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In Reply to: surface area vs. scale: panel and cone posted by Adriel on February 17, 2011 at 22:35:28:
There are several factors at play here.
One is the radiation pattern of the speakers. As much as we may be embarrassed to admit it, the apparent size of a stereo image is extremely dependent on room reflections: in an anechoic chamber, the sound seems to emanate from a barrow band between the speakers, if it emanates from the speakers at all -- the ear has difficulty determining whether the sound is coming from in front or behind. Reflections can increase the sense of depth (from the front wall) or width (from the side walls), but only if they're delayed by a minimum of 10 ms, and don't come from the ceiling. This means that loudspeakers with different radiation patterns interact differently with room and setup to produce differing apparent source sizes. As a general rule, conventional box speakers tend to spray sound on the ceiling (bad) and side walls, which increases the sense of width if the reflections are delayed by 10 msecs; they tend not to radiate well to the rear above a certain frequency, which decreases apparent depth. But it's highly dependent on the design, forex, as someone observed, MTM or D'Apollito designs have vertical directionality so suffer less from ceiling reflections.
Planars, by way of contrast, typically have a figure-8 pattern laterally, so they're less likely to trigger side wall reflections. This means that the sound is more likely to be bounded by the angle subtended by the speakers, but that's dependent, really, on room geometry and placement. They do emit a lot of radiation to their rear, which increases the sense of depth if the speakers are a minimum of 5' out from the wall (10 ms).
So, as a general rule, conventional boxes are better at width, planars better at depth.
The other advantage large speakers have is that infinite line sources don't have an apparent height. The head-related transfer function can localize the height of a conventional box speaker. It can't localize the height of a line source, so there's nothing to override the height cues from the recording. In this, line sources have the clear edge, whether the drivers are dynamic (lots of them -- the limitation here is that as driver separation approaches a wavelength, you get interference between drivers and a ragged frequency response as you move vertically) or planar.
Then there's the question of wave launch. The further away a sound source is from the ears, the more the wavefront at the listener approaches a plane wave. Line sources imitate this in one dimension, vertically, but not horizontally. Can one, in practice, hear this? I don't know, but it's possible that the brain measures parallax as one moves one's head, e.g., by analyzing the comb filtering from reflections.
Finally, the acoustic centers of the drivers matter. Box speakers tend to have their drivers arrayed vertically, and so the sound seems to emanate from different heights depending on frequency -- height is smeared vertically. (Not of course in the case of an MTM arrangement.) Planars typically have their drivers arrayed laterally, so the sound tends to smear laterally in a frequency-dependent way, particularly when you're close to the driver. This can have the effect of creating a large, but diffuse, soundstage, which can compensate in a cheesy sort of way for the width limitations of stereo which are imposed largely by interaural crosstalk.
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Follow Ups
- RE: surface area vs. scale: panel and cone - josh358 17:40:22 02/19/11 (0)
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