Originally Posted by FJF

You're setting off car alarms due to the exhaust's low frequency emissions. In a parking garage, they're amplified to an even higher degree. This is why knowing what is being measured is absolutely crucial. Simply put, the measurement can include low frequencies or not. The typically specified, municipality-driven XXdB limit does not include low frequencies (ie. it's A-weighted). This means that car able to set off car alarms can still have a relatively low dB reading IF it's A-weighted. This is why some muscle cars have quite a rumble stock and are still able to meet Federal guidelines. Same goes for how the sound is measured. The placement of the mic and its distance from the source is a huge variable, as well.

Originally Posted by EVO8emUp

Interesting factoid. Thanks