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MLTL & port length
MLTL & port length
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Old 16th April 2018, 08:03 AM   #1
samuelj is offline samuelj  United States
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Default MLTL & port length

Hi,

I m planning a MLTL with a 45cm long 10cm dia port tuned to 30Hz. Usually I see people using shorter ports, so I did not find anyone asking this question that I am about to. Does the length of the port contribute to the total length of the transmission line for calculation of quarter-wave resonance frequency?

45cm is significantly long and I plan to place the port (maybe slot) outside the TL.

Thanks.
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Old 16th April 2018, 08:22 AM   #2
wonderfulaudio is online now wonderfulaudio  India
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The port is a lumped mass and should not affect length atleast
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Old 16th April 2018, 01:44 PM   #3
pkitt is offline pkitt  United States
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Theoretically, in an ML-TL the system is tuned by two contributors, the 1/4-wavelength resonant frequency from the line's length and the port's dimensions. The length of the line will determine how much it contributes, and the port will make up the difference. For instance if you were building the most simplest form of an ML-TL, a floor-stander where the internal height is the line's length and that was, say, 40", its 1/4-wave resonant frequency would only be about 84 Hz, and if you were needing a system tuning frequency of 30 Hz, the port would be doing the most work. Now, if you had a longer line, say, 60", its 1/4-wave resonance would be ~56 Hz, allowing it to contribute a much larger part of the tuning and the port much less. Based on the length of your proposed port, it would seem there's very little contribution to the tuning from any internal dimension of the cabinet, or that the port's diameter is larger than absolutely necessary.
Paul

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Originally Posted by samuelj View Post
Hi,

I m planning a MLTL with a 45cm long 10cm dia port tuned to 30Hz. Usually I see people using shorter ports, so I did not find anyone asking this question that I am about to. Does the length of the port contribute to the total length of the transmission line for calculation of quarter-wave resonance frequency?

45cm is significantly long and I plan to place the port (maybe slot) outside the TL.

Thanks.
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Old 16th April 2018, 02:48 PM   #4
Scottmoose is offline Scottmoose  United Kingdom
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What Paul said. I'd also add that a duct of that length will have issues with self-harmonics.
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Old 16th April 2018, 05:38 PM   #5
pkitt is offline pkitt  United States
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Just wanted to add that I've never had to use a port in an ML-TL having a diameter larger than 3", and most of them have had diameters of 2" or 2.5" with the system tuned between 25 and 35 Hz, resulting in predicted f3s in the low to mid-30s.
Paul
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Old 17th April 2018, 01:54 AM   #6
samuelj is offline samuelj  United States
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Thank you very much for your help
I will consider port resonance in my design.
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