Box losses: Ql, Qa and Qp

A real enclosure is not a perfect spring. Some energy escapes, and these three numbers say how much. They are Q factors, so the counter-intuitive part comes first: a higher number means a lower loss. A Ql of 100 is nearly lossless; a Ql of 3 is a leaky box.

The three

Ql: leakage. Air escaping where it should not: seams, the terminal cup, around the driver gasket, through the cone itself. This is the big one in a normal cabinet, and the default of 7 represents an ordinarily well-built box.

Qa: absorption. Energy soaked up by damping material inside. A heavily stuffed box has a lower Qa. Stuffing also slows the effective speed of sound, which is a separate effect Baffle models with the stuffing factor.

Qp: port losses. Friction and turbulence in the vent itself. Usually minor unless the port is long, narrow, or running fast.

What they change

In a sealed box, very little. Only Ql applies, and the response barely moves with it: dropping Ql from 7 to 3 changes the deep bass by under a tenth of a decibel. If you are building sealed, you can mostly ignore these.

In a vented box they matter considerably, and all three apply. Losses damp the resonance, so a leaky box has:

• Less output near tuning, because the port is doing less work
• A shallower rolloff below tuning, around 18 dB per octave instead of the 24 dB per octave a lossless box gives
• A smaller dip between the two impedance peaks

That last one is useful, because it means you can measure your box's real Ql after building it: compare the impedance curve you get with the one Baffle predicts, and adjust Ql until they match.

What to use

Ql = 7 is the usual assumption for a competently built cabinet and is Baffle's default. A very well sealed box might reach 10 to 15; a box with a loose driver gasket or an untaped seam can fall to 3 to 5.

Qa = 100 and Qp = 100 mean "negligible", which is right for a lightly lined box with a sensible port. Heavy stuffing in a vented box might justify a Qa nearer 30.

Do not spend long tuning these. The difference between Ql 5 and Ql 10 is smaller than the difference between the Vas on your driver's datasheet and the Vas of the driver actually in your hands; see how well is this known.

A word on Baffle's model

Baffle's leakage enters the response the way a physical leak does, which makes the rolloff below tuning about 18 dB per octave. Some treatments present leakage in a form that keeps the rolloff at 24 dB per octave whatever Ql is. That difference is the one measurable disagreement found against an independent implementation, and the evidence for it is written up beside the comparison data.