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Speaker Asylum General speaker questions for audio and home theater. |
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Speaker Asylum General speaker questions for audio and home theater. |
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In Reply to: Re: If designed well, can wide speakers image just as well as narrow ones? posted by bare on September 22, 2001 at 20:01:14:
Hi Folks,While this opener is quite simplistic, it does illustrate a fundamental truth. As Duke mentioned, the tendency of a baffle to direct radiated energy into the forward hemisphere is both frequency-dependent and baffle-size dependent. At higher frequencies, where the baffle dimensions are comparable or large in comparison to the radiated wavelength, the baffle will function well to restrict radiated energy into the forward hemisphere. At lower frequencies, where the baffle is small in relation to the radiated wavelength, the radiation becomes essentially uniform into full space. This is the so-called "diffraction step" phenomenon - which results in an asymptotic 6dB change in SPL (3dB from a change in efficiency, and 3dB from a change in directivity) measured in the forward hemisphere.
It is true for transducers as well as for baffles, that the shortest dimension of the surface under consideration tends to dominate the speaker's spatial dispersion of energy. For this reason, narrow speakers tend to have a high frequency of transiton between half and full-space radiation, rather regardless of the speaker's height. That's why even very tall but narrow speakers (such as the Pipe Dreams, to name but one of an inumerable pedigree,) have a high 2-pi/4-pi transition frequency.
But now to get back to narrow speakers, and their imaging properties: one consequence of a high half-space/full-space (i.e., 2-pi to 4-pi steradian space) transition frequency, is that much of the speaker's power spectrum diffracts around the enclosure and is radiated to the rear of the speaker. Such energy typically encounters a reflection from a rear surface (such as the wall behind the speakers) within several milliseconds of emission. This reflected energy reaches the listener's ears and is perceived in combination with the speaker's direct radiation by the ear-brain mechanism, as an apparent net increase in the depth (and possibly width when one considers tertiary and higher-order reflections) of the recreated soundstage.
The overall effect can be quite remarkable, especially if the rear surface is a "good" reflector, such as a bare wall. Of course, the interference between the direct and rear-reflected sound will create a comb-filtering filtering effect too, which has been shown in and of itself to have "interesting" soundstaging properties in many cases.
In this respect, narrow speakers are similar to bipolar or dipolar speakers: much of their imaging characteristics can be attributed to their pronounced rearward-directed radiation.
As an aside, another factor which may contribute to the advantage of a narrow baffle, is that it's easier to design a "quiet" narrow baffle than it is a wide one. Secondary radiation from the vibration of the baffle itself tends to degrade the speaker's impulse response, which in turn makes for poorer "microdynamic" acoustical performance, which yet in turn tends to degrade the precision and specificity of the recreated soundstage.
Hope this helps - best regards,
MAP
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Follow Ups
- Narrow speakers image better due to comb-filtered rear reflections... - MAP 22:53:14 09/25/01 (0)
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