Correct the Instrument Before the Claim

Ada re-checks the co-located offsets, the grid coverage, and the commuter-street concentration before the aggregate is allowed to claim anything

ADA · CALCULATION AUDIT

Correct the Instrument Before the Claim

A crowd-sourced noise map is only as trustworthy as the phones that feed it, and every phone is a different, uncalibrated instrument. Placed beside a reference meter reading 64 dB, family A reads 65 dB and family B reads 68 dB — offsets of +1 and +4 dB — so a later B reading of 72 dB should be corrected to 68 dB before it joins the aggregate. This audit works that co-located calibration step to ask why you must correct the instrument before trusting the claim.

Companion to the chapter Participatory Sensing — every number here comes from that chapter.

Check the offsets, the coverage, and the concentration

• Family A offset: 65 dB - 64 dB = +1 dB, so A is about 1 dB high in this check.
• Family B offset: 68 dB - 64 dB = +4 dB, so B is about 4 dB high in this check.
• Later B adjusted value: 72 dB - 4 dB = 68 dB. The raw 72 dB remains stored, but the aggregate uses the corrected 68 dB value.
• Coverage touched: 4 cells / 12 cells = 0.333..., about 33%. Repeated windows: 2 cells / 12 cells = 0.166..., about 17%.
• Commuter-street concentration: 150 records / 180 records = 0.833..., about 83% of the accepted records from two streets in one hour.

Those checks explain the physics and the statistics at the same time: decibel readings need instrument correction before comparison, and a large sample count still cannot support a citywide claim when coverage is only about one third of the grid and repeated evidence covers about one sixth.

Every number above is taken from the chapter’s own worked example and re-derived step by step.