Sound differences between series and parallel drivers

Hi all, first post.

This is an acoustics question, not an electrical one(I'm an amp designer so circuits are fine).

Situation 1: single (say) 4 ohm driver/bass cab driven by good amp, the damping factor of the amp being (say) less than 0.1 ohm, gives a damping factor of about 40 (ignoring the cable resistance).

Situation 2: Take 2 of above cabinets and connect, correctly phased, in series (R now 8 ohm). Connect to the same amp and total load impedance is 8 ohm and now damping factor is 80.

BUT.... now each cab is 'seeing' the impedance of the other as a source. i.e. if we consider one of the cabs, it is 'seeing' the other cab in series with the amp output; this means that the damping factor will be 'about' 2.

The Question: There is much talk about damping factor etc to keep tight bass, but if 2 drivers (or separate cabs) are connected in series then that goes out the window. Is there a significant sonic difference between the two arrangements (assume amp is happy to drive either and output adjusted down so SPL is same in both arrangenents). I'm particularly thinking of Martin Audio AQ115 vs AQ215s; I notice they connect 8 ohm drivers in parallel.

Ideas, opinions, views, and insults welcome.... smiley1

Hi Duncan

duncan_b wrote: BUT.... now each cab is 'seeing' the impedance of the other as a source. i.e. if we consider one of the cabs, it is 'seeing' the other cab in series with the amp output; this means that the damping factor will be 'about' 2.

This is where you have been caught out. For a single driver in a series pair of 2 (driven by a perfect amp) the source impedance is indeed that of the other driver.

But if the two drivers are identical, then their impedences are identical - that is, the dependence on frequency and the reactive lead/lag will be completely identical. Therefore, they still function as a potential divider, and the voltage across the terminals of one driver is half the total voltage coming from the amp.

I guess what I'm saying is that identical speakers connected in series (or parallel) can "share" the amplifier nicely, always matching each other's impedences and back emfs so that they keep behaving the same assuming the amp is ideal. But a "foreign object" in the circuit won't behave the same way, and its impedence will interfere with the interactions between speaker and amp that produce electrical damping.

That said, I usually ignore damping factor specs on amps. Why? Because the resistence of cables, manfacturing variations among drivers, voice coil temperature and even atmospheric humidity and pressure can all have a bigger effect. Decent modern transistor amps have "good enough" damping factors, and your problems, if any, lie elsewhere.