|
Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
|
|
|
Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
|
In Reply to: Euphonic Compression posted by RandyB on November 11, 2002 at 18:24:44:
Something to keep in mind is that direct-radiator drivers operate in constant-acceleration mode. Stated another way, excursion increases at a 12dB/octave rate as the frequency is lowered.For example, a tweeter crossover at 3kHz at 12dB/octave yields the *same* diaphragm excursion at 1.5kHz as it does at 3kHz. The diaphragm excursion will stay the same, in fact, until the tweeter encounters its own self-resonance (for a 1" dome, around 600 to 800 Hz), and the net acoustic rolloff increases from 12dB/octave to 24dB/octave. Unfortunately, there is usually a rise in response around the self-resonance, so excursion at resonance may actually exceed the level at 3kHz.
Will you hear this increase in unwanted excursion? Not directly, no. The acoustic response at 750Hz still some 24dB below the nominal, but the contribution to IM distortion is substantial. The spectral center of energy of most music is around 500 Hz, so the 500 Hz to 1kHz region intermodulates with the music that the tweeter is intended to play, above 3kHz.
To a first approximation, driver distortion is proportional to excursion, although this changes as the acceleration limit of the diaphragm is approached, then exceeded. Exceeding the acceleration limit of the diaphragm is classical "breakup", and can happen slowly or suddenly, depending on the cone material. More rigid materials like metal, carbon-fiber, or Kevlar tend to enter the breakup region more abruptly than softer materials like paper or polypropylene.
As you can see, first-order (6dB/Octave) crossovers actually increase cone excursion until first resonance, below which the net acoustic response drops at a 18dB/Oct rate. Second-order crossovers have constant excursion, and third-order crossovers decrease cone excursion at a 6dB/Octave rate (until resonance).
The tradeoff between tweeter excursion (and increased IM distortion) and degraded phase linearity is one of the more difficult choices facing the speaker designer. The only way to make an end-run around this is a digital crossover - but that opens another set of problems all on its own. (How do the early adopters of digital speakers feel about DVD-A and SACD?)
At any rate, getting back to the original posting, I would be wary of the tweeters entering an abrupt break-up region, with the onset masked by the louder output of the woofers. This can be quickly debugged by disconnecting the woofers and listening to the offending musical passages with the tweeters and associated tweeter crossovers alone. You maybe surprised at how much distortion you hear without the woofers masking the distortion.
This post is made possible by the generous support of people like you and our sponsors:
Follow Ups
- Watch out for tweeter distortion - Lynn Olson 17:18:20 11/16/02 (1)
- Re: Watch out for tweeter distortion - RandyB 07:23:48 11/19/02 (0)
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2024, Audio Asylum, all rights reserved ]
