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-   -   baffle Step (http://www.diyaudio.com/forums/full-range/46455-baffle-step.html)

rgrayton 24th November 2004 07:15 PM

baffle Step
 
MarkMck, Why do you say this, "Now, I really do not want to get into baffle step correction. Such a discussion may be of limited value. "?
You kinda hint it does not exist under certain measurents. Care to elaborate?
BG

MarkMcK 26th November 2004 03:55 PM

BG,

I think you want to help me get myself into trouble here. Let me just say that there is a world of difference between the theory and what one needs to do in practice to compensate for enclosure/driver interactions. I have heard, even from proponents of baffle step correction, that the amount of correction the theory predicts and the amount needed in practice can vary. In addition, as detailed in a couple of my TB mods posted to this forum, I have found optimal transient response (and flat frequency response) requires correction in the opposite direction from that predicted by baffle step theory. When testing theories it is never a good thing to ignore falsifying experiments.

In addition, the original theory lumps what is a progressive series of acoustic events as if they all happened at once. Attempting to correct for something that can only happen late in any sort of transient event (and the music we listen to is full of such events) in the fashion suggested by baffle step theory, will overcorrect onset and reduce the transient quality of the sound.

I have also previously stated that if you simply want a sound you prefer and if something you do makes the sound more preferable to you, then no discussion is needed. I am interested in more than preference and so am involved in a different conversation. At times it may seem that preference conversations and opinion conversations overlap, but they really don't.

Good designing and good building,

Mark

rgrayton 29th November 2004 12:35 PM

Mark,
You said,"I think you want to help me get myself into trouble here."
Not really, you just hinted about something I have never heard stated, at least that way.

You referenced theory a lot in your response. For the sake of argument, let's forget that. Let's say I have a calibrated mic and an MLS measurement system, and a good measurement down to 200 HZ. The measurement shows a frequency content that includes a step and even a "hump", centered around 800 HZ (somewhat consistent with diffraction simulators I have used). The better transient response is the one where this "hump" is removed -- baffle step correction. Your response even said this:

"I have found optimal transient response (and flat frequency response)..."

So, by correcting baffle step, wouldn't I be producing a better transient response? Wouldn't a perfect transient response be one with infinite and flat frequency response? What am I missing?

I do not view this from the narrow view of preferred sound, only. Most of the time I relax the 2-4K region, or at least make that the lower point of my response -- that, I prefer.
BG

MarkMcK 29th November 2004 06:32 PM

BG,

By definition, by theory, baffle step occurs at some point after the transient (information change). Any electrical filtering will impact all of the signal. It cannot differentiate between onset or after onset.

From just a transient response consideration, when the problem is baffle step, and you try to apply an electrical filter approach you will always be overcorrecting onset.

Next, if the hump you are correcting is not baffle step but some vibration mode or resonance in the driver system, then it will impact both onset and decay. If this is the case, then a prefilter (notch filter) will correct the problem will little acoustic consequence.

Indeed, I am working on a loudspeaker design that has a hump at 800 Hz. The hump exists both in infinite baffle and conventional loudspeaker enclosure set ups. If the hump I measure in this system at 800 Hz were a baffle edge or baffle step problem, then it would not exist when mounted on a baffle where the pressure wave propogation over the baffle never reaches the edges before the sample window ends. As a result, I doubt that it is a product of baffle step or baffle edge diffraction.

I also said that the response variations measured in loudspeakers in the so called baffle step region are of complex origin. Part of that may or may not be baffle step. Blind following of a particular theory when application of that theory produces many falsifiying results is not recommended.

Good designing and good building,

Mark

rgrayton 29th November 2004 08:24 PM

Mark,
Not sure I follow your argument, completely. A transient perfect/near perfect/better perfect... is the one, measured at my listening position with the flattest response (let's not include typical transient imperfect crossovers and limit this discussion to a single driver bandpass). So, if I measure at 2m and sit at the same location (let's disregard room, too), and measure 2 drivers in the same box, one with baffle step filter, one without. Wouldn't the one with, the flattest response measured, have a better transient response?
Regards,
BG

MarkMcK 30th November 2004 11:20 PM

BG,

No. While frequency response can be derived from transient response, they are not one and the same thing. If there is a problem that occurs late after a transient event (such as that caused by an internal cabinet reflection or bounce off of some external object, or the edge of a baffle but within the test window), then correction of that by some alteration of the driver signal will "distort" the sound that proceeds away from the loudspeaker but occuring before the discontinuity of whatever cause.

Without doing my own testing, I cannot say, but I suspect that your 800 Hz bump is not a baffle problem if using a notch filter makes it test and sound more accurate (real, lifelike and so on).

The test of a theory is not how many postive predictions it makes but how many false predictions. Mere coincidence can account for agreement of theory predictions and measured results. Mere coincidence cannot account for when measured results contradict theory prediction.

Best,

Mark

Pan 30th November 2004 11:34 PM

Hi... do you mind? :)

"No. While frequency response can be derived from transient response, they are not one and the same thing."

Sure they are, in a minimum phase system.

"If there is a problem that occurs late after a transient event (such as that caused by an internal cabinet reflection or bounce off of some external object, or the edge of a baffle but within the test window), then correction of that by some alteration of the driver signal will "distort" the sound that proceeds away from the loudspeaker but occuring before the discontinuity of whatever cause."

I think your mental model is wrong here.

A simple BSC made by a resistor and inductor in paralell with eachother and then in series with the driver will only "step in" after some time has elapsed from the beginning of the impulse. The delay of the energy in the inductor is set to match the point of BSC, so the problem you are afraid of does not exist.

You say that the filter will affect all frequencies, well more or less, but the high frequencies and the start of the impulse (same thing) will not care about the inductor but simply goes thrue the non-reactive resistor more or less.

That´s my mental model. :)

/Peter

MJK 30th November 2004 11:55 PM

Mark,

Quote:

While frequency response can be derived from transient response, they are not one and the same thing.
The last time I looked, if you are operating a speaker in the linear range of motion then the transient response (time domain) and the frequency response (frequency domain) are related by the Fourier transform. One can be calculated from the other, you only need one to describe the system. I have to admit that your statements seem pretty far out from an engineer's point of view. Could you provide some reference and further explanation for your theory? Is you Phd a technical degree or something else? I am afraid I am having a hard time acceptiong your position, maybe some further explanation and some references would help.

rgrayton 1st December 2004 12:14 PM

Mark,
I am not sure I follow your coincidence reference. The last time I checked, a theory was just that, a theory, no matter how many times it is reinforced by observation and or experiment -- that includes e=mc**2. It only takes one confirmation of a false result to disprove a theory.
Regards,
BG

jdybnis 2nd December 2004 09:06 AM

Correct me if I'm wrong. A thought experiment shows that a real driver is not a minimum phase system:

A real driver has harmonic distortion. If you input a sine wave to the driver, some of the acoustic output is in the form of the the fundemental and some is in the form of harmonics. The magnitude of the fundamental must be reduced because the energy to generate the harmonics must come from somehere, but the phase of the output at that frequency does not need to change because it has not been delayed.


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