I've heard it said that it is not a good idea to tune a box below the FS of a sub. A few days ago I spoke with a tech at MTX looking for the specs of the MTX TC710 . . . It has an FS of 47!

Yet, it has an EBP of 59.5, so it can be used in a ported.

When I spoke with the tech at MTX, I mentioned to him that I wasn't comfortable tuning a box lower than the fs of the sub, and he said that "The suspension on the sub allows the sub to play much lower than the fs with no problems." He then told me "You run into problems is when the sub plays lower than the tuning frequency of the box." He was working with a box building program and told me that this sub can easily handle 400WRMS tuned to 33hz without any excursion issue.

I am so confused about how these things all work together right now . . . He recommended tuning it to 35 in a 1.45 cu ft box . . .

Here's the specs:
qes: .79
qms: 4.95
QTS: .681
FS 47hz
vas 12.6 l
xmax: 16mm

So, with that being said by the MTX tech guy, how can I know that its okay to tune below the FS of one sub, but not the FS of another?

 

Fs------This parameter is the free-air resonant frequency of a speaker. Simply stated, it is the point at which the weight of the moving parts of the speaker becomes balanced with the force of the speaker suspension when in motion. If you've ever seen a piece of string start humming uncontrollably in the wind, you have seen the effect of reaching a resonant frequency. It is important to know this information so that you can prevent your enclosure from 'ringing'. With a loudspeaker, the mass of the moving parts, and the stiffness of the suspension (surround and spider) are the key elements that affect the resonant frequency. As a general rule of thumb, a lower Fs indicates a woofer that would be better for low-frequency reproduction than a woofer with a higher Fs. This is not always the case though, because other parameters affect the ultimate performance as well.

 

Helpful . . . I assumed what you stated about the fs, and don't see why a sub couldn't play lower than that.

What are some of those other factors for discerning if a sub can be ported lower than its fs?

I also have some pioneers with an FS of 39.7 that I really want to port and tune to 32hz.

 

i've never had an issue tuning below the FS of a woofer. i just made sure that my subsonic filter was set accordingly, as well as my gains.

 

At 59.5 EBP, that sub will not perform as well in a vented enclosure. The cut off is usually about 50 for ONLY sealed. If the EBP was closer to 80-90, vented would be the best choice.

In addition to what Thumpper wrote, the air pressure that will develop in the enclosure acts as a "shock absorber" for the suspension of the driver. Tuning below the FS (read a box with larger airspace) will have the driver act as if it is not in a box at all. It will play, but may become damaged from over excusion.