Q-factor of bass reflex port

[SSassen]

Just a quick question; I'm simulating a few enclosures using Speaker Workshop, but I need to know how to calculate the Q-factor of the bass reflex port as that variable needs to be entered for Speaker Workschop to do the simulation.

Any suggestions?

Best regards,

Sander Sassen
http://www.hardwareanalysis.com

[richie00boy]

If you mean Ql then I'm not aware of a way to calculate it, it has to be determined experimentally. 7 is a typical value. My last sub measured at 5.3.

[SSassen]

No, not Ql, that's obvious.

Best regards,

Sander Sassen
http://www.hardwareanalysis.com

[Ron E]

Originally posted by SSassen
No, not Ql, that's obvious.

Obvious? So what value did you use

The only reference I know of that walks you through the calculations to calculate Ql Qp and Qa - losses due to leakage, port and box, respectively - is JE Benson's "Theory and Design of Loudspeaker Enclosures". The methods for measurement and calculation are not trivial and subject to a great deal of uncertainty if you use the 3 point methods described in the book..

The simplest way to "know" the values you have entered are "correct" is to compare a measured vs simulated impedance curve. Ql (as it descreases) causes the minimum at port resonance to rise - Qp and Qa, respectively, cause the lower and upper peaks to fall as they are decreased.

Proximity of damping material to the speakers and unmodeled effects can have a significant effect on the shape of the curve - so in the end, it is good that the bass response shape is not greatly affected by changes in these parameters over their typical range of values.

Qp, interestingly enough, is variable with level - perhaps obvious to you as well.

[SSassen]

Ron,

Be nice, he was stating the obvious. As I'm sure you'll know for Ql you can use a rule of thumb, for Qp I don't know of any ballpark figures I can use. It isn't important really, but I'm just trying to have as few unknowns as possible when I design these enclosures, saves me a lot of work afterwards.

Best regards,

Sander Sassen
http://www.hardwareanalysis.com

[catapult]

The defaults in WinISD are Ql 10, Qa 100, Qp 100.

Ql, leakage losses. These are produced by leaks in enclosure or in driver itself. Generally, this is most dominant loss type in vented boxes. Typical value is 5-20. This is quite impossible to predict before building the actual box. For reasonable quality box, WinISD pro uses Ql of 10 by default.

Qa, absorption losses. These are produced by losses in enclosure. Any stuffing will increase the absorption loss. With no stuffing inside the box, 100 is typical. Heavily stuffed enclosure has this about 3-5.

Qp, port losses. These are produced by port. Port has some resistance (air doesn't actually flow through the port without some friction). Actually, by setting this into very small value, you can turn vented box into closed one!

[SSassen]

Thanks catapult,

Just as a FYI, I use the following rule of thumb for Ql:

Closed box, small < 30-liter : Ql = 20
Closed box, medium < 100-liter : Ql =10
Closed box, large < 300-liter : Ql = 5

Vented box, small < 30-liter : Ql = 7
Vented box, large < 300-liter : Ql = 5

For Qa (box stuffing) I use the following:

No fill : Qa = 100
Minimal : Qa = 50
Normal : Qa = 10
Heavy : Qa = 5

Best regards,

Sander Sassen
http://www.hardwareanalysis.com