Understanding cabinet pressure

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dcibel
Posts: 39
Joined: Sun Dec 13, 2020 6:39 am

Understanding cabinet pressure

Post by dcibel »

Hello,
SoundEasy is the only software I know of that will plot enclosure pressure. I am plotting this result and you can see an example in the attached image for a random 12" driver in 60L ported cabinet tuned to 30Hz.

I just want to be certain of what I'm looking at, below the tuning frequency I would expect cabinet pressure to drop towards zero, as the port starts to act as just a hole in the cabinet, at low frequency the cabinet should not hold any pressure and the speaker driver becomes "unloaded". However the plot shows the cabinet pressure remaining constant below tuning frequency. Can anyone explain this result, or is the plot in SE not entirely correct?

FYI, the comparison tab here doesn't show the scale for cabinet pressure, it is plotted at 0.5kPa per division, so the peak here is about 1.8kPa with 100W input power.
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Bohdan
Posts: 38
Joined: Mon Dec 07, 2020 8:31 am

Re: Understanding cabinet pressure

Post by Bohdan »

Hi,

I will have to dig into my source literature, if you need the exact formula.
For now, internal box pressure is derived from Vent Air Excursion and Cone Excursion data.
Both of these quantities remain greater then zero for the low frequencies.
The vent still tries to pass compressed air, and the cone still tries to compress the air.

At frequency = 0 Hz, formula for Vent Air Excursion would not work, because it includes division by frequency.

Best Regards,
Bohdan
dcibel
Posts: 39
Joined: Sun Dec 13, 2020 6:39 am

Re: Understanding cabinet pressure

Post by dcibel »

Thanks for the explanation. I will have to review the data in a bit more detail and see if I can make some sense of it.

If I find some free time, I'll maybe grab a differential pressure sensor from work and see if I can reproduce some cabinet pressure readings at various frequencies to confirm this plot. I'm just having a hard time understanding the plot for ported systems, I would expect to see cabinet pressure approaching zero below the tuning frequency, but perhaps I have a flawed understanding of how vented systems operate.
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