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Old 20th December 2001, 02:23 PM   #11
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Quote:
plz don't post obvious or stupid answers
Are you ever going to take people's advise and BUY A BOOK or GO TO THE LIBRARY? Or are you going to continue wasting bandwidth by posting elementary school level questions?
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Old 20th December 2001, 02:44 PM   #12
ucla88 is offline ucla88  Tahiti
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though thomas may be a little harsh, his point is on target.
frequency response testing is well covered in LDC, and even more detail in D'Appolito's Testing Loudspeakers. you really should plow through these books-it's not a slam. Look, i'm on my second read of D'Appolito's book, rederiving most of the relevant equations for my own understanding. i can say this is more educational that lurking the forums (usually, though i too, can't help lurking now and then)



Now, do manufacturer's fudge? that's why we test drivers ourselves, isn't it.
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Old 20th December 2001, 04:49 PM   #13
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though thomas may be a little harsh
Yep he sure is....... and with good reason. You can't imagine how many emails clog my mailbox with how-to questions regarding the various Klone-Audio projects. Many of the authors of these letters expect support for their projects, but haven't a clue as to how to make a good solder joint, let alone know what T/S parameters are.

The internet generation expects answers to every online question they post; and are just along for a free ride, letting others do the work.
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Old 21st December 2001, 01:23 AM   #14
Ignite is offline Ignite  Canada
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Quote:
Originally posted by ThomasW


Yep he sure is....... and with good reason.
True, although I imagine I've done it a lot here. (heheh)
What I find frustrating is how much DIY sub talk there is here that is exactly the same thing every few days:

Which is better? Shiva or Blueprint/Stryke/Dayton/etc whatever.
or
Is a ported sub good?

Especially annoying when www.diysubwoofers.org exists.
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Old 21st December 2001, 02:28 AM   #15
pkgum is offline pkgum  Australia
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"Are you ever going to take people's advise and BUY A BOOK or GO TO THE LIBRARY?"

thomas W

I have a loudspeaker design book
The public libraries don't have any specific books on Audio Engineering...etc
Even I'm having trouble finding a loudspeaker design book in a University Library, let alone a public one

Unfortunately this brief book doesn't cover everything in detail and I even misunderstood some of the misleading information, some jargon that I don't know what the heck they mean the first time I read it. Diyaudio.com serves the purpose of clarifying any problems....
Want me to buy 'the loudspeaker cookbook'? I'll go to Jaycar and have a look...In any other store besides Jaycar the book costs $150AU...A LOT(WASTE) OF MONEY FOR JUST ONE MEDIUM SIZED PAPERBACK BOOK...while I can construct a basic loudspeaker right now using basic knowledge.



In this thread I just want a simple answer how do manufacturers determine frequency response.....whether they tell -3Db, -6Db...etc bass cutoff frequency. I DON'T THINK THATS TOO MUCH TO ASK
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Old 21st December 2001, 03:39 AM   #16
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...sigh...
This is one of those frustrating times where everybody's right...and everybody's wrong...and it's difficult to unravel the two.
pkgum,
I confess that I, too, have seen you ask a lot of basic questions that might be best studied on your own. No, it's not illegal, and no, it's not going to get you kicked off the site (assuming that such a thing were possible). However, it does get frustrating to be asked so many 2+2=4 questions. I'm human, and to the best of my knowledge, most other members here are human (although I suspect that there are a few aliens thrown in as ringers, just to see if everyone is alert). We get tired. As yet, there's no FAQ section here at diyAudio, though I suggested the idea once upon a time. There are other sites on the web that have a lot of basics available simply for reading. No one will think that you're disloyal to diyAudio if you pick up some basic information elsewhere. A number of people who come here regularly visit other sites. That's how they keep finding all these wild and weird new links that you see.
Speaking purely for myself, I've never regretted the money spent on a book. This probably won't be the only speaker project you ever attempt. Think of the money as being spread out over every speaker you will ever make throughout your entire life. Hit used book stores. Try to pick up something used on E-bay. Go in with a friend and split the cost. Think creatively. Books are basic, and once they're on your shelf, you can look up the answers as many times as you wish.
ThomasW, ucla88, Ignite,
Guys, you're being ugly. None of us were born knowing this stuff. We all had to learn it somewhere, someway, somehow. Okay, I kinda agree with the essential point, but...tone it down a bit. I mean, cripes, I thought it was my job to keep people around here irritated, and you're elbowing into my terrority, ya know?
Okay...
Back to the original question.
Speaker response can be measured in several ways.
--The traditional way is to use an anechoic chamber...a room that absorbs all sound...and an accurate microphone. The problem here is that the room has to be large enough to allow the full wavelength of the lowest frequency you're trying to measure, and for anything below, say, 50Hz, that starts meaning a pretty decent sized room. Then you have to put the entire room on springs to keep outside vibration out, etc. etc. etc. Difficulties multiply like rabbits in the spring time, but it's still arguably one of the best ways to check the basic response of an entire speaker.
--Oddly enough, if you don't mind a half-space measurement (envision a speaker suspended in mid-air--that's a full-space, cut it in half with a wall, the floor, or in this case the ground, and it becomes a half-space, thus the speaker only has to fill half of the original space with sound) you can take the speaker outside and drive the neighbors crazy with test tones. The trick here is to get completely away from any houses or other reflective surfaces. Use very, very long extension cords.
--You can take a near-field measurement, which I believe was described above. This, however, is a compromise, as you can't get a very good measurement from higher frequencies this way. If all you're interested in is the (sub)woofer, it can give fairly good results.
--You can use pulses of sound. A fast burst of white noise that's measured before the reflections from the walls get to the microphone can give you a very accurate reading on a speaker. (This requires recording the data and feeding it into a computer program for analysis. No one can read a couple of milliseconds worth of hash on an oscilloscope.)
--I'm sure I'm forgetting a few methods; let this stand as a placeholder for whatever they are.

So how do you interpret all this, at least from a consumer standpoint? You don't. No two manufacturers use the same methods, so you're comparing apples and oranges. Then you get into the question of whether you're going to say +-3, 6, 10 dB. (Jeez, the things guitar amplifier manufacturers quote...) There's no industry standard, per se. It's customary to use +-3 dB, but keep in mind that 3 dB is twice (or half) as loud. Suppose you were to go to buy a new car, and the salesman told you that it got 30 miles per gallon, plus or minus 50%. What would your reaction be? Right. Using 3 dB limits is pretty lame, but it's also a measure of how crude speakers are, overall. Worse yet, there are ways to fiddle the limits. Suppose we were still to stick to +-3 dB. Suppose the speaker was flat as a ruler right down to the rolloff, and I mean flat. Then all you have to do is put the flat part of the response curve slap up against the top part of the limits (the +3dB line) and presto! you just bought yourself a half octave of bass extension.
Note that we haven't even mentioned averaged or smoothed curves, or...oh, never mind. Take my word for it, it's a jungle out there.
And people wonder why I don't chase specs any more...
Anyway, the point is that you're going to have a hard time comparing two manufacturers' products, and you'll have an even harder time measuring your own. You simply do the best you can and then you go on to the next project. Obsessing over it will get you free meals for life--in a mental hospital.

Grey
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Old 21st December 2001, 02:31 PM   #17
ucla88 is offline ucla88  Tahiti
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Quote:
Guys, you're being ugly.
i'm somewhat suprised by this. i think your posts are usually well thought out and your composed replies in one of the other threads (...vintage speakers) is even more impressive.

i did not think i was being that harsh, and if that's the way it sounded, then i apologize for that.

back when i was still at the university, occasionally i would ask a question the nature of which would make painfully obvious my lack of a deeper, conceptual understanding of an equation (say maxwell's equations for an example.) and, occasionally, the professor would answer "gee mark, it sounds like you really don't understand these concepts fully. why don't you reread chapter 23 as well as so-and-so referenced at the end of the chapter."

my reply was certainly not, "gosh professor, your book is expensive, and the library is a long way off. you don't have to get ugly!" i would have thanked the professor and skulked away with a slightly bruised ego. but i would have gone to the library, read multiple chapters and references, and cranked through the problems until the light went on. this is the price of understanding. if i reference a book, that's because the book can explain it much better than myself, or an faq, or a brief post.

if somebody with more experience, say thomasw or jon hancock takes the time to reply, i am appreciative. if it's just a terse reference or reply, well, so be it if the knowledge is there.

my interest in speakers isn't so much to build the "best design" whatever that might be, but to understand the theory and what it means on a practical level. when i see a impeadance graph, i want to know that the blip at 400hz is from my cheesy cabinet design. i'm happy to bolt a cheap $5 woofer into an ugly test cabinet and see if i can massage good sound out of it.

there is an idea floating out there that all you have to do to get a truely high end speaker is just buy the "best" components, use the "best" software, and build away. as if all you had to do was plug the specs of your scan speak into leap and viola, you're done. what you end up with is a working prototype, which likely has some warts. if you have a good grasp of the theory, and at least some basic test equipment, you can then figure out why your prototype isn't up to snuff, and run through a couple of iterations. even then, you might not like the sound, but that's a separate issue.

if you're not willing to learn the theory and test your prototype, you're probably better off using a well tested design and sticking to it exactly.


just my two cents

{edited for spelling and punctuation}
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Old 21st December 2001, 03:53 PM   #18
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Mark,
Yea, verily, I agree about looking things up, and by extension, owning the books. Man, I've got a mess of them lying around here.
Libraries are good (assuming that you've got a good one nearby--not everyone has that luxury, unfortunately).
Books are good.
The printed word is good.
(But you knew I'd say that, being an author. If I was a painter, I'd probably say that diagrams are good...we all see things according to our aptitudes.)
The one caveat about working from a published project (I 100% agree with your points--use a project as a stimulus for your curiosity--if you aren't curious, you aren't learning, and if you aren't in the mood to learn, use a cookbook, don't just start throwing ingredients in a pot) is that projects out on the web are highly variable in their quality. There are few trustworthy sources for projects. This has been brought home forcefully in another currently active thread where someone is trying to build an amp from a schematic from a site that I regard as being pretty good, yet there seem to be errors in the schematic. To make matters worse, the schematic appears to have been published in a magazine at some point, so that lends an air of authority to the layout. If I'd found the schematic and decided to build the circuit, I'd have looked it over and noted a few suspicious things, but someone who's never done this before isn't going to be savvy enough to recognize potential trouble points.
(For the record, it's possible that the next issue of the magazine printed an erratum pointing out the errors in the circuit, but if so that didn't make in onto the web.)
One of these days, the web will get its act together.

Grey
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Old 21st December 2001, 04:24 PM   #19
ucla88 is offline ucla88  Tahiti
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grey,

point well taken. not just web based projects, but even other highly regarded sources can be less than perfect. a while back speaker builder would have someone build a kit which dr d'appolito would then test. despite being highly regarded kits, he often found fault, at least in the measurements, sometimes suprisingly so. not sure if they still do this, having let my subscription lapse (and now i guess SB is folded into a different publication), but it was very interesting. i suspect this goes for virtually anything else on the web. i'm sure you see a fair bit of inaccuracy in many of the posts-sometimes obviously wrong, other times subtly incorrect. hey, sometimes i come back to a post i made awhile back somewhere and i realize i wasn't quite right!

hence my push for understanding. i see guys with a thousand posts, and it seems like "sound and fury..."well, you know the rest
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Old 27th December 2001, 05:08 PM   #20
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This is straight out of Vance Dickason's famous book;

Quote:
You can make reasonably accurate frequency response measurements using a 1/3-octave pink noise source and an inexpensive sound level meter.... If you own a computer you can utilize one of the available FFT (fast fourier transform) programs, you will have the very best setup possible for making response measurements.
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