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Old 28th October 2013, 08:40 AM   #1
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Default How to design a loudspeaker

What goes through an Engineer's mind upon designing a loudspeaker? Surely there must be some general guidelines, no? Perhaps. Although this is most likely dependent upon the circumstances (cost, enclosure, available power, etc).

If these guidelines do exist, what are they. If you could order them, how would you go about doing it?




I need to design a loudspeaker around these circumstances:

Cost: FREE
Frequency Response: 20hz-20khz
Enclosure: Infinite Baffle (is it baffle or baffled?)
Available Power: .5 RMS


Okay, so this is a bit unrealistic. A full range driver capable of playing the whole spectrum? I'm not sure if that exists. Why IB? This speaker will be for a demonstration, so no enclosure! The .5 wrms is from the LM386. I'll just drive it with my Ipod.


Okay! So I need a loudspeaker with great sensitivity. Wait though, I thought sensitivity was how loud a drive could play without audible distortion (vaguely speaking). When people say "I need a driver that's really sensitive," does that mean they need one that's like 90db+ or a speaker rated at like 5wrms?

I'll stop here. I have no idea how large to make the cone, what angle should it have, what material to make it out of, etc. I assume that last part is up to the individual's taste (I prefer paper!). Btw, I have no idea why cone material affects the sound. I just see sound as a vibration of a medium. Regardless of how they're pushed & pulled, atoms are atoms, right? I have no idea how many turns I need, what AWG to use, the size of the voice coil. I have no idea what magnet to use, & what type (neo or that other one). I don't know whether I need a dust cap or not. I'm not sure how large to make the pole piece (or vent if you will).

^& why stop at the geometry of the cone in relation to the desired frequency response? You don't want a 40oz magnet with a 2" cone. ...lol, a ratio of a ratio.

Maybe I should just start with whatever kind of relation I need between the voice coil & the magnet?

I'm lost!

Thanks


*It seems to me that there are a LOT of give & takes when it comes to designing loudspeakers. For instance, you don't always need a 15" cone to get 30hz. Enclosure type plays a role here. I know there are 5" drivers capable of 30hz in the correct enclosure (like transmission line).

Last edited by TylerTraulich; 28th October 2013 at 08:49 AM.
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Old 28th October 2013, 09:40 AM   #2
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You have too many conflicting constraints. I'd suggest you take a look at what is achievable with a few different designs then see how you can best meet a compromise of your needs.

And to answer one of your questions, a sensitive loudspeaker is one which does not require much electrical power for acoustic output, i.e. the dB per watt figure is high.

You are not going to be able to make a drive unit to compare with anything reasonable that you can buy, just build something that is easy for you to finish and see it as a learning excersise. Then build a bigger or smaller version, or use a different material or bigger magnet and see how that makes it perform differently.
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Old 28th October 2013, 09:43 AM   #3
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And what about internal ( spider ) and external ( surround ) suspensions ?
The first is for making the former always in line; the second also keeps the cone centered and also follows the movement. Basically, the membrane should be stiff & light; most phase alterations that happen to be in a fullrange -though well engineered & executed- exist because the membrane isn't capable of following the signal and some modes of resonance take place along the cone. The very central part of the cone emits the treble because it's direcly attached to the former, going away from the center it's difficult for a big mass to vibrate at high frequencies. Then you see the pistonic working range given by the diameter and how it acoustically applies, so the dB and W and distance can relate each other.

I would start with a 5-10 W amplifier and a existing speaker.
Distortion made from an amplifier are unavoidable and a power reserve of 10 X would be welcomed- Same for the speaker.
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Old 28th October 2013, 05:15 PM   #4
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I recently wrote a paper on the history of electricity. I broke it down like this,

1.) Thales discoveries with amber & lodestone

2.) 1st true electrical storage device (Galvani VS Volta)

3.) Discovery of electromagnetism (Faraday/Orsted)

4.) My conclusion was to give a practical application. I did this by explaining the workings of a crystal set.

I stopped just before the audion tube. We live in a wireless age. I wanted to show how it (wireless communication) all got started (radio).


I'm taking this a step further & giving a presentation. I figured I could bring in a microphone & show how mechanical waves (sound) can be turned (via mic) into an electrical signal (using o-scope) & how this electrical signal can thus be turned back (via loudspeaker) into a mechanical wave. The point I'm trying to get across is simplicity; anyone with a few household items (paper plate, fridge magnet, etc) can toss one together. Not sure how many people have magnet wire, ha.

I do not wish to purchase a completed loudspeaker. I'm an engineering student & am more interested in functionality.

Last edited by TylerTraulich; 28th October 2013 at 05:19 PM.
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Old 28th October 2013, 06:25 PM   #5
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Yes...and Ohhh now I understand why you want to use the LM 386
Great amp BTW

So...talking about sensitivity and efficiency...the latter has got to be kept under control, as all the various parts have to be assembled and work at best.
All the assembly's got to be with minimum gaps and tolerances.
So, make the basket ...
Personally, I would disassemble some units from radios etc and use the already assembled former and voice coil > corresponding magnet, too.
Then, I would mix from other cones and baskets and see how it comes.
You can try also with old cheap headphones, using the membrane as the spider and tryng to attach it to a cone and a basket. You can substitute the spider with 3-4 tensioning rubberbands.
There was a thread some years ago with a chinese forumer showing this - he was very good with paper, makink very deep cones and using HP's magnet.
I lost the thread, then I recovered it some years ago ( a german member remembered it) then I lost it again
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Old 28th October 2013, 06:37 PM   #6
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Sounds fun. Is there any way of keeping it so simple that you can actually assemble it during the demonstration?
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Old 28th October 2013, 10:03 PM   #7
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Hi,

Imagining something is very simple is not the same as knowing it simply isn't.

Try knocking up a speaker with no cabinet, a paper plate, some wire, a
fridge magnet etc and see how you get on. Very badly I would assume,
as I would and I've got a very good idea of what I would be doing.

No enclosure is open baffle, infinite baffle is wall mounted between rooms
and also a misnomer applied to describing sealed box loudspeakers.

You can't just toss a speaker together, or a microphone for that matter.

The history of Communications, which leads to our "wireless world" is
not a simple as the history of Electricity, though very intertwined.

rgds, sreten.

BTW - 3) Discovery is Orsted And Ampere, Faraday is a decade later.
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Last edited by sreten; 28th October 2013 at 10:30 PM.
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Old 29th October 2013, 07:26 AM   #8
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LM386; you should have seen the line array I completed with the $1 Parts Express buy outs. Those things dropped bottom while bottoming out, lol.

I have a Lepai 2020 that I will be utilizing for my demonstration. I believe it puts out 15wrms/ch which should be sufficient.

Confining 2000 years of electrical & magnetic phenomena into a 2 page double spaced paper isn't an easy task. I'm really struggling to put together a thesis statement,

"...spark a revolution in wireless communication..."

What would? Electromagnetism of course. & where does the invention of the battery fall into place? How about hertzian waves?

I recently read a book called "The Battery." I believe his thesis went something like this,

"What began as a long running dispute in biology over a dead frog's leg would...."

I'll let you assume the rest. I thought about starting with Thales, but that wouldn't be fair to state amber & lodestone were just two manifestations of the same thing. Amber; that's the electrostatic force. I'll just leave it as them being both problematic to the ancients.

ugh, well here's the start of my paper,



We're all familiar with it; the hair on your hood straight from the dryer, a tingle on the tongue, the sudden discharge of a door handle after a swift walk across the wintry carpet floor. It's a mysterious force -electricity. Tales of it's (blank) date as far back as Ancient Greece.


^Now I'll briefly go over Thales & his discoveries with amber & lodestone. Then I need a thesis. Again, my paper needs to be short.

Any ideas on a thesis? Again the point of my essay is to give a little back round on how radio (wireless communication) all got started.

Last edited by TylerTraulich; 29th October 2013 at 07:36 AM.
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Old 29th October 2013, 07:35 AM   #9
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The first things that come to mind are piezo electricity ( for pick ups)
and quartz for oscillators.

For loudspeakers and their origins, take a look here http://www.leradiodisophie.it/Spillo.html

Last edited by picowallspeaker; 29th October 2013 at 07:49 AM.
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Old 2nd November 2013, 02:06 AM   #10
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Quote:
Originally Posted by TylerTraulich View Post
(snip) Amber; that's the electrostatic force. I'll just leave it as them being both problematic to the ancients.
You're inadvertently on the right track. You could research what the not-so-ancients (Kellogg and Rice at Bell Telephone in the early 1920's) had to choose between when they were looking to design the first loudspeaker and found that they were doomed to go down the black magic path of magnetics and cones...?
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