At its simplest, composition is the process of choosing sounds and arranging them in time. This process might vary depending on what kind of music you’re making, what instruments you’re using, how many people are involved, etc… but the fundamentals of composition are the same for everyone.
When choosing sounds for loudness, you have to understand what kinds of sounds and instruments sound loud. When arranging sounds for loudness, you’ll have to understand how to combine sounds in ways that maximise the desired effect. As discussed earlier, there are two fundamental attributes of sound relevant the way we perceive loudness – length and frequency.
For sounds of equal recorded volume level, longer sounds are generally perceived as louder than short sounds. The effect isn’t linear, however. It’s true for very short sounds (i.e. less than about 500ms). For sounds longer than about 500ms, however, additional length doesn’t sound louder. You know this yourself – if you have a snare drum and an organ in your song and they’re both hitting the same peak level on the meters, the organ will sound much louder than the snare drum. That’s because the snare drum is very short and the organ notes are much longer. The effect only works for short sounds though – an organ note that lasts four beats will sound just as loud as an organ note that lasts eight beats.
It’s a similar story for frequency. Again, you know this from experience. If you have an instrument where all notes hit the same levels on the meters (such as an organ or a synth with an open filter), you’ll know that in the mid to upper-mid range (e.g. around middle C and above), these notes sound louder than notes in the bass (e.g. a couple of octaves below middle C).