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High Efficiency Speaker Asylum Need speakers that can rock with just one watt? You found da place. |
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In Reply to: Re: Karlson tube not Karlson box posted by eric degny on January 31, 2001 at 06:56:22:
Hi Eric!You wrote:
Hello Wayne
> > My sumary is that the theoritical analysis and measurements
> > of M. Popper seems to prove that a K. Tube could be compared
> > to an exponential horn and provides "equivalent" acoustical
> > impedance transforms.Impedance is not the only measure of a horn and impedance changes do not provide a direct indication of efficiency. Filters also provide impedance changes.
One of the common analysis mistakes made in the early part of the century, was to assume that a horn's gain was directly related to the impedance load presented to the amplifier. From this, one would have expected 15dB gains from basshorns of even moderate size. But it can be shown how (and why) this is not the case.
Filters - both electronic and acoustic - also present an impedance change to the system. In the case of the K-Couplers, higher impedance is presented at the frequencies where energy is reflected back towards the diaphram. This raises impedance without a corresponding increase in efficiency. It is not unlike the effect caused by an electronic filter - where the action of the device raises impedance of the system at the filtered frequencies.
I have seen a few types of overgeneralizations that are relatively prevalent. The most common offenders are this one - an overestimation of horn gain based on impedance measurements - and miscalculation of reactive components, both for dB attenuation and phase angle.
Many expect twice the attenuation at the place where reactive impedance equals load impedance, and that's because they forget that reactive impedance and resistance are not combined as two resistances would be. They must be calculated with vector or polar (2D) math. This often also causes phase miscalculations, and just like the "time offset" baffle spacing issue - we come up with the wrong values as a result of this over generalization.
Phase differences between 0 and 90 are additive, at least partially, and those between 90 and 180 cause at least partial cancellation. The "magic number" here is 45 degrees - hence 135, in the case of cancellation - because that's the 3dB point. At 180, we're at the limits of couplingf - where if we were perfectly coupled, we'd have complete cancellation and two diaphrams in a waveguide will come very close to this at the waveguide frequency. Baffle mounted drivers, less so, but still significant cancellation at 180 degrees.
Phase angles are constantly changing in a reactive circuit, as a function of frequency. Likewise, two point sources at different locations in space have constantly changing phase angles as a function of frequency. That means one cannot expect to set a proper distance or a proper reactance to "correct" phase. There are only acceptable amounts of tolerance.
Anyway, these are the most common offenders. Filters and horns. Impedance and phase angles.
Wayne
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Follow Ups
- Re: Karlson tube not Karlson box - Wayne_Parham 01/31/0113:23:53 01/31/01 (0)
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