There are many different kinds of whistles made for many different activities and jobs. Whistles are not just used to officiate sports games, but they are also used for calling the end of the work day, and even providing a warning of an oncoming train. While whistles still have many uses, they've also become collectable items for those who like their unique qualities.
These whistles, formerly known as metropolitan whistles, are known for their tapered mouthpiece and high pitched sound. Police whistles are primarily used when police need to get someone's attention. Although police cars and sirens are more common in some areas, many police officers still use whistles.
The dog whistle is usually a slender cylindrical-shaped whistle that provides an extremely high pitched sound inaudible by human ears. It's primarily used to train dogs and sometimes even cats, since they can also hear it.
Original train whistles were mounted onto trains and activated by a pull cord. The train whistle is a loud, low, billowing pitch that is actually made up of three different frequencies. The noise is created with air compression, the same type of system used by big-rig trucks. These whistles are meant to warn people that the train is coming.
While referees officiating different sports use many types of whistles, the most popular for professional sports is the Fox 40 whistle, which makes sound only using air pressure and does not have a pea---a small metal ball that fits inside the whistle's air chamber and vibrates when the whistle is blown---inside. Professional refs prefer this type of whistle because it's one of the loudest and can be heard well over the noise of an arena.
Factory whistles are used primarily in industry jobs to indicate various times---start of a work shift, break, lunch and end of the work day. These whistles are usually steam powered and vary in sound, depending on the factory. Similar to the train whistle, these whistles are large and can be heard over a large surface area.