Tuesday, December 2, 2014

Exercise 8


Please read chapter 7 from Miller's book.

Problem Set:

  1. A sinusoidal plane wave at 40 Hz. has an SPL of 80 decibels. 
    1. How far does the air move? (I.e., what is its RMS displacement?)
    2. What is the change in SPL (in dB) if the wave maintains this RMS displacement, but the frequency changes to 80 Hz?
  2. A rectangular vibrating surface one foot long is vibrating at 1000 Hz. Assuming the speed of sound is 1000 feet per second, at what angle off axis should the beam's amplitude drop to zero?
  3. How many dB less does a cardioid microphone pick up from an incoming sound 90 degrees (pi/2 radians) off-axis, compared to a signal coming in frontally (at the angle of highest gain)?
  4. Suppose a sound's SPL is 0 dB. What is the total power that you ear receives? (Assume that the opening is 1 square centimeter).
Home Lab:

Work on your final projects! Remember, they're due next week!

Tuesday, November 18, 2014

Exercise 7


Please read chapter 6 from Miller's book.


For these exercises, assume the speed of sound is 343 m/s, and the air density is 1.225 kg / m^3 .

1) A car is moving toward you at 83.71 miles per hour. Its horn is blowing at 440 Hz. By how many half-tones does the sound rise above 440 Hz because of the car's motion?

2) Suppose the same car has the same horn blowing, but now you hear a pitch of 405.64 Hz. Assuming the car is moving directly away from you, how fast is it moving (in miles per hour)?

3) A sound has a wavelength of 5.7167 meters. What is its frequency?

4) If two sounds are a perfect fifth apart, what is the ratio of their wavelengths?

5) Suppose that 2 furlongs away from a loudspeaker the SPL is 60 dB. The speaker is the only object making sound nearby, and is a point source. What is the SPL 4 furlongs away from the loudspeaker? 1 furlong = 201.168 meters.

6) A 171.50 Hz. sinusoid is traveling in a plane wave in the +x direction. Two microphones are placed at x = 0 and x = 1 meter, respectively. What is the phase difference, in radians, between the signals picked up by the two microphones? 

Home Lab: 
This project shows how to measure the frequency response of a filter, whether it's a designed one (as in this case) or it's something that acts as an unintentional filter (such as a loudspeaker that doesn't have a flat frequency response--and, in fact, none of them do.)
The filter we'll measure is the bandpass filter supplied in the music 170 library (called ``bandpass"). It's a classical filter design that appears often in digital audio applications.
To measure it, make a ``sinusoid" object and pass it through a ``bandpass" object. Make two ``meter" objects, and connect one to the output of the oscillator (so that you see what you're inputting to the filter) and one to the filter output so you can see how the two levels differ.
If you want to save time later, you can slightly complicate the patch by inserting a multiplier between the oscillator and its two connections (with a constant to multiply it by) so that you can adjust the oscillator's output to a round number in dB; but this isn't necessary to finish the project.
We're interested in two settings of the filter: the center frequency should be 1000 Hz, and the value of ``Q" set to 10 and to 20. For each of these two filter settings do the following:
Set the oscillator's frequency to a series of values separated from 1000 by half octaves:
31, 44, 62, 88, 125, 176, 250, 353, 500, 707, 1000, 1414, 2000, 2828, 4000, 5656, 8000
With these numbers evenly spaced on the horizontal axis (a logarithmic scale), plot on the vertical axis the gain in decibels (the output level of the filter minus the input level). These numbers will all be negative. (Suggestion: find all the 34 values first--each filter's gain at each of the 17 frequencies shown--so that you will know what the bounds of the graph should be.) Draw two traces, one for each of the two filters. Enjoy the fact that at high frequencies you get two nearly parallel lines. How many decibels per octave do the filters' frequency responses drop off by at frequencies above about 2000?

Monday, November 17, 2014

Acoustics Animations by Dan Russell

Here are some wonderfully demonstrative animated gifs by Dan Russell, acoustics professor at Penn State:


I find these to be helpful when I'm trying to picture simple wave interactions. Hours of wavy entertainment!

For example, here's what breaking the sound barrier looks like:

Thursday, November 13, 2014

Exercise 6: Final Project Proposals

Due Tuesday, 11/18.

Please write a description of your proposed final project. Each team will submit a single document. The target length is 1 page, double spaced, 12 point times font, normal margins (no funny stuff).

Things to consider when writing this document:

  1. Summarize the core goals - What deliverables are you proposing to have completed by the deadline? 
  2. Explain your reasons - Why is this an interesting and relevant final project for the class? Why is it complex enough to be worth 1/3 of each of your grades?
  3. Explain your team's methods - How will you complete the project? Why is it a realistic undertaking given the timeline, budgetary considerations, and skill-set of your group? 
  4. Bonus - What specific roles will each team member perform for the completion of the project? 
  5. Bonus - What is your team's project timeline?
  6. Bonus - What is your team's budget? Where will you find materials?
The questions marked "Bonus" are largely there to get you thinking. Furthermore, answering all of these questions in a satisfying way will increase the likelihood of your project being accepted (and possibly your grade on the assignment).

We will give feedback on the project proposals. If your team's idea is not accepted or requires revisions, we will let you know. This will be handled on a case-by-case basis.

You and your team have probably been thinking about the final project a good deal by now. However, if you need to see the prompt again, please refer to the syllabus, linked here.

Sunday, November 9, 2014

DIY Resources

Here are some resources, both online and local, that might be helpful as you plan your projects.  There are plenty of avenues to explore here, so we caution you to be realistic in evaluating what you’ll have the time and ability to pull off.  Happy adventuring!

Electronics hobbyist stores:


Electronics parts suppliers:



Local Hardware/Surplus/etc.
www.marshallshardware.com (in Miramar)
www.murphyjunk.net (El Cajon)

Embedded computing platforms:


Beginning synth DIY:


Guitar FX DIY:


Random DIY:


Learn to solder in 1980:

Wednesday, November 5, 2014

Yaybahar by Görkem Şen

Someone brought this gentleman's novel musical instrument to my attention today. I thought I'd share it with you folks:


He's using membranes to amplify the sound of the springs, rather than a pickup. The whole thing is acoustic. It sounds lovely!

Monday, November 3, 2014

Exercise 5

1) A stretched violin string has 50 N tension and 1x10^(-2) kg / m linear density. What is the propagation speed of a transverse wave traveling on this string?
2) Violin strings are typically 60 cm long.
a) What is the frequency in hertz of the first mode on the string?
b) How about the third?
3) Another string on the violin has the same length and tension, but its linear density is instead (4.44x10^(-3) kg / m).
a) What is the propagation speed of a transverse wave on this string?
b) When played together, what interval (in semitones) will separate the two strings?
4) What is the frequency of the second mode on the string from question 3?