off. In my case the main system cuts off with 6th order slope, -6 dB at 32 Hz. Therefore I use 6th order filters that acts like (more on this later) filters with Q=0.5 at 32Hz for the sub's, and no filtering for the main system. The main system and the sub will then have the same slope, same Q, same -6 dB point, and the same phase shift (180 degrees) at the crossover frequency. This gives a flat summed response and well behaved phase response. Although this works well in my system, some degree of filtering of the main system is recommended. Most speakers will be happy to get rid of the low bass.

An other example: The main system is a closed box, a box type which have 2nd order cut-off. The -3 dB point is at 60 Hz. We can then choose to cross at 60 Hz, and build a 2nd order filter at 60 Hz with Q=0.7 (-3 dB at 60 Hz) for the main system. This makes the main system fall off 4th order from the -6 dB point at 60 Hz. The subs must then be filtered with the same slope to get a flat summed response, meaning a 4th order filter at 60 Hz, with total Q of 0.5 (two filter stages with Q=0.7).

The two examples shown above will work perfectly if the loudspeaker box itself has no phase shift at the desired crossover frequency. This is never the case. A crossover which shifts the phase 180 degrees, will not make the whole system shift the same, because the loudspeaker box's own phase shift is added to the total phase response. In addition the EQ filter also shifts the phase. It is therefore necessary to alter the crossover frequency and Q of the filter in order to get the desired 180 degrees phase shift at the crossover frequency. An analysis of this is very impractical to do by hand, only a speaker crossover CAD system can do this effectively. I use CALSOD for my simulations.

These problems can be avoided to some extend by using 4th order or steeper filters, and by placing the crossover point well above the main speakers' cut-off and in a region where the sub's have low phase shift. This means a crossover frequency around 100-150 Hz. However, crossing that high is a compromised solution.

Level control must also be implemented in the filter electronics. It will most certainly be differences in sensitivity between the main loudspeaker system and the subwoofers, and between the amplifiers.

All this may seem difficult, but it is absolutely necessary to do a thorough investigation of the whole concept to get the desired result. Most subwoofer systems do not at all sound integrated in the total soundstage, mostly because of inferior filters. Remember this!

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