4th order-care to explain?

[Triticum Agricolam]

1 hour ago, dsw422 said:

Do all sealed chamber sizes reflect the same resonant tuning no matter the sub? Is it unusual to see sealed chamber size larger than the ported side? Can tuning the ported chamber higher than the sealed increase ouput? Thanks for any responses

 

No, sealed chambers don't have the same resonant frequency no matter the sub.  The sealed chamber itself doesn't actually have a resonant frequency at all (excluding things like standing wave frequencies), what is does is it raises the resonance frequency of the sub that is placed in it.  Every different sub put into a sealed chamber is going to have it resonance frequency raised a different amount.  

Its uncommon (but not unheard of) to see a sealed chamber larger than the ported side in car audio applications, but its not uncommon at all when 4th order bandpass boxes are used in applications other than car audio.  

Tuning the ported chamber higher will move the frequency that output peaks higher (at the expense of low frequency output), it will also most likely peak at a higher SPL number (just like a ported box does when tuned higher).  Its not going to get louder overall though. 

[bluegmcjimmy]

23 hours ago, Triticum Agricolam said:

Here ya go:

First start with the rear chamber. The rear chamber determines what output you are going to get and controls cone excursion below the tuning frequency of the front chamber. The bigger you make the rear chamber, the more low end output you will get and the lower your sealed resonant frequency will be, but as you make it bigger your cone excursion goes up too. It's a balancing act to try to make the rear chamber as big as you can to get the most output, but without having it be so big your subs destroy themselves the first time you play a low note at war volume. The manufactured recommended sealed box size is a decent starting point for rear chamber sizing, but you still need to check and see what you get.

 

The front chamber size determines what output you will get around the tuning frequency.  Bandpass boxes let you trade efficiency for bandwidth and vice-versa.  The bigger you make it the more output you get, however you only get output over a limited frequency range. Make the front chamber too big and you get a one-note-wonder. Just how big you should make it depends on the parameters of your subs.  Some subs will give you wider bandwidth than others will using the same size front chamber. Subs with lower QTS and/or Vas will be more peaky than subs with higher QTS and/or Vas, this will make some subs inappropriate for 4th order bandpass box usage. Just how peaky you want the output to be depends on your personal goals and listening tastes.  Everything is a trade off. 

 

When it comes to the tuning of the front chamber you can move the tuning around a bit to get the frequency response you want, but generally you will get best performance with it being near the sealed chamber resonance, this is another reason why its a good idea to keep an eye on your sealed chamber resonance when sizing your rear chamber. Its a good idea to make your front chamber port adjustable so you can tweak the tuning a bit when you get it in the vehicle. Tuning frequency on paper doesn't always work out to what you get in practice and you may want to shift your passband up or down a bit to fit your listening tastes.

 

In terms of port area for the front chamber, you need to have a lot.  All of the output has to come out of that port, so you don't want to lose output to air resistance due to an undersized port.  At 30+ m/sec you can lose half, or more, of your output to port compression  I try to keep port velocity under 20 m/sec if at all possible, though it may not always be.  Depending on how much power is going to be used, this can require ports as big as 1/2 the cone area.

 

A lot of people try to really over-simplify bandpass box design, this leads to shortcuts like using "ratios" that will often bite you in the ass. In my opinion, acoustic modeling software (WinISD, BassBoxPro, TermPro, HornResp, etc) is a must for bandpass box design. Otherwise you are just taking shots in the dark, you may be know which direction you are shooting towards, but without software you can't see where you are hitting until the box is complete. It takes some experience to use the software well and to know how to interpret the results, but this still a whole lot better than trial and error.  

thanks I'm gonna give it my best once it gets warmer outside.

[dsw422]

22 hours ago, Triticum Agricolam said:

No, sealed chambers don't have the same resonant frequency no matter the sub.  The sealed chamber itself doesn't actually have a resonant frequency at all (excluding things like standing wave frequencies), what is does is it raises the resonance frequency of the sub that is placed in it.  Every different sub put into a sealed chamber is going to have it resonance frequency raised a different amount.  

Its uncommon (but not unheard of) to see a sealed chamber larger than the ported side in car audio applications, but its not uncommon at all when 4th order bandpass boxes are used in applications other than car audio.  

Tuning the ported chamber higher will move the frequency that output peaks higher (at the expense of low frequency output), it will also most likely peak at a higher SPL number (just like a ported box does when tuned higher).  Its not going to get louder overall though. 

Thanks for the response.. so how would I be able tof test resonant frequency of a sealed chamber if the subs in this case do not come with TS Parameters (re audio MT 18s)

[Triticum Agricolam]

1 hour ago, dsw422 said:

Thanks for the response.. so how would I be able tof test resonant frequency of a sealed chamber if the subs in this case do not come with TS Parameters (re audio MT 18s)

You could do an impedance sweep. 

[Joe X]

This is what you get with a SA-12 on a 1 net sealed box:

 

 

You can see the impedance peak at about 44Hz, you will need a smartphone, your amp, a multimeter, and a few other inexpensive items to do the measurement.