Why do planars have poor bass extension?


I've been wondering about this. Is it because the excursion is limited because of the tension of the diaphragm & low BL limits its excursion?

On a related note, what techniques do companies like Magnepan use to get more bass out of their low-frequency drivers? Do they put bigger magnets in the grid than would be feasible for the midrange? Heavier diaphragms?

Finally if anyone knows a good reference for learning about the TS parameters of planars and if/how such calculations differ from dynamic speakers, that would be awesome!

Thanks all!
sideshowgabe
I think you're right -- the diaphragm's tension limits its excursion so that it can't move enough air to generate adequate volume at lower frequencies.

Solutions seem to include bigger panels to move more air and augmenting lower frequency response with dynamic woofers. In the latter case, driver integration is a significant challenge, since the panels tend to be very fast while the dynamic woofers can be slow.
Good bass extension can be achieved with adequate power.
If the bass area is large enough they can have a lot of bass. My Apogee Duetta Signatures have very impressive bass and the bigger Apogees had even more.
The 20.7 (and the prior 20.1) has a magnet structure on both front and back of the diaphram to improve bass performance and slam. (lesser models have the structure on only one surface)
The main reason for lesser bass performance is the fact it is a dipole. The bass from the rear meets the bass from the front at the sides and self cancels a lot of the bass.
Any full dipole is gong to have the same issue. That is why you see so many with a cone woofer stuck on a dipole.

One trick is to add 'wings' to the sides and the bass gets better, as it has more distance to go to self cancel at the sides.
I use a 6" panel addition on my Magnepan 3.6s just off the bass side (none on the tweeter side)

Also the panels flex, and the various stiffening stands also improve the bass response.
I've read accounts that make sense to me that often the support structure is the weak link in the bass of planars. The frames tend to not be very massive and there is little inertial foundation for the drivers to work against. Adding mass and/or rigidity to the frame helps. At least that's what I've read that seems to make sense.

Also, many do not realize that most planars like Magnepan forexample require lots of power (though not necessarily current) to be driven to the max. A 40-80 watt SS amp might sound OK, but does not hit the mark. 200W+ SS or a larger tube amp is usually what is needed to wake things up.
Not all planers have poor bass. My Apogees have plenty of bass, but it is different bass than a cone speaker. It is tight and defined and tuneful and goes deep, but it just doesn't have that hit you in the chest slam of a good cone woofer.
Why do planars have poor bass extension?
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Sideshowgabe
maybe because you've never heard a good planar setup?? I'm with Ptmconsulting on this matter - my bass is fast, tight, tuneful & goes as low as the room will support (about 40Hz) but it does not sound like a cone woofer boom-boom-boom bass.
2nd reason you could be disillusioned with planar bass is that planar bass sounds different from cone boom-boom-boom bass which is the only thing that 99% of the people know. So, when you hear planar bass, you ask 'where's the bass?' It's all in there just that it does not call attention to itself like most cone woofers do.
I'm in agreement with PTM and Bombay, except that I would not describe good cone bass necessarily as "boom-boom-boom". I realized after owning planars for a while that "cone" bass can sound similar to planars when done right, for example with good monitors set up and driven properly.

However it may be easier in general to coax better dynamics out along with good bass articulation ala planars using many common amplifiers out there with cone bass done well than with planars, if that is something that matters to you. I find it does, especially for more rock/pop types of music and large scale orchestral works and big band music, for example. I settled on the OHMs largely for their planar or electrostatic like sonic attributes combined with cone bass type dynamics (albeit with a different, more omni-like spatial presentation).
except that I would not describe good cone bass necessarily as "boom-boom-boom"
Mapman, I suppose your key phrase here is "good cone bass"! How many such speaker systems exist today? Most of the speakers in the market are indeed boom-boom-boom. In relative terms only a few do bass well/correctly & many of them are from yester years....

However it may be easier in general to coax better dynamics out along with good bass articulation ala planars using many common amplifiers out there with cone bass done well than with planars...
you mean it's easier to match an amplifier to a cone driver speaker than a planar speaker?
"Mapman, I suppose your key phrase here is "good cone bass"! How many such speaker systems exist today? "

I suppose what is considered good is subjective, but I would say many if set up well and driven well with the right amp.

"you mean it's easier to match an amplifier to a cone driver speaker than a planar speaker?"

I'd say smaller, less powerful, and accordingly less expensive amps in any particular line might be found to fit the bill better in general with many box designs compared to planars. At least that is what I have experienced.

Newer high performance amp technologies like Class D switching amps where you essentially get a bigger high performance amp in a smaller package might help render that distinction as more of a moot point though I suppose.
You need to hear some Apogees driven well. No bass problems there...
You can have low bass with proper room placement, no piles of equipment and racks, behind, and between the speakers.

Also an adequate sized, but not large room, with not a lot of furniture, and sit in the right spot.

Read "Get Better Sound" by Jim Smith for great info.

My Magnepan 3.6's go down to about 30Hz in the several different rooms I have had them in!

It takes time and careful listening, if you take the effort.
Dipole panels have the same problem with bass as open baffle cone speakers. The technology is less relevant than basic acoutics. As an experiment, try 1' deep plywood "U-frames" on panel speakers.
Due to the energy lost into the acoustic short circuit between front and back surfaces in a domestically friendly 14.5" wide planer speaker or deep W/H frame you need 4X the displacement compared to a monopole and 12dB more raw output compared to the same speaker at 160Hz (so total excursion is 64X what it is at 160Hz).

It's better to roll-off the response than have the speaker encountering its mechanical limits.

If you want dipole bass (it couples differently to room modes, and sounds subjectively better with Siegfried Linkwitz hypothesizing it's due to preserving the bass signal's envelope better) with low extension and the ability to handle music at less than scale model levels in a spouse friendly package the only practical solution is W or H frame woofers with electronic boost at low frequencies.

The Orion designed by Siegfried Linkwitz (http://www.linkwitzlab.com) and sold by Wood Artistry (Starting at $14,750 - http://www.theorionspeakers.com/speakers-and-cabinets/release-orion-4.html) does this. Gradient sells the SW-63 sub-woofer for the classic Quad ESL 63. Martin Logan has a few speakers with physically opposed cone woofers that start out as dipoles and transition to monopoles as excursion becomes a problem and you're getting below the room's fundamental resonance where it's not buying you anything.
At 40Hz (oops, omitted that). At 20Hz it's 8X more than the monopole, 18dB more raw output (64X) than at 160Hz, and 512 times the displacement.

04-15-12: Drew_eckhardt
>Due to the energy lost into the acoustic short circuit between front and back surfaces in a domestically friendly 14.5" wide planer speaker or deep W/H frame you need 4X the displacement compared to a monopole and 12dB more raw output compared to the same speaker at 160Hz (so total excursion is 64X what it is at 160Hz).
Thanks all! I'm particularly interested in the canceling effects inherent in the dipolar radiation pattern – could anyone point me to some more in-depth literature along the lines of Drew_eckhardt's response -

"Due to the energy lost into the acoustic short circuit between front and back surfaces in a domestically friendly 14.5" wide planer speaker or deep W/H frame you need 4X the displacement compared to a monopole and 12dB more raw output compared to the same speaker at 160Hz (so total excursion is 64X what it is at 160Hz)."

References are greatly appreciated!
>04-16-12: Sideshowgabe
Thanks all! I'm particularly interested in the canceling effects inherent in the dipolar radiation pattern  could anyone point me to some more in-depth literature along the lines of Drew_eckhardt's response -

My comparison was at 40Hz (it's worse at low frequencies and better at high with equal output between dipole and monopole in the hypothetical 14.5" example around 160Hz); sorry for not proof reading that.

Siegfried Linkwitz

http://www.linkwitzlab.com/models.htm

and John Krevosky

http://www.musicanddesign.com/Dipoles_and_open_baffles.html

have written extensively on dipole speakers.

Their focus is open-baffles with cone bass and midrange drivers and dome or ribbon tweeters but the same principles apply to planar speakers especially at low frequencies.
Aside from the Infinty IRS V and the Genesis 1,2 and 3 series, can't think of any other planar using either flat or folded "wings" and they all had sealed woofers and somewhat different bass expectations.

Getting beyond theoretical but...

http://www.quarter-wave.com/OBs/U_and_H_Frames.pdf

http://www.musicanddesign.com/u_frame.html

There's also A,B,C,Dipole freeware.