Fs and Port Tuning

Just doing a little design work here on a port and remembered something about the free air resonance of the sub (Fs) and how it relates to the "Proper" frequency one should tune their ports to. Let's say I have a 12 inch sub in a 2 cu. ft. enclosure and I want to tune it to 30 Hz but the Fs for this hypothetical sub is 25 Hz. Does this mean I should tune it to 25 Hz for the best performance? If so, why? What happens if a choose a frequency other than the Fs?

Thanks,

Mike


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2000 F150 Supercab Lariat, Alpine CDA-9831, Polk MMC570 in Doors, One Polk MM2104 Sub, One Polk Carbon C400.4 Amp.

John 3:16

errmm, look Fs is the lowest resonant frequency of a sub when it is out of the box, basically any frequency under this in a free-air setup will not be heard. this shud have nothing to do with your port tuning. if it is a normal daily drive sub keep the tuning between 28-34hz. The port tuning just extends a sub response in that particular range, again the Fs shud have nuthing todo with the port tuning

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6* Rockford T2's
6* POWER AMPs
BLACK CORRADO
BAANNGGIINN

a subs response in free air will be different to that in a box

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6* Rockford T2's
6* POWER AMPs
BLACK CORRADO
BAANNGGIINN

Yes, but it still does factor in. The free-air resonance will be the lowest frequency the woofer can play well. The closer to this frequency you tune the port to, the further down the Fr is moved.

I here what you are saying but what does Stevdart's statement mean as quoted here,

I wouldn't. Fs is 40 Hz and this is a subwoofer meant for SPL, not SQ. Use the guidelines found on the website. They must be referring to a 4" round flared port at the given length, which seems to me must be tuning to about 45 Hz or so. I couldn't find enough parameters on the site to work it up. You should be able to use a flared port like this one and customize it to the length given on the site. This came from this thread

It seems like Fs is to be considered to some extent, right? My question is to what extent should it be considered. Why is it important? If it is not important then why is it published at all?


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2000 F150 Supercab Lariat, Alpine CDA-9831, Polk MMC570 in Doors, One Polk MM2104 Sub, One Polk Carbon C400.4 Amp.

John 3:16

You want to tune the enclosure to a frequency that the driver can play well.  Look at Fs defined HERE (and any other descriptions you want to search for).  You can tune the box to a frequency slightly below Fs but you have to bear in mind that the benefit of tuning that low will be lost because the driver is not performing well at that tuned frequency.  The enclosure doesn't create a frequency response, it amplifies it (in my words).  You also run into serious problems with the driver flapping out of control below its Fs if the port tuning is too low, which can damage the driver as well as sound terrible.

You can tell a lot about the intended use of a subwoofer when you see specs like the example.  An Fs of 40 and an EBP of 100 tells you right away that the sub was designed to be installed in a ported enclosure at a fairly high SPL tuning frequency.  In contrast, a sub with an Fs of 23 Hz and an EBP of 65 would let you know that it would also use a ported enclosure but that you could tune the box in the high '20s for extended bass (amplified) response.  That is, the box itself would create a sound centered on the tuning frequency that would add about 3 db to the sound produced by the driver.  At such a low frequency, the driver's output would be attenuated, so the effect would be that the combination of driver and box would bring the lowest frequencies up, and the result would be extended sub bass response.  That would be considered a SQ setup using a vented box.

I don't have the time right now to look up data to suggest for you to read, but this is just off the top of my head right now based on what I've picked up about box tuning.  Anyone can feel free to correct me if I didn't get it right  :)  (...as long as you're right!)

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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

Please bring be up to speed Stevdart. What is "EBP". I did not see it in the list of T/S parameters.

Mike


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2000 F150 Supercab Lariat, Alpine CDA-9831, Polk MMC570 in Doors, One Polk MM2104 Sub, One Polk Carbon C400.4 Amp.

John 3:16

EBP is a function of Fs and Qes. It tells you wether a sub is ment to be ported, sealed, or can be used for either.

Paul

EBP (Efficiency Bandwidth Product) is calculated by dividing fs by Qes.  An EBP close to 100 usually indicates a speaker that is best suited for a vented enclosure, while an EBP closer to 50 or less indicates a speaker best suited for a sealed design. 

In general, a loudspeaker has its least amount of output at fs.  A ported enclosue has its greatest output at fb (tuning frequency) but dangerously unloads the loudspeaker below that frequency, requiring a high-pass (sub-sonic) filter at approximately the same setting.  Generally creating fb = fs results in a good compromise, assuming a good sub-sonic filter is used.  In car audio a sub with a high fs and high tuning frequency (above 40Hz) is generally acceptable since cabin gain will help compensate for the lack of deep bass (and there really is not that much musical information below 40Hz anyway.)

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