How can I tell by a motorcycle's specs if its loud?

You can tell by the fact that it’s a motorcycle. By design, motorcycles are loud, since being seen and, in this case, heard, is what allows a motorcyclist to navigate through traffic filled with SUVs with the windows, AC, and stereo all the way up, and still arrive in one piece.

If you’re wanting to alter it, then the first thing you’ll want is some sort of muffler for the exhaust… but I’m not entirely sure they even make those. In a collision between motorcycle and car, car almost always wins, and so most motorcycles will be very loud, even at the cost of seeming like “that noisy jerk.”

If it says Harley-Davidson it is tremendously loud, and your neighbors will hate you.

If it says Honda, it is so quite you can hardly hear it.

All others fall somewhere in between.

It’s a motorcycle. Motorcycles are loud. You’ll be hard pressed to find one that isn’t louder than a car, and as @dUc0N said, for damn good reason. If it’s too loud for you, get a bike and learn to pedal really hard. Or just deal with a car.

My brother has a Yamaha V Star, it is louder than his Honda Shadfow was, but not as annoying loud as one of those over-priced Harleys. You can always fit a motorcycle with different pipes, as it is the pipes (or mufflers) that regulate how loud the bike is. Loudness depends simply on preference.

The first rule of thumb for any motorcycle rider is to assume nobody can see you and that every other driver around you is drunk or stoned. Most people barely pay attention to their surroundings, and many of them simply do not ‘see’ motorcycles. This is the main reason keeping me from buying a bike.

That said, you can get special lights for the rear of your bike that flash and flutter for when you brake to make your tail end more visible.

I have a SUV and a Honda, with them running side by side, you cannot hear the bike unless you turn off the SUV. (they are both stock from the factory)

Thanks @evelyns_pet_zebra, for the only actual answer to my question. Motorcycles are not just loud, why should a motorcycle, by nature, be any louder than a car? Maybe I don’t want a motorcycle just because it’s loud and obnoxious, what a thought.

You can’t tell from the specs how loud the bike is. You must kick it over and rev it up to hear for yourself.
Not all motorcycles are loud. Many are actually very quiet, with excellent muffled pipes.
Harleys are made to be loud on purpose.
I wouldn’t get anything smaller than a 500 for your daily 80 mile commute, and would preer a 750 or an 800 or bigger (It is nice to be able to get out of the trouble you can get into out there, and smaller engines are asking for trouble).