>-----Original Message-----
>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] 
>Sent: 06 September 2006 17:20
>To: Microcontroller discussion list - Public.
>Subject: Re: [OT] Transmission tower noise
>
>
>>Set governors of each to exactly the same engine revs.
>>Natural differences will cause a beat frequency between
>>the two engines.
>>Engine shed will sum air pressure waves and produce a
>>high amplitude somewhat subsonic signal.
>>Whump a Whump a Whump a .....
>
>I have experienced something similar during the day with a 
>single engine. It was idling along as it went past my 
>grandfathers house, but you couldn't hear it, but you could 
>sure feel the air pressure change. I suspect the engine was 
>idling with the absolute minimum of fuel combustion to keep it 
>idling over, and this meant the thing was behaving as an air 
>pump, and not making any noticeable exhaust noise.

Possibly it was resonance (e.g. between two buildings) and you happened to have you head at one of the peak pressure nodes. It's quite uncomfortable in one room of my house if a car is waiting outside with it engine idling at the wrong speed, the whole room resonates, windows rattle etc.

Regards

Mike


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