benefit of shifting port resonance location

[AllenB]

Isn't a resonance a property of the mechanical system and even if the crossover cancels woofer electrical energy at port resonance frequency, it still gets excited because of the frequencies below that which build up the resonance?

Yes, the resonance covers a band, outside this it is OK. You just need some space.

 

[sheeple]

The frequencies below the port resonance might not excite it by themselves but their harmonics which are caused be the woofer's nonlinearities might do so.

Hello Charles,
may I read this as another argument in favour of a higher space in the passband where one wants to store the resonance energy, instead of a lower? With the reason being when the fundamental is higher, the harmonics are more likely to fall out of the passband of the woofer and because they also have less energy, might not excite, reversly, the fundamental?

In my practical example, i.e. fb = 35 Hz in the given system described in the original post, the higher option results in a fundamental port resonance of 683 Hz, 2nd harmonic at 1366 kHz, 3rd harmonic at 2049 kHz, while LR4-crossover is acting acoustically 1.25 kHz. With this option the 3rd harmonic is pushed outside the passband and the 2nd gets attenuated, whilst in the lower option, both are well within the passband of the woofer.

 

[sheeple]

Maybe you can high-pass the signal going to your speaker such that the low-end influence of the reflex port is minimised and then you listen whether you can hear differences in midrange reproduction between open and closed reflex port.

Took me a while to understand what you mean, but now I got it. And yes, this is absolutely possible and I may give it a try, although I am not sure to what degree the results of such a high-passing can tell me something about actual full range FR, where port resonance is a result of total bandwidth response. Still an interesting way to isolate.
Best
m.

 

[phase_accurate]

The undesired port resonance will mostly be triggered by midrange frequencies. I assume that it might also be triggered by wideband noise caused by air friction due to high velocities at high SPLs but I would say that the triggering by midrange frequencies is the dominant part here.

Regards

Charles

 

[SpeakerBob]

Hi Sheeple,

There is some interesting talk in this thread.

I posted a couple of measurements of my BP subs which have very long ports and resonances starting quite low and close to its passband, with the effects of parametric EQ.

I have an old 12" no name car sub that I plan to build and test to destruction wrt Charlielaubs comments that resonances are caused by the port itself and not the input signal, but I haven't had time yet.

Cheers,
Rob.

 

[sheeple]

Thank you Rob,
post #40 is particularly insightful.

So it seems that as long as one cannot push the resonance considerably outside the passband, which isn't possible in a 2-way, it cannot be avoided that it gets excited. Then, one can go and manipulate the port to surpress resonance with a secondary device attached to it, but this is much effort, basically a project for itself. It is possible to choose which part of the passband will harbour the resonance and some have argued to favour a higher frequency, mainly because a lower frequency is supposed to be more prone to result in a port resonance which is audible. This seems to be a general rule of thumb. This is the technical side. From a perceptive approach, whether a frequency range is more prone to audibly deteriorate by port resonance over another, I am still up for further inputs.

 

[tmuikku]

Hi,

Instead of hightech port one could use passive radiator or closed enclosure, or just go for three way system. Three way system solves some other problems as well, trade off being mainly in cost and complexity.

Two way is somewhat cheaper and easier and can be smaller than a three way speaker, but if you are after audio quality go for three way system. Audio quality if mainly the frequency response consisting from bass and treble extension as well as good on axis, listening window and power response. By three way speaker system I mean a full range speaker with ported bass, or two way with sealed bass + sub.

If you allow yourself for the extra bass driver compared to a two way system you potentially get more SPL capability, less distortion and can optimize enclosure construction further than with a two way system (including the port dilemma you are having). Some of the acoustics stuff can be addressed a bit better, mainly within the bass region.

 

[AllenB]

Choosing to compromise some frequency is disappointing. Some would rather not do what it takes to damp a higher frequency in the box, for fear of losing a little bass. Damping material does damp higher frequencies better, so try to do enough for that.

 

[sheeple]

This is by the way another good argument to locate the resonance higher, because a reasonable thick damping is more effective with higher frequencies.

 

[SpeakerBob]

Thank you Rob,
post #40 is particularly insightful.

So it seems that as long as one cannot push the resonance considerably outside the passband, which isn't possible in a 2-way, it cannot be avoided that it gets excited.

Well I've yet to test that theory so the jury is still out for me. The plots I posted were done with pink noise, so why after EQ are they gone? Surely if they are caused by the 'wind noise' of the air in the port itself then they would still show up after EQ? (which is why I plan to do a test box and run it up until the driver fails to see if the wind noise thing happens near xmax)

Also depends on box tuning as to whether you can get a high 1st resonance.. I'm planning a 12" with CD horn 2 way where the 1st resonance is 7089Hz. (XO at 1200Hz) Tuned at 56Hz it will be high passed and used with subs.

Sealed box, maybe a 2.5 way would be my choice over a box with port resonances in the listening band.

Cheers,
Rob.