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Editor's Forum: Planar speakers, dipoles & more
#34958 02/24/04 09:26 PM
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alan Offline OP
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A post from BigFoot_Lives in the Advice from Axiom Owners forum got me thinking about planar speakers, dipoles and bipoles.

"Planar" refers to any speaker that uses a flat diaphragm instead of a cone or dome as the radiating element. In conventional, non-planar speakers like Axioms, a rigid but lightweight cone (for midrange and low bass tones) and a dome (for higher frequencies) move rapidly back and forth to generate the sound pressure waves that strike our eardrums. The back waves from a cone woofer are either absorbed inside a sealed box (acoustic suspension) or funneled out a hole or port in the enclosure (bass reflex) to reinforce and smooth the deep bass response of the speaker system. (The back waves from tweeters and smaller cone drivers are usually absorbed in the speaker enclosure and do not contribute to the speaker's overall output. In the case of the Axiom M60ti and M80ti tower speakers, the tweeters are sealed so that no internal cabinet waves can affect them. The small woofers in the M60 and M80 are mounted in their own internal enclosures so as not to be affected by the bass drivers' internal cabinet waves.)

By contrast, a planar speaker uses a large, flat film diaphragm (often a mylar or plastic composite) stretched tight in a frame and usually suspended between two mesh grids that are electrically charged. As the audio signal is applied, electrostatic attraction and repulsion alternately move the flat film back and forth, generating sound waves that radiate to the front and the rear of the speaker.

Planar-magnetic speakers use a flat diaphragm as well, often a foil ribbon suspended between permanent magnets, or some variation of this principle. The audio signal is applied to the ribbon, and the positive or negative magnetic forces move the ribbon back and forth. These planar speakers are almost always dipoles, because the pressure waves radiated from the back of the film diaphragm are out of phase with those coming from the front of the panel. This becomes problematic with bass frequencies below 100 Hz, because the deep bass waves, being non-directional, migrate around the panel and cancel out those from the front. The liability of these dipole planar speakers is that no deep bass is possible unless a subwoofer or auxiliary conventional woofer is used. Moreover, because the range of movement of a flat film diaphragm is more restricted than the excursion of a cone driver, the maximum peak sound levels are usually somewhat limited. If pushed too far, the flat film or ribbon may be damaged.

The virtue of dipoles is that with careful placement and a single known listening position, the midrange and treble sounds from the rear of the speaker will be reflected from the back wall and adjacent room boundaries to supply an enhanced sense of spaciousness to the soundstage.

Conversely, some of these midrange and high-frequency sounds may cancel or interact in a manner that typically make imaging somewhat vague and indistinct.
Nevertheless, a well-designed planar speaker--electrostatic or planar-magnetic--may produce very smooth and neutral midrange and high-frequency response. I say "may" because there are plenty of good and bad examples of each type, just as there are with forward radiating cone-and-dome systems.

Don't confuse dipole speakers with bipoles. A bipolar speaker radiates sound in several directions--typically towards the front and the rear (or in other directions as well), but here the outgoing pressure waves at the front and the rear are in phase, so bass cancellation is avoided. Like dipoles, a bipolar speaker can produce greatly enhanced spaciousness for stereo listening, but again the downside is a lack of distinct and precise imaging. The latter liability makes them unsuitable for use as front speakers in a home theater system. On the other hand, bipolar speakers like the Axiom QS8s (which are really multipolar in nature) used in a surround-sound application can be extremely effective in producing a diffuse and enveloping sense of immersion in the surround sound field. Even dipoles, properly set up as surrounds (and that can be quite a challenge) are also capable of yielding a diffuse and enveloping ambience.



Alan Lofft,
Axiom Resident Expert (Retired)
Re: Editor's Forum: Planar speakers, dipoles & more
#34959 02/24/04 10:35 PM
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In reply to:


These planar speakers are almost always dipoles, because the pressure waves radiated from the back of the film diaphragm are out of phase with those coming from the front of the panel. This becomes problematic with bass frequencies below 100 Hz, because the deep bass waves, being non-directional, migrate around the panel and cancel out those from the front. The liability of these dipole planar speakers is that no deep bass is possible unless a subwoofer or auxiliary conventional woofer is used.




This is true. I have noticed with 3 Magnepan models from the 1.6QR and lower that it would benefit having a subwoofer when listening to 2 channel audio. There is also a bump in the 60-80hz range which almost seems like there is a lot more bass but dwindles fairly quickly from the 60hz and lower.
One can treat planer speakers like a standard monitor where it is necessary to pair it with a subwoofer to get the full frequency range. If it is acceptable to have a monitor like the M22 to be paired with a good sub then I would presume it is no different to pair planer speakers with a good sub. Would I dare say the M22 is lacking because it can not produce lower frequencies?

I am no expert and please correct any of my assumptions if invalid. Planer speakers emit a nice clean tight bass since there is no break in the frequency spectrum. On normal box speakers the woofers play as certain range as do the midrange driver and the tweeter. Careful adjustments have to made to crossover and overlap (but not too much) to get a nice clean flat frquency spectrum. Axiom's are king on this as many people gospel about.

In reply to:

because the pressure waves radiated from the back of the film diaphragm are out of phase with those coming from the front of the panel.




Although out of phase from the back...by the time it deflects of your back wall and other things it will cancel out/absorbed out of phase signal or will be in-phase by the time it reaches your ear. Someone correct me if I am wrong with this assumption.

In reply to:

the maximum peak sound levels are usually somewhat limited. If pushed too far, the flat film or ribbon may be damaged




That is true ...that is why planer speakers are built with the fuse that breaks if those levels are achieved. I have had my planers at really really high level ... 80% on my volume level pushed by a Bryston 4B at 600watts at 4ohms. Mind you of course we are peaking at lower levels than 600w and only for a spilt second or so but I have seen my 4B red circuit protection hit 3 times.

In reply to:

Conversely, some of these midrange and high-frequency sounds may cancel or interact in a manner that typically make imaging somewhat vague and indistinct.




That is true. But proper placement and tweaks can be done. I find by having objects or sound absorbers such as having a bookshelf with books and stuff behind the radiating area of the planer speaker would absorb some of this out-of phase sound wave. What you get is more pinpoint imaging. A lot more focus without losing the expansive sounstage. (Hey Mike how are those sleeping bags behind your Maggies working out.

In reply to:

Conversely, some of these midrange and high-frequency sounds may cancel or interact in a manner that typically make imaging somewhat vague and indistinct




The midrange of the planer speakers are amazing. Placement is key and certain things need to be done to achieve a sound that is focused, detailed and expansive at the same time.

I enjoy the sound planer speakers present. It is the closest simulation of any speaker that I personally have heard being in a auditorium listening to Frederick Fennel and the Dallas Wind Symphony Orchestra.

As with varied different opinions, some do not like the sound of planers or electrostats. (My dad...he is Proac this and Proac that..blah blah blah)


Saturn

ps: if you have a chance give Magnepan a listen. You'll either like it or not. When our fearless leader was up and about (He seemed to have disappeared for some reason) he would give high praise to the sound quality a planer speaker would present.

Re: Editor's Forum: Planar speakers, dipoles & more
#34960 02/25/04 07:03 AM
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Well said Saturn. BTW, I'm no longer using the " dimpled foam sleeping bag "sound diffusers" that you refered to (yes, my wife is very tolerent of my little obsesion, bless her heart).

I've learned that with a bit more judicious placement, it sounds just as good without them and really even much better. I've moved the speakers further apart and closer to the corners of the room and nearer to the side walls (where the side wall, back wall and the entertainment center form a sort of alcove). The result is that the upper bass is slightly more emphasized yet still tight and clean and the speakers now utterly dissapear into an even wider and deeper (didn't think it was possible) sound stage with pin point imaging. Sound stage dimensions with these things still amaze me even after a year with them (especially after moving them further apart). But I didn't know what I was missing until I took a bit more of a radical approach to placement.

I had a preconcieved notion (that I picked up from internet audio "experts") that the speakers needed a lot of distance from the side/back walls to perform thier best. Just goes to show that even the so called "experts" aren't right all the time. Once someone has a good system put together, the key to getting maximum performance is experimentation with speaker placement and room treatments no matter which driver design your speakers use.

P.S: Saturn, I still treasure the "Silver Surfers". Thanks again!

P.S.S: A personal message to all: Even real audio "experts" have biases. Listen for yourselves.

P.S.S.S: IMO, most Maggieheads don't buy them expecting bottom octave bass.


I live the life I love and I love the life I live.
Re: Editor's Forum: Planar speakers, dipoles & more
#34961 02/25/04 08:03 AM
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Saturn,

I love the Magnepan sound, but I can't resist correcting you here:

In reply to:

Although out of phase from the back...by the time it deflects of your back wall and other things it will cancel out/absorbed out of phase signal or will be in-phase by the time it reaches your ear. Someone correct me if I am wrong with this assumption.


This is entirely dependent upon the distance from the back wall and the frequency of the sound. For example, in order for the sound emanating from both the front and the back of the speaker to arrive at your ear in-phase, the distance of the speaker from the back wall needs to be 1/4 of the wavelength of the sound. (To find the wavelength of a tone, divide 1130 by the Hz value of the tone. The result is distance in feet.)

Because the speaker must reproduce a wide range of sounds (on average from say 40 Hz to 20 kHz), you cannot place it at an ideal distance for all frequencies. Every distance that exactly reinforces a particular Hz value will exactly cancel out a value with half the wavelength (i.e. double the hertz).


I can explain it to you but I can't understand it for you.
Re: Editor's Forum: Planar speakers, dipoles & more
#34962 02/26/04 12:11 PM
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fhw Offline
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Alan, could you please settle something for me?

I'd understood that the QS series had the tweeters wired out of phase, but the woofers wired in phase. All of your recent posts (and Newsletter articles) state the tweeters are also wired as bipoles.

Are the tweeters wired as bipolar or dipolar, and is this something which has changed since the switch to the SE designation (along with, for example, the TWWWT array on the VP150)?

Re: Editor's Forum: Planar speakers, dipoles & more
#34963 02/26/04 06:42 PM
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Hi Alen,

Thanks for the great post outlining the virtues and problems associated with planar speakers. Comparing and contrasting traditional speaker design with planars is very usefull and I appreciate the details offered. In the end I realized that the planar speakers in my price range would just not offer the full range that I was looking for. I ended up getting the M60Ti speakers and could not be happier.

Keep up the great technical discussions. I really like that Axiom shares their expertise for all to read and debate. It makes me feel confident that my purchase is going toward a top notch company that cares about speaker design and not to pay a huge marketing department peddling inferior speakers.

- BigFoot_Lives

Re: Editor's Forum: Planar speakers, dipoles & more
#34964 02/27/04 03:26 AM
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Excellent. I didn't know you could actually calculate that. I'm impressed.




Re: Editor's Forum: Planar speakers, dipoles & more
#34965 02/27/04 03:28 AM
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Have you ever listened to planers or electrostats?

Re: Editor's Forum: Planar speakers, dipoles & more
#34966 02/27/04 03:36 AM
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Ever found a planar or electrostat that compared to M60s for $800? I doubt it.

Mind you, I love Magnepans, but they just aren't price comparable to Axioms.

And dude, you need to lose that chip on your shoulder about this. I understand where you're coming from, but....


I am the Doctor, and THIS... is my SPOON!
Re: Editor's Forum: Planar speakers, dipoles & more
#34967 02/27/04 05:20 AM
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In reply to:

Ever found a planar or electrostat that compared to M60s for $800? I doubt it.




No. The M60 are excellent for its price. The Magnepan MMG is only $550. The M60 does perform much more spectacularly. The MG12 is can be had for $1050. I personally prefer the MG12 over the M60.

In reply to:

but they just aren't price comparable to Axioms



So...you don't all see all of us buy Cambridge Soundworks because it is the best sound for the price

In reply to:

And dude, you need to lose that chip on your shoulder about this. I understand where you're coming from, but....




I am just strongly opinionated...as the rest of the people on here with their Axioms. I don't just preach the same dogma.

Maybe Sushi got it right. Its time to do the same.




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