I know how engines work, what makes them work etc. but this question has been on my mind for a while. How can a bunch of controlled explosions create the sound characteristic for a specific engine type? For example, a V6 engine sounds distinctly different than a V8 or I4, but how do they get their sound & why can’t the sound be different, for example an I4 engine with the sound of a V12?
In: Engineering
1) If you think of a car engine and car exhaust as being akin to an instrument, then you can see that the tone, timbre, and harmonics of a car’s engine noise will change depending on the precise geometry and materials of the exhaust side of the engine.
2) There’s actually an interesting thing going on with respect to human perception which is fundamentally why you hear a continual “buzzing” tone from your car’s exhaust despite the fact that combustion is actually a large number of discretized combustion events.
In music theory, there’s a thing called the “inter-onset interval,” which is basically the time between discrete notes, measured in milliseconds. The fastest reliable inter-onset interval is generally held to be around 100 ms, and something weird happens as you push beyond it to 50 ms; instead of hearing the discretized notes, you begin to perceive the notes as singular constant tone. This is because and inter-onset interval of 50 ms corresponds with a signal at 20 Hz, and *20 Hz is the lower limit of human hearing*. 20 Hz also corresponds with an engine operating at 1200 RPM, which is basically a high-idle engine speed. As a result, despite the fact that you’re hearing individual combustion and exhaust events coming down the exhaust pipes, because these individual sounds are coming at you at such a high rate of speed and high frequency, your brain interprets them the same way it would a continual soundwave. This is also why engine sounds change pitch linearly with RPM; RPM converts linearly to exhaust event frequency, which corresponds directly to a “tone” coming from your cars exhaust.
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