[edstrelow]

I have owned a set of the original Polk SDA-1's and have never been tempted to change them for anything else, although I did come close to getting some Quad electrostatics. They are fairly big, each speaker uses 2 tweeters, 4 midranges and a bass radiator and weighs about 80 lbs. Each is essentially a double speaker.

These designs are one of the best-kept secrets in audio. I notice that when these come up for sale on Ebay they go for top prices.

They produce remarkable images, again, I agree with the commentator who noted that when the umbilical cord between the speakers, and which generates the cancellation signals, is connected, the imaging jumps out at you in a fairly convincing, even spectacular fashion, and the speakers go from being a conventional set of speakers to being something quite unusual.

No it's not a gimick, it's an attempt to eliminate an ARTIFACT of stereo speakers, namely that with speakers, each channel feeds both ears, after a slight time delay. The phonomenon is termed "PHANTOM CHANNELS." They are phantom, because they don't exist in the signal, being solely a creature of speaker placement.

I have always found that many people have hard time accepting the artifactual nature of stereo speakers reproduction, I guess because it has been around for so long now. Yet I have seen articles from the early 50's which critiqued stereo speakers for precisely this reason.

The way I look at it is this. If you were present at a musical performance you would have a left signal in your left ear and a right signal in the right. If you record in stereo with 2 microphones, you will still have only a pure left and right signal. If you listen through headphones you will still have a pure left and right signal. However, if you play back through speakers, you now have 2 additional signals. The pure left signal is still going to the left ear and the pure right going to the right. However your left and right ears now also hear the opposite speakers. Their signals arrive slightly delayed in time to the opposite ears due to the extra time needed for the right speaker signal to get across to the left ear and VV. These are the phantom channels.

The Polk arrays recreate the time delay in reverse in an attempt to eliminate the 2 phantom channels.

One can argue whether this is a perfect solution to the crossfeed conundrum, but then neither are stereo speakers a perfect solution to stereo reproduction.

I think it is a misnomer to say that cross-feed in headphone amps attempts to make headphones sound like speakers. Sure they blend the left and right channels, but I do not believe that many of these systems attempt to recreate time-delayed phantom channels as occurs in normal speaker listening.

Cross-feed in amplifiers is not new nor did it originate with headphone amps. Numerous preamps over the years have had crossfeed features, I had an old Crown that did this for example. The only difference I see now is that some of the newer headphone amps use tonal modifications in addition to the blending of channels.

I also find the argument that one should listen to sounds as the producers heard them to be misleading. I doubt that accurate sound quality is the overwhelming consideration in commercial pop/rock music. Rather the sound is tailored to some estimate of what sounds good on the most common systems out there, as would only make sense if you want to sell a lot of records. The Beach Boy re-issues stated that their recordings were made to sound good on monaural car radios back in the day!

The second misleading corollary of the producer's sound argument is that producers only listen to speakers in the recording studio, yet every photo I have seen of sessions shows a lot of headphones in operation. Yet crossfeed advocates routinely assume that the sound is mixed for speakers! This is even more peculiar coming in a headphone forum.

 

[Scott45]

Quote:

Originally Posted by AdamWill I just read this thread, and there's one thing in the initial post I still can't shake off. I still can't figure out why this system requires one speaker to be labelled Left, and one to be labelled Right. If they each output a negative image of the other, why does it matter which one is left and which is right?

Polk's SDA is actually a lot more complicated that this thread might lead one to believe. Nspindel wasn't trying to do more than give a very general overview, which he did quite well actually.

My first post, so be patient. The SDA dimensional effect is actually NOT complicated at all. In fact it is quite simple. It is the same as the Early Hafler Circuit to create artificial rear channel information. The Dimensional speakers on the outside edge of each speaker enclosure, are simply ALL wired across the positive terminals of the amp, and that is why it need the cord connecting the left and right speakers, to create one big loop from Left positive through left speaker, to right speaker and back to Right positive on amp. It is a VERY simple circuit that simply derives L-R and R-L signals and plays them on both stereo speakers. The reason there are left and right speakers, are so the Dimensional speakers are on the OUTSIDE edge of the box and not the inside edge.

This effect make it appear as the sound from out of phase or left or right only sounds are farther apart. Polk marketed it as being more realistic, but in reality, it creates a fake extra wide sound, that only works correctly on binaural recordings. Since the studio creates the master using normal stereo speakers, and NOT SDA speakers, everything will be a quite exaggerated stereo effect, that was not originally intended.

 

[ultimatesynth]

I've been asked to comment here, so I will comply. Mods please don't read thi as advocacy but rather as an opinion contrary to the existing Polk design.

I'v long thought that this design was based on faulty thought bordering on being rediculous.

Lets assume that all recording engineers mixed on headphones without crossfeed or on these Polk speakers. In this case, it would be possible for the recording engineer to design a soundscape that was natural and immersive. But it's very likely that to do so they would have to synthesize what you hear on two normal speakers to some degree. Here's why:

Assuming the goal is to replicate what you would hear from 5 or 10 discrete sound sources. Sound from each source makes it to both ears with varying interaural time differences. Those time differences asociated with each sound source would be the primary localizing audiotor cue with which you would manifest a perception of an acoustic image.

In audio engineering there are ways to not only pan a signal from left to right with volume changes when sending the signal to left and right channels, but to move it left and right by delaying the signal on one channel. Because the left speaker is louder in the left ear (even though the right ear still hears some of it) and the right speaker is louder in the right ear the added delay to one chanel will move the image of the virtual source toward the undelayed side. However, it will be somewhat smeared by the acoustic cross talk of the speaker-to-ear relationship that is actually there---despite what the sound engineer is trying to paint. Therefor the holy grail of speakers is to somehow dissapear, and recreate the whole of the origional acoustic wavefront of 5 people playing instruments.

There is evedence to these ears that what you really want more speakers to recreat this wavefront as opposed to supply isolated channel signals to each ear. I will point to Bob Stuart's Meridian products that synthesize the appropriate signals for 3 or 5 speakers as an array across the front of the room. His stuff (as I recall!?) supplies the normal left and right signals to the left and right speakers, and creates a signal for a center and extreme left and right speaker channel. You would have to research the details, but I can tell you that I have been uniformly impressed by the stability and refinement of the image his systems make. Not only in the exact center but from your entire walk from the door and pretty much anywhere in the room.

Which bring me back to the Polks. Correct me if I'm wrong, but I understand they feed an inverted phase to an an additional speaker array in the opposite speaker in hopes of cancelling out the acoustic cross talk signal from the far speaker. Well what about the mono-componant of the stereo signal. That part that is the same on both channels. Left and right channels can easily be converted into sum and difference signals (with the left and right direction stored in the phase of the difference signal; this is also how "mid-side microphone techniques work with a omni or wide cardiod and figure 8 microphone) and the "sum" signal is large. In Polk speakers you have the
sum signal comming in phase from the left and right channel and out of phase from the compensating speakers. This causes chaotic (because all these speakers are spacially arrayed and also interact in a reflective and reverberant acoustic environment) cancellation of the central image and an unrepresentative of the origional intent broadinging outward.

Also, the Polk principle ONLY works when your head is at the exact center between the speakers. I don't like sitting there with my head in a clamp. (With headphones I can walk around wherever I want

) Any where else in the room the image is completely askew.

I think it's bull$#1t.

The trick, whether it's on speakers or headphones, is to recreate a wavefront that's natural and believable to your ears. I think a significant and accute understanding of the acoustics/psycoacuostic involved and subtle and exquisite tweeks to the audio signal are the right way to go.

With all due respect, have you fully read Matthew Polk’s Whitepapers about how SDA Technology actually works? Because what you describe here is an over-simplification of what’s actually happening. The average human head measures 6-7 inches in width and 8-9 inches in length. It’s Not Zero Inches.
But in particular for a Mono Signal played back with an SDA system, the Left Dimensional Driver is fed L-R, which will be Zero signal output. Likewise the Right Dimensional Driver is fed R-L which will also be Zero signal output. The result being that for a Mono Signal (or Mono components of Signals) the SDA Loudspeaker System will output the Left Signal from the Left Driver(s) in the Left Loudspeaker & the Right Signal from the Right Driver(s) in the Right Loudspeaker — just like every other typical Stereo Loudspeaker System. No Acoustical cancellation even comes into play for a Mono signal (or Mono components of Signals) played back by an SDA Loudspeaker System.

 

[ultimatesynth]

My first post, so be patient. The SDA dimensional effect is actually NOT complicated at all. In fact it is quite simple. It is the same as the Early Hafler Circuit to create artificial rear channel information. The Dimensional speakers on the outside edge of each speaker enclosure, are simply ALL wired across the positive terminals of the amp, and that is why it need the cord connecting the left and right speakers, to create one big loop from Left positive through left speaker, to right speaker and back to Right positive on amp. It is a VERY simple circuit that simply derives L-R and R-L signals and plays them on both stereo speakers. The reason there are left and right speakers, are so the Dimensional speakers are on the OUTSIDE edge of the box and not the inside edge.

This effect make it appear as the sound from out of phase or left or right only sounds are farther apart. Polk marketed it as being more realistic, but in reality, it creates a fake extra wide sound, that only works correctly on binaural recordings. Since the studio creates the master using normal stereo speakers, and NOT SDA speakers, everything will be a quite exaggerated stereo effect, that was not originally intended.

Again with all due respect, Polk Audio’s SDA is Not the same thing as the Hafler Circuit—it Does Not Create artificial rear channel info. Have you fully read & understood the design of SDA—how it works? Because it actually solves issues with playing back 2 Channel / Stereo Recordings over a 2 Channel (Stereo) System. It’s Not a gimmick! It’s based on an understanding of the Physics of Sound, Audio Reproduction, & Psychoacoustics.