voice coil inductance VS transient response

[sreten]

Originally posted by DanWiggins
Hi all, perhaps a bit of clarification...

Many times, people bring up the old myth about the "acceleration factor", BL/Mms, as proof of why one driver has better transients than another. In fact, this factor is nothing but efficiency, which has ZERO relationship to transient response. This is why the paper was done in terms of acceleration and how accleration is SPL, not transient response (rather, the rate of change of acceleration is transient response).

As far as subwoofers go, many of the higher excursion subs out there have inductances in the 5-6 mH range. With a 3 Ohm DCR, this puts the inductive roll-off around 100 Hz. Sure enough, the inductance of these drivers definitely affects the transient response, even with a crossover at 100-150 Hz (in essence, you have another order of crossover).

Anyway, of course the crossover can have a major impact on the transient response of the system; that is not challenged at all. if the crossover is within the pistonic passband, then it is the main limiter of frequency extension (transient response). However, when talking about the raw driver itself, it's not mass or BL that is the limiting factor (and hence not that BL/Mms thing) - it's strictly the inductance that sets your bandwidth/transient response limitations.

Please understand the paper is written for your average car audio person, who probably never stopped to think that a voice coil is essentially a high DCR inductor! Thus the simplistic terms and method to explain why it's inductance that matters, not mass or BL or any other factor.

Dan Wiggins
Adire Audio

In these terms your article does make sense in trying to debunk
one myth, which is fair enough, but these terms were not given.

But a right doesn't justify a wrong, and talk of infinite transient
response etc is entirely misleading, and only possibly applicable
to tweeters or the like.

The simple fact you appear to be alluding to is many subwoofers
have an intrinsic bandwidth, and any attempt to use them outside
this bandwidth, or lack of consideration of this bandwidth, will
impair the resultant frequency response and by definition transient
response.

Its extremely true a driver with a bandwidth far exceeding its
required range will perform within range, converting this to
transient response arguements is pointless and misleading.

Finally it is simply not true the inductance is the only indicator
of transient response capabability, but this is entirely irrelevant
for subwoofers.

Also I'm not aware of the arguement you are trying to debunk,
but it seems to allude to efficiency, which like it or not, is nearly
always associated with good transient response, high efficiency
= good dynamics, which is most of the time true.

sreten.

[DanWiggins]

Sreten posted:

In these terms your article does make sense in trying to debunk
one myth, which is fair enough, but these terms were not given.

Fixed now...

But a right doesn't justify a wrong, and talk of infinite transient
response etc is entirely misleading, and only possibly applicable
to tweeters or the like.

Which is why we were careful to make it a theoretical extension - IF you could slew the current at an infinite rate, you'd have infinite transient response. Driver size/application is irrelevant - is the operation/relationship of current changes to transient response that are being described.

The simple fact you appear to be alluding to is many subwoofers
have an intrinsic bandwidth, and any attempt to use them outside
this bandwidth, or lack of consideration of this bandwidth, will
impair the resultant frequency response and by definition transient
response.

Correct. I don't see the problem here... Operating outside of bandwidth inherently means you don't have the transient response to provide the bandwidth?

Its extremely true a driver with a bandwidth far exceeding its
required range will perform within range, converting this to
transient response arguements is pointless and misleading.

I'm not sure I follow... If you're within the bandwidth of the driver, then all is OK. If you're outside the range (past the inductive roll-off), then you don't have the bandwidth. And you will also be limited in transient response (Fourier transform). Of course, the paper was more about to describe WHAT limited transient response, rather than what wide transient response implied.

Note that many times manufacturers - especially in the prosound and some parts of the high end world - will "fudge" bandwidth by publishing highly smoothed response curves. What you see is not only the bandwidth of the driver, but it's often artifically extended by cone breakup. Smoothing out cone breakup can make the driver appear to have more bandwidth than it really does.

Finally it is simply not true the inductance is the only indicator
of transient response capabability, but this is entirely irrelevant
for subwoofers.

For a raw driver, I don't know what else limits transient response. If you could shed some light on this, I'd be grateful!

Also I'm not aware of the arguement you are trying to debunk,
but it seems to allude to efficiency, which like it or not, in nearly
always associated with good transient response, high efficiency
= good dynamics, which is most of the time true.

Dynamics aren't transient response, however...

I think many times people associate dynamic drivers/light drivers as "fast" for a couple of reasons:

1. They tend to often have hotter top ends than low ends (biased towards the higher end of the spectrum) which can make it sound wider in frequency response than it really is.

2. Often the inductance of high efficiency drivers is low, because voice coils are heavy so they are wound as short and small as possible. It's the search for high efficiency (low moving mass) that lowers the inductance, and as a result bandwidth is increased. It's not because of the efficiency itself, but an side-effect of the need for low mass that gets you the low inductance.

Thanks,

Dan Wiggins
Adire Audio

[sreten]

Hi DW,

I don't really want to go into whats relevant as an arguement.

You are correct the transient response arguement is only a
transform away from the bandwidth arguement, but I'd suggest
for your less than well informed types the article is supposedly
intended for the bandwidth version is a lot more straightforward.

My point about the bandwidth of subwoofers is that most
do have the bandwidth required, except for the silly ones.

I quite agree dynamics isn't the same as transient response ,
but again I'd suggest your misleading your less than well
informed types with an arguement they will misunderstand.

Finally a voice coil has an effective mass. So does a driver cone,
but importantly this reduces with frequency, the high frequency
roll-off is simplistically defined as when these are equal.

So the cone / voice coil interface defines hf bandwidth and
transient response, not the simple voicecoil inductance,

sreten.