Oct-14
Performances from Student Etudes (HW2)
Stations for Wave study
1) Impulse response, speed of sound, reflections for echo-length delays
Determine speed of sound in feet/sec using 1 mic, 1 table, tape measure.
Set up a second table behind and reaching above the first, measure the distance between the tables by reflection time differences.
Create an “infinite” echo train by positioning the tables differently.
What is an impulse response, and what determines if it is finite (FIR) or infinite (IIR)?
2) Air column reflections for pitch-length delays, pitch changes with length
Find a way to record the impulse response of a tube, and consider that the highest quality impulse is as short and loud as practical
Measure the timing between reflections and predict the pitch. Confirm with an FFT, and confirm again with calculation from the speed of sound and tube length.
Is the system FIR or IIR? And reflectors – any?
3) Stretched-string reflections for pitch-length delays, pitch changes with string tension
Using a string, a doughnut magnet, a bass guitar pickup, and Snd see if the speed of sound changes with string tension.
Frequency-wise, how low can it go? “Pitch” is the perception of oscillation frequency – so “pitch-wise,” how low can you go?
4) Visualization of wave motion in 1D (string) and 2D (checkered tablecloth)
Using a digital camera, a long rope, and a strobe light, try to capture a shot of string motion at the beginning of a sharp impulse.
As the tone decays, how does the motion change?
Try to get similar shots of motion in two dimensions using a checkered tablecloth.
Back to the string – is it oscillating in more than one dimension? How many dimensions are possible?
5) Real-time spectrum analysis of theremin and digital oscillator (graphical program)
Back out of the mechanical world to the electronic world. Setup a real-time analyzer (freqTweak) and send it signals from the theremin and from a real-time digital oscillator (Pd). These two applications are great for playing around in real time and will be used in the next homework projects.
How many harmonics in the theremin sound?
Try holding the theremin as still as the digital oscillator.
jack is an application which provides audio for running multiple, simultaneous audio applications. Until now, we've only used audio one app-at-a-time.
starting jack: [I run this command in its own terminal and without the backgrounding `&' just in case I want to kill it to run audacity or Snd, etc.]
jackstart -R -d alsa -d hw -p 1024 -r 48000
stopping jack: [in case jack gives you any grief, you can directly kill the “daemon”]
killall jackd
The first of these runs only on top of jack, the other provides that as an option.
FreqTweak
FreqTweak is a real-time spectrum analysis / resynthesis enging. FreqTweak transforms incoming audio signals into their constituent frequencies, allows certain modifications and then transforms the signals back into the time domain for listening.
Modifications are filtering (graphical equalization), pitch shifting, gating (threshold), delay, and feedback (output mixed back to the input).
starting freqtweak: [after starting jack]
freqtweak &
Pd
Pd stands for Pure Data (or possibly, public domain) and provides a graphical programming language useful for building real-time music “patches.” An appendix in the Pierce text describes its predecessor and colleague, Max.
starting Pd: [if jack is running it will automatically choose to run on top of it]
pd &
For kicks and to prove to yourself why jack makes a difference, you can start both programs and interconnect their audio. For example, start jack, pd and freqtweak. Open pd's
Help: Pure Documentation: 07.frequency.mod.pd and get it started playing a 1kHz sine tone. Then, in freqtweak choose input 1 to be pure data output 0. Have fun carving FM sounds with Pd as you see and hear them.