Electric Eel - Introduction

Below, Arthur is playing the Electric Eel.  It's an electronic music instrument I designed to be like an acoustic instrument..  The things that  slow me down when playing most electronic instruments are having to plug into speakers and find batteries, so this instrument has its speaker and generator built in.  And as a bonus, the sound varies with how hard you play and how you move the generator, because the synthesizer can detect your playing movement and the amplifier gets louder when you player louder!  So let's get started and build one like this lovely Exertion Violin below. 

Step 1 Electric Eels - How to Build Your Own

To explain how it's done, we'll use this prototype Exertion Violin.  From the outside, it has three main components.  These can be thought of as similar to a violin:

1. Expressive Generator / Strings
2. Speaker / Violin Body
3. Keyboard / Fret Board

In the the next picture, you can see the electronic insides.  The electronic insides have three major jobs:

1. Rectifier - Rectify and buffer power from generator
2. Synthesizer - Synthesize musical instrument sound
3. Amplifier - Amplify the sound to be as loud as an acoustic instrument

In the next picture, you can see the beginnings of an instrument body.

Comments

[gmoon]

These things are great! I love the concept--and the expressive quality, unlike any simple electronic instruments I've ever seen! Please-embed the video--you can add it to the instructable itself.

One small point--the speaker graph on step 3--I believe that's the speaker impedance graphed to the frequency, not the frequency response itself.

I.E., it's graph of the input impedance change, not the output frequency. A large hump in the impedance around 100+ hZ is pretty typical (but a freq response like that is not).

[shifty]

aah thanks! I've been wondering about the details of that stuff. I'd love to ask you more questions about it!

How can we use that impedance graph to choose the frequency range to run the speaker at its most efficient?

If it's necessary to know the acoustic structures in which it would be placed, the two most important are the fixed-end cylinder and the exponential horn.

What other information do we need to combine with this graph to estimate how good it would be at e.g. 200 Hz vs. 400 Hz?

thank you for helping out!

[Cheyannelovesham]

omg thats hot lol

[chello2k9]

"the Electric Eel. It's an electronic music instrument I designed to be like an acoustic instrument.. The things that slow me down when playing most electronic instruments are having to plug into speakers and find batteries,"

Great idea, but a useless one...just play an acoustic instrument

[shifty]

Acoustic instruments are old-fashioned and don't sound good to me. I want synthetic timbres because I like the sounds of synthesizers.

[gmoon]

For some reason, I cannot reply to your comment--the button is missing...

Anyways--the thing is, the impedance graph doesn't really tell you much about frequency response. Look at the specs of any good guitar speaker that shows both freq response and impedance vs. freq (PDF--Eminence Red Fang shows both) and there's no correlation between the two graphs. I.E., one cannot be used to predict the other--despite the LARGE peak in impedance around 100 Hz, the freq response is approx linear.

The impedance graph just illustrates that impedance changes with the frequency--but in a very non-intuitive way. That's because speakers are physical transducers, and their impedance is effected by mass, magnetism, inertia, materials, etc.

But that's OK--they work. The impedance value stamped on the speaker is an average, and that's adequate. Their general design characteristics (woofer, horn, tweeter) dictate approx how they respond.

So if you can find the frequency response graph, that would be the most useful of two metrics.

Beyond that, the resonance, etc., would be much effected by the acoustic structure--but that's another thing entirely...which I suspect you're already coming to grips with...

[shifty]

Heya gmoon!

I've been looking into this more. I found one student's paper pretty useful. It's at: http://online.physics.uiuc.edu/courses/phys498pom/Student_Projects/Spring01/PPoongbunkor/Piya_Poongbunkor_TS.pdf

I took a look at that spec from Eminence, too!

If I understand this right: The numbers Dayton Woofer Tester 3 gives are the Thiele & Small Parameters (T/S). T/S are a set of parameters used to describe speakers minimally. They mainly describe the speaker operating in free air as having a resonant point based three things: the springiness of its surround, the mass of its cone, and the inductance of its coil. They can be used to estimate the response of speakers inside an enclosure.

However, the effect on the response of the enclosure can be be considerable. For example, it tends to make the speaker resonant at a lower frequency than in free air. Also, if it the enclosure is a cuboid shape, it will have sets of additional resonances related to the dimension of that shape. Other complex shapes like violins will be nearly impossible to adequately represent. Plus, the walls of the enclosure will have an effect, too.

So, there are probably too many variables needed to get completely precise. In the Eminence Red Fang spec, the note next to three asterisks (!) mentions the test conditions used to generate the frequency response. They used a medium-sized baffle in a medium-sized room with slightly reflective walls. That's probably a much more accurate representation of what the speaker will sound like in actual use. To predict those graphs from the T/S parameters that Woofer Tester 3 gives, one would have to be apply several transformations to get to the most accurate picture.

There are so many interesting things I've picked up. Did you know that batches of the same speakers can vary considerably in their sound? Some of the designers cherry-pick their woofers! I also found that when using wide-diameter tubes as resonant bodies, I could create "ideal" duty cycles for pulse waveforms that made them maximally loud!

If this part of speaker theory can be mastered, the bounty would be significant: one could effectively use an acoustic horn, like old turntables used to use or trumpets have, with a speaker in a musical instrument. I have tried using emergency siren horns, like public address systems, but their design attenuates ALL of the bass in exchange for efficiency in the higher frequency region. And you can hear it: it makes the music sound like it's coming over a public address system! fun once in a while, but not classical, like an acoustic instrument.

Fwiw, I have developed some techniques to build horns from sheet metal, but getting the right parameters and the perfect speakers to match I have not yet achieved. One of these days :) The horns I've been looking at building are "bass" exponential horns. In place of normal compression drivers, for example, they have an additional resonant space behind the driver.

[gmoon]

OK, several things...

Don't get caught up on "impedance." Impedance is just a fancy name for resistance--resistance to AC signals, so it changes with frequency. It's not an acoustic metric at all.

Thiele/Small parameters are a host of variables, LOTS of stuff.

Note that the T&S impedance "resonant" point in the Red Fang is around 100 Hz, but the acoustic "sweet spot" is between 2KHz and 5Khz  In fact, the speaker is just beginning to be linear above 100 Hz (that's useful info).

People who obsess about T&S parameters usually are looking for more bass response from a given driver. That's well and good--it's much harder to transfer energy below 80 Hz or so, than above it. Have you noticed how ultrasonic transducers (above human hearing) are tiny, but sub-woofers are huge? It's MUCH tougher to move a speaker cone at low frequencies. Much more energy is required (and wasted) in transferring low freqs.

But the overall response of most speakers is wide, despite the narrow electromagnetic resonant point that's reflected in impedance measurements. Any just about all general-purpose speakers have an impedance response that's similar to one's we've looked at. Taken by itself, It's not as significant as you're thinking...

Do cabinets matter. YES.

ALL Speakers MUST have some kind of enclosure to sound good...but it doesn't have to designed by an acoustical engineer. In fact, may fancy "ported" speakers don't sound good at all, and are just a marketing gimmick.  Others sound very good indeed; check out Bose for great audio engineering. But they design EVERYTHING from the ground up, including the transducers...

Fancy porting IS useful for getting more bass from a given driver. But it's very tricky.

The most common speaker enclosures are pretty simple. And they sound great--because they are designed to be used with simple cabinets.

Will different speaker designs have different freq responses (and impedance peaks)? Sure. They are designed that way. That's why they make woofers, tweeters, horns, etc.

Still, the reason I wrote this...
Beyond that, the resonance, etc., would be much effected by the acoustic structure--but that's another thing entirely...which I suspect you're already coming to grips with...

...is simple--you'll need to design speaker enclosures that enhance the frequency response of the particular instrument. I suggest you start by mimicking the structures of acoustic instruments--large tube/horn for the bass register; smaller oboe-like tubes for midrange, etc. Remember that a fixed length tube can have a very narrow resonant frequency, so play around with designs until you like the results.

[Robot Lover]

Is that Zoz from prototype this! looks like it

[rimar2000]

Very interesting!

[shifty]

Thanks! And I almost forgot, there's a wiki for these instruments over at:
http://exertion.pbworks.com

[craig3]

Will you be bringing out more aswell as example of what they sound like?

[shifty]

Ah yes, sound examples! Thank you for asking!

I've got a youtube channel at http://www.youtube.com/user/ExertionMusic

Let me see what videos best demo the instruments... Here you go:
http://www.youtube.com/watch?v=BNgyMfH3E9Q
http://www.youtube.com/watch?v=xbUjkeaTjxo
http://www.youtube.com/watch?v=SwNCqfY_jds
http://www.youtube.com/watch?v=xDScGokDaO8
http://www.youtube.com/watch?v=ZU7vgmtbKBU

And as school requirements die down, I will be bringing out more, one at a time from my studio, selling through http://synthshopping.com