How the numbers are made

Measuring

The phone sits on the centre armrest and records the cabin's A-weighted level and its 1/3-octave spectrum every second, with GPS speed. Only steady driving counts, and every second is set to the centre of its 10 km/h speed band with the usual speed law (about 9 dB per doubling of speed).

Cleaning

Seconds with speech, music, blinkers, door slams or a handled phone are cut before anything is averaged. A drive with more than a quarter of its seconds cut isn't counted.

Phones and microphones

Each shared drive carries the exact calibration it was recorded with, so it can be corrected later when a better calibration for that phone arrives. Drives fall into tiers:

• Reference microphone: a measurement microphone (for example a UMIK-1) with its calibration file.
• Measured phone: a phone measured against a reference microphone or meter.
• Verified default: a phone model whose sensitivity was measured.
• Assumed default: kept, but not counted until enough cars link it to measured phones.

Pooling

A car is one unit: its drives are pooled first, then cars are pooled into a model with a random-effects model, robust to odd drives. Intervals are 95%. A model page appears from one car (marked Early); rankings need three cars, and class bands five.

Privacy

Shared drives contain no location, no timestamps and no audio. Before sending, the phone keeps only steady driving, drops a random few minutes at each end, cuts the rest into 30–60 second pieces and shuffles them, so a route can't be rebuilt from speed. This site only ever shows numbers pooled across cars.

Limits

Roads, tyres, phones and placement differ between drivers, so small differences (under about 1–2 dB) aren't meaningful. Wet roads are kept apart.