I've been working on modeling some subwoofers. I've never designed or built one, but I've been using WinISD Pro to model drivers in both sealed and ported configurations. Each driver has an X-Max or amount of excursion it has available. For subwoofers this is in the range of 10-40 mm one way. If given the same amount of power, the excursion increases as the frequency decreases. A driver that moves 2 mm at 50 Hz might move 18 mm at 10 Hz with the same amount of power.
In order to keep from damaging your subwoofer at low frequencies either a high pass filter is used to filter out lower frequencies, or the available power is kept to a level that it won't push the driver to much. For example, I can design a sealed subwoofer with a 15" driver that has 23 mm of X-Max. To reach 23 mm of X-Max at 5 Hz, it takes 800 watts. However, to reach X-Max at 20 Hz it takes 1200 watts (which is the maximum power the driver can handle). The higher the frequency, the more power the driver can take before reaching X-Max. There is usually a trade off between dynamic range in the mid-bass vs allowing the driver to reach maximum excursion in the low bass.
I was wondering if it was possible to create a Limiter DSP that allows the driver to always maximize the power available at any frequency. I don't know for sure of anyone that has one, but I think some of the OEM subwoofer amps with DSP have limiting capability (Seaton Sound, SVS). There has also been a request for this with the MiniDSP. If MC could provide the DIY subwoofer community with some more advanced tools, I think that would be great.
Here is what I think might work:
1. Measure distortion with REW (or other measuring software) at increasing volumes at set frequencies to determine the maximum output you can have at each frequency.
2. Once the maximum SPL is recorded for the frequencies you could plug these numbers into the limiter DSP. For example 79 dB at 5 Hz, 93 dB at 10 Hz, etc at 11, 12.5, 14, 16, 18 Hz. The limiter DSP could use these to create a limiting curve.
3. You would need to play a test tone at a certain frequency and record its dB in the Limiter DSP so MC would have a reference between its volume control and your output (unless there is a better method).
4. The Limiter DSP could then make sure the maximum output was never exceeded for each frequency range.
Is this something that is even possible? Is there a way to do it based on the electronic signal?
What this does is give a lot more power available to your mid-bass while protecting the driver from over-excursion. In my 15" driver example, if you wanted it to play to 10 Hz, you would need to limit your power to about 770 watts. This only gives you output at 30-100 Hz of 105 dB. The LFE channel can have output up to 115 dB and higher when combined with redirected bass. Therefore, this sub isn't capable of reference levels. However, if I can give it the 1200 watts the driver can handle, then I can get output levels of 113 dB or greater over 30 Hz. Now the sub can almost reach reference levels for most material while still providing a lot of low output. In other words, you are giving yourself the maximum headroom that your driver will allow. This seems to be a much cheaper method than adding more subs. It is also of greater benefit when using a less expensive driver with less X-Max.