2. Duration is the key to the volume
Here’s another problem for you. There are two identical sounds with a volume of 65 dB, but one of them lasts for 7 ms, and the other one lasts for 40 ms. Which one is louder? The answer is the second one, of course. We are talking about the subjective volume, as heard by our ear. Between the two identical sounds, we think that the longer one is louder. This fact is ubiquitously used in audio mixing and mastering, particularly in sound compression. So, since the hardware has a volume limit and there’s always a temptation to crank it up even higher, the compression becomes a solution, because making the sound louder means making it last a little longer. By suppressing sound peaks with a compressor, the engineer changes the dynamic pattern of the musical instrument, so the parts that faded quickly now sound more distinct and… a little longer! That means the subjective volume of the instrument will be higher.
In addition to that, the compression (and/or limiting) provides a better audibility for the texture and tone of the instrument, makes it sound more dense and punchier, which only improves its overall attractiveness. This is particularly true for hip-hop, because this style is all about the punchiness! In this case, you simply can’t do without compression, because the natural kick has a rather short attack that lasts only a few milliseconds and has a quickly fading body. Compression changes everything, as it adds sustain during mixing and strictly limits the attack. As a result, we get a rich powerful bass drum that has a soft, barely audible attack, but a strong tone and character. But the main thing is that it’s noticeably louder!
You also might be interested to learn how to mix vocals to rap/hip-hop beats.