Art Welter wrote on Thu, 04 November 2010 12:19 |
Phillip Graham wrote on Thu, 04 November 2010 09:47 |
Cabinet impedance testing at different values of large signal V_in is the most straightforward way, in my mind, to see the port compression effects, and well as shifts in port tuning frequency due to air behavior in the port with increasing Reynolds number.
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Phillip,
Could you clarify a few points:
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Sure
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How do you define "port compression effects" ?
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It is a collection of effects. Let me see if can mention a few:
First effect:Lets call the acoustic impedance of the port is R + j(w)M where R is acoustic resistance, (w) is angular frequency, and M is mass.
Mass of air in the port resonates in concert with the compliance (1/stiffeness) of the air in the box as a mass/spring system. The mass of air in the port is the port's reactive component, and R is the lossy component.
As the Reynolds number increases above about 2000, there is a transition range to fully turbulent flow and eventually at very large Reynolds numbers (200,000+) to a turbulent boundary layer, too. R increases as the Reynolds number (Re) increases.
Second Effect:The dramatic increase in R in effect one reduces the suppression of the driver excursion. Ideally the port air mass resonates out of phase with the driver at Fb, suppressing its movement. If R is high, the effect is damped and the driver moves more. This leads to the driver itself producing more harmonic and IMD components.
Third Effect:Flow out of the end of a open tube does not behave the same way as flow into the end of a open tube, so if you feed, say, a sine wave to a driver, the airflow "waveshape" from the port can be distorted asymmetrically on each half of the waveform.
Fourth Effect:Vortex formation at the edges of the port to cabinet transition, while technically a subset of the first effect, are significant enough to mention separately. These contribute to increasing the R in effect one.
Fifth Effect:The port itself makes extraneous noises (e.g. "chuffing" and "whistling") that contaminate the original acoustic output.
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My understanding is the port tunes the enclosure fB (box frequency), when you say "shifts in port tuning frequency" are you referring to a fB shift or something else?
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Yes, the reactive component of R + j(w)M above can change with different Reynolds numbers, and can change the apparent box Fb slightly at different output levels.
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Does that frequency shift go up, or down with increased turbulence (higher Reynolds numbers) ?
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While technically you might be able to make boxes that exhibit either a raising or lowering effect, generally the behavior of highly undersized ports is to have M decrease as a function of Re, which has the effect of raising the box Fb.