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Flat Speaker vs. Conventional Speaker  

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The prototype Whisper system had been using a unique flat speaker that offered a small form factor, a lightweight design, and a broad spectrum (for its size). However, during experiments comparing the HiBall tracker with Whisper, Nick Vallidis discovered that the speaker was causing problems. This prompted the change to a more conventional speaker.

The Flat Speaker

Figure 1 below provides information about the flat speaker. Note the language about "four small speakers for high-frequency sounds." Looking at the diagram of Figure 2, the four separate areas of the speaker (one in each corner) can be seen. It is believed that these four "separate" speakers are actually seen as physically different emitters by the Whisper system. Thus, for higher frequencies, four different points in space are "battling" to represent a single point. (Note that future work could investigate using this feature to our advantage. But for now, it's a problem).

flat speaker info

Figure 1: Information about flat speaker from Digi-Key catalog.


flat speaker diagram

Figure 2: Diagram of flat speaker.

Figure 3 below shows a manifestation of the problem. This plot resulted from rotating the flat speaker slowly away from the microphone. The vertical axis represents chips (i.e. range). The horizontal axis can roughly be thought of as milliseconds. To exaggerate the problem, the speaker was held such that two opposite corners were perpendicular with the axis of rotation. Thus, as the speaker is rotated, one of the "mini-emitters" gets closer to the microphone as a second one gets farther away. The remaining two emitters are roughly centered on the rotational axis. Note that the speaker was handheld and hence the range drifts slightly as the speaker is rotated.

flat speaker range plot with error

Figure 3: Flat speaker range plot with errors. Left of plot shows speaker orientation.

The shaded area highlights the problem. As the difference between the near and far emitters increases, the range fluctuation grows. It appears that the near emitter (which would presumably be louder wrt the microphone) eventually "wins," causing the sharp decrease of over one chip slightly before the 6000-millisecond mark. Figure 4 below shows a sample correlation result for the experiment. Note the two peaks. This is undesirable to say the least.

flat speaker correlation with two peaks

Figure 4: Correlation with two peaks.

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Last updated on:
Monday, June 24, 2002