Multiple ports with different tunings.

[gedlee]

Originally posted by djk
So the staggered ports act as one as a lumped mass, as far as the box tuning goes.

Say we use four smaller ports vs one large one of the same area. The four smaller ports will have a circumference of twice that of the one large port. Is this a potential source of port compression at high volumes?

When calculating the box tuning just use the total area and the average length. Its simple.

[djk]

So you are saying there is no loss at high volumes from the extra surface area inside multiple smaller ports?

[gedlee]

Originally posted by djk
So you are saying there is no loss at high volumes from the extra surface area inside multiple smaller ports?

No, I'm not saying that, I misuderstood your comment.

Yes, there is more loss, more damping, with multiple ports. But I even add foam in my ports to dampen them, because I want some damping in bandpass designs. But SPL loss from turbulence is a quadratic thing and isn't a factor until the velocities get quite high. SO I design the ports to keep them in the low loss (turbulence), but well damped region of operation.

[djk]

Thank you.

[xpert]

Originally posted by gedlee




xpert

I said that the average was not exact, but when the deviations are a small part of the total lenght, as they should be, then the average is quite accurate.

And the statement about higher Q for narrower ports is not theoretically correct.

I wrote before:


due to the square in Fr^2 ~ Aeff/Leff small errors in port length or area won't do to much. A deviation of 10% in Leff or Aeff will only cause Fr changeing about 5%. That is neglegible compared to atmospheric influences as air pressure, humidity etc.


and


The above correction to Mr. Geddes' note was done in case of someone building a double port of lets say 1" length + 10" length. The effective length will be much smaller than the average of both! It will be more around 1.1" instead of 5.5".


It's not as easy as Mr Geddes advises You. If the difference of length is small, I told You You could take the average. Small deviations to the correct measures don't harm at all. To be right if the lengths differ much please take the "x-perts" formulas.

Thats all folks!

The Q-thing: take a port of 10 x 10 dimension or take 4 ports of 5 x 10. Experience the difference!

[WaVeInFoRm]

one thing that may work, lets say the driver is in a box and the port in this case is simply a shelf at the bottom of the speaker.
if you added a second driver and attatched a piece of wood to it on a slider to block and unblock the port.
then theorietically the port would change its tuning frequency dependent on the note being played.

what i am basicly saying is if you had a port that could somehow alter its tuning to match the frequecy being played, it may well enable the a tuning drom 100hz-20hz

be funny to watch a port changing shape when different bass notes hit. im not sure how it woyld work in reality, but in imagination it is possable

or lets say a slider mechenisom where you have a load of ports like organ pipes, and when different notes hit, then via a computer or summin, the port that matches the frequency opens up.

not explaining to well, but its food for thought

[revboden]

Originally Posted by WaVeInFoRm

one thing that may work, lets say the driver is in a box and the port in this case is simply a shelf at the bottom of the speaker.
if you added a second driver and attatched a piece of wood to it on a slider to block and unblock the port.
then theorietically the port would change its tuning frequency dependent on the note being played.

what i am basicly saying is if you had a port that could somehow alter its tuning to match the frequecy being played, it may well enable the a tuning drom 100hz-20hz

be funny to watch a port changing shape when different bass notes hit. im not sure how it woyld work in reality, but in imagination it is possable

or lets say a slider mechenisom where you have a load of ports like organ pipes, and when different notes hit, then via a computer or summin, the port that matches the frequency opens up.

not explaining to well, but its food for thought

That would sound awful. The group delay alone make it a bad idea. A port should never be tuned in the functional range if fidelity is wanted. Say you want a sub that will function from 25Hz to 80Hz It should be tuned ~18Hz to keep the output coherent with the first harmonics. If the delay is too high in the audible range then the low end will sound "slow" as it is lagging behind. More than ~30ms delay is noticeable.

Now if all you want is loud "burps" at a specific frequency, tuning within the functional range is used.

[imix500]

Just to add some real world experience. Both the Meyer 600HP and 700HP use two pairs of differently sized ports.
I'm sure there are others out there, but those are too I'm very familiar with.

[weltersys]

Originally Posted by imix500

Just to add some real world experience. Both the Meyer 600HP and 700HP use two pairs of differently sized ports.
I'm sure there are others out there, but those are too I'm very familiar with.

The EAW SB1000z use a large central duct between the "V" loaded speakers and smaller ducted ports on either side of the "V", probably reduces the port resonance peaks that have been previously mentioned in this thread.

Even when a sub is band passed well below the port resonance, upper harmonics generated by the speakers when run hard can still be amplified by the port resonance.

In other words, the "optimally tuned large very low turbulence port" honks .

Art