Just wanted to get the community’s thoughts on why we don’t we see more (any?) dipole subwoofers in outdoor/large-venue pro audio.
To ground the discussion, here’s a basic example:
A 21” subwoofer driver mounted in the center of a round, 8’ diameter sheet of something sufficiently stiff (maybe sandwich composite to save weight). I.e. open baffle.
Siegfried Linkwitz (RIP) has a great page on this, but to summarize, a dipole’s “…frequency response has a characteristic 6 dB/octave roll-off towards lower frequencies…”. The larger the baffle, the lower in frequency this roll-off begins. Dipole’s have a figure-8 radiation pattern, but this breaks down above a certain frequency, so in practice dipoles have a limited bandwidth which is generally fine for a subwoofer.
This chart shows a case that is very close to my example (where b=4.57a):
So for my example, ka=1 would be about 200hz. Compared to an identical driver in an infinite baffle (i.e. monopole), the response would be the same at about 60hz, and the dipole would be 6db lower at about 30hz. The dipole has fairly consistent figure-8 dispersion up to 200hz (30-200hz is about 2.74 octaves). Please jump in if my math is wrong here.
I see a few big reasons for using dipoles instead of traditional sealed/ported/horn-loaded subwoofers:
Potential downsides:
I know that frequency response is not equivalent to efficiency, but wanted to explore a simple example. I would welcome any comparisons between sealed/ported/horn-loaded subwoofers and dipoles.
Thanks for reading, and excited to read what everyone thinks.
To ground the discussion, here’s a basic example:
A 21” subwoofer driver mounted in the center of a round, 8’ diameter sheet of something sufficiently stiff (maybe sandwich composite to save weight). I.e. open baffle.
Siegfried Linkwitz (RIP) has a great page on this, but to summarize, a dipole’s “…frequency response has a characteristic 6 dB/octave roll-off towards lower frequencies…”. The larger the baffle, the lower in frequency this roll-off begins. Dipole’s have a figure-8 radiation pattern, but this breaks down above a certain frequency, so in practice dipoles have a limited bandwidth which is generally fine for a subwoofer.
This chart shows a case that is very close to my example (where b=4.57a):
So for my example, ka=1 would be about 200hz. Compared to an identical driver in an infinite baffle (i.e. monopole), the response would be the same at about 60hz, and the dipole would be 6db lower at about 30hz. The dipole has fairly consistent figure-8 dispersion up to 200hz (30-200hz is about 2.74 octaves). Please jump in if my math is wrong here.
I see a few big reasons for using dipoles instead of traditional sealed/ported/horn-loaded subwoofers:
- Portability: While you need a huge baffle for low frequency extension, if you could detach the driver, and stack the baffles in a truck, they would take up much less volume than traditional subwoofers
- Directivity: Cardioid subwoofers are already used to great effect in pro audio, often at the expense of efficiency. The dipole pattern offers an even tigher pattern which could make it easier to ensure consistent bass coverage over a crowd.
Potential downsides:
- A given driver would not be able to produce as much low bass as the same driver in a ported/horn-loaded subwoofer. But this could be mitigated by simply using larger cones/ more drivers. There is no enclosure so you’re not contrained by enclosure size. Surely the increase in driver cost would be less than the money saved through increased portability.
- There is a back-wave: This is prevents use in small/medium indoor venues.
- The baffles would block view of the stage. The baffles could be clear (plexiglass), or even covered with displays.
I know that frequency response is not equivalent to efficiency, but wanted to explore a simple example. I would welcome any comparisons between sealed/ported/horn-loaded subwoofers and dipoles.
Thanks for reading, and excited to read what everyone thinks.
Pro audio is all about delivering a lot of sound power to the audience. They deliver almost 5dB less total radiated power compared to a closed box, and a CB sub is not suitable for PA and pro audio sound reinforcement. Dipole subwoofers are probably one of the worst types of configuration for high SPL in the lower octaves unless they are huge, and also have a fixed directionality that is too low to be useful.
The baffle still needs to be relatively stiff and with some mass. So not all that portable in the end if very large and planar. Folded systems could work to some degree but you lose portability and on a size basis will be less useful for pro audio compared to e.g. a folded horn.
Finally, there are the driver related issues. High SPL implies high motor strength and this in turn leads to low Qts. The dipole configuration does not benefit from this at all, in fact the low Qts creates a drooping low end that comes on top of the dipole losses. It just makes the situation worse.
I am sure that others can add to what I have written above.
The baffle still needs to be relatively stiff and with some mass. So not all that portable in the end if very large and planar. Folded systems could work to some degree but you lose portability and on a size basis will be less useful for pro audio compared to e.g. a folded horn.
Finally, there are the driver related issues. High SPL implies high motor strength and this in turn leads to low Qts. The dipole configuration does not benefit from this at all, in fact the low Qts creates a drooping low end that comes on top of the dipole losses. It just makes the situation worse.
I am sure that others can add to what I have written above.
The equal energy to the rear of a dipole is of no benefit in most prosound applications. It's actually a detriment from a noise pollution and feedback standpoint in many cases, which is why cardioid is more typical for directivity control.
Happy to try.
In no particular order:
- With the advent of very-high-end drivers, sealed boxes are just about becoming useful for PA. I have a few Faital 18XL1800s in sealed boxes. Their 20mm of Xmax means enough air can be shifted to create plenty of useful output: https://www.diyaudio.com/community/threads/fun-in-a-theatre-3x-faital-18xl1800.386725/
FWIW, last night was a festival stage for a few hundred people in a marquee, outdoors. I took two subs EQ'd them to be flat to 20Hz, and then added a BW24 highpass at 33Hz. ie, my -3dB point was 33Hz. Standing in the power alley (the stage layout meant the subs had to be under the mains), I had feel-the-bass levels past 10m away, with the amp reporting 6dB of headroom. Program material was live music, and the kick drum RTA was showing strong 40Hz content, with the 31Hz band coming in at -10dB.
- I suspect this has been covered elsewhere, but low-Qts drivers tend to have a very broad impedance peak, meaning they draw relatively little current. Large voltage swings will still be required, though, but I suppose it does mean that many drivers could be used in parallel. Careful investigation would be required here.
Overall, I think it would be possible to use dipoles for PA. The side nulls would be very useful for reducing the LF on-stage, but the obvious compromise is the lack of output. Lots of good drivers (and enough amplifier to drive them) would be needed, meaning the initial costs would be very high.
While I agree that the rearward sound would also be a problem, that probably won't be any worse than any omnidirectional subwoofer.
Chris
The question was about practicability, and so I replied with no. The amount of drivers and the size of the baffle required would be huge.
They are not sensitive enough (aka need a lot of cone surface and amp power to get the power that is needed) and the directivity is not controllable. So not practical at all for P.A.
P.A. need to be light, practical in handling, and use the least power for a certain volume possible and preferable with controlled directivity (cardioid). Sound quality is not the most important thing (altough for most it stays important) and comes behind those needs.
P.A. need to be light, practical in handling, and use the least power for a certain volume possible and preferable with controlled directivity (cardioid). Sound quality is not the most important thing (altough for most it stays important) and comes behind those needs.
That is not the case for many applications either. Using least power is only needed for big rigs or poor grid. Otherwise the power density might win over efficiency, in the name of lesser box volume. These things are not black and white. That's why we have so many products. Ditto for the distortion and quality...
In my experience as former sound engineer it's just reverse. Small rigs in bars and pubs often are limited to the local power availeble, that is often one or two 16A 220V plugs. That means per plug. That means 3500w per outlet maxmimum. At least here in Belgium, 3phase AC systems are not that common in non-industrial envirroments so the power availeble is often very limited. So in the time i was sound engineer (1998-2005) we often had a generator as backup for when the AC system was not enough.
No denial here. Poor grid is quite prevalent in pubs. But on many places, grid is also good and strong, yet there is a requirement for least distraction while maintaining SPL output. And there are products with good size and great output, but these eat a lot. You have to see it with the trend of speakers between 2000-4000W of peak power, fed accordingly.
Again, the aim was to not paint the reality as black and white. PA can serve many different purposes, and very different product lines are available for that reason.
Again, the aim was to not paint the reality as black and white. PA can serve many different purposes, and very different product lines are available for that reason.