Comparison of Frequency Response
This page presents a comparison of the frequency spectra of two
conventional speakers and the flat speaker. The conventional speakers
used were the Calrad 20-224A and 20-175A. In the plots below, the 20-224A
is referred to as the "28mm Round Conventional Speaker" and the
larger 20-175A is referred to as the "40mm Round Conventional Speaker."
Figure 8 shows the results of separate experiments where each speaker was
placed in roughly the same position and a random code from the Whisper system
was played for several seconds. The volume was adjusted for each speaker to
be comparable across the experiments. The 20-224A results appear in the top
plot, the 20-175A is in the middle, and the flat speaker is on the bottom.
The horizontal axis is scaled by 104, so a
value of 2 (for example) represents 20000 Hz or 20 kHz.
Figure 8: Power Spectral Density (PSD) comparison.
Note the familiar "terrible null" at about 5 kHz for the flat
speaker. Note also the spike that appears in all three plots at about
38 kHz. This resulted from the Whisper system picking up equipment in the
lab (particularly monitors). When all extraneous equipment was turned off,
the spike disappeared (see Figure 9 below). As to why the system was picking
this up, there was a test wire coming from the breakout box that may have
been acting as an antenna, though this was never verified.
Figure 9: Power Spectral Density (PSD) comparison (without spike).
Informally comparing the spectra, the smaller conventional speaker
(top plot) appears to be the winner. Its response is (fairly) flat from
about 3 kHz to 18 kHz. The larger conventional speaker (middle plot) has
a wider response, as expected. However, its spectrum
seems to be less flat (e.g. the drop off at about 12 kHz).
At this point, the smaller conventional speaker was chosen, and
the experiments comparing the HiBall tracker with Whisper were repeated.
More details on these experiments can be found here.
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