Personally, some of my favorite albums to listen through are those that feature seamless transitions (also known as “gapless playbacks”) between songs. A few notable albums that I often revisit just because of these transitions are Coldplay’s “Mylo Xyloto”, Angels & Airwaves’ “We Don’t Need to Whisper,” and Aries’ “Believe in Me, Who Believes in You.” This effect is done in many different ways at a wide range of complexity, though it ultimately boils down to tempo/rhythm, the piece’s key, and the volume/balancing of the audio itself.
The simplest and most common way a transition (or even more generally, a song’s ending) is executed is by fading out the final segment of the piece. This can be a simple held note, chord, or even a diminishing vocal track on top of other elements. In the case of a transition, once the final “fade-away” has concluded in its duration, the next song’s intro will slowly crescendo from the previous silence, creating a “crossfade” effect. This effect is what Spotify’s titular “crossfade” feature utilizes when a user toggles it on and sets the duration of the crossfade, allowing songs that were not originally mixed to transition into each other to do so in a more natural way. From what I understand, the key to executing a crossfade well is having an “unheard downbeat” related to the tempo of the first piece to the next that marks where the transition between the two should begin/end. Otherwise, the effect will either be too long to notice its presence or be too short and as a result, potentially jarring to the listener.
The balance of the sound between the zones of hearing within the speakers also can be an effective way to transition between songs. Typically, a song’s audio is balanced equally between the left and right channels to create a “centralized” listening experience, also known as a “mono” balance. However, it is common for instrumentals and sometimes vocals (especially when layered for additional depth) to be “offset” into different zones of hearing, such as front-left, back-right, and so on. 8D audio utilizes this to create the feeling that the song is circling around your head, smoothly moving between these zones for its duration. Much in the same way that fading an audio track can be used to create a transition, shifting the same final segment from the left to the right zone of hearing, for example, can create a similar effect. In this case, the unheard downbeat marking the transition will likely be when the audio is balanced evenly between the beginning and ending zones.
As mentioned previously, the key a piece is played in determines how well a transition will work. Transitioning between two that are both written in the E minor key, for example, will be much easier to accomplish than if one was in B major. However, if the pieces end and begin on the same note/chord, respectively, a clean transition can be accomplished. In this case, the ending note or chord of the first piece can be held into the beginning of the second to create an ideal, gapless bridge between the two. Though it sounds simple in this specific context, in practice it usually is much more complex, especially when coupled with a variety of instruments, tempos, and other factors that will also need to be balanced accordingly to create the desired effect.
I have personally tried to curate a playlist of my own that is not only seamless when played from beginning to end, but is also made up of copyright-free music (at least as far as YouTube’s copyright detection system could tell) for the purpose of livestreaming. It was no easy task, as it meant being creative with how the songs each fed into each other, and what I found was that linking similar elements was what created the most consistent transitions. For example, if a jazz-inspired piece ended with a sharp snare drum “stinger,” the next piece would need to feature a kind of drum at the beginning. The best illustration I can give of this is with the start of the playlist, which features Toby Fox’s “Hopes And Dreams” piece for the game Undertale, which ends on a guitar chord decrescendo. The next piece, 94Stones’ guitar-heavy cover of Dragon Ball Z’s intro “We Gotta Power,” picks up a similar chord in its opening, creating a believable transition. From there, genres blend, with rock giving way to jazz, electronic, city pop, lo-fi, and everything in between, proving it is difficult, but not impossible, to make transitions between these seemingly drastically different genres work with the right approach.