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Port placement in a straight MLTL
Port placement in a straight MLTL
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Old 7th October 2013, 08:17 AM   #1
tomchr is offline tomchr  Canada
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Default Port placement in a straight MLTL

Folks,

I apologize if this question has been asked ad nauseam, but searching got me nowhere.

Much inspired by Bob Brines' work, I am toying with a straight MLTL for an Alpair 10P-A - the version with the paper cone rather than the metal cone. I'm playing with Martin J. King's MathCad scripts. I'm puzzled by a few things:

- How is the distance from the bottom of the TL to the port center determined?
- How does one decide on a resonance frequency for the TL and the port?

The f0 of the Alpair 10P-A is 42.4 Hz. Qts = 0.330. So I aim for a TL resonance frequency in the high 30ies.

By empirical design (i.e. trial & error) I arrived at an enclosure which provides a nice flat response and f3 of 33 Hz. I noticed that the resonance of the enclosure is very sensitive to the port diameter and length but not very sensitive to port placement (within reason, of course). The resonance frequency depends on the length of the TL, but the port dimensions seem to have the most impact.

It seems there are two knobs for tweaking the TL resonance: The length of the TL and the port. I think my question boils down to this: What's a systematic way for turning those knobs? Which tradeoffs are associated with each knob? I'd like to understand how this works rather than rely on trial and error.

Thanks,

~Tom
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Old 7th October 2013, 12:38 PM   #2
xrk971 is offline xrk971  United States
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Port placement in a straight MLTL
I have also found that port placement is not that sensitive either when I tweak parameters in akabak for a mltl. Look for the resonance peaks and dips and see if moving the port position can smooth those out. In general, I find that placement of port at 2 to 4 diameters from the bottom makes little difference. If you will be using a slotted shelf type port, placement at the bottom is natural and the preferred location based on ease of construction. The placement of the driver, on the other hand, can be used to reduce the resonance peaks and dips with great effect and is very sensitive to small movements of even a half inch. However, driver placement between 0.2 and 0.33x from the top usually provides a reasonable place to start before tweaking to optimize. In fact, you may find that a quite reasonable and well-performing mltl can be designed based on rules of thumb alone without any tweaking using a simulation as I did - and describe in the Accidental MLTL technique thread: Accidental MLTL Technique
Certainly use MJK since you have it, but try the AMLTL as a starting point and see how close to a solution it is.
Can you please post your sim results for the A10p or at least the dimensions of the design?
Good luck.
X
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Old 7th October 2013, 01:07 PM   #3
sreten is offline sreten  United Kingdom
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Hi,

The AMLTL is a long thread but makes a lot of sense. I'd add
depending on the driver you can play around with the vented
box alignment a little before you convert it to the MLTL.

e.g. whilst WINISD suggest 16L tuned to 52Hz,
20L tuned to 42Hz looks quite good to me.

Port placement is not critical at the end of
the box, driver placement is more critical.

rgds, sreten.
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Old 7th October 2013, 07:22 PM   #4
tomchr is offline tomchr  Canada
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I followed Bob Brines' design procedure. If I recall correctly, I ended up with about 135~140 cm for the internal height. The cross sectional area was 4*Sd. I started with the driver at 0.2*L and tweaked the position slightly to remove the slight SPL wobble caused by the 5th harmonic.

I forget the exact port dimensions, but I seem to recall using either 2.5" or 3" diameter; 3" length.

~Tom
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Old 7th October 2013, 08:07 PM   #5
tomchr is offline tomchr  Canada
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Mr. Brines explains it well: Nature of the Quarter Wave Resonator.
On that page the tradeoffs between TL area, port area, and driver placement are explained. No word on port placement, though. I'm getting the feeling it isn't overly critical.

Thanks Bob for making that information available.

~Tom
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Old 8th October 2013, 12:56 AM   #6
Bob Brines is offline Bob Brines  United States
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Port size and length are both significant. You want to use the absolutely smallest port diameter that will support the SPL required. Determine this by looking at the port velocity. Port length is important because , as mentioned above, the are two degrees of freedom here. You can find an infinite number of pipe lengths and port lengths that will give the same Fc.

Driver and port position are both important and rather touchy. After setting the stuffing to zero, move the driver to get the best suppression of the first overtone.

Now the port. There are three rather large harmonics between 700-100Hz. Move the port up from the bottom until it suppresses the middle harmonic. This produces the best FR plot. Now stuff the pipe until you get the degree of smoothness you desire.

Bob
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Old 8th October 2013, 02:35 AM   #7
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by tomchr View Post
Mr. Brines explains it well: Nature of the Quarter Wave Resonator.
On that page the tradeoffs between TL area, port area, and driver placement are explained.
No word on port placement, though. I'm getting the feeling it isn't overly critical.

Thanks Bob for making that information available.

~Tom
Hi,

Though that page mentions Mass Loading by tapered lines,
it is not the same as a MLTL, and they are not discussed.

Hence nothing on port placement and nothing on if the
the port actually suppresses the higher modes output.

rgds, sreten.

Last edited by sreten; 8th October 2013 at 02:58 AM.
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Old 8th October 2013, 03:14 AM   #8
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by Bob Brines View Post
You want to use the absolutely smallest port diameter that will support the SPL required. Determine this by looking at the port velocity.
Smaller port diameter --> higher velocity. From what I understand, higher velocity creates turbulence in the port, which adds distortion. I don't know if this is science or internet hearsay, though. What's a good tradeoff between "as small diameter as possible" and port velocity? In other words, what's the speed limit or the limit on the minimum port diameter?

Quote:
Originally Posted by Bob Brines View Post
Driver and port position are both important and rather touchy. After setting the stuffing to zero, move the driver to get the best suppression of the first overtone.

Now the port. There are three rather large harmonics between 700-100Hz. Move the port up from the bottom until it suppresses the middle harmonic. This produces the best FR plot. Now stuff the pipe until you get the degree of smoothness you desire.
Awesome. Thank you!

Another question: How do you model the port placement in Martin's "Sections" sheet? Do you just assume that the port is placed at the end of the TL? After all, the "Sections" sheet is intended for use with a TL that's open in the end. A TL with a port located 5" up from the bottom, is closed in the end, but open 5" up. I don't see a logical way to include this in the "Sections" sheet, yet, on your website you mention that you use it.

~Tom
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Last edited by tomchr; 8th October 2013 at 03:18 AM.
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Old 8th October 2013, 03:33 AM   #9
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by sreten View Post
Though that page mentions Mass Loading by tapered lines, it is not the same as a MLTL, and they are not discussed.
Isn't MLTL = Mass Loaded Transmission Line?

I believe the port mass loads the TL, so a straight pipe with a hole in it would qualify as a MLTL. No?

~Tom
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Old 8th October 2013, 03:40 AM   #10
tomchr is offline tomchr  Canada
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Note the comments about port sizes and bandpass boxes near the bottom of this page:
tech note 14

~Tom
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