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Frequency dependent image wander
Erin (Erin's Audio Corner) just posted this speaker review (AsciLab C6B) in which he talked about how a somewhat narrowing directivity with frequency can affect stereo imaging. If the directivity is not constant, musical instruments or voices will appear to be in two places at the same time. This, in turn, will confuse the listener because it does not sound natural. What are your thoughts on this concept?
And would it be possible for the average DIY'er to build something similar to this? My thinking is that you would need a small diameter tweeter (maybe 1/2" to 3/4" or so) placed on a very shallow waveguide and then cross to a somewhat small 5" diameter mid-bass. If you wanted deep bass, a subwoofer would be necessary.
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I though Apogee ribbons had a very narrow dispersion since you have to basically listen with your head in a vice for them to work their magic, and magic they are when it comes having a huge, deep soundstage with everything in its place.
I thought that all planar speakers had a very narrow sweet spot
I had a pair of Maggie IIs that had incredible imaging but in a very small sweet spot
Isn't the waveguide supposed to help with wider directivity? Sorry, I went through the video but did not get the context of directivity in it and it looks like it is a regular big 6" diameter waveguide
https://www.stereophile.com/floorloudspeakers/911/index.html
Would these be the Apogee Caliper ribbons you posted pictures of a while back? I did some google searches on Apogee ribbon directivity measurements, but came up empty. I don't know if it would be possible to measure the full horizontal and vertical directivity of a speaker this large. Be interesting to see the data, if someone could do it.
I'm not exactly sure. If you were to compare the directivity of this tweeter on a flat baffle verses the directivity in the shallow waveguide that was used, then the flat baffle measurement would have a very wide directivity at lower frequencies and a much narrower directivity at higher frequencies, especially above 10kHz. When you mount the tweeter in the shallow waveguide, the directivity narrows at lower frequencies but stays the same at the higher frequencies. The net result is a tweeter/waveguide combo that has a somewhat "constant" directivity pattern from low to high frequencies, but the overall directivity index at all frequencies has been reduced. This is why horn loaded speakers sound better in a highly reverberant listening room, such as the Harry Potter room at Grinnel. One of the best sounding speakers that year was a large horn loaded system.
Watch the video again and pay close attention to the horizontal contour plot that Erin shows at timeline 3:16 and the vertical contour plot that he shows at timeline 6.09. These plots show how the speaker's horizontal and vertical directivity tend to be relatively constant and even expanding a little bit at higher frequencies (above 10kHz). Most loudspeakers do not do this. Most loudspeakers tend to have a narrowing directivity at higher frequencies, especially above 10kHz.
That's not exactly how I would describe it, but Eric isn't directing his videos to a speaker designer or pro audience. Directivity is effectively determining the proportions of direct vs reflected energy in what you hear at the listening location. Changes in proportions of direct vs reflected energy at different frequencies simply throws off any sense of "natural" sound, the soundstage / image or whatever you want to call it gets a bit lost. This is a basic description of "good" vs "bad" but of course reality is a bit more complex than those two choices. Slope of power response matters as well, high slope and shallow slope can both be "constant", one may be preferable in acoustically lively rooms, one will be preferable in acoustically dead rooms. Then there's the debate that a constant slope may not be "best", however I think anyone with ears should agree that the mushroom bloom typical of many speakers with flat faceplate dome tweeter and 7" midbass is not great, with excess energy in 2.5-4kHz range.
Have a pair of those. Large bass ribbon mated with long tall m/t ribbon. One has a torn bass ribbon. Also have a pair of centaur major w/10" woofer,40" ribbon. Both require head in a vice. Not sure I would even want to attempt a full set of measurements on either of them. Here's some stereophile data of the apogee slant 6 https://stereophile.com/content/apogee-centaurus-slant-6-loudspeaker-measurements
Strange thing ...
Going to the way back machine, I had one of the calipers playing out in the shop. Door to the shop was open and the caliper was pointing out towards the yard playing music. Neighbor came out and I walked over to his yard and we talked a bit. Direct line of sight to the speaker. I was thinking how loud the music must be in the shop because I could easily hear it playing over into the neighbor's yard, but when I get back to the shop, the music is most definitely not loud. If it had been one of the cone n domes, the shop would have been rocking. There's something different in how the sound decays from a ribbon.
Stumbled on this short video, by Paul McGowan, on the subject of stereo loudspeaker "image wander" Very interesting and also very confusing.
What I meant by instruments and voices being in "two places at the same time" was that the phantom image created by the high frequencies of a voice or instrument would appear to be in one physical location while the phantom image created by the lower frequencies of the same instrument or voice would appear in another physical location. Your mind computes the location of the phantom image based on an analysis of both the direct and reflected sounds, compared to what your mind "thinks" should be the proper location. You become confused, so the image appears nebulous. This would be the case for a speaker with a narrowing directivity pattern with frequency verses one with a relatively constant directivity. This seemed to be what @erinh was describing in the video, as he talked alot about the phantom image placement and how it seemed to be more stable with the tested speaker compared to other speakers that he had tested in the past.