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Low Earth Orbit
DIY Audio
Cardioid using aperiodic ports (a la Fulcrum Acoustics)
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<blockquote data-quote="David Gunness" data-source="post: 207193" data-attributes="member: 1032"><p>Hi Kevin,</p><p></p><p>The difference between an "aperiodic port" design and our passive cardioids is the acoustical mass of the ports and the amount of resistance. The acoustical mass in our passive cardioids puts a second-order low-pass filter a little below the frequency below which we want cardioid behavior. The resistance is just enough to damp the helmholtz resonance to "slightly underdamped" (Q of about 1). An aperiodic port design has almost no acoustical mass and is dominated by resistance, so there is essentially no helmholtz resonance and the resulting first-order low-pass corner is well above the passband; so it operates simply as a resistive leak that damps the sealed box resonance. If you look at the impedance of our passive cardioid subs, you'll see a trace of the double peaks of a ported box, but the dip between them doesn't go nearly as low. That's the effect of the port resistance.There is still significant reduction of excursion around the helmholtz frequency, and the summation of the rear radiation and front radiation increases the output over about 1 1/2 octaves. If you try to stretch the cardioid behavior beyond 1 1/2 octave, there will be a penalty in low frequency extension and maximum output.</p><p></p><p>Adding "conventional" ports to the front of the cabinet would essentially short out the rear ports, which would destroy any cardioid behavior.</p><p></p><p>Cheers,</p><p></p><p>David Gunness</p></blockquote><p></p>
[QUOTE="David Gunness, post: 207193, member: 1032"] Hi Kevin, The difference between an "aperiodic port" design and our passive cardioids is the acoustical mass of the ports and the amount of resistance. The acoustical mass in our passive cardioids puts a second-order low-pass filter a little below the frequency below which we want cardioid behavior. The resistance is just enough to damp the helmholtz resonance to "slightly underdamped" (Q of about 1). An aperiodic port design has almost no acoustical mass and is dominated by resistance, so there is essentially no helmholtz resonance and the resulting first-order low-pass corner is well above the passband; so it operates simply as a resistive leak that damps the sealed box resonance. If you look at the impedance of our passive cardioid subs, you'll see a trace of the double peaks of a ported box, but the dip between them doesn't go nearly as low. That's the effect of the port resistance.There is still significant reduction of excursion around the helmholtz frequency, and the summation of the rear radiation and front radiation increases the output over about 1 1/2 octaves. If you try to stretch the cardioid behavior beyond 1 1/2 octave, there will be a penalty in low frequency extension and maximum output. Adding "conventional" ports to the front of the cabinet would essentially short out the rear ports, which would destroy any cardioid behavior. Cheers, David Gunness [/QUOTE]
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Cardioid using aperiodic ports (a la Fulcrum Acoustics)
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