Just a little house-keeping;
Quote:
Magnetic Compression
More typically in bass drivers, at high levels, stuff happens to the magnetic flux fields that make the power response nonlinear.
Also, the woofer can be driven so hard that it's out of the linear portion of the magnetic gap, and the field weakens.
I still don't know if that typically happens if you stay within the speaker's max peak power rating.
From the compression thread, I submitted;
Quote:
This is an aspect that rarely gets discussed, however power compression isn't only due to the thermal aspects of the heating of the voice coil. Yes, the majority of compression as we know it results from the ability of the voice coil to dissipate heat, but another component to compression is magnetic flux compression. Although the typical thermal compression most discussed is more long term, the effects of magnetic compression are instantaneous. These magnetic compression distortions, can dull transients, and render reproduction as lifeless, dull and unrealistic.
Certainly, thermal compression is a big issue. The vast majority of mfrs. have no idea of their products compression characteristics, other than their likely not so good. This is why typical residential speakers are inadequate for the huge dynamic swings associated with realistic HT. Many enthusiasts merely discount the effects of compression, because they are mostly subtractive in nature. But subtractive they are, compression robs the playback of life/realism. These effects are insidious because it's not an offensive distortion. It just thins out the presentation,...no life or snap. Oftentimes, without a direct comparison, the un-initiated have no idea of what they're missing.
The magnetic issue lies in the fact that when the drummer hits the kick drum pedal, and the beater strikes the head, the signal path results in the voltage is impressed across the loudspeaker's terminals. This results in the current flow in the VC, and subsequently the driver attempts to track the signal accurately. The problem is that when current flow begins in the VC, there exists three separate, and entirely different sources of magnetic flux in the gap. The permanent magnet's flux, the signal voltage VC flux, and lastly the flux that's generated by the varying eddy currents in the pole pieces. This is the problem, the flux modulation compression.
Whereby the thermal compression effects are a function of time, the magnetic flux compression is instantaneous. Magnetic saturation of any of the motor elements needs to be entirely avoided, as it's effects instantly impact waveform shape, the peak, and peak capability.
So just as reducing drive levels to avoid thermal buildup and subsequent losses, avoiding magnetic compression should be a goal to be mindful of even if it's only for the split second of the top of any given waveform. Depending on the frequency, we can reduce drive levels via spreading the signal over multiple drive units, ie multiple subwoofers, mid-bass units etc. Fortunately this flux modulation compression is most prevalent with high current levels that are typically associated with low frequencies,...thus enabling implementing multiple drivers covering the given passband without acoustic interaction issues that would accompany multiple drivers at higher frequencies.
I've experimented with the effects of compression, and be it thermal or magnetic, and spreading the drive signal across multiple drive units, thereby reducing the effects of potential compression, can be quite dramatic.
I just wanted to take a moment to clear this ambiguity up. It's not anything to do with driving the VC out of the linear component of the gap. It's the complex effect whereby the quickly changing
* magnetizing force of the high currents through the voice coil, interacting with the permanent magnet magnetic field of the driver, then throw in the non-linearity element of the magnetic field set up by the ever changing eddy currents in the pole pieces.
(
*Say for example a 41.2 hz E string on a bass guitar, and for simplicity we'll forget about the large amount of all it's harmonics. Remember, there exists two magnetic reversals per cycle.)
So the initial magnetic field is established, hypothetically, in a couple cycles the magnetic field attains full strength. Magnetically, all is relatively linear as long as the peak stays within the capability of the materials, or stays low enough to avoid the effects of saturation. Saturation is the condition attained when the increase in the applied magnetic field, doesn't increase the magnetization of the material further. Thereby the rapidly rising effect is dampened somewhat, and the rise is leveled off to a degree.
This is the compression, the magnetic compression, that I was referring to.
Clearly, driver mfrs can make the magnetic motors that are robust and avoid some of the non-linearities associated with compression,...thermal or magnetic. Enthusiasts can spread the high drive signals across multiple drivers, mitigating much of the negative effects. Keeping the drivers as centered as possible in the magnetic gap further reduces the associated effects of the non-linear Bl curve, and the third interactive magnetic effect.
The cool thing is that this flux modulation compression typically only comes with the higher currents that accompany the lower frequencies. So as long as it doesn't present negative acoustic interactions, multiple drivers/magnetic motors covering any given passband is helps in a multitude of ways, thereby rendering the effects of any potential compression of little consequence.
I know, what a yawner
I'm just sayin',
... but all said,
once again, headroom is the answer
Eyleron, quite an undertaking, you're to be commended. Great job.
