Trick to test for cabinet leaks

[Mos Fetish]

If you're building a 3-way where you have a sealed midrange chamber and want to test if there are any air leaks with the bass chamber (which will influence the fidelity of the mid), try this:

Put a voltmeter across the terminals of the midrange unit (via its leads) and then gently move the woofer with your hand. If your voltmeter shows any voltage it means the movement of the bass unit (via air pressure) is leaking into the midrange chamber and moving the midrange driver, thus producing a voltage in its coil which you are reading.

You could try it with the tweeter as well although I think the danger there is more from mechanical vibration through the cabinet than from air movement.

[DigitalJunkie]

I usually connect up the woofer,and leave the mid (+tweet) disconnected,then feel the mid cone with my hand,if it's moving/vibrating,there's a leak.. Usually the cabinet vibration is pretty well damped,so you don't really feel anything if it's sealed,but it's pretty easy to feel a leak.

[Mos Fetish]

Sure, but the voltage fluctuation in the mid can be tiny and may not be discernible with the hand.

Now, we could debate at which point a voltage becomes audible... but I'm not going there

[despotic931]

Originally posted by Mos Fetish
If you're building a 3-way where you have a sealed midrange chamber and want to test if there are any air leaks with the bass chamber (which will influence the fidelity of the mid), try this:

Put a voltmeter across the terminals of the midrange unit (via its leads) and then gently move the woofer with your hand. If your voltmeter shows any voltage it means the movement of the bass unit (via air pressure) is leaking into the midrange chamber and moving the midrange driver, thus producing a voltage in its coil which you are reading.

You could try it with the tweeter as well although I think the danger there is more from mechanical vibration through the cabinet than from air movement.

Awesome tip/trick! That is a great idea, one I will have to remember!

Justin

[Svante]

... and the voltage is the velocity of the cone, multiplied by Bl.

v=U/(Bl)

So if Bl is known, the cone velocity can be calculated.

And from the velocity, the sound pressure generated by the cone can be calculated as:

p=Sd*v*rho0*f/(2*r)

where rho0=1.2 kg/m³, f is the frequency and r is the distance.

The sound pressure level generated by the mid driver is:

Lp=20*log10(p/pref)

where pref=20 µPa.