Trafica and Nightwatch

Engines, Wednesday 23 September 2026

Recorded on a 360 camera in Meaford with a microphone on the hat. At 15:12 the question was put plainly: determine what engine it is, give a top three with a confidence on each, and let Nightwatch supply the hum and the levels. At 15:35 the two were asked to share one page. This is that page.

Nightwatch's half: the levels

Clip 019, from 15:28:40, 216 seconds. Nightwatch measured the level and the low end every half second and found four smooth, bass heavy swells. It measures sounds and does not name what made them, so it left them as engine or wind.

0:000:301:001:302:002:303:003:30
LevelHum, the low end under 200 HzNightwatch's four smooth low swellsEngine lines heard, 0:46 to 1:25Car in frame, driving awayDashed: street ambient, -27.2 dB
First swell, 0:05 to 0:16

No engine lines anywhere in it. Wind or handling on the hat microphone.

Swells two to four, 1:02 to 1:36

A car's engine is audible from 0:46 to 1:25 and it is in frame, driving away, at 1:18. The last two swells also cover the walker running from the trail to the road, so they are engine and wind together.

Trafica's half: which kind of engine

An engine's sound sits on multiples of its crank speed. Which of those lines are strongest depends on how many cylinders fire per turn: a four fires twice per turn, a six three times, a V8 four times. So the spacing of the lines gives the revs, and the pattern of strong and weak lines gives the layout.

The car on the road, clip 019, 15:29:57 to 15:30:04

Spectrogram of clip 019 from 1:10 to 1:30, engine lines slowing, then rising as the car pulls away60 Hz120 Hz180 Hz240 Hz300 Hz1:101:121:141:161:181:201:221:241:261:281:30
Seconds into the clip, which started at 15:28:40. Brighter is louder. A steady horizontal line is a tone; an engine makes a stack of them that rise and fall together. The lines at exactly 60, 120 and 240 Hz run through the whole recording and are electrical hum on the microphone, not traffic.

Slowing from 1:12, then pulling away from 1:17 to 1:23. At 1:20 the lines sit on every multiple of 35.3 Hz: 70.6, 106.7, 141.6, 177.0, 212.2, 247.8, 282.7, 318.4 and 352.8 Hz. The even ones stand 8 to 20 dB above the odd ones beside them. That puts the crank at about 2,100 rpm, rising to 2,300.

  1. 1Four cylinder80%

    A four fires twice per turn, so its lines at two, four, six, eight and ten times the crank speed dominate. That is the pattern here.

  2. 2Three cylinder10%

    There are a few in-between lines a three makes, but its main line, near 53 Hz, is missing.

  3. 3V85%

    A V8 would make the 141 and 283 Hz lines stand out from the rest. Here 70, 212 and 353 Hz are just as strong.

Accelerating through the gears, clip 016, 15:13:13 to 15:13:21

Spectrogram of clip 016 from 0:15 to 0:27, engine lines rising and dropping at two gear changes60 Hz120 Hz180 Hz240 Hz300 Hz360 Hz0:160:180:200:220:240:26
Seconds into the clip, which started at 15:12:56. Brighter is louder. A steady horizontal line is a tone; an engine makes a stack of them that rise and fall together. The lines at exactly 60, 120 and 240 Hz run through the whole recording and are electrical hum on the microphone, not traffic.

Heard from the woods, not seen. The lines climb, drop at a gear change near 0:20, climb again and drop at 0:24.5. The two strongest lines keep a ratio of 1.6 to each other the whole way (73 and 118 Hz at 0:21, 92 and 146 Hz at 0:23), which fixes them as the 2.5th and 4th multiples of a crank at about 1,750 rpm, rising to 2,350 before each change.

  1. 1V860%

    A V8 fires four times per turn, so the 4th multiple should be strong, and it is the stronger of the pair in four of five checks. The 2.5th is the uneven bank beat a V8 makes.

  2. 2Five cylinder25%

    A five fires 2.5 times per turn, which fits the lower line, but then that line should be the stronger one.

  3. 3V105%

    Fits the 1.6 ratio, but would mean changing gear at about 1,200 rpm, which is unlikely while accelerating.

How far to trust it