Dual coil drivers - parameter conversion

[Collo]

I'm currently comparing drivers from the catalog of an Aussie components supplier. Some drivers are dual voice coil units, but for most of them they dont give specs for the three possible configurations (single coil / both in series / both in parallel)

Has anyone discovered a "rule of thumb" that allows you to convert parameters. Most stay the same, but the following change:
impedence and power handling - OK I can work those out
sensitivity, Qms, Qes, Qts - all affect the design

I know some drivers are rated for resistive braking by loading one coil with a resistor or short, but those drivers generally publish their figures.

I would also like peoples opinions on the wisdom or not of putting my recommended designs for Home Theatre for each driver on my website. I don't want to get sued.

Thanks,
Collo

 

[Ron E]

Specs are almost always for the parallel-coil config:

Bl doubles for a series coil arrangement, but Re quadruples, so specs are the same series or parallel - except that voltage sensitivity is 6dB less for the series case than for the parallel case

Bl stays the same for a single coil as for a parallel coil, and Re doubles so Qes is double, Qms stays the same and Qts is calculated from Qes and Qms. Voltage sensitivity is also -6dB from parallel case....

 

[Collo]

thanks Ron...

any rules for the sensitivity expressed as [email protected] as opposed to the voltage sensitivity of 2.83v into 8 ohms @ 1m?

From the sample I came across it seems that parallel connection was 1.5dB greater than single coil, and series connection was 3dB greater than single coil.

Collo

 

[Ron E]

Voltage sensitivity (2.83V) and "power sensitivity" (1Watt into Re, the DC resistance of the coil) are different by 10*log(8/Re)

1W/1m specs are useless IMO, because your amp doesn't really output watts, it is an approximate constant voltage device. When comparing sensitivity figures for building a system you should always use the 2.83V figure. The 1W figure is too subject to interpretation.

1W/1m spec - "Power available efficiency" from Beranek is the quantity inside the log below. Taking the log and adding 112.2 gives halfspace sensitivity at 1W into Re.

dB=112.2+10*log(9.614e-10*fs^3*Vas/Qes) [Vas in liters]

for the 2.83V spec you simply add 10*log(8/Re) to that.

 

[simon5]

I want to add my 2c here hehe!

It's true that an amplifier is a voltage source, but it's so easy to get a high dB rating at 2.83V RMS if you got a coil with very low impedance, that's why I prefer the 1W/1m rating, power in for SPL output.

One 80W amplifier rated at 1 ohm will get the same SPL with a 99 dB/2.83V RMS 1 ohm speaker compared with a 80W amplifier rated at 8 ohms with a 90 dB/2.83V RMS 8 ohms speaker.

Both will play at 109 dB SPL.

 

[Ron E]

I want to add my 2c here hehe!

It's true that an amplifier is a voltage source, but it's so easy to get a high dB rating at 2.83V RMS if you got a coil with very low impedance, that's why I prefer the 1W/1m rating, power in for SPL output.

One 80W amplifier rated at 1 ohm will get the same SPL with a 99 dB/2.83V RMS 1 ohm speaker compared with a 80W amplifier rated at 8 ohms with a 90 dB/2.83V RMS 8 ohms speaker.

Both will play at 109 dB SPL.

Yet most amplifiers are rated at either 8 or 4 ohms so your example is a straw man. What if your speaker Re is 2.3 ohms and you don't have a 2.3 ohm rating - do you use the 4 ohm rating for your amplifier?

You can think of the 2.83V rating as cheating, but it is less ambiguous than 1W - frequently when mfg rate their driver at 1W they mean 2.83V anyway. There is no way to be sure what "watt" they are talking about unless you go through the calculations. I always calculate the sensitivity myself.

Thus for me, for calculation and comparison purposes it is easiest and least ambiguous is to use the voltage sensitivity calculated with the formulas. There are other reasons why using voltage is more convenient, like when calculating excursion in a program or when matching two drivers for sensitivity which have different DC resistances.

If you plugged the two drivers into the same amp with the same volume control setting, the 99db/2.83V driver would play 9dB louder than the 90dB/2.83V rated one. If you were using the two in the same system, it might be nice to know about that discrepancy.

 

[simon5]

If you plugged the two drivers into the same amp with the same volume control setting, the 99db/2.83V driver would play 9dB louder than the 90dB/2.83V rated one. If you were using the two in the same system, it might be nice to know about that discrepancy.

True, under a certain threshold the 99 dB/2.83V driver will be 9 dB louder, but the maximum SPL achievable will still be the same because the amplifier will clip early with the 99 dB/2.83V driver.

2.3 ohms DC is a bit rare... I would use three drivers to get this to 6.9, so then I use a 8 ohms rated amplifier.

 

[Collo]

Thanks again guys!

The reason I'm chasing the Power sensitivity rather than the voltage sensitivity is that I want to model the drivers using WinISD and it likes the former measure


RonE, I'm almost there; at the risk of incurring mathematics burnout, let me follow your equations longhand:


for Power sensitivity:

dB=112.2+10*log(9.614e-10*fs^3*Vas/Qes)

For both series and parallel conections Qes is half that of a single coil connection (or so I have been led to believe)

so the equation becomes:

dB=112.2+10*log((9.614e-10*fs^3*Vas/Qes)*2)

which simplies to:

dB=112.2+10*log((9.614e-10*fs^3*Vas/Qes)*2)+ 10*log(2)

thence to

dB=value for single coil connection + 3 dB
------------------------------------------


as a check, do the conversion to 2.83v spec using a pair of 4ohm coils

add 10*log(8/Re) to the Power sensitivity figures


series case (total Re 8ohms)
-----------

dB = (value for single coil connection + 3 dB ) + 10*log(8/Re)

dB = (value for single coil connection + 3 dB ) + 10*log(8/8)

dB = value for single coil connection + 3 dB


Parallel case (total Re 2ohms)
-------------

dB = (value for single coil connection + 3 dB ) + 10*log(8/Re)

dB = (value for single coil connection + 3 dB ) + 10*log(8/2)

dB = (value for single coil connection + 3 dB ) + 6

dB = value for single coil connection + 9 dB

To summarise
------------

"Power" sensitivity SPL [email protected]

series sensitivity = single coil sensitivity + 3dB
parallel sensitivity = single coil sensitivity + 3dB

Voltage sensitivity [email protected]@1m

series sensitivity = single coil sensitivity + 3dB
parallel sensitivity = single coil sensitivity + 9dB


which contradicts your earlier figures for voltage sensitivity of
series equal to single and
parallel 6dB more than single

I seem to have picked up 3dB too much somewhere. Is it related to the "half space"?

although my head is spinning, I do appreciate the help!

regards, Collo

 

[simon5]

He never said that series was equal to single coil if I'm reading correctly.

You've calculated correctly.

 

[Ron E]

something seems a little weird. I'll look further tonite...

 

[Ron E]

I didn't go through your calculation because I am not entirely sure where you are going with them. Here's a concrete example which I hope will clarify things.

Let's say we have a driver which when the VC's are in parallel it has the following parameters
Fs=20
Qts=0.38
Qes=0.4
Qms=7.6
Vas=100
Dia=24.75cm
Re=1.5 ohms
Mmt=206g
Bl=9.85
92.13 dB/2.83V
84.86 dB/1W

If you connect the coils in series you get:
Fs=20
Qts=0.38
Qes=0.4
Qms=7.6
Vas=100
Dia=24.75cm
Re=6 ohms
Mmt=206g
Bl=19.7
86.11 dB/2.83V
84.86 dB/1W

If you connect a single coil and leave the other open, you get:
Fs=20
Qts=0.724
Qes=0.8
Qms=7.6
Vas=100
Dia=24.75cm
Re=3 ohms
Mmt=206g
Bl=9.85
86.11 dB/2.83V
81.85 dB/1W

So as I said:
Voltage sensitivity of series connected and single coil are the same. and parallel is +6dB.

Or you could say - power sensitivity of series and parallel are the same, but single coil is -3dB.

Clear as mud, right?

The reason there is no change in the power sensitivity is because the parameters are the same except for impedance between parallel and series, and the -3dB for the single coil case is caused only by the Qes change.

 

[Collo]

Thanks RonE

that answers my question. I'm not sure where the maths went astray, so I'll stick with your info ie:

Voltage sensitivity of series connected and single coil are the same. and parallel is +6dB.

Or you could say - power sensitivity of series and parallel are the same, but single coil is -3dB

which lets me get on with my project

Collo