Linkwitz has an excel sheet (spl_max1) that will show you the required displacement to get a given SPL - sort of backs you into it.
As Linkwitz points out, diplacement increases at 12dB per octave. If you filter it electrically at 6 dB/octave you're going to have increasing displacement at lower frequencies.
As Linkwitz points out, diplacement increases at 12dB per octave. If you filter it electrically at 6 dB/octave you're going to have increasing displacement at lower frequencies.
That doesn't sound like what I need. While the midrange will be mounted in an OB, I will not be using it below the frequencies where output drops off. Here's the modeled OB response:
The midrange will cover 200-2000hz.
In an OB, the fs of the driver will be around 45hz. Since the impedance peak is two octaves below the crossover point, I was hoping a single capacitor would be sufficient to protect the driver from overexcursion.
Maybe a single 80uF cap for a 1st over highpass xover at 160hz would do the trick. That would give me about -6db at 200hz, and keep the phase shift below 45*. Driver is the Vifa XG18.
Dan
An externally hosted image should be here but it was not working when we last tested it.
The midrange will cover 200-2000hz.
In an OB, the fs of the driver will be around 45hz. Since the impedance peak is two octaves below the crossover point, I was hoping a single capacitor would be sufficient to protect the driver from overexcursion.
Maybe a single 80uF cap for a 1st over highpass xover at 160hz would do the trick. That would give me about -6db at 200hz, and keep the phase shift below 45*. Driver is the Vifa XG18.
Dan
owdi said:
Using a very large box should simulate the cone amplitude of the OB correctly, but not the response.
Edit:
Here is the response of a made-up driver in an OB and in an infinitely large closed box. The red curve is the cone excursion for both systems.
An externally hosted image should be here but it was not working when we last tested it.
Notice that the excursion at constant voltage drive increases by what looks like a nice 12 dB/octave slope right up to resonance. So reduce the drive by 6 dB/octave you're still going to get excursion that increases down to drive resonance. With 2 octaves between crossover and resonance, it is likely to limit your output.
Aha.. now I understand. Thank you both.
Running a few more models in WinISD shows a 1st order filter would keep the driver within it's excursion limits, but max excursion would occur at fs, where the OB respons would be about -16db. That's a waste of energy, and it would cause additional distortion.
With a 2nd order highpass filter, max excursion is just below the xover frequency. Much better, but it will cost more to build.
Dan
Running a few more models in WinISD shows a 1st order filter would keep the driver within it's excursion limits, but max excursion would occur at fs, where the OB respons would be about -16db. That's a waste of energy, and it would cause additional distortion.
With a 2nd order highpass filter, max excursion is just below the xover frequency. Much better, but it will cost more to build.
Dan
I am not sure I fully understand what you do with those filters, but I think taht you should optimize for the response first. If the driver limits the output, then you should add more drivers instead.
A driver that is suited for OBs typically have a high Qts, which will resiult in that the bass response is extended, but also that there will be a higer cone amplitude around fs. Reducing that amplitude by means of a filter counteracts the purpose of having a high Qts in the first place.
...or maybe I did not understand what you are doing.
A driver that is suited for OBs typically have a high Qts, which will resiult in that the bass response is extended, but also that there will be a higer cone amplitude around fs. Reducing that amplitude by means of a filter counteracts the purpose of having a high Qts in the first place.
...or maybe I did not understand what you are doing.
Let me clarify. I decided to try an midrange OB for aesthetic purposes, and to "get rid of the box." The baffle will be shaped to give me exactly the response I need without any filters (when combined with the response of the driver).
Since I already have the response I want, the purpose of the highpass filter is only to increase power handling. I want to do this with minimal phase distortion, and minimal cost. That's why, initially, I wanted to use only a single series cap.
I'm concerned about power handling because this speaker will be used for music and HT. HT is so important to me, I will not use a dipole for bass.
I got this idea after modeling OB response using TheEdge, which has been an incredibly useful program.
Dan
Since I already have the response I want, the purpose of the highpass filter is only to increase power handling. I want to do this with minimal phase distortion, and minimal cost. That's why, initially, I wanted to use only a single series cap.
I'm concerned about power handling because this speaker will be used for music and HT. HT is so important to me, I will not use a dipole for bass.
I got this idea after modeling OB response using TheEdge, which has been an incredibly useful program.
Dan
Hello Dan, i m new here and very intrigued with open baffles. I can relate with ur concern and can see what u r aiming for.
It is a situation very similar to high-passing a tweeter using a single capacitor 2 octaves above its Fs. - Without a 1st order high pass, the mid would have to sustain an increase of 24dB equivalent of excursion at its Fs; and with a 1st order high pass 2 octaves above Fs, the mid driver will sustain a net increase of 12dB equivalent of excursion at its Fs.... considering that the capacitor-aided signal would be 12dB lower 2 octaves below Xover.
* That means, at Fs, with the capacitor, the mid would be handling only 1/16 of the uncensored power, right?
** The issue then remains is whether even at 1/16th power, is the excursion at Fs still going to be a matter of concern? (And for this, i suppose the issue is - even in my mind as of now - as to what is the Pmech. mechanical power handling of a given driver at its Fs in free air? - With a 1st order highpass 2 octaves above Fs, i would multiply the derived Pmech. value into 16 to get my effective power handling at Fs.)
I hope i making sense with my amateurish understanding.
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