Pipe resonant frequency

Can someone answer this, please!

Take the example of a cabinet with an identical driver mounted on both the front and rear..one connected in reverse polarity and therefore a dipole.

Obviously limited benefit over one driver except the benefits of only using the front of the cone and in push/pull to cancel non linear distortion. Downside, the distance between the two limit the bandwidth you can use.

But you still have an almost free air condition..and consequently the steep impedance spike at resonance....I'm getting there..be patient!!

One could just divide the cabinet and fit acoustic resistance (felt) to increase the Qt of the system and damp the impedance...

Ok..here's the real question. If I put a port in the partion, it would be seen by both drivers which are in push/pull..so I am thinking we still have isobarik condition in the cabinet so there is no pressure change across the open ends of the port (or is there? )..Would the port still resonate at a defined frequency and be able to be tuned to damp the driver resonance..

Ok, that's it...thanks for reading!

Cheers....
 
Hi, I haven't thought such setup before and as first thought it would not work. Helmholz resonance works with the inside air of box as spring and air in the port as mass making a resonator. I think the spring is not excited if you have this kind of isobarik setup, and there is no resonance.

You could put the drivers on a baffle as dipole, which would double up effective cone area and still have at least some reduction of 2nd harmonic if you put one driver cone toward you and the other magnet toward you and reverse polarity.
 
  • Like
Reactions: 1 users
Well, yes..that's true if the target is dipole only. In truth we have 2 drivers on the rear baffle connected in series and 1 on the front. This close spacing of acoustic centres vertically provides an even dispersion pattern to a higher frequency than a simple 2 x 15" which lobe around from around 350Hz upwards due to the cone centre distance of a pair.

The application of a low pass filter to the rear pair (with known GD), when combined with the cabinet depth creates a cardioid response which becomes monopole as the front driver takes over...with the benefit of the dispersion of a single driver up to around 800Hz for around a 90deg conical dispersion.

When similated it appears to be possible to achieve full cancellation to the rear at very low frequency with a smooth and gradual transistion to monopole ...in other words...cardioid through super cardiod to monopole without side lobing..

Well, that's the plan ...and I have been able to juggle the dimensions and LP filter order and type to produce the required result..an arduous task as I had to produce polar response plots at frequency intervals from 40Hz - 1Khz, taking into account the GD of the filter and the attenuation at each chosen frequency. (40, 50, 64, 80, 100, 125, 160, 200, 300, 400, 500, 630, 800, 1kHz). The two rear drivers in series have the same efficiency as the single front driver of course.

And other benefits include...Swicth the LP filter out and you have a dipole (only usable up to around 450Hz due to cabinet depth in this case). Or put them all in "like polarity" and you have a 3 x 15" linkwitz transform monopole sub (usable up to around 200Hz before polar pattern anomolies occur.

Some method in my madness I hope. My OP was concerned, as all drivers share the same cabinet..if I could tune the Qt of the system by partitioning the cabinet between the front and rear drivers and fitting an acoustic resistance ...in other words, add a damping system? The Fs should still be near free air but "could I change the Qt of the system" was the issue I was struggling with! In monopole of course the acoustic resistance would have no effect...
 
  • Like
Reactions: 1 user
Try it, divider with lossy port, whip up a prototype box with detachable panel to experiment with. GM might know if it has been done, or would it work, or some way how to achieve you goal.

Why you want to adjust Q, whats the goal? dampin would lower Q, making more gradual acoustic rollof. Is there a peak you want ro tame, or does it affect the pattern or something?

You could likely emulate this in vituixcad and see polar pattern, play with positioning of the elements and get polar graphs with reasonable accuracy for the usable bandwidth. You could generate responses in enclosure tool, exploit driver TS parameters to modify Q some, or perhaps there is directly a leak parameter you can use, to see whether it could work, before making a prototype. Point is to test your idea with idealized situation to see if it worked for what you are trying to achieve.
 
Hi, thanks for the recommendation..I use an EV programme to simulate polar response which seems to give relable results. Community also do an excel spreadsheet. You have to cheat and just use their cabinets but the polars simulations are about driver spacing....

You are correct of course in that a damper would reduce Qt...I was thinking, if the partitions were solid for instance, we would have two back to back IB's which would raise both the Fs and Qt. As a path between the two sections was opened, the Fs and Qt would start to move towards the driver free air specs? So say your driver has a Qt of about .5 and fs around 40Hz..in a small IB it would be closer to around 80Hz and much higher Qt around 1 ..

So I guess I'm trying to create a system Qt around .7 and as close to free air fs as possible. One solution would be to just have the drivers share the cabinet volume and use resistance values to increase the Qt...or just apply eq!!
 
  • Like
Reactions: 1 user