Home Theater Forum and Systems banner
1 - 5 of 5 Posts

·
Registered
Joined
·
255 Posts
Discussion Starter · #1 ·
Hi fellas,

Got a box design with a first port resonance is 122Hz. I have read several topics on here itself about this issue, but have not come across a definitive "this is the rule of thumb you should go by" or a final value. Those were a few years back, and would rather not resurrect an old thread. I was wondering if any further testing has been done since to verify the uncertainty of such a scenario and the outcome.

The box is 5 cubic feet net, 10 cubic feet gross. 18.5" x 3" x 55" long slot. Tuning is 21Hz-ish. There will be 2 bends in the port, as opposed to one where it reaches the end and turns and just finishes. It is a long port. Just thought I'd mention as apparently more turns/bends can reduce resonances. Could be wrong.

I was planning on using a very steep (8th order, if possible) LPF to roll this off at a considerable rate - at about 60-80 Hz. I have read Steve's view and Collo's view on this, and I am not sure what to expect if a box whose fpr is 120Hz-ish. Steve says use a port no longer than 36", with a resonance no lower than 190Hz. I have seen Mike P. suggesting a 125hz resonance will be fine with a 4th order @ 80Hz.

What are some of your thoughts/opinions on this?
 

·
Premium Member
Joined
·
19,397 Posts
I have seen Mike P. suggesting a 125hz resonance will be fine with a 4th order @ 80Hz.
That would be for strictly home theater and not for critical music listening.

The box is 5 cubic feet net, 10 cubic feet gross

Your port displaces 2.2. cu.ft., that leaves 2.8 cu.ft. for bracing which is excessive.

Lowering the port height to 2.75" will raise the FPR. and shorten the port length.

 

·
Premium Member
Joined
·
1,923 Posts
In full range speaker design they say that unwanted resonances need to be reduced by -40dB or more to be inaudible. So that means a 120Hz port resonance would need no less than a 4th order LPF at 60Hz, and that would be on "the edge." A 4th order at 80Hz might not do it.
 

·
Registered
Joined
·
255 Posts
Discussion Starter · #4 ·
Thank you for clearing that up Mike, For now this will be more music than HT.

See here Mike:


Yes, that port is huge. It needs to be as it is going to be housing a custom built Z v.4 15". With 2000w input the port airspeed still tops 26 m/s, and Collo's flare it program indicates I will still hit chuffing. I am of course taking into account a real world situation - I will not be sending a 2000w 20Hz sine wave for any prolonged period of time, so I concluded I will be okay. If I drop the width to 2.75", it almost hits 30 m/s. Slightly OT but what would you do here?

fusseli: Thanks, that figure helps. If I use a 8th order LPF, assuming 70hz, 122Hz is -38dB. This is close...
 
1 - 5 of 5 Posts
Top