why does the engine of motor verhicles emit a buzzing sound instead of continuous small booms?



why does the engine of motor verhicles emit a buzzing sound instead of continuous small booms?

In: Engineering

Well, it does. Sort of. At low RPM, you can hear individual pops. [Here](https://youtu.be/HsccG5Nh3kw) is a video I found from a quick YouTube search of a super low-RPM engine. However, especially in cars and other vehicles, you will likely also hear various other moving parts. Belts, transmissions, etc.

At higher RPM, your explosions blend together into a buzz. The sound that we recognize as a buzz is really just a buh-buh-buh-buh sound but really fast.

It’s because most car motors are so called 4 stroke motors. Only one of these strokes contain the explosion the ignites the fuel. The other three strikes just prepare the cylinder for this. As a result, what you really have is a rapid series small booms. HOWEVER, since most motors have 4 or more cylinders, in any given point in the cycle, you have a small explosion. And when you consider how fast a car motor spins, normally between 1000 and 5000 revolutions per minute, all those many small booms come together to form the buzzing sound. Think of a bee. You don’t hear individual flaps of the wings, but the constant fast beating of the wings makes a buzzing sound. This is also how speakers and most sound works.

Most passenger automobiles idle at between 600 and 1000 rpm. That is revolutions of the crank shaft so in a 4 cylinder engine each cylinder will have a detonation within it for every revolution. This means 2400 to 4000 explosions a minute, or 40-66 per second.

That speed of little booms tends to blend together into a buzzing or hum.

It would be hilarious though if we invented an ultra low rev motor with say 20 explosions per minute that would go like

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.