You might be thinking of room resonances, or of a dipole in an infinite space. It's important to consider how a dipole generates sound and how the dimensions of the room can limit the low frequency response. Let me give some background on that:
A dipole is a velocity source, which is very different from a monopole (a pressure source). In a dipole, the front of the cone is pushing air while the back is pulling it. The air is flowing/sloshing back and forth in the room between the front and rear "sources" and this air "movement" generates sound that we can hear. I do not know the exact physics of it, however, for a dipole to create a wave of a certain wavelength, that wavelength or a good portion of it must "fit inside" the room dimensions. So, for example, the wavelength of a 20Hz tone is almost 60 feet long, 30Hz is about 38 feet long, 40Hz is about 29 feet long, etc. Do you have that much listening room at your disposal? If not, I would advise against trying to deploy a dipole subwoofer, or at least that a consideration of the room size should be used to determine how low the dipole sub can be expected to operate. Unlike a monopole sub, making a "bigger dipole" will not improve the situation because it is limited by the room itself.
One way to get around this is to place a large dipole subwoofer very near you, so that you are within the nearfield of one side of it and farther away from the other one. For example, you could set up a U-frame with the "Front" driver right behind your listening chair, and the "rear" line of the U-frame extending away from you. But you will still need a certain amount of space behind the listening chair for this to work properly.
Yes, my Dipole HT sub's in-phase output (from it's front vent's) is about 2 feet from the front row listener's heads and the out-of-phase output (from it's rear vents) is about 3 feet from the rear row listener's heads (..this is when the seats are typically reclined).
-still, it's not nearly enough for a good tactile result (..hence the actuators in the seats).
(..ironically the floor-bounded dipole-sub platform that the rear row seats are on top of provides a great deal of FLOOR tactile sensation, so it's still pretty good if you have your feet on the platform, but the second you recline the seats and your feat are off the floor - it's gone.)
-still, it's not nearly enough for a good tactile result (..hence the actuators in the seats).
(..ironically the floor-bounded dipole-sub platform that the rear row seats are on top of provides a great deal of FLOOR tactile sensation, so it's still pretty good if you have your feet on the platform, but the second you recline the seats and your feat are off the floor - it's gone.)
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There is no way that you are going to get the infrasonic "tactile" response of a large monopole (e.g. closed box) subwoofer when using a dipole sub. The monopole can pressurize the room and the dipole cannot. Case closed!
A dipole subwoofer is only going to get you down "so far". That CAN be 30Hz, or even 20Hz, if you have a really large room. But never lower than that, and often a dipole starts to poop out at higher frequencies in medium and smaller sized rooms, or when not designed or EQ'd properly for the application.
Since even relatively large monopole subs can operate up to almost 100Hz nicely, why not take advantage of that fact to anchor the bottom end, and then cross over to the dipole in the 80Hz-100Hz range? This will work best for "most" DIYers IMHO.
You can even operate the dipole and the monopole sub together over some band to get a cardioid (NOTE that they must be operating at the same SPL over that band for form a cardioid). So for example, you could operate the closed box sub up to 120Hz and the dipole down to 60Hz, and between 60Hz and 120Hz you get a cardioid pattern. By using a flexible and capable active crossover, you can try out various options by changing crossover frequencies and so on.
The other advantage that a monopole sub has over a dipole one at low frequencies is that, for the same on-axis SPL, the total radiated acoustic power is 4.77dB higher for the monopole, e.g. it will seem "louder". Once you get below 30Hz you need all the output you can get to make up for the dropoff in hearing sensitivity.
I think one issue that is plaguing this discussion is that one person's idea of "bass" is vastly different from another one's. Some people might feel that if their system extends down to 30Hz then the bass is great. The next guy might want to "feel" 10Hz output. Stomping dinosaurs versus Stomping at the Savoy.
A dipole subwoofer is only going to get you down "so far". That CAN be 30Hz, or even 20Hz, if you have a really large room. But never lower than that, and often a dipole starts to poop out at higher frequencies in medium and smaller sized rooms, or when not designed or EQ'd properly for the application.
Since even relatively large monopole subs can operate up to almost 100Hz nicely, why not take advantage of that fact to anchor the bottom end, and then cross over to the dipole in the 80Hz-100Hz range? This will work best for "most" DIYers IMHO.
You can even operate the dipole and the monopole sub together over some band to get a cardioid (NOTE that they must be operating at the same SPL over that band for form a cardioid). So for example, you could operate the closed box sub up to 120Hz and the dipole down to 60Hz, and between 60Hz and 120Hz you get a cardioid pattern. By using a flexible and capable active crossover, you can try out various options by changing crossover frequencies and so on.
The other advantage that a monopole sub has over a dipole one at low frequencies is that, for the same on-axis SPL, the total radiated acoustic power is 4.77dB higher for the monopole, e.g. it will seem "louder". Once you get below 30Hz you need all the output you can get to make up for the dropoff in hearing sensitivity.
I think one issue that is plaguing this discussion is that one person's idea of "bass" is vastly different from another one's. Some people might feel that if their system extends down to 30Hz then the bass is great. The next guy might want to "feel" 10Hz output. Stomping dinosaurs versus Stomping at the Savoy.
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@keithj01:
A kick drum doesn't really have "low" frequencies, or as low as you would imagine. I think this is a common misconception, because it is the "big" drum in a drum kit. Here are some frequency ranges for drums, according to this web page:
Percussion Frequencies Part 1 - Drums | Musical U
There is a nice chart showing frequency ranges for drums and instruments, from this page:
Bass Frequency Range | StudyBass
I couldn't embed it directly, so I attached it. Despite the "kick in the chest" sensation, the chart lists the kick drum range starting at 50Hz.
I believe you need a very large (e.g. Kodo) drum to generate very low frequencies.
I also poked around some recording/mixing posts from pro audio sites, and some people actually highpass the kick drum between 40Hz and 70Hz to get rid of infrasonic stuff they don't want in the recording! See:
The Ultimate Kick Drum EQ Beginner's Guide (With Steps)
How to Use EQ to Improve Your Kick Drum Sound : Audio Issues
A kick drum doesn't really have "low" frequencies, or as low as you would imagine. I think this is a common misconception, because it is the "big" drum in a drum kit. Here are some frequency ranges for drums, according to this web page:
Percussion Frequencies Part 1 - Drums | Musical U
There is a nice chart showing frequency ranges for drums and instruments, from this page:
Bass Frequency Range | StudyBass
I couldn't embed it directly, so I attached it. Despite the "kick in the chest" sensation, the chart lists the kick drum range starting at 50Hz.
I believe you need a very large (e.g. Kodo) drum to generate very low frequencies.
I also poked around some recording/mixing posts from pro audio sites, and some people actually highpass the kick drum between 40Hz and 70Hz to get rid of infrasonic stuff they don't want in the recording! See:
The Ultimate Kick Drum EQ Beginner's Guide (With Steps)
How to Use EQ to Improve Your Kick Drum Sound : Audio Issues
Attachments
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I should also point out that the actuators that I use and are commonly available don't do much below 30 Hz and are likely "centered" about 60-70 Hz. It's where you get most of your tactile sensation.
A good IB subwoofer setup can on rare occasion provide quite a lot "more" tactile response, but it's very much signal dependent.
Of course it also "loads" the space and as a result the room itself becomes more identifiable (which is where the dipole is kind of magical - because it doesn't and the difference is easily audible), additionally (and perhaps most importantly) - a good IB subwoofer presents substantial leakage to other rooms unless very serious room construction is done to isolate that room from adjoining rooms (..interior doors in particular are difficult to get properly sealed, but its a rather large problem overall - with typically a great deal of pressure loss from walls into other spaces like adjoining rooms and attic spaces).
A good IB subwoofer setup can on rare occasion provide quite a lot "more" tactile response, but it's very much signal dependent.
Of course it also "loads" the space and as a result the room itself becomes more identifiable (which is where the dipole is kind of magical - because it doesn't and the difference is easily audible), additionally (and perhaps most importantly) - a good IB subwoofer presents substantial leakage to other rooms unless very serious room construction is done to isolate that room from adjoining rooms (..interior doors in particular are difficult to get properly sealed, but its a rather large problem overall - with typically a great deal of pressure loss from walls into other spaces like adjoining rooms and attic spaces).
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Thanks Charlie. That's a bit surprising, but helps explain why kick drums are effective in relatively small spaces...
Thanks for that. I was thinking of the waves reflecting around the room. As you say it's the difference between a velocity source and a pressure source in a room, I'm not sure how that works though.
Hey frangus. Thanks for the interest in helping!
I do have a dipole system except for the subs, including 18" baffleless midbasses that get to 65Hz. I was thinking about your same lines, but after considering several inputs from knowledgeable dipole users such as Charlie and Danny Richie (GR Research), the room I have and the constraints (WAF, furniture, etc) I am steering towards adding a couple of sealed servo 15" and a MiniDSP to set up a distributed bass array.
If I had less constraints I would probably turn around the setup firing into the 10m length and try OB subs as you mention, and maybe use the existing sealed subs to supplement the lowest octave.
But have to work with what I can do, and it's really not bad at all!! Dipole midbass from an 18" is pretty good, and so is midrange and treble. Glad I tried them!! Also glad I tried MTM with 8" mids and a large tweeter, despite common wisdom suggesting lobbing would be awful, but it isn't.
Trying news things is good!
Cheers!
Sounds like you are on track to getting great results with that plan.
Are you using the Beyma AMT with waveguide? Any photos of your system?
Cheers
Certainly! Here's a thread on the evolution of the dipole test to where I'm now. 3-way open baffle plus subs
It's 22 pages long...in the works for the past year. Still a prototype (made from scrap wood I had, mostly), but an evolved one. Started with twin 10" on a small baffle, added an 8" mid, a Beyma TPL with waveguide and back cover. Eventually took out the back cover (post #57) and made a rough back waveguide with similar dimensions to the Beyma waveguide (#120), which measured fairly well past 7kHz which was indicated as a good high frequency reach to aim for at the back wave. XO started at 1.5kHz and is now 1.7kHz NT2. Still want to try going higher.
With the mids I went naked single 8" on a swing for a good while, eventually decided the swing wasn't doing much for mids and tried MTM on a baffle (#159) mostly because I would like to drive the mids with a 2A3 SET so higher sensitivity would be nice. Initially thought I would add an inductor to take the lower 8" out at higher frequencies to mitigate lobbing, but I am not noticing it so haven't tried the inductor yet. I did add wings to the lower mid to allow it to reach lower and could cross the 18" at 275Hz (#194). Been playing with baffle shapes, minimizing baffle around the top mid (#210). Want to try a much narrower baffle and the TPL without waveguide too.
Added the 18" without a baffle (instead of twin 10") (post #73), on a swing, then improved the stand (#154), now on my 3rd iteration of the stand. Swing is great! No vibrations transmitted to the MTM section.
Also went through room treatments (#140). BTW, I also live in an apartment so have added constraints both for the room and place to work with tools, etc.
Got a lot of great input from many forum members!
It was fun to revisit the journey to write this post. A lot has happened in the last year. I'm waiting for a 45 SET to drive the tweeters. Right now I'm short one stereo amp so running speakers in mono. Also part of the prototyping!
cheers!