SubSonic

Subsonic is a 2.1 Extreme Demon mega-collaboration created by Terron, Ggb0y, Xcy-7, Evasium, Dudex, Viprin, Rustam, TheDevon, Panman, Sumsar, and Jeyzor, and verified by Combined, just like its predecessor (Hypersonic). This is the official 4th level of the Sonic Series. It is currently #30 on the Official Geometry Dash Demon List, above Furious Flames (#31) and below Audio Expulsion (#29).

Gameplay
Terron: This part starts with a normal speed cube sequence, where the player needs to hit jump orbs with some difficult timing. The player then goes through a short UFO and robot section, then goes into a ship sequence with numerous tight spaces, and ends with one final cube segment that features even more jump orbs.

Ggb0y: The theme of this section is purple, blue and pink much like his parts in Supersonic and Ultrasonic. This begins with a x2 speed robot with hazards covering a low ceiling, forcing the player to control their jumps well. This then leads into an extremely tight straight flying section and then another robot. This then leads to a somewhat tight wave section and then a x3 speed mini spider and a ball. The part ends with a confusing memory cube section.

Xcy-7: This part starts with a quadruple speed mini wave segment with gradually tightening spaces and an abundance of gravity portals. Next, the player enters a ball section with difficult timings in the gravity swaps and jump orbs, and there are also some size changing portals throughout the section.

Evasium: Still in x4 speed, this begins with a cube that features many moving blocks that the player must land on, or avoid. This then leads to a short spider section, and then another cube with some more moving blocks. This then goes into a short ship section and then a UFO with size changes and teleportation portals. This then goes into another cube with dash orbs, and then a robot with some more moving hazards. The parts ends with a ship that features Evasium's name, and then a tight wave.

Dudex: This continues from Evasium's wave, still in quadruple speed. There is a small bit that features the use of D Blocks which allows the player's wave to touch the floor and ceiling, but there are still spikes to avoid. This then leads to a cube with some jump orbs, and then another wave with more D Blocks. After this is a spider segment which contains numerous speed portals, changing the pace of the level in sync with the music.

Viprin: Following the rest of the Sonic series, Viprin's part is a memory ball. It is similar to his part in Hypersonic, but features tons of moving orbs, platforms, and even the player constantly flashing invisible and back. Throughout the memory ball, the speed constantly changes, mostly to quadruple speed. It ends off with a short auto ball part. Viprin's name is seen at the beginning of his part.

Rustam: This part begins with a triple speed cube section with numerous moving hazards and jump orbs. This leads to a ball section with the same concept. Next, the player enters a ship sequence where the player needs to enter a teleportation portal. This goes to a dual cube segment with Rustam's name and art of Sonic The Hedgehog. After this, the player enters a tight wave section and another cube section, and the part ends with a final ship sequence.

TheDevon: This part begins with a fast-paced cube section full of slopes and jump orbs to transport the player. There is then a UFO segment with many gravity changes. After this is another cube that is slightly slower paced with more jump rings.

Panman: This begins with a mini dual cube and ship. The cube is required to follow a fairly linear path of simple jumps and orbs, and the ship is required to stay close to the platforms that do not have any spikes in order to avoid crashing. The dual then ends, leaving the player as a singular cube having to hit more jump orbs. The end of the cube features Pan's name with wings. The part finishes with a slow wave having to avoid pillars topped with spikes.

Sumsar: This part begins with a very short but auto ball part, featuring Sumsar's name. It then transitions into a triple speed spider segment with several quick timings. This then transitions into a ship segment with several mini and regular-sized portals all at triple speed. It then transitions back into another triple speed spider segment, much like the first one. After this, it transitions into a quadruple speed robot segment with some difficult jumps and a short auto part at the end.

Jeyzor: This part starts with an incredibly complex dual segment in quadruple speed, hinting towards Jeyzor's "Machine" series which also features many complicated dual segments. The player needs to hit jump orbs, activate triggers to open doors, straight fly and many more things that are hard to keep track of. The dual ends with a singular ship where the player needs to hit more orbs to fly over hills lined with spikes. This then leads to a ball section with somewhat difficult timings, and then a spider with more hard timings. The part ends with the player becoming a robot, going through a brief auto before hitting some more orbs, and then a double speed mini wave. Then, the level ends.

Trivia

 * After numerous popular complaints about Hypersonic, Viprin brought back many of the original creators from Supersonic, including Xcy-7, Sumsar, Ggb0y and Jeyzor.
 * Most of the parts are x4 speed, a request many fans wanted.
 * Namtar originally had a part, but he could not finish it due to issues with his computer. He has been replaced by TheDevon.
 * As tweeted by Viprin in his Twitter account, Glittershroom can't finish his part because of personal issues, which was the reason why the level ended at Jeyzor's part.
 * Xcy-7's part was redecorated for the final level. His old decoration was much more simplistic, and it can be seen in the preview on Viprin's channel.