The Scary Part I Filtered Out
Here’s an image of the frequencies of the Echo idling, unfiltered. I’ve circled and labeled the area that I filtered out to make it less scary.
This is interesting and heartening for Fitz. All of those peaks are overtones of the 45.5 Hz fundamental that the motor generates. At first it is surprising that the fundamental frequency of a sedan would be such a low frequency (probably subaudible for a dog, thought we can hear it just fine). But the fundamental relates to the speed of rotation of the engine, so the fundamental for a truck wouldn’t necessarily be lower than that of a car. Trucks can rumble for other reasons, though.
That 45.5 Hz fundamental shows low amplitude in part because of the limitation of my speaker, but if I do an FFT of the digital file directly, it does come out low compared to the others. In any case, it **should** be below Fitz’ hearing range.
Images That Show Lower Frequencies Being Added Back In
I had speculated that I may have done too many splits in the first part. I’m providing these images to help you decide whether to skip any. The four peaks that range from 91 Hz to 228 Hz combine to make the scary engine noise. Looking over these seven different filters (corresponding to Sounds #1-7 on the main page), you can see that each peak gains about 10 dB in amplitude as the filter is modified. You can also see from the blue number in the upper right-hand side that the aggregate amplitude is gaining about 1 dB each time.
I leave that to you to decide whether you want to skip any of the early filters or do fewer exposures. Things definitely move faster as we get to lower cutoff frequencies. You may need more splits in that area, which I can provide.