City night sounds

Machines are awake and aware - while the humans are sleeping. This sounds like science fiction but it is easy to realize. Just switch on a sound recorder in your hotel room and open the window. The recorded sounds are mundane and predictable: car traffic - airplane traffic - birds - hotel guests walking - you snoring - people talking on the sidewalk - streetcars - ambulances. But the overall statistics give a unique profile of each place.

1

The first time I did this in Rotterdam - in a quiet suburb. The Rotterdam sound map says the level is less than 50dB at night. The steady hum of the highway can be heard all the time. No individual sounds are visible in the graph, everything is smoothed by distance from the sound source.

2

The second time was in London – in a quiet corner of the city. My room was on the ground floor and anyone could have walked in from Regent's Park while I slept. I could not open the windows. But you can see the cars driving by in the recording.

3

The third time was in Berlin, next to a very busy street, the Prenzlauer Alee. Maybe 65dB. The sound level is changing periodically. Traffic lights form clusters of cars that stay close together. This causes high peaks in the sound level.

Could we resolve one of these peaks into the different cars that cause the peak? I didn't succeed. Even spectrum analysis is unable to separate the cars. Their sound looks too much like white noise.

Some things are constant in every location:

• Cars dominate the urban soundscape. Interesting sound drowns in boring noise.
• Night is much quieter than day.
• The quietest part of the night is around 3:00 AM. The best time for sound recordings.

Marcel Minnaert has not written a specific chapter about night sounds. But he mentions them often while writing about other phenomena:

• At night sounds can travel much further because of temperature inversion in the lower atmosphere. This is an omnidirectional effect. Sounds can also travel much further downwind, but this is a unidirectional effect. So it is not only the greater silence at night that makes some sounds more audible. 
• In nights with copious dew (= inversion) one can hear the sea-waves at distances of 13 km inland.
• Measuring the speed of sound using a thunderstorm works better at night, because the lightning is more visible and the silence makes observation of the thunder easier. In a silent place outside the city lightning and the corresponding thunder can be studied up to distances of 100 km (30-40 km is easy to do).
• Echoes are observed best at night. In a silent night the echo produced by a row of beech trees could be heard up to a distance of 500 meters. A special kind of "whistling" echo is produced by corrugated iron walls or roofs. This is also best observed at night.
• At night one can hear the difference in the sound of one's footsteps while walking along walls or bushes. And one can hear the rustling sound produced by the blood flow in one's own ears.

And ... Marcel Minnaert did not find car sounds boring. He wrote a whole chapter about listening to machines and analysing which "part" of the sound is produced by which "part" of the machine. More about that in a future blog entry.

Sources:

Rotterdam sound maps

De natuurkunde van 't vrije veld. Deel II, Geluid, warmte, elektriciteit (Part 2: Sound, temperature, electricity), Marcel Minnaert