Electrical Engineers confirm my plan...or not.

[AVB]

I have four 4 ohm tactile transducers (aka bass shakers) wired in series parallel attached to my couch. I'm thinking of adding 3 more seats to the home theater and 3 more transducers. My plan is to hook up the original 4 in series and parallel them with the 3 new transducers also connected in series. This should give me a ~6.9 ohm load. (Series 1 total times Series 2 total divided by Series 1 total plus Series 2 total) or (16x12)/(16+12) or 192/28 = 6.85

What is the downside to doing it this way if any and is there a better way to do it?

 

[BlindedByScience]

Problem #1 - the voltage won't divide equally across the loads, but it may not matter much in this application.
Problem #2 - transducers, especially "bass shakers" and other very low frequency drivers, are a long ways from being a constant impedance over frequency. You may have frequencies where the impedance is down to an ohm or two, and depending on the amplifier driving them, you may be in trouble. Even more so at low frequencies / high output.

I might try series / parallel the first four (~4 ohms nominal), then parallel two in series with one for ~6 ohms nominal of load on the other. If you get an amp that's very stable at 4 ohms, you should be fine. By breaking them up and putting some on one channel and some on the other, you minimize the reactance of the load, rather than having them all on one output. For this load you don't need a fine amplifier, just one that's really stable into 4 ohm loads. Or, depending on the cost of the loads, add four more so you have two, 4 ohm nominal loads (series / parallel four on each side of the amp) where the voltage will divide equally across all the loads.

This is one of those 'try it and see' types of applications, but the biggest worry I'd have is the impedance of the loads when things get moving. A whole bunch of them tied together probably isn't the best way to go.

JMHO, YMMV, etc.

 

[AVB]

Problem #1 - the voltage won't divide equally across the loads, but it may not matter much in this application.
Problem #2 - transducers, especially "bass shakers" and other very low frequency drivers, are a long ways from being a constant impedance over frequency. You may have frequencies where the impedance is down to an ohm or two, and depending on the amplifier driving them, you may be in trouble. Even more so at low frequencies / high output.

I might try series / parallel the first four (~4 ohms nominal), then parallel two in series with one for ~6 ohms nominal of load on the other. If you get an amp that's very stable at 4 ohms, you should be fine. By breaking them up and putting some on one channel and some on the other, you minimize the reactance of the load, rather than having them all on one output. For this load you don't need a fine amplifier, just one that's really stable into 4 ohm loads. Or, depending on the cost of the loads, add four more so you have two, 4 ohm nominal loads (series / parallel four on each side of the amp) where the voltage will divide equally across all the loads.

This is one of those 'try it and see' types of applications, but the biggest worry I'd have is the impedance of the loads when things get moving. A whole bunch of them tied together probably isn't the best way to go.

JMHO, YMMV, etc.

Thanks Tom, I only have one channel available unless I get another amp, it is stable at 4 ohms and has been driving the current 4 shakers for some years now and handles low impedance and reactive loads very well. The shaker channel has an electronic crossover on it set for 80hz so the amount of load change due to frequency is somewhat limited. For some of the reasons you stated I was thinking of getting 4 more transducers instead of 3 and going to an 8 speaker series parallel circuit just to make everything even plus it would give me an 8 ohm load. Really no extra space on the chairs so I'd add the extra to the couch...it couldn't hurt