Do T/S parameters predict low end FR and rolloff?

When I look at FR in a program like WinISD, do the T/S parameters for a driver help model the low end FR/Rolloff, or is the program assuming a flat-ish response driver and the box response is more of a contributor to low end roll off?

For example, when I take a driver, the manufacturer FR is this:

And when I do a basic modeling of that driver in an enclosure in WinISD, I get this:

So, at 25 hz, the driver in free air is down almost 10db from 100 hz, but in WinISD in a vented box example, the reponse is down only 5 db.
What's closer to reality?
The reason this matters to me: I'm looking at port velocity in a flared 2 inch port, and WinISD and Flare-It show I'd be in danger of chuffing at the power levels I anticipate, but at VERY low frequencies, like <35 hz. So I'm wondering if the port and driver will really be as excited below 35hz as WinISD models, or if the driver's roll off down there will make it lower than predicted?


They are accuarate up to the mass corner or so

OK, digging into this, I assume we're talking that the T/S parameters can define the driver response from the lower to upper mass corners? Or from 1 hz to the upper mass corner?

In reference to horn design:
T/S theory peters out at the driver's effective upper [Fhm], lower [Flc] mass corners, so any bandwidth response plot beyond these points are strictly due to [inputted] inductance [mH] and/or mass roll off, with on-line freeware generally just flatlining it:

Fhm = 2*Fs/Qts' end of acceleration BW

Flc = Fs*Qts'/2 end of roll off BW [normally never used]

Qts' = 2*Fs/Fhm

Which, with an Fs of 37.2 and a Qts of 0.49, I believe would be from 9 to 152 hz on this driver? Thus, the WinISD prediction of the driver/port behavior at 25 hz should be fairly accurate.
Do I have this right?


Joined 2003
Which, with an Fs of 37.2 and a Qts of 0.49, I believe would be from 9 to 152 hz on this driver? Thus, the WinISD prediction of the driver/port behavior at 25 hz should be fairly accurate.
Do I have this right?
Not quite, Flc in an infinite baffle and below vented Fb is 4th order Vs sealed's 2nd order.

Note that if designing a reactance annulled BLH, Flc is the effective mass corner for design purposes as I understand it, i.e. only loads between Flc - Fs.
Default leakage or Ql for WinIsd is 10

7 being more realistic so that will drop port velocity.
can be changed in advanced settings

Port efficiency or Qp be 100
can drop that to 90 or 80
Velocity also go down.

Using Max SPL setting just applys full thermal power you enter
in the driver info.
So if your looking at port velocity with max SPL ,it is higher
than real life. since very rare youll get full thermal power into
a speaker

I just enter power manually till it starts hitting xmax
Below tuning or Fb. Basically look at when the unloading
hits Xmax
Then look at velocity. it usually will be lower.

18 to 20 ms usually acceaptable for velocity.
  • Like
Reactions: 1 user
Joined 2004
Paid Member
I should add that you can lose 2 dB efficiency due to voice coil heating - plus all the T/S electrical parameters shift. So if you are planning high power operation you might want to run a raw driver up and measure the T/S parameters yourself at the power levels you will run after it has warmed up. Then you can correct the box design and get the predicted response.

Don't forget to add volume for everything inside the box, including the port volume. If your port velocity is too high, increase the port size and flare the edges, that will greatly reduce noise.

These programs are accurate - if you correct for everything to make the box volume the same as the program assumes. I used to calculate this stuff by hand, it works.
  • Like
Reactions: 1 user
Thanks all!
For reference, I'm looking at an 8 ohm sub driver driven with 15 watts... Not huge power or super high SPL. (Modeling about 90 db at 1.5 meters)
Kind of a design influenced by Carmody's "Burro" or "Turtle" design. Had to alter dimensions to fit my situation, and then the driver too, since an 8" wouldn't fit the baffle size I need. Carmody talks about being concerned about port noise with his Burro design using a 2" Precision Port (even with their flares, etc). I read enough to see that many think my port velocity would be acceptable for most designs/situations. But then I came across Collo's info and Flare-It program.... And seems his research, even with some padding for seating distance, masking, etc..shows that a 2" port with flares and all will chuff at 11 m/s and 15 m/s at 25 and 30 hz.

In my case, I'm front porting, AND putting a cloth grille in front of the driver and port. So I have risks of chuffing at low frequencies, AND trying to oscillate air at high velocities through the grille fabric... that was making me re-think my port choice. So I wanted to confirm that the WinISD port velocities wouldn't be way off, and that the driver response modeled would be close at LF. Interesting on the advanced parameters one could alter in Win ISD...moving port efficiency to 90 didn't change much.

So, I'm looking hard at going to a 2.5" or 3" port instead of the 2" model shown in my port velocity screenshot above. I have about 18" of internal depth, so a 2.5" port could be 1 port diameter off the back and be right about where I want it tuning-wise....but still might have some chuff risk according to Collo's program. A 3" port I'd have to start adding bends, but it is possible to fit.... but also starting to worry about port resonance there coming under 300 hz.... The 2" port peaked at 19 m/s, 2,5 at 12 m/s, and 3" gets me down to about 8 at these power levels.