18 inch pro woofer for passive radiator

[muddasirwaheedmalik]

Hi,
I have JBL 18 inch pro woofer. i do not have any utility for it except i want to use it as passive radiator instead of using a ported box to eliminate the port noise for my 12 inch HT woofer.

It has a Xmax of 6 mm so it will get displacment equal to 6mmx2.25 (Sd surface area ratio of 18 inch to 12 inch) 13.5 mm which i assume would be reasonable.

Now if i short the the coild of 18 inch woofer being used as woofer it would act as a brake on coil movement due to back emf so by varying the resistance of passive radiatior voice coild i hope to easily vary tuning to my preference.

My question is that has anyone else done it before? is it a good idea? any clues how to achieve it?

[CharlieLaub]

Originally Posted by muddasirwaheedmalik

Hi,
I have JBL 18 inch pro woofer. i do not have any utility for it except i want to use it as passive radiator instead of using a ported box to eliminate the port noise for my 12 inch HT woofer.

It has a Xmax of 6 mm so it will get displacment equal to 6mmx2.25 (Sd surface area ratio of 18 inch to 12 inch) 13.5 mm which i assume would be reasonable.

Now if i short the the coild of 18 inch woofer being used as woofer it would act as a brake on coil movement due to back emf so by varying the resistance of passive radiatior voice coild i hope to easily vary tuning to my preference.

My question is that has anyone else done it before? is it a good idea? any clues how to achieve it?

You are by far NOT the first person who thought of such a thing. Here are my thoughts. Not good ideas:
*short the coils: NO
*use pro woofer as passive radiator: NO

The main problem is that the suspension of the pro woofer is only designed to hold the existing cone, which is very light weight so that the driver can have high SPL. Because the driver diameter is relatively large, you will need to add quite a lot of mass to make a useful passive radiator (e.g. minimum 500g to over 1kg). This will cause the suspension to sag significantly, probably making the driver unusable.

Shorting the coils, or adding a resistor across them, is only changing the damping. This does not effect the resonant frequency of the radiator - only mass and compliance (of the suspension) controls that.

-Charlie

[moray james]

yes but I cannot recall where I read about it or the value of the pot used to make adjustment easy to do. If the JBL that you have has a free air resonance in the range that you need there should not be an issue. Perhaps someone who recalls will let you know.. Best regards Moray James.

[muddasirwaheedmalik]

thanks for reply
I intend to apply layers of fibre coatings in resign and glue to add stiffness and eliminate flexing hence reducing distortion . It would significantly add weight and reduce Fs.

Now my question is that how Qts of the system would vary if i add resistive load or short the voice coil of passive woofer ?

[muddasirwaheedmalik]

btw I THINK THAT Fs of the Passive should also change as by shorting the coil we are passing current through it due to back emf which tries to hold back the motion of the main coil hence fs may change ? any one knows any site or link providing more detail to it.
regards

[ashok]

Didn't Sonus Faber ( Extrema?) make a book shelf speaker with rear facing radiator that had a coil with switchable damping ?

[muddasirwaheedmalik]

just an extension of the idea....if we short or add resistive load we would be converting the accoustic energy to electrical which would have been wasted as noise and kienetic energy to move air mass, now this electrical energy instead of wasting as heat can be put to some good use??? like feeding it to another woofer for more bass?????just an idea.

I am sure someone would have worked on it already.

[ashok]

The conversion efficiency is very low.

[muddasirwaheedmalik]

Ok. then lets suppose we have two similar woofers back to back in one sealed enclosure , if i feed one woofer with one watt i.e. 2.83V at 8 ohm and measure current through the 8 ohm second woofer shorted through ammeter i can get output power added to the passive radiator accoustic advantage without using a port. any idea what figures to get?

10%???

[sreten]

Hi,

If you can get the in box driver resonance to match the design target
for the ABR (by adding cone mass very probably) then it will work.

Open circuit the Q of the ABR will be Qm, closed circuit Qe comes
into it, so you would be able to tune the ABR's effect see :

Sonus Faber Extrema loudspeaker | Stereophile.com



Fig.3 Sonus Faber Extrema, nearfield ABR response, 1/3-octave
smoothed, showing effect of LF Contour control (2dB/vertical div.).

rgds, sreten.