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In Reply to: RE: About that bass... posted by Doug Schneider on July 18, 2024 at 22:19:36
As far as I know, the sounds of all non-electronic musical instruments are seriously colored by physical or mechanical factors.
One example: The lowest string of a 4-string electric bass guitar is E, usually tuned to around 41Hz (the numbers to the right of the decimal point don't make a difference). 41Hz has a wavelength of 27.5 feet. To begin to do full justice to the frequency, you need a string length of 6.7 feet. Not gonna happen!!!
So, in real life, the low E of an electric bass is a "Weak Fundamental," which means that the octave harmonic of Low E, E = 82Hz, is 6dB louder, which is perceptually twice as loud. But even that harmonic is not "full voice." Not until you get up to 164Hz does the speaking length of the string get longer than the quarter-wave of the frequency.
Of course, it is a lot more complicated than that. When you hear an electric bass Low E, or a grand piano Low A = 27.5Hz, you don't viscerally feel "Wow, that sucks--so WEAK." That's because our ear-brain system, for reasons of its own, "fills in" the weak fundamental.
(I am trying my best to give the shortest possible version.)
If you give the matter some thought, it might become apparent that the problem of the Weak Fundamental has no choice other than to be inextricably intertwined with the problem of Baffle Step Compensation.
Then why, you might ask, would a loudspeaker designer omit BSC?
I can think of a few reasons.
One, both for ideological as well as expense reasons, wanting to keep the "parts count" of the crossover as low as possible.
Two, voicing a speaker entirely by ear and emotional responses, and for some designers, that is not a bug and it is not a feature. It's the farking entire Raison d'Etre!
Three, development costs. As far as I know, there is no crossover-design software that can compute BSC. Or, if it is, it's the kind of software that costs $2,500 per module to rent per year.
So, engineering a BSC network is as far as I know a trial and error iterative process, with lots of measurements. The designer of the final crossover for my current project put more than 50 hours work into it.
JA, please comment!
Thanks,
John
PS: That inductor is what it is for reasons of electrical efficiency.
Follow Ups:
Basic rule (for many decades before design software existed) is that the -3dB point of the baffle step is 4,560 divided by baffle width in inches. So for a 15" wide baffle, that will be 304Hz. This certainly can (and should) be refined with measurements and computer modeling, but it gets you in the ballpark at the prototyping stage. In a 3-way design, the correction can be incorporated easily into the woofer-to-mid crossover, by having the lowpass transfer function down -9dB (as opposed to the usual -6dB) at the crossover frequency, and then padding down the midrange output accordingly.
In a 2-way, many designers have had good results with a "staggered" lowpass filter: an initial 3dB shelving filter of paralleled L and R, followed by the higher frequency lowpass to cross to the tweeter. Or by simply adjusting the L to C ratio in a second order (or higher) filter, usually with an oversized L1.
Prototype, measure, listen, adjust, and repeat until satisfied.
Hi,
That all seems about right. But there are also many tools, including simple online tools like the one below I quickly looked up. It's a very well-known and definable thing, obviously.
Doug
SoundStage!
The whole site is packed with great info and useful calculators.
Not surprising! What was more surprising was John Marks's comments about this being so difficult to calculate. Like, experimentation is one thing, but the tools for getting you in the ballpark are everywhere.
Doug
SoundStage!
It's often said that if you can blow out a fundamental premise of an argument, then the rest of the argument falls away: "As far as I know, there is no crossover-design software that can compute BSC."
Transitioning from a 4pi to 2pi environment is well known and is easy to computer model these days.
There is absolute no reason to not remove the baffle step if you want a proper response in a room.
Doug
What one can do is take an anechoic measurement or one far enough from reflections for a crossover use, exported as a text file (freq/mag/phase) and use a program like LSPcad.
With a corresponding impedance measurement, one can use the computer to iterate to a target you set (like say flat response). This is the best tool for passive and some active crossovers i have run across and it can make this kind of passive filter's values a breeze.
Really.
Hi,
Yes, interesting approach -- and it makes sense. Thanks. Obviously, crossover work takes a lot of work to dial in the sound just right, but the 50 hours to iron out the baffle step that John Marks talks about -- as well as the lack of tools to do so -- just isn't true.
Doug
SoundStage!
I said "As far as I know," because I am familiar with only three such programs. "Auto-BSC" was not on any of the lists of features. I was talking about "Auto-BSC" and perhaps I was not emphatic enough on that. (I have no experience with the very expensive modules from Loudsoft.com. I am told that Harbeth uses those.)
I had a phone conversation with the designer of the program I believe to be the most respected package (X-over Pro version 3) and he said that it was not part of his software and that he did not envision it as a possible feature of any "Insert the data, press 'Process' and you get a crossover design" packages, because there are too many cabinet-related variables.
Regardless, my point was that Baffle Step Compensation, as important as JA and I (and apparently you) think it is, is not something that every loudspeaker designer thinks is worth the trouble.
I was surprised as all get-out when JA commented upon the apparent lack of BSC on a very expensive solid-rock-enclosure loudspeaker he was measuring. I thought that with all that money on the table, why no BSC?
"Though the response in the crossover region is flat, there is a broad peak in the upper midrange, which implies a lack of baffle-step compensation in the crossover. The tweeter's output between 5kHz and 20kHz is 3-5dB higher than it is in the presence region, almost matching the level of the midrange peak."
john
"...is not something that every loudspeaker designer thinks is worth the trouble."
I'm not sure how you can make a statement like that. That a designer doesn't think it "is worth the trouble"?
It's about the behavior of the driver when it is attached to a box. Baffle step isn't some magic -- it's an acoustic result as the driver goes from radiating from 4pi (free space) to 2pi (half space), because of the baffle it's now attached to.
On a freestanding loudspeaker, the choice not to compensate indicates not a design compromise, but a fundamental misunderstanding of how drivers work with their enclosures. Compensating is part of designing. And once again, this is an easy thing to model, even with a calculator and a piece of paper.
Doug
SoundStage!
You complained about:
"I'm not sure how you can make a statement like that. That a designer doesn't think it "is worth the trouble"?"
Did you read my reply, in the context of JA's statement about the Acora Rock Box???
Are you privy to some double-secret explanation why the Acora Rock Box had no BSC?
john
Nope, didn't read your reply in the context of anything but what baffle step compensation is and why it has to be dealt with.
Doug
SoundStage!
> One example: The lowest string of a 4-string electric bass guitar is E,
> usually tuned to around 41Hz (the numbers to the right of the decimal
> point don't make a difference). 41Hz has a wavelength of 27.5 feet. To
> begin to do full justice to the frequency, you need a string length of
> 6.7 feet. Not gonna happen!!!
>
> So, in real life, the low E of an electric bass is a "Weak Fundamental,"
> which means that the octave harmonic of Low E, E = 82Hz, is 6dB louder,
> which is perceptually twice as loud.
Fig.3 at the link below and the table below the graph show the levels of
the harmonics of the bass guitar's open E string. The second harmonic is
almost 12dB higher in level than the fundamental.
> So, engineering a BSC network is as far as I know a trial and error
> iterative process, with lots of measurements. The designer of the final
> crossover for my current project put more than 50 hours work into it.
> JA, please comment!
The baffle step compensation can be achieved in a second-order low-pass
crossover filter by using an inductor with a relatively high series
resistance. However, this reduces sensitivity. But yes, optimizing BSC
is very much a matter of trial and error.
John Atkinson
Technical Editor, Stereophile
Thanks, John
Sorry for the memory glitch. I don't have time to fact-check it, but I vaguely now remember that it was the Stand-Up Acoustical String Bass that had the octave harmonic 6dB louder. And yes, the electric bass's harmonic series is even more sharply divergent.
My crossover designer Curt Campbell told me that the BSC on my current design was -6dB.
So, if the efficiency had been 84dB before BSC, after BSC it was... 78dB.
I can't imagine any magazine doing that measurement, and then not making a major issue of it.
Which is to say, that can be Reason #4 why a loudspeaker designer might make the Executive Decision not to implement BSC.
john
"I can't imagine any magazine doing that measurement, and then not making a major issue of it."
"Which is to say, that can be Reason #4 why a loudspeaker designer might make the Executive Decision not to implement BSC."
Are you sure you understand "baffle step"? From the comments above, saying "that measurement," perhaps no. It has to do with frequency response and simply shows up as a rise on the frequency response if not dealt with in the crossover.
Also, in another post you talked about the money involved in compensating for it -- and companies not seeing it being worthwhile because of that. It becomes part of the crossover network, and if you know how much a crossover takes to make, well, it's not exactly all that much money.
Did you say you "design" speakers?
Doug Schneider
SoundStage!
Hello,
I came back to this comment and realized it indicates that you might not understand sensitivity well. If not, you're not alone, I made a whole video about it when Robert Harley got it wrong.
How "sensitive" a speaker is depends on the frequency or, preferably, frequencies at which you measure. The baffle step varies per speaker, and in the case of that Franco Serblin speaker, it starts happening just shy of 1000Hz. So if someone where to take a sensitivity measurement at, say, 400Hz, it would be unchanged. Instead, the rise of 6dB would be at its fullest by 1000Hz. So the entire speaker didn't change by 6dB if you compensate for the baffle step -- 400Hz remains the same, the frequencies just below, at, and above 1000Hz get reduced.
We average sensitivity over a range -- 300Hz to 3000Hz -- so by doing so, once again the sensitivity wouldn't change by the full 6dB.
Finally, if you have a speaker that's 78dB, you've got a real problem with it -- that low sensitivity will likely mean it'll blow up if someone plays it loud. It's too low.
Doug Schneider
SoundStage!
The usual reason for not eliminating the baffle step typically completely is because you don't even know it's there. As soon as you measure something properly -- anechoically -- it sticks out like a sore thumb.
Furthermore, if you understand the interaction of loudspeaker and rooms -- read any version of Floyd Toole's book -- you'll know if has to be ironed out to get a respectable response.
Otherwise you'll have one of the worst sins a loudspeaker can commit: NO BASS. And if you do further reading on the importance of bass response based on the research of Toole and later Olive, you'll find out that, all things being equal, the speaker that produces deeper bass is the one favored by listeners.
In fact, Toole talks about that very thing in our latest podcast episode, linked below.
Doug Schneider
SoundStage!
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