I'll add to this list from time to time, as I think of other items.
You might consider a single-line melody and some chord symbols to be "a song". But there are a huge number of "devices" you can use to enhance your music and increase it's emotional content. Here are some of the fundamentals:
- Free-floating (non-rhythmic) music is completely valid, but listeners can instinctively follow music better if it's rhythmic
- Rhythm can be used to "trap" or to "fool" listeners. For example, establish a repetitive pattern then break it, either suddenly, or gradually. A simple adjustment of accents, for example, can cause some listeners to perceive a song in 3/4, and others, in 4/4.
- Knowing the above, keep the simple "math" of rhythm in mind, as a tool. One example is that "4 bars of 3/4 takes the same amount of time as 3 bars of 4/4". (12 beats)
- If the rhythmic accuracy of the players is obviously-poor, listeners will have a harder time understanding the music.
- Recapitulation (or "the final chorus") is the perfect time to spring a surprise on the listeners, because they expect something familiar, and you can alter that to something fresh.
- Repetitive rhythmic parts are "missed" by the listeners when they stop unexpectedly. It's a great way to indicate a mood change, just as beginning a pattern after silence signals a mood change.
- Several melodies an be combined into a recapitulation. The same is true for rhythmic qualities, if they're established like the melodies, earlier in the composition.
- Whether in an orchestra or a nightclub dance band, the percussion instruments have a wide tonal range, and should be used with some thought, to support and embellish the music.
- All things being equal, high-frequency instruments have more clarity than low-frequency instruments. Those in the middle share attributes of both..
- Like melodies, percussive parts can be "stacked", so that a "slow-moving" part, such as a cymbal roll, can be given a "flavor", according to the supporting part. For example, a cymbal roll with a military-style snare drum part under it sounds entirely different than a cymbal roll combined with slow timpani rhythms, or no accompaniment at all.
- Increasing tempo conveys a rising energy. Decreasing tempo conveys either a diminishing energy, or in the case of the finale of a song, conveys a feeling of finality.
- Interpretation in jazz matters a LOT. The way people think of "swing eighth notes", versus "triplet-based" patterns, versus "dotted-eighth/sixteenth" patterns should be addressed when required. (!)
- See all the "Rhythmic Devices" info above.
- There is probably no more dramatic effect than a moment of unexpected silence. It's nearly as shocking as the opposite... a loud blast, after silence.
- Gradual increases in volume convey a building emotion. Gradual decreases in volume convey a subsiding emotion.
- Multi-layered dynamics are important at times. For example, a light string background combined with a loud brass melody
- The perception of any melody can be greatly altered by simply moving the accents to unexpected places at unexpected times.
- "The harmonic series diminishes as the fundamental generating tones ascend." It's important to select instruments for the desired emotional effect. For example, a tenor trombone playing C2, the octave above middle C, sounds quite strident, because that's at the top of most trombonists' range. A trumpet playing the same note sounds quite ordinary, by comparison.
- Instruments played loudly, but into mutes have an energetic effect without being too loud. Mutes can also help convey a "distant" feeling. Opening and closing mutes can be used for the "wah wah" effect.
- Using crescendo and decrescendo during long held notes can drive the emotion up or down. This is even more attention-getting when it's done note-by-note, not just in major sections.
- Like empty space, quiet music focuses the audience's attention, so use it when focus is important.
- Diatonic melodies with little or no harmony convey a childlike innocence.
- Minor chords typically invoke a "darker" feeling.
- Chords that imply both major and minor sounds, like #9 chords, have elements of both..
- Melodies that use the upper components of the chords (9ths, 11ths, etc.) can imply a depth of harmony that doesn't really exist, without cluttering the composition.
- Chromatic movement and tri-tone movement always draws attention, because of it's unsettling nature. Chromatic movement of a half step away from the arrival chord begs for resolution, and is a popular device for emphasizing local 2-5 progressions. Chromatic movement that exceeds a half step tends to invoke the feeling that we're "leading to a new place" in the music. When the V7 chord moves chromatically, it tends to signal a key change.
- If any note or chord is held long enough, (usually just a few seconds) it will sound "resolved", to the listener's ears. (Play C, then play F#, and simply let the F# sustain, to hear this.)
- See the "Building Backgrounds" article below for much more.
Notes from William Fowler's article, "Building Backgrounds"
- Fast background motion imposes energy on a lethargic melody line.
- Fat background motion intensifies an already-active melody line.
- Slow background motion further lulls a slow-moving melody.
- Against a fast melodic motion, a slower background can establish solidity.
- Background harmony chord changes refresh repeated notes in the melody, especially at the end of phrases. ("turnarounds")
- Conversely, by making certain melodic notes sound dissonant, a harmonic background can urge or force melodic resolution.
- Diatonic triads behind a non-urgent melody force simplicity. (i.e., a child-like innocence)
- The background can arpeggiate chords, or "suggest" harmony, while also serving as a rhythmic counter-line.
- Most harmony should be at or above middle C, to keep it from being muddy.
- The amount of sound below middle C that any melody can tolerate diminishes as melodic delicacy increases.
- Using both massive timbre (wide range) and full volume, block chord motion (unison, rhythmically) can deliver "sledgehammer blows" musically.
- Syncopated sectional riffs, applied against a strong block chord "beat" can intensify and stabilize rhythmic flow.
- Arpeggiated versions of the clock chord structure thin out the background while still furnishing rhythmic stability. There are a dependable way to enhance melodic delicacy.
- When the melody outlines or identifies the chords, or defines the rhythm, the need for a background and/or harmony diminishes, and makes a monophonic structure feasible. Here, the background duplicates the melody in different pitch registers. In this monophonic structure, it's ok to load up the bass area. It won't detract from the tune, but rather, adds power. When used briefly, it's a good attention-getter, and "refreshes" the listener's ears.
- Melody emphasizes sustained notes - Needs background beat
- Melody emphasized rhythm - Needs sustained background
- Melody moves, then pauses - Needs counter-melody
- Melody emphasizes pitch variety - Needs background harmony
- Melody emphasized bass register - Needs higher background pitch