18 inch pro woofer for passive radiator

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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?
 
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
 
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 ?
 
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.
 
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%???
 
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

Sfefig03.jpg


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


rgds, sreten.
 
I built a passive radiator speaker system using two cheap dual cone 8 inch drivers that sounded remarkably good (there was a tweeter for above about 3kHZ too). One driver was active, and the other had a shorted out coil, which caused substantial damping (minimal ringing). If I remember correctly, I did glue a somewhat heavy washer to the center of the non-active driver (with contact cement), but I was truly just guessing at how much weight would be right. This was a very long time ago (the 70's). The box was about 10 x 19 x 9 internal. I gave them to a Dentist friend for work he did on my teeth. Then I moved 100 miles away for a job. About five years later I returned to his town, had him do some work on my teeth, and I had totally forgotten that I had made these speakers for him, which now hung from the ceiling in one of his dental rooms. I was totally proud of how great they sounded. Amazing low end. No sense of screwed up frequency response anywhere. I wish I knew for sure if I actually shorted the coils of the inactive driver, or id I used an 8 ohm resistor across the terminals. and I'm not sure about the details of the washer I may have glued to the inactive drivers coil. I'm thinking that it's probable that I glued in some maybe 1 inch thick steel washers. What I do clearly remember is that they sounded superb. No audible ringing in the bass. It may be that the presence of an inactive driver in the box greatly reduces the peak pressure levels inside the box, which may diminish any internal box resonance effects... just a theory, not something I've verified. I plan to try another one of these active/passive radiator systems after all these years. There may be more magic to it than has been documented. The EE that was with me agreed that they sounded amazing. On paper it's easy to criticize such a method, but when it sounds great, it is great.
 
I know someone who changed his DIY Manger box from a twin-woofer closed-box speaker into a reflex speker with passive radiator just by using this method. He disconnected the second woofer from the X-OVER (which had to be altered as well). And he also used a damping resistor accross the terminals of the second woofer. The resistor value was chosen by response measurement and listening test.

Before damaging this expensive 18" driver by glueing stuff to it: Did you ever think about using that one as subwoofer driver ? You may probably not be able to use a "textbook tuning" to reach the desired cutoff frequency. But with modern 18" PA drivers there are some choices you have to get good low-end extension.

Regards

Charles
 
I guess the main advantage of using some variation of a passive radiator is to keep the system simple and small. All my speaker projects since the early 1980's have been at least biamp'd, with active EQ on the woofers to get the response I want (flat acoustically to 20HZ). Lately I'm thinkin' simple would be nice too.

I noticed that Jamo has a woofer called the model 800, that has one active 6.5 inch woofer, and two passive radiators on the sides of a cube that is 8 inches on all sides. It's got phase and rolloff freq. adjustments and a built in poweramp), and is even allegedly wireless (not sure if that requires additional product)(for about $500 at Amazon). I wouldn't be surprised if it competed well for it's physical size. I wonder if their passives are actually regular drivers with shorted out coils for tighter bass (?).
 
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