I know in most audiophile cirlces these are frowned upon for critical listening...but the deep, room filling, somewhat relaxed presentation can be addicting...however the slightly diffused or blend of instruments can be a turn off for those used to razor sharp imaging...the trade off being a very large sweet spot..often large enough to accomodate two individuals...any thoughts?
1) Dipoles and bipoles are extremely different things. An acoustically small dipole has 4.8dB of directivity which means it's radiating 1/3 the total power of an omnipole for a given on-axis SPL. A bipole tries to approximate a monopole with 0dB of directivity.
2) Most "dipole" speakers are planers which are not acoustically small at higher frequencies and don't do a good job approximating an acoustic dipole with directivity breaking 10dB at high frequencies and all sorts of lobing that make for a narrow sweet spot for both imaging and correct tonality. You need to differentiate between those and speakers which act more like dipoles. Panel resonances are also an issue.
3) Conventional wisdom which holds that dipoles add more reflected energy to the sound than conventional speakers is incorrect. A conventional speaker starts off omnidirectional with 0dB and has directivity increasing with frequency to perhaps 10dB at the end of it's tweeter's pass-band. It generally doesn't surpass the dipole's directivity until 1-2KHz which we could approximate as the end of the human vocal range.
That's because although dipoles have more energy headed out the back they have less to the sides (-3dB at 45 degrees, -6dB at 60, -12dB at 75, etc.) which is friendlier for many listening rooms. When I eyeballed the numbers for one listening room I noted a 7ms delay for the front wall reflection that was -6dB for distance, 4ms off the ceiling at -3.5dB, and 3.5ms at -3dB for the side wall. Mix in dipole attenuation of -3.7dB off the front wall, -3dB off the ceiling, and -8.2dB off the side wall and it's obvious the dipole can color the sound less than a conventional speaker.
Using a decent dipole implementation (not a planer) you still get whatever sound stage is recorded without embelishments. You can use the off-axis roll-off to compensate for speaker distance as you move off one side so you can have an imaging sweet spot for several people. You also get no bass boost below the room's fundamental resonance and better modal behavior.
It works _spectacularly_ well for critical listening.
With a well executed design, I suspect that there is less fatigue factor in general whenever all the sound is not being fired directly at your ears in the sweet spot. I think this generally lends itself to longer listening sessions, especially at higher volumes. At lower volumes, listening fatigue is less of an issue and it is generally easier to handle all the sound firing directly at you. At least, this would seem to be consistent with my experience.
08-03-12: Phasecorrect Off axis dispersion is one of the great attributes of rear driver systems...one can be almost 90 degrees off center and still hear the separation of instruments, etc...I also enjoy the increased depth and relaxed presentation which lends itself to vinyl reproduction... ...these systems tend to focus on what the recording is doing well.
Maybe that's part of why I can spend hours on end spinning vinyl through my Mirage OMD-15s. They are also the first speakers I've had that reveal inner detail well while keeping it in musical perspective. Separation of instruments and voices is probably behind that. In particular I noticed the subtle blending of wordless vocal backup effects that I often didn't know were there.
Anyway, the idea that rear-firing and omni speakers create an indistinct but large smear of what's on the recording doesn't match my long-running experience. You can hear deeply into the soundstage--and therefore--the recording.
Off axis dispersion is one of the great attributes of rear driver systems...one can be almost 90 degrees off center and still hear the separation of instruments, etc...I also enjoy the increased depth and relaxed presentation which lends itself to vinyl reproduction....in other words...instead of brutally punishing a recording...these systems tend to focus on what the recording is doing well..
I think we need to make a distinction between soundstage and imaging. Soundstage is the overall width, depth, and height of the stereo image thrown by the stereo pair. I *do* like the soundstage the best from omnidirectionals because the soundstage stays stable relative to the 2 speakers even when you move about the room. Unlike most forward-firing speakers, it doesn't collapse when you move outside the width of the stereo pair. You can still hear what both speakers are doing, but it's like if you're at a live concert and you're sitting in one of the side sections. I was just auditioning a pair of nice forward-firing speakers a few hours ago and I noticed that when I moved to the left of the left speaker, the left speaker was all I could really hear. Not so with omnis and bi-polars.
Many people distinguish imaging from soundstage, being that imaging is how separate and holographic the individual images within the soundstage pop into sonic focus. In other words, the soundstage defines how big the soundfield is; imaging defines the elements within it. For imaging, definitely you get more finite images with good panels or mini-monitors properly set up and with you listening in the sweet spot. However, omnis and bipolars image better than most people think they would. In fact, I find that omnis image to about the degree I hear in live concerts. In a live concerti don't really hear everybody's presence etched in 3D space as precisely as I do when sitting in the sweet spot focused on mini-monitors.
However, given the omni/bipolar advantages in power response, stable, realistic and scaling soundstage, and truth of timbre, I can comfortably give up that last bit of imaging for all the other advantages I hear.
If you have a forward-firing pair of speakers and un uninterrupted home life where you can consistently sit in the sweet spot, you might as well enjoy the enhanced imaging of forward-firing speakers. However, since I'm often puttering, cleaning, doing dishes, or typing on my laptop to the left of the left speaker, I'm happier with omnis. And when I *do* finally settle into the sweet spot for a focused listen, I'm plenty happy with what I hear.
I think controlled radiation yields better imaging and that the flooding figure 8 effect of dipoles or bipoles to slightly smear image but at the same time give it more depth. So to some the depth of image behind radiator would equal better imaging but its not really when you factor in loudspeaker placement. But it can be musically enjoyable I spent much time with both. Today if ruling out stats etc. Which to me makes little sense if wanting a dipole. I would suggest the fine designs of Duke at AudioKinesis.
Phasecorrect, I have yet to run into a situation where a dipole or bipole was less precise about imaging information. If you are a speaker manufacturer, essentially this is an easy way to improve imaging of the speaker, by taking advantage of the human hearing/perceptual rules.
However you achieve it, a larger soundstage is like a big screen TV. Assuming all else is well, there is less congestion and more "room to breath" which makes it easier to focus in on specific elements of the recording as desired and also enhances the overall musical experience in that sound is a 3 (actually 4) dimensional phenomenon and requires space in order to be reproduced accurately.
Actually...or ironically...part of the appeal of large rear driver systems and others is you DON'T know where the sound is coming from since one is engulfed in it...the scale of imaging, increased soundstaging, improved dynamics, and large sweetspot to me far outweigh the marginal advantages in detail, transparency, etc say found in good monitor/ stands set up...plus no need for sub
The principle applies to all speaker designs and home sound reproduction in general and is affected by both room acoustics, dispersion patterns and speaker placement relative to walls.
Bipoles/dipoles/and omnis just have different dispersion patterns and leverage it to a greater extent.
The idea is that with all the reflections the ear/brain system can get a better idea of where the sound is coming from. So dipoles and bipoles should be capable of greater imaging than speakers that radiate only from the front.
I have an entire 7.2 HT system with bipolar LCR and surrounds, with omnidirectional rear surrounds. The L-R speakers are Mirage M5si floorstanders with identical woofers, tweeters, and ports firing both front and rear. I originally used them for a couple years as a stereo pair and expanded it into a surround system. I love these speakers either way. What I really like them about bipolars and omnis in a surround system is that it provides a 360 deg. seamless soundfield. You never hear channels handing off from one speaker to another. It's continuous like real life.
My living room 2-channel system is anchored by a pair of Mirage OMD-15s, which are more omnidirectional, but with a 60/40 tilt towards the listener. It won't create the etched 3D sonic hologram of mini-monitors, they create a realistic sounding--and acting--soundstage that remains stable when you move around the room, just as it would in a live concert. The speakers scale very well too. Solo guitar or voice centers between the speakers with a realistic size image. Similarly, when you play big band or full scale orchestra, the soundstage expands accordingly. The lack of suckouts and hot spots help create an even power response. Personally I find the timbre-correctness and realistic soundstage make it easier to losse myself in the music than if the primary strength is an imaging precision beyond what I can hear in most live concerts.
Read reviews of Ohms, Mirage, MBL, and dipoles such as Magnepa, Martin Logan, and Quad. You'll seldom see a criticism that the speakers smear the sound or sound artificially large. Quite the contrary; most reviewers find the soundstages thrown by omnis, bipoles and dipoles to be more involving. One difference; the bipolars and omnis tend to be more placement friendly, whereas the side-canceling nature of dipoles makes them more placement sensitive. However, although the bipoles/omnis are more placement friendly, their presentation improves considerably with thoughtful placement.
I own both . One pair are Von Schweickert VR2s which has a rear firing tweeter/midrange cone "ambience" driver. They sound good thats all I care about. My Jm Lab Focal Electra series speakers only have front firing drivers and ports, they sound even better to my ears. I drive the VR2s with a Jadis DA-60 so they are being used with good electronics, that's not the issue.
Prepare for an onslaught of responses from Maggie lovers. Boy, are you asking for trouble. I attend at least a dozen live performances a year. I find that Maggies do a credible job of reproducing symphonic music.
I would use the terms focused versus unfocused imaging to realistically assess imaging performance.
Bipoles and dipoles are capable of a focused presentation I believe as are omnis, however the presentation is often (not always) different from say a good pair of monitors.
Ratios of direct to reflected sound is a discriminating factor. Bipoles/dipoles are different than most conventional box designs in this regard but setup including distance to walls also comes into play (except perhaps in an anechoic chamber) and might be done differently for different speaker types depending on the target results desired.
Time coherency across the audible sonic spectrum at any particular instant is a related but different sonic artifact that factors into quality imaging. Not sure bipoles/dipoles ae categorically any better or worse than other designs at this aspect either.
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