Waveform Measurements
Abstract
Sound waves produced by the guitar were recorded and analyzed via computer to give a benchmark
for what the final output of the pickup should be.
Measurement
An impromptu sound room was established in the Senior Design Lab by placing two white-boards in
a corner of the room. Care was taken not to make measurements during particularly noisy moments, such as
when a plane was flying overhead. Using the open source program Audacity
(http://audacity.sourceforge.net/) and a desktop microphone, each
note from B2 to E5 (the range of notes playable without placing
fingers below where the fret joins the body of the guitar) was
recorded. An approximately one second period of time passed
between the pressing of the record button and the playing of the
Figure 1: Original Audio File of Middle C (C4)
note. This delay proved to be useful for noise canceling.
Processing
Figure 2: Spectral Content of C4
Figure 3: Edited version of C4
With the original files left preserved, copies of them were
modified using Audacity. Noise removal is a built-in feature of
Audacity. Using the period at the beginning of the file to define the
noise, Audacity automatically removed noise from the waveform.
Amplification was performed, with each note being normalized to
have a peak amplitude of 0dB. This caused noise to rise to
noticeable levels once again, so the noise filtering process was
repeated. In some cases, additional iterations of noise removal were
deemed necessary. Additionally, some files were found to contain a
significant amount of "unused space" at the end, and were shortened
accordingly. A small amount of empty audio was left on either end,
however, as their presence has proven useful in the removal of
noise. These sound files exist both as Audacity project files (.aup)
and standard wave (.wav) files. This allows future analysis of the
signals using Matlab, through the use of the wavread function.
Results
An example of the original sound clip can be seen in the
time domain in Figure 1, and the frequency domain in Figure 2.
Figures 3 and 4 represent the same signal, having been cleaned
through the use of Audacity.
Conclusion
A benchmark, to help understand what fundamental and
harmonic frequencies are present in each note, has been established.
These benchmarks have been captured digitally, and can be
analyzed using Audacities suite of tools, as well as Matlab.
Figure 4: Cleaned Spectral Content of C4