Instructables

Measure Muscle Sounds! Part 1: Electronic Board

This is a step-by-step guide on how to make your very own prototype acoustic myography sensor. That means a sensor you can use to measure muscle sounds and other physiological vibrations. What is this good for? well... it is a more affordable alternative to the electrodes used in powered prostheses (specially if you make your own). To see what else you can do with these sensors, check out these videos.

The short paper below describes briefly how this sensor works and the diagram illustrates how it is built. For more details, check my MASc thesis. There are also some videos of the sensor in that page that you can check out.

Have fun!

Step 1: Bill of Materials

Here is a detailed bill of materials with suppliers and part numbers. The circuit schematics are also included at the bottom.

Part: printed circuit board (PCB)
Supplier: Any PCB manufacturer... or make your own!
Qty: 1 per sensor

Part: Knowles Acoustics BU-27135 Accelerometer
Supplier: Digi-Key BU-7135-ND ($39.85 for 1 - minimum order 500)
Qty: 1 per sensor

Part: Panasonic WM-63PRT Omnidirectional Microphone
Supplier: Digi-Key P11961-ND ($3.47 for 1)
Qty: 1 per sensor

Part: SMD Resistor 33.0k 1/10 Watt 5% 0805
Supplier: Digi-Key RR12P33.0KDCT-ND ($0.14 for 10)
Qty: 2 per sensor

Part: SMD Resistor 5.6k 1/10 Watt 5% 0805 SMD Resistor
Supplier: Digi-Key RR12P5.6KDCT-ND ($0.14 for 10)
1 per sensor

Part: SMD Resistor 2.2k 1/10 Watt 5% 0805 SMD Resistor
Supplier: Digi-Key RR12P2.2KDCT-ND ($0.14 for 10)
1 per sensor

Part: Ceramic capacitor 1uF 16 Volt 0805
Supplier: Digi-Key PCC2249CT-ND ($0.13 for 10)
2 per sensor

Part: Ribbon Cable
Supplier: Any surplus electronics store
Qty: 2.5 Feet

 
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m.hussien4 years ago
first i want to thank you as this demonstration helped me allot during my work ..but i need to know what kind of signal processing is used in order to view the signal ..or where can i get that kind of information.
thepelton4 years ago
What kind of sounds would you get if you ran it through an amp?  Could you make your own precussion backup to music by dancing or moving with the sensors on?
endolith5 years ago
Would it make sense to use a digital accelerometer and subtract the waves in software? Is the amplitude of the audio signal much smaller than the amplitude of the acceleration signal? Maybe something like the DRL+ diff amp circuit in an EEG would be useful, to cancel out the acceleration electronically even if the person is moving around a lot. Were you able to find any recordings?
Biotele7 years ago
maybe you can hack a Wii joystick it has a three axis acc. the heart sounds are fantastically clear! It is better than the commercial electronic stethoscopes.
nice instructions. where do you buy an Accelerometer?
endolith7 years ago
The accelerometer does not appear to be accessible to the kinds of hobbyists who are going to follow these directions. What properties are important for the accelerometer and microphone to have, so we can suggest substitutes? Low-frequency response seems to be the major factor. What else? Sensitivity? Noise?
The ADXL32x series is cheaper ($8.60) and more obtainable (Newark and Digi-Key sell them in single quantities), and go from DC to 2.5 kHz. They are smaller, too.

The three models have different noise, sensitivity and range, though. Which would be best for this application?

Part#   Range Sensitivity Noise Density (ug/rtHz)ADXL320 ±5g   174 mV/g    250ADXL321 ±18   57 mV/g     320ADXL322 ±2g   420 mV/g    220

Operon (author)  endolith7 years ago
In principle, none of these accelerometers are appropriate for the application, unless you are willing to place the sensor perpendicular to the board which would make the whole thing bulkier. The direction of measurement for the ADXLXXX sensors is along their long sides rather than the short one as it is the case with the Knowles accelerometer. If you surface mount these flush to the board, they will give you the movement parallel to the skin rather than perpendicular to it. You may still want to try it, since it may be easier to at least detect "noise" this way, but I doubt you will be able to filter it out.
endolith Operon7 years ago
Yes, but if they were mounted perpendicular to the microphone, in the correct orientation, which would be the most correct for this application? I'm not an expert on accelerometers and their specs, in other words.

Just guess at the maximum acceleration that would ever be seen on the surface of a wildly moving arm and get one that can measure that much? Go for the one with the greatest sensitivity? I don't know what's important for this.
Operon (author)  endolith7 years ago
I don't imagine an arm could normally generate much more than 1g of acceleration. I would go for sensitivity and noise. The ADXL322 seems to be best.

I just found another page with a bunch of acc that may be useful too. You could try to get the price on those: http://www.meas-spec.com/advizia/?v41=Vibration
endolith Operon7 years ago
I would go for sensitivity and noise. The ADXL322 seems to be best.

Excellent. Thanks.

There's also the ADXL330 for an application that requires the chip to be mounted parallel to a PCB (though the Analog chips are only 4 mm wide, so the height in perpendicular orientation isn't a big deal):

Part#   Range Sensitivity Noise Density (ug/rtHz)ADXL320 ±5g   174 mV/g    250ADXL321 ±18   57 mV/g     320ADXL322 ±2g   420 mV/g    220ADXL330 ±3.6g 300 mV/g    280

It has three axes (with Z perpendicular to the package surface) but is less than a third of the price of the Knowles (one for $9.99 Newark or $11.58 Digi-Key), so the extra axes aren't costing anything. I think I'll get some of these and see what I can do with them.

Might also find it helpful to combine vibration information from the other axes anyway? I'm not sure if there's a direct relationship between the vibration signal and the microphone signal, or if you're just using the time-averaged amplitude of vibration to ignore microphone signals during that period. I guess I should read through the thesis more carefully. :-)

I just found another page with a bunch of acc that may be useful too.

Ok. They look kind of big. Here are some other companies that make ones that can be bought through Digi-Key:

Analog Devices
VTI
Freescale
ST
Operon (author)  endolith7 years ago
I should point out that the signal-to-noise ratio of the ADXL sensors is worst than the Knowles sensors... we tested and abandoned them because of this. If you actually fill the form in the page I posted, you will get the smaller sensors, including some that are surface mount. Try 0-10g, single axes, amplified.
endolith Operon7 years ago
And "cheaper" is an understatement, since the minimum order for the Knowles accelerometers is $19,925. :-)
endolith7 years ago
Do you really need a circuit board to attach three SMT components? Why not just solder them directly to the component leads? It would be cheaper and smaller. If the board is needed for some kind of mechanical structure or acoustic baffling or something, why not just a piece of standard prototyping board? That would be cheaper and easier than creating a custom board for such a simple circuit.
vector-board.jpg
Operon (author)  endolith7 years ago
Well, I wanted to have a bit more control over the final size of the sensor (including thickness of the board), but you are right, the printed board is not essential and there may be better/easier ways to do it. If you come up with something better you can include it here... I have added you as a collaborator.
These "surfboards" (not a very useful term for search engines) are like perfboards but for surface mount components. I'm sure there are even more appropriate prototyping boards for surface mount components, but I can't find them right now.
brdsurf.gif
endolith7 years ago
It would be good if you could list typical prices here, and acceptable part substitutes.
jfk7 years ago
Operon - Thanks for putting this together. We've been thinking for a while that MMG might be a low cost alternative for UE prosthetics. We'll have to add building some of these and trying them out to our list of stuff to do. Jon Kuniholm Shared Design Alliance/Open Prosthetics Project
Operon (author)  jfk7 years ago
Not a problem at all... let me know if there is anything else I can do. I will be posting the second (and trickiest) part soon.
sumguysr7 years ago
mind explaining just what the accelorometer does?
Operon (author)  sumguysr7 years ago
Well, besides measuring acceleration, this particular arrangement makes the acc more sensitive to movement than to muscle sounds... movement induces lots of noise in the microphone, so you would use the acc to measure this noise and filter it out from the mic signal (more detail on the attached paper). The fact is, the acc is not absolutely essential, but it would help you a lot if you were to try to use the sensor in something like a hand prosthesis.
Measures acceleration.