I am asking for any and all insight and ideas for designing a SIMPLE Infinite Baffle Subwoofer Driver Excursion Limiter within the design parameters shown below. I appreciate all respectful comments and questions!
Existing equipment;
Four FICar Audio IB318 V2 in a single plane array. One QSC RMX 4050A. One MINIDSP 2X4HD.
Design parameters of the limiter;
NO DSP. NO MODELING. NO ACCELEROMETERS OR HALL EFFECT SENSORS. NO SENSING OR MEASURING OF AMPLIFIER'S VOLTAGE, CURRENT, OR FREQUENCY. Nothing attached to any portion of the cone. NO concern whatsoever for driver thermal limits or amplifier distortion/clipping. Only one of the four drivers will be sensed; that driver's cone position will represent that of all four drivers.
Concept:
-Two phototransistors, one PNP, and one NPN, mounted on the frame of the driver (behind the cone), hooded to prevent light other than from the corresponding LED, from entering.
-Two LED lamps, also mounted on the frame of the driver (behind the cone), emitting continuous light in a wavelength optimized for the phototransistors.
-A fast-acting 10 dB attenuator circuit that opens (disconnects) a direct path to the amplifier's input, thereby re-routing the audio signal through a potentiometer that is shorted by the phototransistor when it is closed (connected). A dwell period keeps the phototransistors open until 100 milliseconds after driver excursion is less than 90 percent of Xmech.
-The cone itself blocks the light of the lower LED from the lower phototransistor at cone positions greater than 90 percent of negative (driver pulled in) Xmech, de-energizing the phototransistor in the attenuator circuit. Conversely, the cone unblocks the light of the upper LED from the upper phototransistor at cone positions greater than 90 percent of positive (driver pushed out) Xmech, energizing the upper phototransistor in the attenuator circuit. Please note that I am not experienced in circuit topology...as is evident in the attached block diagram😳.
Is this concept viable????
Existing equipment;
Four FICar Audio IB318 V2 in a single plane array. One QSC RMX 4050A. One MINIDSP 2X4HD.
Design parameters of the limiter;
NO DSP. NO MODELING. NO ACCELEROMETERS OR HALL EFFECT SENSORS. NO SENSING OR MEASURING OF AMPLIFIER'S VOLTAGE, CURRENT, OR FREQUENCY. Nothing attached to any portion of the cone. NO concern whatsoever for driver thermal limits or amplifier distortion/clipping. Only one of the four drivers will be sensed; that driver's cone position will represent that of all four drivers.
Concept:
-Two phototransistors, one PNP, and one NPN, mounted on the frame of the driver (behind the cone), hooded to prevent light other than from the corresponding LED, from entering.
-Two LED lamps, also mounted on the frame of the driver (behind the cone), emitting continuous light in a wavelength optimized for the phototransistors.
-A fast-acting 10 dB attenuator circuit that opens (disconnects) a direct path to the amplifier's input, thereby re-routing the audio signal through a potentiometer that is shorted by the phototransistor when it is closed (connected). A dwell period keeps the phototransistors open until 100 milliseconds after driver excursion is less than 90 percent of Xmech.
-The cone itself blocks the light of the lower LED from the lower phototransistor at cone positions greater than 90 percent of negative (driver pulled in) Xmech, de-energizing the phototransistor in the attenuator circuit. Conversely, the cone unblocks the light of the upper LED from the upper phototransistor at cone positions greater than 90 percent of positive (driver pushed out) Xmech, energizing the upper phototransistor in the attenuator circuit. Please note that I am not experienced in circuit topology...as is evident in the attached block diagram😳.
Is this concept viable????
While something like this could be built, I'd advise against it. There are a lot of reasons, but the main on is that it would sound terrible and might not even safely prevent max excursions (there is inertia).
Basically what you are suggesting is electro-mechanical clipping instead of purely mechanical clipping.
I have not built a limiter, but I have written code for it, and it is not easy to make it sound transparent and not distort. You need to reduce gain softly before the excursion/amplitude reaches the wanted max, and release softly as well.
Does the minidsp not have a limiter, or do you not like the sound of that? Or do you not trust that it works? A display for max excursion based on optical control similar to what you described would help with that.
Basically what you are suggesting is electro-mechanical clipping instead of purely mechanical clipping.
I have not built a limiter, but I have written code for it, and it is not easy to make it sound transparent and not distort. You need to reduce gain softly before the excursion/amplitude reaches the wanted max, and release softly as well.
Does the minidsp not have a limiter, or do you not like the sound of that? Or do you not trust that it works? A display for max excursion based on optical control similar to what you described would help with that.
I agree, possible, but probably not as easy as you envision or as effective.
I think you would be way ahead to just use a high pass filter for subsonics and a notch filter in the DSP for Fs.
Is there a module for the MiniDSP that can do a soft clipping to a calculated voltage? As mentioned, not easy code. You need to manage how you turn the current back on or might easily get an unstable transient loop. Servo is not an easy subject.
With 4 18's, you would blow out your ear drums before exceeding the drivers capability. Have you done the calculations to see what kind of SPL vs frequency you expect?
You can model the expected behavior with WinISD by just picking an excessive sealed box size. Then you can see the expected excursion over frequency, input power and SPL.
LED's frequency is selected, not optimized. They are very narrow band.
I think you would need some very focused lenses for the LED. Or might I suggest laser diodes. ( even pointers.
You say IB, but this would require being in a light sealed environment.
Have you built the basic system to see if you have a problem to start with?
What is your use case? HT, Stereo, 500 seat auditorium?
Putting the drivers in an array increases the efficiency, but limits the ability to distribute room modes.
4 18's are enough to fill a movie theater. So in a HT or something, they would only move a fraction of an inch before excessive levels. Good for low distortion, though probably way overkill for human sensitivity in bass. That low distortion and linearity is why IB has fans, so I get that. But to then dismiss any distortion effects seems contradictory.
You said no DSP, I think you meant in the sensing circuit as you define a MiniDSP as I assume your crossover and equalizer. The MiniDSP already can measure the frequency and amplitude of the input signal.
No modeling? Of what? Do you expect to just build something like this and it works first time? Somewhat unrealistic I suggest.
No harm in trying I guess, Unless you cause hearing damage while testing it, which with that much cone and 5000 W, would seem quite likely.
I think you would be way ahead to just use a high pass filter for subsonics and a notch filter in the DSP for Fs.
Is there a module for the MiniDSP that can do a soft clipping to a calculated voltage? As mentioned, not easy code. You need to manage how you turn the current back on or might easily get an unstable transient loop. Servo is not an easy subject.
With 4 18's, you would blow out your ear drums before exceeding the drivers capability. Have you done the calculations to see what kind of SPL vs frequency you expect?
You can model the expected behavior with WinISD by just picking an excessive sealed box size. Then you can see the expected excursion over frequency, input power and SPL.
LED's frequency is selected, not optimized. They are very narrow band.
I think you would need some very focused lenses for the LED. Or might I suggest laser diodes. ( even pointers.
You say IB, but this would require being in a light sealed environment.
Have you built the basic system to see if you have a problem to start with?
What is your use case? HT, Stereo, 500 seat auditorium?
Putting the drivers in an array increases the efficiency, but limits the ability to distribute room modes.
4 18's are enough to fill a movie theater. So in a HT or something, they would only move a fraction of an inch before excessive levels. Good for low distortion, though probably way overkill for human sensitivity in bass. That low distortion and linearity is why IB has fans, so I get that. But to then dismiss any distortion effects seems contradictory.
You said no DSP, I think you meant in the sensing circuit as you define a MiniDSP as I assume your crossover and equalizer. The MiniDSP already can measure the frequency and amplitude of the input signal.
No modeling? Of what? Do you expect to just build something like this and it works first time? Somewhat unrealistic I suggest.
No harm in trying I guess, Unless you cause hearing damage while testing it, which with that much cone and 5000 W, would seem quite likely.
Bad idea for the reasons stated above. If you don't want to use a high pass filter side chain limiting is an alternative approach:
https://www.diyaudio.com/community/...or-excursion-and-excursion-measurment.375676/
Here is a non contact method I used to measure excursion:
https://data-bass.ipbhost.com/topic/760-measuring-excursion/
this is not suitible for real time control of the cone though as the sensor has too low sample rate and has delay.
Depending on the driver it may also be very difficult to reach XMECH as the drivers suspension becomes very tight and the coil leaves the gap beyond XMAX. So if there is a large difference between XMECH and XMAX the driver itself will mechanicaly limit excursion (and sound BAD).
https://www.diyaudio.com/community/...or-excursion-and-excursion-measurment.375676/
Here is a non contact method I used to measure excursion:
https://data-bass.ipbhost.com/topic/760-measuring-excursion/
this is not suitible for real time control of the cone though as the sensor has too low sample rate and has delay.
Depending on the driver it may also be very difficult to reach XMECH as the drivers suspension becomes very tight and the coil leaves the gap beyond XMAX. So if there is a large difference between XMECH and XMAX the driver itself will mechanicaly limit excursion (and sound BAD).
Thank you ALL for these very salient points! THIS is why I chose this site for a sanity check.
Efluon; "it would sound terrible"
I believe it would indeed sound terrible, but by design; it would be an audible alarm, telling me to back off. The limiter would be benign until driver excursion approaches the limit, at which point it is like a speaker protection relay. I would never intentionally allow continued operation in this condition. It is a firewall; a last stand against damage from an unforeseen L.F. transient event.
"and might not even safely prevent max excursions (there is inertia)"
This is of deep concern to me. The QSC's damping factor isn't great (250 at 8 ohms), and I will be operating it at 2 ohms per channel in stereo parallel (not bridged), decreasing counter EMF even further. It is a high current amplifier, but then the mass of four large, heavy cones operating without the damping of an enclosure (this is an infinite baffle, after all) would not stop on a dime. That could be a show-stopper...
The MiniDSP 2x4 HD would be upstream of this limiter, and thus isn't part of it. It does have an onboard compressor, but no limiter. I plan to set a 24 dB/octave high pass filter at 12 Hz using a linear-phase filter (biquad) in the MiniDSP optional plug in, but that still leaves a vulnerability between 12 and 24 Hz. A display would be fun and interesting, but not particularly effective as a protective device.
An unstated design parameter (the elephant in the room) is that I insist on the ability to reproduce 16 Hertz fundamentals, being that I am a classical pipe organ nerd! Once I heard the effect in a real cathedral (no one actually hears 16 Hz), I became obsessed with it.
By the way, I love your term "electro-mechanical clipping"!
TVRgeek "LED's frequency is selected, not optimized. They are very narrow band. I think you would need some very focused lenses for the LED. Or might I suggest laser diodes. ( even pointers. You say IB, but this would require being in a light sealed environment."
Optimized was a poor choice of words on my part. Selected is a much better choice! The back of the drivers will be in my attic, which remains VERY dark on the brightest of Summer days. Still, I was thinking about black hollow tubes on the LEDs pointed at the corresponding phototransistors (hooded?) to prevent crosstalk between the two light-paths. While lasers would certainly generate a bright, focused coherent beam, can they be operated continuously for hours or days without a cooling system?
"Have you built the basic system to see if you have a problem to start with? What is your use case? HT, Stereo, 500 seat auditorium?"
I have all of the equipment, but have not started building yet. This will be an HT in a great room of 24 feet wide by 36 feet long by 20 feet tall or 17,280 cubic feet.
"That low distortion and linearity is why IB has fans, so I get that. But to then dismiss any distortion effects seems contradictory."
By "distortion". are you referring to the audible effects of the limiter upon reaching it's attack threshold? It is akin to the overload protector on an electric motor; I would never intentionally trip it, but certainly need to notice it. An insurance policy if you will.
"You can model the expected behavior with WinISD by just picking an excessive sealed box size. Then you can see the expected excursion over frequency, input power and SPL."
I would LOVE to do exactly that!! I must say though, that I am quite intimidated by WinISD. I am only just now able to put this project on the front burner after my disability retirement was approved by my employer.
Kipman725 "this is not suitible for real time control of the cone though as the sensor has too low sample rate and has delay."
Exactly! This limiter MUST operate real time to be useful.
"Depending on the driver it may also be very difficult to reach XMECH as the drivers suspension becomes very tight and the coil leaves the gap beyond XMAX. So if there is a large difference between XMECH and XMAX the driver itself will mechanicaly limit excursion (and sound BAD)."
In an infinite baffle subwoofer, there is very little to no enclosure-based damping to help limit excursion. It is almost free-air operation, not unlike a dipole, but with an "infinite" baffle separating the front of the driver from the back.
I did NOT know that the coil could leave the gap BEFORE Xmech! I foolishly assumed that the positive Xmech limit was predicated on the outward excursion distance at which the coil could leave the gap. Quite an epiphany for me!
To ALL
I am not an electrical engineer, so I am infinitely grateful for the time and effort each of you put forth in your considered replies. The difference this site makes can be the difference between the bliss of a successful and fulfilling project, and the wasted time, expense and bitter discouragement of a failed one.
Efluon; "it would sound terrible"
I believe it would indeed sound terrible, but by design; it would be an audible alarm, telling me to back off. The limiter would be benign until driver excursion approaches the limit, at which point it is like a speaker protection relay. I would never intentionally allow continued operation in this condition. It is a firewall; a last stand against damage from an unforeseen L.F. transient event.
"and might not even safely prevent max excursions (there is inertia)"
This is of deep concern to me. The QSC's damping factor isn't great (250 at 8 ohms), and I will be operating it at 2 ohms per channel in stereo parallel (not bridged), decreasing counter EMF even further. It is a high current amplifier, but then the mass of four large, heavy cones operating without the damping of an enclosure (this is an infinite baffle, after all) would not stop on a dime. That could be a show-stopper...
The MiniDSP 2x4 HD would be upstream of this limiter, and thus isn't part of it. It does have an onboard compressor, but no limiter. I plan to set a 24 dB/octave high pass filter at 12 Hz using a linear-phase filter (biquad) in the MiniDSP optional plug in, but that still leaves a vulnerability between 12 and 24 Hz. A display would be fun and interesting, but not particularly effective as a protective device.
An unstated design parameter (the elephant in the room) is that I insist on the ability to reproduce 16 Hertz fundamentals, being that I am a classical pipe organ nerd! Once I heard the effect in a real cathedral (no one actually hears 16 Hz), I became obsessed with it.
By the way, I love your term "electro-mechanical clipping"!
TVRgeek "LED's frequency is selected, not optimized. They are very narrow band. I think you would need some very focused lenses for the LED. Or might I suggest laser diodes. ( even pointers. You say IB, but this would require being in a light sealed environment."
Optimized was a poor choice of words on my part. Selected is a much better choice! The back of the drivers will be in my attic, which remains VERY dark on the brightest of Summer days. Still, I was thinking about black hollow tubes on the LEDs pointed at the corresponding phototransistors (hooded?) to prevent crosstalk between the two light-paths. While lasers would certainly generate a bright, focused coherent beam, can they be operated continuously for hours or days without a cooling system?
"Have you built the basic system to see if you have a problem to start with? What is your use case? HT, Stereo, 500 seat auditorium?"
I have all of the equipment, but have not started building yet. This will be an HT in a great room of 24 feet wide by 36 feet long by 20 feet tall or 17,280 cubic feet.
"That low distortion and linearity is why IB has fans, so I get that. But to then dismiss any distortion effects seems contradictory."
By "distortion". are you referring to the audible effects of the limiter upon reaching it's attack threshold? It is akin to the overload protector on an electric motor; I would never intentionally trip it, but certainly need to notice it. An insurance policy if you will.
"You can model the expected behavior with WinISD by just picking an excessive sealed box size. Then you can see the expected excursion over frequency, input power and SPL."
I would LOVE to do exactly that!! I must say though, that I am quite intimidated by WinISD. I am only just now able to put this project on the front burner after my disability retirement was approved by my employer.
Kipman725 "this is not suitible for real time control of the cone though as the sensor has too low sample rate and has delay."
Exactly! This limiter MUST operate real time to be useful.
"Depending on the driver it may also be very difficult to reach XMECH as the drivers suspension becomes very tight and the coil leaves the gap beyond XMAX. So if there is a large difference between XMECH and XMAX the driver itself will mechanicaly limit excursion (and sound BAD)."
In an infinite baffle subwoofer, there is very little to no enclosure-based damping to help limit excursion. It is almost free-air operation, not unlike a dipole, but with an "infinite" baffle separating the front of the driver from the back.
I did NOT know that the coil could leave the gap BEFORE Xmech! I foolishly assumed that the positive Xmech limit was predicated on the outward excursion distance at which the coil could leave the gap. Quite an epiphany for me!
To ALL
I am not an electrical engineer, so I am infinitely grateful for the time and effort each of you put forth in your considered replies. The difference this site makes can be the difference between the bliss of a successful and fulfilling project, and the wasted time, expense and bitter discouragement of a failed one.
The MiniDSP has a compressor/limiter. The difference between compression and limiting is semantic, anything above a 4/1 ratio is generally considered "limiting". A ratio of 20/1 would reduce a 20dB increase above threshold to just 1dB output increase. You should be able to use it for voltage/ excursion limiting and have a margin of safety, with out the complexity ( and trashy sound 😉 ) of what you have proposed.
You want a damping factor of about 50 for critical listening.
The amplifier itself has almost nothing to do with the system's damping factor equation, which is largely controlled by the resistance of the loudspeakers and cabling attached to that amplifier.
Damping factor 500 with:
50' of 12AWG cabling, 8-ohm speaker: 49.14
Drop amp DF to 100, DF with cable is 44.74 , raise the DF to 5000, DF is 50.25
50' of 12AWG cabling, 4-ohm speaker: 24.54
50' of 12AWG cabling, 2 ohm speaker: 12.29
Worry about the cable AWG and distance, not the amp DF.
IIRC some of the largest pipe organs actually only put out a bit over 100dB at 16 Hz in the audience center. At Xmax, your IB set up should be able to put out over 120dB at one meter at 16Hz, which would sound (or feel) around 4 times louder.
Even at 20 feet from the subs, level may be upwards of 108dB at 16Hz.
Hornresp is probably better than WinISD, here is what I get at 49 volts (around 600 watts into 4 ohms).
Hey, these FICar Audio guys designed an IB driver that hits Xmax at rated power!
There was no Le (driver inductance) listed in the FICar Audio specs, so I made two guesses, response should be somewhere between the two.
That said, in room response can vary considerably.
Another consideration is ceiling shake, mounting opposed pairs in a plenum slot (a box with drivers on 2 or four sides, ) would reduce that vibration considerably.
Hornresp can also model the effect the plenum would have, a bit of an upper response boost, easily equalized with DSP.
The FICar Audio specs don't specify Xmech. You should push the drivers by hand to hear whether the suspension or the voice coil hitting the back plate is the limit.
If the suspension allows the voice coil to leave the gap, looks like travel may only be about 4mm, not much compared to 34mm..
You definitely have a project with many outcomes possible.
If you have options available, before committing to a ceiling location, you should experiment with testing the response of different locations of pairs of speakers.
Good luck!
Art
WOW😱! Thanks Art! This is great stuff💪, and you should be a con$ultant!
You clearly have in-depth knowledge of the MiniDSP 2X4 HD. Do you work for them? Are YOU an engineer? How would one confine the compressor's frequency range to the octave from 15 to 30 Hz? Is side chain eq. (or dynamic eq.) possible in a MiniDSP?
You clearly have in-depth knowledge of the MiniDSP 2X4 HD. Do you work for them? Are YOU an engineer? How would one confine the compressor's frequency range to the octave from 15 to 30 Hz? Is side chain eq. (or dynamic eq.) possible in a MiniDSP?
I have been paid as a consultant, and after over 40 years mixing and designing sound systems, I'd consider myself a sound engineer, though have retired for the most part.
Just read a portion of the MiniDSP 2X4 HD manual, never used that particular DSP.
There is no need to confine the compressor's frequency range to the octave from 15 to 30 Hz, the output limiter for whatever range chosen for your sub will react to input voltage, and there won't be a high enough voltage to cause excursion problems as long as you limit the amp output to below approximately 49volts.
This is a pretty interesting thread. I can't help but mention that I have personally heard a pair of IB3 18s hit the backplate HARD. It was while watching this Falcon Heavy launch clip with a group of folks in a dedicated HT room I designed for a friend (women and children present as well).
One of the most terrifying things I have experienced, with those hitting the backplate 3x in a row. BAP, BAP, PAP! Then the 20A circuit breaker trips and we are all sitting there in a pitch-black room immediately afterwards! LOL, excursion limitation is indeed necessary with some content.
