Matthew J Poes
AV Addict
Thread Starter
- Joined
- Oct 18, 2017
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i very much hope this turns into a real project to share and not just an exercise in theory and modeling. None the less, I’m going to describe and foreshadow a planned future project and explain a little of why.
For those familiar with Hornresp, you may have noted an enclosure option called an ABC box. You probably thought it was a joke, some kind of placeholder for something. It’s actually the name given by the software author for a kind of series tuned dual reflex enclosure known as an Aperiodic Bichamber enclosure.
What is it:
It’s a box with a response similar to a 4th order reflex enclosure but dividing the internal volume into two chambers. On the larger chamber is a port of a given length. Between the chambers is another port of the same given length. Finally that second chamber has a third port of again, the same given length. That means we have two chambers, a direct radiating driver, two external ports (one in each chamber) and one internal port (series loading the first chamber to the second chamber/port.
What does this complex setup do, it creates two minimum motion notches roughly one octave apart of similar Q (which can be varied by the internal damping).
I believe Don Keele was the first to discuss this kind of enclosure, and it’s primary purpose is excursion control. In fact, if the ports are linear, it would thus also provide a reduction in distortion.
Why use it?
Distortion reduction and excursion control, and the distortion reduction is a direct result of the excursion control.
Here is why that is important, if a driver has high linear xmax and modest power handling, then such drivers would not exceed their xmax before exceeding their thermal limits around and above the port tuning frequency. As such, it provides no advantage for such drivers. I see no value in using this with typical home theater subwoofer drivers of say 20mm xmax one way (or more) and 1-2000 watts power handling (or often less).
So when do you use this and why am I bringing it up?
Because I define a subwoofer by its ability to meet certain performance characteristics with a minimum of distortion. For example, to produce 20hz to 100hz up to 120db at 80hz, with little distortion. There is a complete misnomer that we are insensitive to distortion at low frequencies, which is not true. In fact some forms of distortion at certain volumes are very audible at very low levels (fractions of a percent). Because we are dealing with non-linear distortion it’s inportant to remember that smaller amounts of these distortions are audible when at higher volumes. With subwoofers the mechanical limits of the driver can contribute to these distortions so controlling high excursion with ports can be a good thing, as long as you haven’t traded other problems.
Knowing all that, I like pro audio drivers with modest xmax compared to pro drivers but often larger coils, lower inductance, and higher power handling. Not all pro drivers can replace typical HT subs so this isn’t a blanket statement, but many can.
Take an 18” driver with 2000 watts rms power handling and 15mm xmax. This would be a very high performance pro subwoofer but those specs might seem modest at best compared to an HT driver. Now imagine that it’s got a RE of 2ohms, a BL or 32, and xmech of 60mm. It’s sensitivity is 99db (but not at the low frequencies for which it will be used). If we stick this driver in a modest/large ported box and tune it to 20hz it may actually come very close to the maximum output of a typical 18” HT driver. However it may be limited below 35hz by exceeding its linear excursion. While it may need less power for a given output and have lower distortion in that range, it would probably fall short by a factor of 2-4 of those HT subwoofers at 35hz and below (3-6db). Some say, well that’s the trade off. You can have loud and low distortion low bass or loud and low distortion upper bass, but hard to get both.
here comes the ABC box. Now with a ported box of typical construction a minimum motion notch would exist centered at the tuning frequency. Tuned to 20hz it wouldn’t provide much of any support in controlling excursion at 35-40hz and yet the driver may very well be exceeding xmax by that frequency. Now imagine two minimum motion notches centered at the same 20hz and at 30hz (or maybe 35hz). Now you have the exact same response shape as before, but you can gain significant output before exceeding xmax.
The Keele rule for these boxes is that volume is taken as the normal volume for the bass reflex you intend to design. You then divide it in 3rds. You then make the driver chamber 2/3 total, and the second chamber. Port as mentioned previously. Port dimensions are all identical but obviously port tunings are not.
So you can use very efficient and very linear bass drivers in very high performance subwoofers and gain additional headroom in the low frequencies allowing you to tune often a full octave lower while maintaining good excursion control.
When I have time I’ll try to post models comparing a common driver in these two boxes to show what happens. You will have to use your imagination a little bit as Hornresp doesn’t represent reality correctly. The internal port is represented as nothing more than a minimum motion notch in the response with no port contribution. I think it’s a mistake but not being as smart as Marten King, maybe just a limitation of such a modeling approach. None the less these have been built and measured by Keele and others (they became popular in car audio too) and do work as advertised.
For those familiar with Hornresp, you may have noted an enclosure option called an ABC box. You probably thought it was a joke, some kind of placeholder for something. It’s actually the name given by the software author for a kind of series tuned dual reflex enclosure known as an Aperiodic Bichamber enclosure.
What is it:
It’s a box with a response similar to a 4th order reflex enclosure but dividing the internal volume into two chambers. On the larger chamber is a port of a given length. Between the chambers is another port of the same given length. Finally that second chamber has a third port of again, the same given length. That means we have two chambers, a direct radiating driver, two external ports (one in each chamber) and one internal port (series loading the first chamber to the second chamber/port.
What does this complex setup do, it creates two minimum motion notches roughly one octave apart of similar Q (which can be varied by the internal damping).
I believe Don Keele was the first to discuss this kind of enclosure, and it’s primary purpose is excursion control. In fact, if the ports are linear, it would thus also provide a reduction in distortion.
Why use it?
Distortion reduction and excursion control, and the distortion reduction is a direct result of the excursion control.
Here is why that is important, if a driver has high linear xmax and modest power handling, then such drivers would not exceed their xmax before exceeding their thermal limits around and above the port tuning frequency. As such, it provides no advantage for such drivers. I see no value in using this with typical home theater subwoofer drivers of say 20mm xmax one way (or more) and 1-2000 watts power handling (or often less).
So when do you use this and why am I bringing it up?
Because I define a subwoofer by its ability to meet certain performance characteristics with a minimum of distortion. For example, to produce 20hz to 100hz up to 120db at 80hz, with little distortion. There is a complete misnomer that we are insensitive to distortion at low frequencies, which is not true. In fact some forms of distortion at certain volumes are very audible at very low levels (fractions of a percent). Because we are dealing with non-linear distortion it’s inportant to remember that smaller amounts of these distortions are audible when at higher volumes. With subwoofers the mechanical limits of the driver can contribute to these distortions so controlling high excursion with ports can be a good thing, as long as you haven’t traded other problems.
Knowing all that, I like pro audio drivers with modest xmax compared to pro drivers but often larger coils, lower inductance, and higher power handling. Not all pro drivers can replace typical HT subs so this isn’t a blanket statement, but many can.
Take an 18” driver with 2000 watts rms power handling and 15mm xmax. This would be a very high performance pro subwoofer but those specs might seem modest at best compared to an HT driver. Now imagine that it’s got a RE of 2ohms, a BL or 32, and xmech of 60mm. It’s sensitivity is 99db (but not at the low frequencies for which it will be used). If we stick this driver in a modest/large ported box and tune it to 20hz it may actually come very close to the maximum output of a typical 18” HT driver. However it may be limited below 35hz by exceeding its linear excursion. While it may need less power for a given output and have lower distortion in that range, it would probably fall short by a factor of 2-4 of those HT subwoofers at 35hz and below (3-6db). Some say, well that’s the trade off. You can have loud and low distortion low bass or loud and low distortion upper bass, but hard to get both.
here comes the ABC box. Now with a ported box of typical construction a minimum motion notch would exist centered at the tuning frequency. Tuned to 20hz it wouldn’t provide much of any support in controlling excursion at 35-40hz and yet the driver may very well be exceeding xmax by that frequency. Now imagine two minimum motion notches centered at the same 20hz and at 30hz (or maybe 35hz). Now you have the exact same response shape as before, but you can gain significant output before exceeding xmax.
The Keele rule for these boxes is that volume is taken as the normal volume for the bass reflex you intend to design. You then divide it in 3rds. You then make the driver chamber 2/3 total, and the second chamber. Port as mentioned previously. Port dimensions are all identical but obviously port tunings are not.
So you can use very efficient and very linear bass drivers in very high performance subwoofers and gain additional headroom in the low frequencies allowing you to tune often a full octave lower while maintaining good excursion control.
When I have time I’ll try to post models comparing a common driver in these two boxes to show what happens. You will have to use your imagination a little bit as Hornresp doesn’t represent reality correctly. The internal port is represented as nothing more than a minimum motion notch in the response with no port contribution. I think it’s a mistake but not being as smart as Marten King, maybe just a limitation of such a modeling approach. None the less these have been built and measured by Keele and others (they became popular in car audio too) and do work as advertised.
