why does the engine of motor verhicles emit a buzzing sound instead of continuous small booms?
In: Engineering
It’s a mix of three things;
1) The speed of the engine.
2) The number of cylinders.
3) How your ear actually interprets sound.
Most of the time, what you “hear” from the engine is actually the expansion of hot exhaust gas when the exhaust valves open at the end of the cycle, which produces a pulse of expanding gas (and thus sound) once every other revolution of the engine (on a modern 4-stroke engine) per cylinder. Thus, if you have a 4-cylinder 4-stroke engine operating at ~3600 RPM (or 60 revolutions per second, or 30 full engine cycles per second per cylinder), you’ll get a pulse of gas (and thus a sound) 120 times per second.
Now the neat part is how your ear interprets sound; remember sounds are waves, with individual peaks and troughs of that wave hitting your ear in quick succession. But that also affects your ability to hear distinct sounds in rapid succession; if a given noise is happening often enough within a short enough time frame, your ear stops hearing it as distinct notes and *starts* hearing it as a continuous tone corresponding to how rapidly the tones are being produced. Where this transition occurs varies from person to person, but *generally* seems to fall between 15 and 30 Hz for most humans. Thus, on a multi-cylindered engine, you typically will always be above that lower limit unless the engine is idling really slowly.
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