Curious about ZAPH's designs. Seas L18 and P18RNXP
Having NO knowledge in speaker design, I'm doing alot of reading and trying to get a better understanding of what makes a good driver (among other things). I was looking at Zaph's L18 design and looking at his 6.5" measurements and comparison... And can't help but wonder why did he choose the L18 over the P18RNXP?
If I look at the responce curve and the harmonic distortion, They are very comparable (to me). But when I look at the SCD graph, the P18RNXP looks like a clear winner. What am I not understanding or overlooking?
Well, for starters, the P18 didn't exist at the time I designed the L18 system. It came out about 1.5 years after that. But, even if I had to choose today, I'd probably still pick the L18 over the P18 by a teeny margin based on a few characteristics.
You're making a very common mistake about cumulative spectrum decays. But don't feel bad, it's a mistake that even industry professionals make. The CSD is the most over rated and useless form of measurement there is. There is absolutely nothing in there that can't be seen in a response curve. They both are generated from the same impulse and are simply different ways of looking at the same linear distortion information.
One might ask why I provide them on my web site. I often ask myself the same thing. The sad fact is that I think I've been brainwashed by people who think they need to see them. If I didn't provide them, there would be hoards of people saying "yeah, but he didn't provide any CSD plots". I'm guilty of posting CSD plots to avoid having to explain linear distortion to people who aren't likely to understand it. There are people who think a CSD contains info that a response curve doesn't because one has time slices and the other doesn't. Consider this question: If both the P18 and L18 were filtered with a proper crossover to shape the response curves into perfect LR4 rolloffs, and a CSD was then taken of the result, what would the differences in the CSD's of each be? The answer is that they'd be exactly the same. This is hardest thing to grasp about CSD plots.
A response curve and a CSD are just different ways of looking at linear distortion. linear distortion can and should be fixed in the crossover when possible. Most proper crossovers for metal cone woofers do have the the breakup filtered out, and thus ringing in the CSD is not a problem.
Examples with tweeters: when I see this response curve with the sharp peak/dip combo right in the middle of it's bandwidth, even before looking at the CSD I know that it will show a ridge at the same spot like this. What's notable about this is that the ringing anomaly is not easy to fix in the crossover. If you want to view a CSD, this is the kind of thing to look for.
On the other hand, let's look at a tweeter with a high Qts bump in the low end like this. Without even looking at the CSD, we know the low end will have some ringing like this. It's ok though, because the low end can and will be shaped into the required rolloff by the filter, making the low end ringing a non-issue.
So, the bottom line is that nobody needs to see CSD plots, but if you're going to look at them, at least know what to look for. You may simply ignore the CSD plots and look at the response curves for sharp/steep anomolies not easily fixed in the crossover. Also be aware that the same gating issues that reduce the number of data points in the low end response curves of drivers, will also affect the low end of CSD plots. Most of the CSD plots I post are only usefull down to maybe 800hz, after which the mush of gating obscures usefull info. On the other hand, some of the response plots I post (not the 6.5" woofer tests though) have nearfield low end responses merged in. This alone makes the response curves much more usefull than the CSD plots.
I hope this clears things up for you and others.
you are waying that a CSD plot doesn't provide anything more at all than a correct FR graph ?
i might ask here, what is the point of the CSD plot at all then ?
or ..are you saying that a CSD doesn't present much usefull information in what we are using to determine crossovers and such when desinging a particular system??
I do not understand fully here,
is the FR curve a sum up of all ( what we see in the CSD)
waves for a certain period of time summing up together?
Wow, the Zaph-man himslef, my question wasn't as stupid as I thought afterall :)
"If both the P18 and L18 were filtered with a proper crossover to shape the response... ...what would the differences in the CSD's of each be? The answer is that they'd be exactly the same. This is hardest thing to grasp about CSD plots"
Good to know! Thanks for the explanation.
Great write-up Zaph, very informative, I really like the use of examples. :)
This lead me to do some playing on your site to test my new found knowledge and yes, you can see a correlation between the CSD and the freq. response. I think I may have found one example in which it doesn't apply. The Ascendant Audio Poly 6.5 has a raise/hump between 1.5-4 kHz in the freq. response and I except to see some similar information in the CSD, but the CSD looks very clean in that range. Am I missing something? Is this an odd-ball example? Is the hump in the freq. response due to something else that doesn't show up in the CSD?
Again, thanks for the lessons.
Assuming that the frequency curve in your tests on your site includes a certain amount of time in the measurement, is it the same time that is sampled in the CSD graphs ?
I am not sure i completly understand CSD though,
aren't the graph supposed to show a linear decay on time of producing sound vs rising frequency ?
i assume that a "perfect" driver would measure in such a way that the sound wave timing would get smaller with the frequency and thus the graph would show more like a 3d 3-sided pyramid towards high freqs?
Zaph is exactly right.
The CSD provides no additional information than is present in a well done FR curve. Neither do burst/linear distortion graphs. They are all different ways of showing the same phenomena. You have to think about what you can easily fix in the xover and what you can't. That's all.
Although I don't spend much time on CSD here--
This plus the three references should make for a very good set of readings.
I'm 99% in agreement with what is above, but feel it may be worth adding a little.
As Zaph explained there is some information hidden in the frequency response that shows in the CSD.
Generally anything where there are multiple time delays involved (reflections, echoes, cone breakup) introduce the possibility that the resulting peaks or dips cannot be corrected, and that means the driver is hard to use in that frequency range. (Again as explained by Zaph.)
Although not very useful for quantitative information (mainly because it is recorded on one axis and the time evolution often depends strongly on the measurement position), the CSD does hint at whether there is time delay involved.
Another way to provide the same hint would be to record the phase in a (very) near-field measurement (although multipath problems or cavity effects will make this challenging approaching 20 kHz).
In the general electronics and control systems world, it would be normal to plot the amplitude and phase of the response (i.e. a Bode plot or equivalent) to help reveal this information.
(I've deliberately not mentioned minimum phase, and zeros in the postive real half plane, as those who know about that probably won't need the rest of this post.)
ps. I'm still trying to get a feel for what is useful to post, so please forgive if this is not the kind of thing people want to read.
Someone pointed this thread out to me because it was funny. Actually it is a bit sad.
I hate seeing people being mislead by false information.
Some of this I just had to respond to.
QUOTE]There is absolutely nothing in there that can't be seen in a response curve.[/QUOTE]
Actually the truth is quite the opposite. You can't see the stored energy and inertia (CSD) in a frequency response measurement, but you can see a frequency response measurement in a CSD.
If you play a driver to a 90db reference and the break up is down 20db it is still ringing at a 70db level and easily heard.
A crossover will also not filter out problem areas within the drivers pass band.
Don't be fooled. Look very closely at the spectral decay.
Mathematically speaking, there is nothing that isn't in an FR curve, that isn't in an impulse. And the CSD is derived purely from an imulse. So, from an information theory standpoint, the CSD offers no additional information that isn't already in the impulse, and thus, the FR.
What you're saying is it's your preference to look at a CSD. Well, OK. The up side of a CSD is that, by slicing up the time domain response into time delayed chunks, yes, you can see how much energy is left after some arbitrary time more easily than by looking at an FR curve where all time is superimposed. However, the down side is that by successively truncating the impulse you introduct significant artifacts.
Again, there is no new information in a CSD that is not present in the impulse, and, hence, in the FR. This is a mathematical fact of no dispute.
Again, I would recommend the links from my previous reference to john k's and SL's comments on energy storage.
Also, a brief easy read on csd's can be found over at liberty instruments-
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