The added advantage of folding, is that higher frequencies will either be reflected back and/or absorbed.
We are talking about frequencies in the order of 800-2000Hz here.
We are talking about frequencies in the order of 800-2000Hz here.
I am keen to investigate that further.
I have been thinking about it:
if the boundary effect is proportional (or even quadratic) in relation to the air speed, then this would affect the peaks of velocity the most.
it means a non-linear behaviour, so in addition to some reduction in output it would mainly increase H3 (and further odd harmonics).
and as far as I could see and as far as i can imagine the main loss of output is the transition from port to interior/exterior air mass.
I suppose this is the main reason to keep air speed low. the "boundary effect" seems like a simplified explanation - happy to be proven wrong, however!
the turbulent flow caused by abrupt cross section change (or change from velocity system to pressure system) probably also causes all sorts of harmonics. but it is a rather chaotic and mostly noisy distorsion.
highly chuffing small straight tube port in post #165. this also includes harmonic %.
I don't remember the exact formulas anymore, has been to long that I was on the level of equations.
But they can be found in any proper acoustics or thermodynamics book 🙂
Well technically it's flow resistance.
Which comes from the roughness of the surface as well as the boundary layer.
Or basically that is the same thing, the boundary layer profile will change.
There aren't any other variables.
Well yes, having bends and such also adds flow resistance.
When you make a tube smaller, for the same air flow, this boundary layer will get bigger.
It basically works as a compressor/limiter eventually.
The chuffing part is a totally separate thing, which is more connected to impedance jumps and very non-linear
just had a quick look into the salvatti devatier button paper about ports:
(6th page in the pdf, called "page 27", my remark in italics)
so I suppose as long as the dimensions of ports are large compared to the 1 mm boundary layer the flow is not affected much.
however I have to admit I don't really have the acoustics and physics background to fully undestand it!
Martin J. King would know how to demonstrate that - as far as I know...
but guys, let's concentrate on the port research!
Remember that while the port might introduce an unwanted resonance in the mid-band it will reduce distortion in the bass, usually, because driver excursion is reduced.
I do like the latex solution. Especially over a small hole the membrane needs to be rather flexible. The KEF port has an entire large section made of foam rubber so can be a stiffer material.
I do like the latex solution. Especially over a small hole the membrane needs to be rather flexible. The KEF port has an entire large section made of foam rubber so can be a stiffer material.
B-force, how would you choose to determine boundary layer thickness experimentally? Maybe transparent port with a fine powder in the air? Or maybe moving a hot wire flow sensor to different areas? Just curious what you might think of.
In simulations the user needs to define the boundary layer size because different formula are used there, AFAIK. Andy might chine in here.
In simulations the user needs to define the boundary layer size because different formula are used there, AFAIK. Andy might chine in here.
More like tooled around, 'investigation' is too much credit. If I understand the latex membrane application it sort of makes sense that an element of tuning comes into play. Its native sprung resonance intuitively - a risky word - feels like it should match the frequency of the port resonance for optimum effectiveness. Avoiding sharp hole edges inside the port still appears a challenge, if I understand the application right.
Decades ago Martin King licensed his MathCAD sheets to the diy community, I had a paid subscription. A pdf copy of an mltl study I posted way back in 2006 attached.
for lowest level and highest frequency resonances the port should be as short as possible, in absolute dimension but also compared to the diameter.
Port tuning relies (simplified) on a relation between port air weigth (thus: volume) and the surface upon the oscillation force acts.
Thus, when increasing diameter, the surface rises squared and the length must also follow squared.
Therefore a small port has advantages regarding legth-diameter ratio.
Of course there is a limit when the port air speed gets too high.
Dual port has a worse combined port cross section surface/circumference ratio. The port "edges" ( or roundovers) inside and outside which can create turbulences and chuffing is higher than a single port with the same cross section area.
Also eventual viscous flow resistance would be higher due to higher port surface. Not sure how much influence that has.
Of course, if the chuffing and resonance problem is solved it's no problem anymore ... i am trying my best to do that 🙂
Last edited: