Helmholtz resonators tuned as MLTL?

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Hi Folks,

The recent paper with the spiral absorber got me thinking.

Currently, tuned helmholtz resonators for absorbing specific bass frequency are quite large and bulky due to the volume of the box. Knowing how we can easily tune an approximately 4ft tall floorstander sized box to 30-40hz using MLTL, is the reverse possible?

A MLTL tuned absorber? If the box is 48in tall, 4in x 4in cross section, can it be tuned to 45hz with a ML 'port'? Note there is no driver, just the box.

Has anyone tried this before?

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So there is no additional 'efficiency' from the MLTL? I thought the MLTL woofers are smaller, maybe the MLTL absorbers can be smaller too?

Cardboard tube sounds like a good start, how do I size the mass port? What is a good size to start?

Size wise, perhaps you could cover a wall with a 4-6in deep enclosure, with multiple MLTL. So perhaps the benefit is in the slim aspect ratio of the enclosure, in not the absolute size compared to a helmholtz.

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Joined 2003
MLTLs can be for any size driver and it's efficiency is dependent on net Vb same as any box. What the MLTL does is provide more vent damping than a simple reflex, which in turn allows a larger vent for a given length, tuning, increasing vent efficiency.

Designing one requires either lots of higher math not available in one neat 'package' of formulas AFAIK other than in the form of a TL/horn designer.

In short, if you want to damp a high Q peak [narrow BW], then it can be a small diameter pipe, but as the peak's Q lowers [increasing BW], then so too must the pipe area increase too.

Hi GM,

I have a 45hz mode in my room due to the room length.

Also So it is a rather narrow bandwidth and high Q peak. How small can the pipe be? Is 4in diameter, or 4x4in square x 48in long sufficient? What is a good starting approximation for port size and length?

How many tubes do I need? It's a rather small room, 9x12 feet.

I'm already using quite a number of broadband rockwool traps, so I am only looking to target the 45hz with this MLTL absorber.


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Hi All,

FYI; Forget Passive Traps..IMO:Use instead One or Two Active Traps, i.e. an inexpensive 6.5" Driver in a TL-Tube ~LPF cut of at 60-70Hz , Mass-loaded or not to remove the unwanted BW. Terminus or port as near as 0.5 x Pi as possible.



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Hi Bjorno, thanks for the response and info.
Can I get a bit of clarification :
1. The 6.5in woofer in TL, tuned to 47hz. How do I drive the woofer, and what to feed into its input?
2. Is it an active bass absorber, or is it multi sub approach to even out the room modes- geddes style? I'm a bit confused after studying the attachment sims.
3. Maybe if you could explain the sims, that would help.

Thanks again!

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