Microphone sensitivity exercise

I’ve been thinking a bit about improving the microphone lab for the Applied Circuits course.  Last year, I had the students measure DC current vs. voltage for an electret microphone and then look at the microphone outputs on the oscilloscope (see Mic modeling lab rethought).  I still want to do those parts, but I’d like to add some more reading of the datasheet, so that students have a better understanding of how they will compute gain later in the quarter.

The idea for the change in this lab occurred to me after discussing the loudness detector that my son wanted for his summer engineering project.  He needed to determine what gain to use to connect a silicon MEMS microphone (SPQ2410HR5H-PD) to an analog input pin of a KL25 chip.  He wanted to use the full 16-bit range of the A-to-D, without much clipping at the highest sound levels. Each bit provides an extra 6.021dB of range, so the 16-bit ADC should have a 96.3dB dynamic range.  The sound levels he is interested in are about 24dB to 120dB, so the gain needs to be set so that a 120dB sound pressure level corresponds to a full-scale signal.

He is running a 3.3v board, so his full-scale is 3.3v peak-to-peak, or 1.17v RMS (for a sine wave).  That conversion relies on understanding the difference between RMS voltage and amplitude of a sine wave, and between amplitude and peak-to-peak voltage. The full-scale voltage is 20 log10(1.17), or about 1.3dB(V).

Microphone sensitivity is usually specified in dB (V/Pa), which is 20 log10 (RMS voltage) with a 1 pascal RMS pressure wave (usually at 1kHz).  The microphone he plans to use is specified at –42±3dB (V/Pa), which is fairly typical of both silicon MEMS and electret microphones.The conversion between sound pressure levels and pascals is fairly simple: at 1kHz a 1Pa RMS pressure wave is a sound pressure level of about 94dB.

Scaling amplitude is equivalent to adding in the logarithmic scale of decibels, so for a sound pressure level of 120dB, the microphone output would be about 120–94–42±3=–16±3dB(V), but we want 1.3dB, so we need a gain of about 17.3dB, which would be about 7.4×. Using 10× (20dB gain) would limit his top sound pressure level to 117dB, and using 5× would allow him to go to 123dB.

One can do similar analysis to figure out how big a signal to expect at ordinary conversational sound pressure levels (around 60dB):  60–94–42=–76db(V).  That corresponds to about a 160µV RMS or 450µV peak-to-peak signal.

I tried checking this with my electret mic, which is spec’ed at –44±2dB, so I should expect 60–94–44±2=–78±2dB, or 125µV RMS and 350µV peak-to-peak. Note that the spec sheet measures the sensitivity with a 2.2kΩ load and 3v power supply, but we can increase the sensitivity by increasing the load resistance.  I’m seeing about a 1mV signal on my scope, so (given that I’m not measuring the loudness of my voice), that seems about right.

I’ll have to have students read about sound pressure level, loudness, and decibels for them to be able to understand how to read the spec sheet, so these calculations should be put between the microphone lab and the first amplifier lab.  I’ll have them measure peak-to-peak amplitude for speech, and we’ll compare it (after the lab) with the spec sheet.  This could be introduced as part of a bigger lesson on reading spec sheets—particularly how reading and understanding specs can save a lot of empirical testing.

 

What online education cannot teach

In response to yesterday’s MOOC Roundup, one of my fellow faculty members sent me a pointer to an article by Jennifer M. Morton in The Chronicle of Higher Education that was just published today: Unequal Classrooms: What Online Education Cannot Teach.  I would have included the article in the roundup, if it had come out a little earlier.  Here is a paragraph from the middle of the article:

MOOCs would seem like a promising way to increase access to education for those who cannot afford the steep price of a liberal-arts education. And indeed, my students often end up sitting in crowded lecture halls being lectured at by a professor who doesn’t even know their names—as is the case for many students across the country. Many of my students also work, some full time, or have families of their own, and they struggle to fulfill the course requirements for graduation. However, the adoption of online education by large public universities threatens to harm the very students for whom a college education is an essential leg up into the middle class.

Prof. Morton makes a point that several other skeptics of MOOCs have made—that a big chunk of college is the interaction in the classroom (between students as well as between faculty and students), and that is mostly missing from MOOCs.  For students from poverty-stricken communities, college may be the only place to learn the “practical skills to navigate middle-class institutions”, and MOOCs deprive them of this learning.  The learning may not be an explicit part of the curriculum of the colleges, but it is implicit in the use of a college degree as a ticket into the middle class.

While I do not regard social mobility as a primary goal of college education, for many people that is the main justification for public universities.  It seems a bit unlikely that MOOC-based education will serve that purpose well, even if it manages to convey curricular content adequately (which has not yet been convincingly shown either).

The Old Reader disappearing

The Old Reader, which I use for handling my RSS feeds, is disappearing.  More precisely, it is going private, shrinking from 420,000 users to 10,000 users.  I’m not one of the 10,000, since I jumped ship from Google Reader to The Old Reader too late.  I’ve now got a week or two to find a new RSS reader and move my subscriptions over.

I’m disappointed, but not surprised.  The Old Reader did not have a business plan for scaling up and being able to afford full-time maintenance, but was a labor of love.  Such projects either remain tiny or run into a wall at some point, when the tedious maintenance work exceeds the pleasure of producing something good.  The Old Reader just hit that point.

It isn’t clear to me how anyone monetizes RSS readers on the cloud, so I doubt that The Old Reader will find a reputable company to take over the public site from them.  It made sense for a company like Google to provide the service as a PR gesture, but they eventually got tired of it, and few other companies have the spare cash for such a big PR gesture (Apple has the money, but no interest in supporting ideas not tied to their hardware).

Free training on KL processors from Freescale

I got email for Freescale today, advertising a free seminar for designing with their KL series processors.  Looking over the contents of the seminar, I think that my son an I have gotten at least half of what they cover already (and possibly a lot more), just from reading the documentation, experimenting with the KL25Z board, and debugging using the gcc toolchain. I’m not about to fly down to LA and stay 2 nights in a hotel room (for a total cost of over $500) for a one-day free seminar, but someone in the LA area might find the seminar useful.

The seminar seems to be mainly about the Freescale software development tools, which are quite pricey.  We don’t plan to usetheir toosl, but either the MBED tools from mbed.org (in my case) or the gcc tools (in my son’s case), both of which are free.

I’ll be doing more with the KL25Z board this week, once the headers I ordered from Digi-Key arrive.  My son will be using it to prototype his engineering project for the summer, and (I hope) to provide another platform for the Arduino Data Logger, with higher resolution in time and sampling frequency.

Here is the message from Freescale:

Register Now for Designing with Freescale Training in Los Angeles »

Experience the advantages of Kinetis L series MCUs built on the ARM® Cortex™-M0+ processor and discover what makes it the “world’s most energy-efficient 32-bit MCU.” This session includes a focus on the Freescale Freedom development platform, Processor Expert software and MQX™ Lite RTOS to give designers a jump start in developing applications for L series devices. Interested? Good, because we’ll cover that and more in our hands-on training seminar in Los Angeles on August 22.

You will experience:

.

Kinetis L series MCUs deep dive

Freescale Freedom development platform components

Processor Expert software and methods of configuring device drivers for a variety of peripherals including ADC, Timers/PWM and I2C

Principals of the MQX Lite operating system

RTOS concepts

.

The session is free of charge and attendees will receive a complimentary Freescale Freedom development platform.

Session prerequisites:

Knowledge of embedded systems

Basic knowledge of C

Basic understanding of IAR Workbench

Seating is limited. Register now »

Integrated engineering education

Mark Guzdial, in The challenges of integrated engineering education, discusses “integrated engineering education”, a curricular approach to getting engineers to learn the prerequisite science and mathematics better:

The idea of integrated engineering education is to get students to see how the mathematics and physics (and other requirements) fit into their goals of becoming engineers. In part, it’s a response to students learning calculus here and physical principles there, but having no idea what role they play when it comes to design and solving real engineering problems. (Computer science hasn’t played a significant role in previous experiments in integrated engineering education, but if one were to do it today, you probably would include CS — that’s why I was invited, as someone interested in CS for other disciplines.) The results of integrated engineering education are positive, including higher retention (a pretty consistent result across all the examples we saw), higher GPAs (often), and better learning (some data).

But these programs rarely last. A program at U. Massachusetts-Dartmouth is one of the longest running (9 years), but it’s gone through extensive revision—not clear it’s the same program. These are hard programs to get set up. It is an even bigger challenge to sustain them.

…

Overall, I wasn’t convinced that integrated engineering education efforts are worth the costs. Are the results that we have merely a Hawthorne effect? It’s hard to sustain integrated anything in American universities (as Cuban told us in “How Scholars Trumped Teachers”).

I can believe that integrated programs are hard to set up and maintain—any interdisciplinary program that relies on courses offered by other departments is hard to maintain. Even if each individual department has a high degrees of curricular stability, the combination of many fields and many departments can be unstable.

The bioinformatics and bioengineering programs at UCSC rely on courses from about 8 different departments.  Essentially every year one or more of the departments makes a “minor” change to their curriculum that affects our students—nearly always adversely.  The biggest problems come from the Chemistry department, as they keep adding more and more courses in the prerequisite chain to biochemistry—to the point now where there is more chemistry in the bioengineering degree than any two other subjects (and chemistry is not the core science of bioengineering).  The only solution we’ve been able to think of is for the School of Engineering to offer their own abbreviated chemistry sequence (one quarter general chem, one quarter O. chem, one quarter biochem), but we have neither the instructional wet lab space nor the teaching resources to do this currently.  Getting resources from the dean seems unlikely—the dean just gave away the only instructional wet lab space to a researcher (despite the courses already scheduled in the space for next year), and we don’t have the faculty to meet even our current teaching load if any one takes sabbatical leave.

Some of the ideas of integrated engineering education are good: getting students to see the point of learning math and physics before they take the courses, rather than 3 years later, certainly improves their focus and desire to learn the material.  It is not clear that “integrated engineering” is the only, or best, way to do this.  Early design and lab courses may be as effective, without needing such tight coordination among many departments.  (I think that this is the approach that Olin College of Engineering uses, though they are a small enough school that one could argue that they are essentially doing integrated engineering no matter how they structure the curriculum, as long as the faculty talk to each other.)  Of course, lab and design courses are the most expensive ones to teach, as you need competent mentors, and both time and space in the labs and workshops.

Engineering education is properly a hands-on activity, not suitable for large lectures and MOOCs, though those are much cheaper to scale up to large numbers.  I think that a lot of the interest in, and difficulty in maintaining, integrated engineering education is the hands-on nature of most integrated engineering courses.  Physics, math, and chemistry departments are not interested in providing intense hands-on courses for engineering students (though they might produce such courses for their own majors)—at least, not if they can get away with minimally staffing a mega-lecture course.