To achieve "HD" quality (clear, deep, and distortion-free), you can generate your own using free or professional software: Creating Bass Drop SFX in Less Than a Minute
Used during titles or scene cuts to create a sense of scale and "gravity".
In sound design, a bass drop—often called a "subdrop" or "downer"—is a low-frequency sound (typically a sine wave) that rapidly decreases in pitch, creating a falling sensation.