I can offer some insight on this matter; it's a shame I saw this thread as late as I did. I will admit that I have nto had time to read all of the replies, but I will go back and do so.

Once a vehicle has been sold by the manufacturer, there is no federal guideline, law, or rule as to how loud it can be. There may be local statues, but there is nothing mandated by the federal government.

The vehicle has to pass a Federal pass-by test before it can be released to the public. This is done by a "test mule" and is usually tested by the manufacturing company themselves. The limit is 80 dBA at 15 m by microphones on both sides of the bike as it accelerates by.

As long as the vehicle passes that, it can be sold. Once it is sold, there is no regulation for aftermarket pipes, exhausts, or other devices as regulated by the _federal_ government.

There may be local laws or statutes, but "loudness" is very difficult to measure accurately and in a repeatable fashion in the field. From what distance does the police officer take a measurement? From what angle? What's the air speed/humidity? Is this by the side of the highway? How do passing cars affect the measurement?

Because of this, you can get written up by being subjectively being "too loud", but it is often difficult to back this up with real data.

The regulations for noise are somewhat stricter in foregin markets. Europe has a commom procedure in all of the countries, which is slightly stricter than that of the US. It's still only done on a test mule before the vehicle is sold, however.

It's only in Japan that they have a federal noise test that is done on a stationary vehicle (not accelerating) and is done repeatedly and in a closed environment. Similiar to getting your emissions checked here, they recently passed regulations in Japan so that everyone with a licensed motorcycle has to get periodic noise checks on their vehicle at their equivalent of a DMV.

If someone has any questions on the regulations, feel free to ask and I can elaborate.

When a bike is sold from the dealership as new, it has to pass the US federal regulations. There have been plenty of bikes that have been delayed because they do not meet those regulations. But once they are out in the field it is fair game to put on straight pipes and deafen yourself and those around you.