MJ18 Measurements

[Rybaudio 0]

A while back I picked up 4 MJ18s for use in my home system and I was recently finally able to get them on a Klippel analyzer to see how they perform at small and large signals. I'm posting the information here for anyone else who cares.

First off, the unit to unit variation was small. For the linear parameters I'll list the average and for the nonlinear curves I'll show one of them; the rest are similar.

The linear parameters are determined in three circumstances: the driver at very small signal, the driver at large signal but cold, and the driver at large signal and warm (after playing some time). Which of these to use really depends on the application; in my case I use the small and large + cold because in my home setup they don't see much power. If you're main usage is street beating then large+warm is probably better. In any case, here are the 7 electromechanical linear parameters for those cases. Thiele/Small parameters can be calculated from these.

Small signal + cold

Mm: 292 g

Rm: 6 Ns/m

Cm: 110 um/N

Bl: 20.15 Tm

Re: 3.45 ohms

Le: 2.3 mH

Sd: 1030 cm^2

Large signal + cold (same except)

Cm: 150 um/N

Le: 2.92 mH

Large signal + warm (same as Large/cold except)

Re: 4.2 ohms

For the nonlinear performance there is a lot of data available but it can be confusing and/or misleading if you don't know how to interpret it. The basic issues are the variation of parameters over cone displacement and current through the coil (because these things cause nonlinear distortion) and the variation of parameters depending on the power dissipated in the device as this causes thermal compression and performance variation with time.

I'm not going to go into variation with current through the voice coil but I'll tell you that it's a minor factor in this case. For variation versus excursion I'm going to display the Bl, Cms, and Le versus displacement as they give a rough idea of usable limits. These are the three curves below.

Bl is symmetric so the coil is centered pretty well. It's down to ~70% at 10 mm and ~50% at 15 mm, which are roughly the "clean" and "usable" limits. Cms is very assymmetric which reflects the spider being dished by about 10 mm, but it doesn't really matter because the variation over 15 mm each way is fairly small. The Le variation is not very good, but if you're using this as a sub it doesn't matter.

There is various information available for thermal performance but the clearest is the set of two curves depicting input power and power compression. These are shown below:

What I read out of this is the following approximate thermal compression:

.5 dB @ 30w

1 dB @ 75w

2 dB @ 100w

3 dB @ 125-150w

This is OK for a 3" coil sub, but it's not great. Keep in mind this is actual power being dissipated in the driver, which is a very different thing than the size of amp hooked up to it which is what most people mean when they talk about power handling.

Hope this has been helpful. If something is confusing I can try explain it better.

Edited June 14, 2008 by Rybaudio

[DevilDriver 28]

Very interesting results....thanks for sharing. The symmetry on the BL curve is excellent, although a little more extended plateau might be preferable. Le(x) reflects the lack of a shorting ring, while the compliance symmetry could definitely use some work. Still a great value driver at that price.

[Aaron Clinton 2,372]

Thank you for taking the time to test and post results.

[Viperoni 0]

Thank you for this, it's awesome to see these kinds of measurements on the sub

[Rybaudio 0]

IMO, for most applications the most useful and important information is the thermal compression. The variation of parameters with excursion leads to nonlinear distortion, but what a lot of people don't realize is that (1) music spends a hell of a lot more time around 0 than it does at high amplitude and (2), that nonlinear distortion, if it's low order, can be quite high in a sub before it's a problem. On the other hand, even a 1 dB shelf from 80 Hz down is audible and 3 dB will significantly affect the tonal balance of the system.

If you put one of these in a ported box and play music through it, the dominant issue by far is the thermal compression. On the other hand, in some situations the variation with excursion may be an issue. For example, if you're using them IB and for very low frequency, short-duration effects in movies, then they're not seeing a whole lot of continuous power but could be seeing some high excursion.

[TheEAR 0]

Thank you for the testing !

If you ever test the IXL15 or IXL18 it would be the icing on the cake.