I finally got around to a refresh of the pre-build modeling that was needed for a system that I hope to create this winter. The motivation follows these lines:
The attached plot shows how the pattern changes from a dipole at HF through hypercardiod and cardioid before reaching monopole at LF. I have seen this also done another way using two drivers: one is a monopole (for LF) and the other a dipole positioned directly above it. There is some overlap to their passbands in wihch a cardioid pattern is generated, but of course I want to do something different, right? Actually, the solution I am proposing has the advantage that you do not need to have a stack of two drivers, one on top of each other, to achieve the effect. Instead the system can be created using a low-slung box with both drivers firing upwards or front and back, as long as the separation between them remains at the correct distance. Also, both drivers can be used to generate LF output in my setup.
The only quirk is that, at frequencies below 100Hz, the outputs are in relative phase, but offset by one cycle. It will be interesting to see how that sounds. Based on what I know about group delay, I am skeptical that this will sound any different than a single source of the same SPL but hearing is believing...
- A woofer system that can operate up to ~300Hz
- At 300Hz the response pattern should be a dipole, so it can be crossed over to a dipole midrange
- At low frequencies, the output should be a monopole that can pressurize the room
The attached plot shows how the pattern changes from a dipole at HF through hypercardiod and cardioid before reaching monopole at LF. I have seen this also done another way using two drivers: one is a monopole (for LF) and the other a dipole positioned directly above it. There is some overlap to their passbands in wihch a cardioid pattern is generated, but of course I want to do something different, right? Actually, the solution I am proposing has the advantage that you do not need to have a stack of two drivers, one on top of each other, to achieve the effect. Instead the system can be created using a low-slung box with both drivers firing upwards or front and back, as long as the separation between them remains at the correct distance. Also, both drivers can be used to generate LF output in my setup.
The only quirk is that, at frequencies below 100Hz, the outputs are in relative phase, but offset by one cycle. It will be interesting to see how that sounds. Based on what I know about group delay, I am skeptical that this will sound any different than a single source of the same SPL but hearing is believing...
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This is a very interesting technique to combine a conventional bass with a dipole mid/high frequency driver. I look forward to your winter project.
Would you be so kind as to elaborate on your requirements for the bass-to-bass distance and how you manipulate the phase/delay.
Really curious, Bernd
Would you be so kind as to elaborate on your requirements for the bass-to-bass distance and how you manipulate the phase/delay.
Really curious, Bernd
The phase delay is manipulated with an allpass filter. The filter is out of circuit in the HF limit, so the pattern there is determined by the source separation. The "front "woofer has normal polarity and the rear "reversed" polarity, and you get the usual dipole pattern from two drivers (in boxes) set up this way. The source separation determines at what frequency the dipole's on-axis response will start to drop into the first null above the dipole peak. The greater the separation the lower in frequency this will occur. I want to use the system up to around 300Hz, and a 40cm separation is about the maximum I can use in that case. The pattern magic at lower frequencies is all from the filter. I worked up modeling for this system back around 2016 to 2017 and have wanted to actually build it for some time. Unfortunately, I lost all of the original data when I had a hard drive "mishap" and did not have a sufficiently recent backup. Lesson learned! Back then I had created a video that slowly swept through frequency to show how the pattern changes, but you get the idea if you just show the "highlights" using a static plot like I used above, in the first post.