Registering an aural trademark with the United States Patent and Trademark Office can be difficult. Harley-Davidson famously tried for years to get this protection for the purr of their V-twin motorcycle engine, only to be locked in legal limbo for so long that they gave up and withdrew their request.

By 1998, only 23 sound trademarks had been issued in the United States. For reference, around 730,000 total trademarks had been granted by that time. Since then, more and more companies have been able to lock down so-called "sound marks," but the distinction is still relatively rare.

Harley-Davidson might've missed out, but here are 18 examples of sounds that have been successfully trademarked.

1. Mockingjay Whistle

Serial Number: 85409089

Lions Gate Entertainment is understandably protective of its lucrative Hunger Games franchise, so it's no surprise they trademarked Rue's four-note song, described to the United States Patent and Trademark Office as "a human whistling a G4 eighth note, followed by a Bb4 eighth note, followed by an A4 eighth note, followed by a D4 half note, in the key of G minor."

2. Law & Order's "Chung Chung"

Serial Number: 76641094

That iconic two-strike "chung chung" sound was created by composer Mike Post, who also wrote the show's theme song. "I sampled a jail door slamming, I sampled a couple of other things, I put together this ‘clunk clunk,’ ‘ching ching,’ ‘chong chong,’ whatever the hell you want to call it," Post said in an interview.

"It’s not a sound effect," he added. "It’s actually a piece of music that gets a royalty…I call it the ‘ching ching’ because I’m making money off of it."

While Post gets a royalty, Universal holds the trademark, describing the sound as "two musical notes, a strike and a rapid rearticulation of a perfect fifth pitch interval, which in the key of C sounds the notes C and G, struck concurrently."

3. 60 Minutes' Ticking Stopwatch

Serial Number: 85793891

The iconic ticking of 60 Minutes' stopwatch was successfully trademarked by CBS. The above video is a full hour of that sound, just in case you are bored and want to drive yourself insane.

4. Meth Being Smoked

Serial Number: 77268435

Bad news for any companies out there who want to use the sound of someone smoking methamphetamines to promote their product: The Meth Project Foundation, a group that produces anti-meth PSAs, trademarked that sound about a decade ago.

"The sound of burning methamphetamine," according to their application, features "the flick of a lighter, followed by a fizzing sound of a small flame ignition, and high pitched metallic crackling sounds."

You can listen for yourself in the above PSA.

5. “D'oh!”

Serial Number: 76280750

While it may be listed in Simpsons scripts as an "annoyed grunt," the sound of Homer Simpson saying "D'oh!" is now an official trademark owned by Twentieth Century Fox.

6. Tarzan's Yell

Serial Number: 75326989

Edgar Rice Burroughs, Inc. protects the intellectual property of the author and Tarzan creator, and it holds the trademark for his hero's yell, as made famous by actor Johnny Weissmuller. However, in the Tarzan books, Burroughs merely described this holler as "the victory cry of the bull ape." The trademark registration's language isn't quite as concise:

A series of approximately ten sounds, alternating between the chest and falsetto registers of the voice, as follows - 1) a semi-long sound in the chest register, 2) a short sound up an interval of one octave plus a fifth from the preceding sound, 3) a short sound down a Major 3rd from the preceding sound, 4) a short sound up a Major 3rd from the preceding sound, 5) a long sound down one octave plus a Major 3rd from the preceding sound, 6) a short sound up one octave from the preceding sound, 7) a short sound up a Major 3rd from the preceding sound, 8) a short sound down a Major 3rd from the preceding sound, 9) a short sound up a Major 3rd from the preceding sound, 10) a long sound down an octave plus a fifth from the preceding sound.

7. Darth Vader's Breathing

Serial Number: 77419252

LucasFilm trademarked the "sound of rhythmic mechanical human breathing created by breathing through a scuba tank regulator," better known as Darth Vader's robotically enhanced respiratory system. You know he's on the dark side because lawyers got involved.

8. Lightsaber Sound

Serial Number: 77419246

Use legal force, Luke.

The sound of a lightsaber, FYI, is described as "an oscillating humming buzz created by combining feedback from a microphone with a projector motor sound."

9. Pillsbury Doughboy Giggle

Serial Number: 76163189

Pillsbury owns the rights to this "childlike human giggle," but remember, trademark infringement is no laughing matter.

10. Taco Bell Bell

Serial Number: 77805701

This trademarked entity is described as a "bong" sound, but not the one many Taco Bell enthusiasts are familiar with.

11. NYSE Bell

Serial Number: 76344794

That "clang clang" is "the sound of a brass bell tuned to the pitch D, but with an overtone of D-sharp, struck nine times at a brisk tempo, with the final tone allowed to ring until the sound decays naturally."

It's heard every workday morning on Wall Street...except when it isn't.

12. Mimsie's Meow

Serial Number:75143671

Mary Tyler Moore's MTM Productions used Mimsie, a stage cat, for their logo, which was a spoof on the MGM lion's hearty roar (which is itself a trademarked sound). Apparently, the meow had to be added in post-production because Mimsie refused to perform for the cameras. They just captured her yawning and overlaid the now-trademarked sound.

All of MTM's shows, as well as the trademark for Mimsie's meow, are now owned by Twentieth Century Fox.

13. Green Giant's "HO HO HO"

Serial Number: 75821499

This "sound of a deep, male, human-like voice saying 'Ho-Ho-Ho' in even intervals with each 'Ho' dropping in pitch" is used by an exuberant verdant monster to push frozen veggies.

14. Aflac Quack

Serial Number: 76307773

If you're in the park and hear a duck quacking the word "Aflac," please notify American Family Life Assurance Company as you may be witnessing a trademark violation.

15. ESPN's "DaDaDa DaDaDa"

Serial Number: 75676156

When you hear "six musical notes played in a fast tempo: 'D, C sharp, D, D, C sharp, D,'" you know some hot sports action is about to go down.

Grammy-winning composer John Colby created the short jingle in 1989, despite claims to the contrary from David St. Hubbins:

16. "You've Got Mail"

Serial Number: 75528557

In 1989, Elwood Edwards recorded a series of test greetings for Quantum Computer Services onto a cassette recorder at the behest of his wife, who worked for the company. The company soon changed their name to America Online, and those test greetings—"Welcome," "You've got mail," "File's done," "Goodbye"—were loaded into software that would eventually usher millions of people onto the Internet for the first time.

AOL allowed Warner Bros. to use the trademarked phrase for the 1998 Tom Hanks and Meg Ryan movie of the same name, helping make Elwood's voice one of the most famous sounds of the 1990s (not that he needed the help). Edwards spent most of his life behind the camera, though, and he recently retired from his local news production job in Cleveland, Ohio.

17. Tivo's Bloops

Tivo trademarked ten sounds produced by their remotes and digital recording boxes. Every bloop and bleep is accounted for.

18. Ice Cream Truck Music

Serial Number: 85485669

No one can own the cheery melodies that blast from ice cream trucks and bring back memories of carefree youthful summers, right? WRONG. “Turkey in the Straw," “The Entertainer," and “Camptown Races” are now trademarked by Breville, Ltd. for use with their Smart Scoop brand ice cream makers.