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Discussion Starter #1 (Edited)
Everybody had experienced it - that uncomfortable pulsating WHOM, WHOM, WHOM when you drive on the interstate and despite previous memories you decide to open the back window - just to see..

It does hurt the ears, it's loud and you have to fight your reflex back to close that window right away.


Today I got the UMIK and laptop, got in the truck and did RTA on that phenomena at 60mph with both windows down.

My wife drove, I took measurements.

We kept the speed down so the sound would be between uncomfortable and starting to get painful. No need to rip the headliner while taking measurements. :)

Overall SPL was only 123dB but maybe the higher harmonics - all odd order harmonics - made it sound loud and hurting your ears.

Here is the RTA:
 

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Discussion Starter #2 (Edited)
When you add the harmonics the envelope has almost perfect square waveform.

Then you notice it follows the CEA-2010 subwoofer THD threshold of acceptance pretty close .

The THD is 22%.


Hmmm .. makes you think..
 

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Discussion Starter #3
The more I look at the measurements and chart graph the more I see THAT:

"16 Hz, one-note, square waveform output , 22% THD, still passing CEA-2010" SUBWOOFER


:D
 

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Everybody had experienced it - that uncomfortable pulsating WHOM, WHOM, WHOM ...

Thank you for recording and analyzing that sound.
You're right, everyone's heard it before, and yet every once in a while someone comes along and states that human hearing doesn't go below 20Hz. Wow, you just can't ignore that low frequency sound.
You can almost count the number of cycles per second bombarding your ears. (Definitely lower than 48Hz, i.e. the next harmonic)
In my car I found I can reduce the odd harmonic contribution drastically by staying below a certain speed and only having one rear window open.

We should ask someone who owns a large SUV to do the same and see how much lower their Helmholtz frequency is.
 
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