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Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
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Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
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In Reply to: Re: Using transformers to up the Q of extended range drivers ..... posted by hitsware on October 16, 2006 at 15:04:01:
HiI know this will be hard to picture but here is some filter theory which applies to this.
Lets pretend you had an infinite baffle and you were using a speaker with a system Q of .5.
The low frequency corner is governed by the drivers resonance, this sets the slope and “Q†or sharpness of that resonance.
A response shape equaling a Q of .5 give a subjectively “fast†bass as it begins rolling off very early compared to the more common range of .6 to .8 (.707 being a Butterworth I think).
At low frequencies with speakers and in simple passive filters or normal filters in DSP world, one is dealing with a system where if you change something in amplitude, there is also an exactly related change in phase. When this “minimum phase†condition exists, EQ is a perfect fix, it fixes amplitude and phase exactly. The down side is many problems one trys to fix with EQ are not minimum phase, such as room reflections ect. Here EQ is a sonic destroyer by screwing up phase trying to fix the unfixable (like a cancellation notch). EQ is a powerful tool which to quote Firesign Theater, “can only be used for Good or Evilâ€.If your speaker is in an open baffle, you are also fighting self cancellation and so the Qt is higher to reach “flat†measured response.
Your drivers resonance response shape is set by damping from two sources, electrical (Qe) and mechanical (Qm).
By placing a series R in the circuit, you reduce the electrical damping, effectively raising Qe.
The down side is you are reducing the sensitivity broad band by having the R present when its not needed.
One can (sometimes) drive the system with a series C that is in effect a decreasing source impedance with increasing frequency and have enough effect.So far as an optimum solution, the best one is “if†you had the voltage headroom to either 1, eq the system flat OR adjusted the amplifier output impedance to be high enough for the woofer.
The latter choice being semi hazardous as you then add changes in system response according to load impedance (crossover stuff).The suggestion I made is a “trick†which in an over damped sealed box, can lift the low corner to what appears to be a vented box response, without a larger amplifier. The result is much more low end but intimately a steeper final roll off.
Just like adding the vent resonance, this is electrical, when they have the same response, they are identical in many ways. They are different well below band were a vented box driver is fully unprotected from VLF, the electrical version is fully protected.
The key thing here is when reactance’s are coupled properly, there are no “unexpected†problems.If you want, if you can supply the T&S parameters, I’l see if I can derive a high pass network to try.
If it raises the Q enough, consider a lower Fs, there is an octave or more of good stuff below.
Best,Tom Danley
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Follow Ups
- Filtered out..... - tomservo 09:40:33 10/17/06 (2)
- I'd like to hear more Tom - Russ57 12:17:04 10/18/06 (0)
- Re: Filtered out..... - hitsware 16:09:26 10/17/06 (0)
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