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DIY Microprocessor Sound Level Meter Demoed At MIT
An anonymous reader writes "A Piezoelectric Sound Level Meter was demoed at MIT's Battle of the Bands last month, borrowing its display from the do-it-yourself USB LED marquee that was the subject of a previous Slashdot story. This video tutorial describes in detail both the analog electronics plus the C code that runs the system. If this is your first experience at the intersection of digital and analog systems, don't be scared!"
ok so pretty cool, but can someone explain how hooking a mic up to an ADC is worthy of a mention for MIT? It sounds more like a high school project at face value, what am I missing?
It IS a high school project at face value. The kit can be yours for only $80!
Shouldn't ads like this be paid for?
Sound level meter? Did i miss something particularly difficult or innovative about this thing?
Whats the point? Selling MC Kits?
Just goes to show ya that MIT guys will crack a nut using a bulldozer. There's plenty of dedicated level-meter chips around which cost next to nothing and provide a better, logarithmic response, which is what you want for sound.
The LM3915 is an oldie but a goodie, you can even daisy-chain them.
See http://www.national.com/mpf/LM/LM3915.html
LM3914 can handle 10 LED's per chip and can be cascaded for more. Add an amp for signal conditioning on the front end, and then hookup lots of LED's per line if you use a transistor to drive a bank of LED's.
They need a micro because their display is too complex for the job. But, make the display simple and you can make the whole design simple. Yet, since this is MIT's it has to be complex for some reason.
This project is an excellent example of how having a little theoretical knowledge is a bad thing.
They have just enough knowledge to get into complicated and pointless gain calculations, but they miss most of the really important things. Here's a few:
(1) A piezo buzzer is not designed for any kind of flat frequency response. Which is a basic requirement for a sound-level meter. Major fail from the get-go.
(2) We're going on 60 years of having a spec for sound meter weighing curves and envelope filtering characteristics. Yet no mention of that in the article. A randomly designed meter is useless.
(3) They go on and on about calculating the gain of the amplifier stage, and they do it incorrectly. We care not one whit about the DC gain. The AC gain is dependent on the AC impedance of the source and load. Even the DC gain they calculate is useless as those transistors have a huge range of gains. And no analysis of the DC stability, which is harder to get right. Gain just happens, stability has to be designed in.
(4) Biasing the base from a pot in that fashion is never done in practice. A better design would use two resistors and avoid the cost and impedance variations of the one pot "design".
(5) A real design would have the +5 volt line decoupled and filtered to keep microprocessor switching noise out.
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In summary these designers should wait until they get past the first chapter of their transistor class before going out and trying to design anything. Good design requires more than slavish focusing on one small area. An engineer has to have a broad view.
Many of the cheap micro-controllers have ADC's that won't do the job well, at least not well enough to get any kind of dynamic range out of the circuit.
A bigger problem with the MIT design, is that it uses a Piezo-Buzzer for a microphone. This will give a wickedly non-linear frequency response curve. Piezo-Buzzers are designed to have a narrow range of frequencies in which they operate effectively.
The MIT design also uses a single transistor amplifier circuit. It wouldn't surprise me if the harmonics on the output are poor. Specifically, with this circuit, the average sound level can be determined by simply averaging the output of the transistor amplifier. Essentially, the average voltage on both the collector and emitter of the transistor should change if an AC signal is applied to the base. If this average is read with a DC voltmeter, then it should give an approximation of the sound-level, subject to the microphones frequency response curve.
I am not clear why anyone would build a sound level meter without using either a proper microphone or an effective amplifier circuit. A quad op-amp IC, and a few circuits from the web, should give you the average sound level over an extended frequency and amplitude range. It is even possible to do RMS to DC, peak-level to DC, and log-linear conversions in analog. For a retro-look, an old-fashioned voltmeter or amp-meter can be used for a display. For a more modern look, it is possible to use a cheap micro-controller with a slow ADC (or an LM3914) for the analog to digital conversion. Historically, this was the way it was done in many stereos, and the same circuit is probably still in use in many professional recording labs.
The advantage of implementing a proper micro-phone is the much flatter frequency response curve. The advantage of the log-linear conversion, is that most sound meters read in dB, which is a logarithmic scale. It takes a very good linear ADC to implement the same conversion digitally. A 5-bit (32 count) ADC reading a log input has more dynamic range than a 24-bit ADC reading a linear input (2^32 >> 2^24). Although in practice, I wouldn't recommend using less than an 8-bit ADC on an analog circuit.