Longer, lower, louder, at the start of systole (contraction).
The "lub-dub" sound is the sound of the heartbeat, which is produced by the closing of the heart valves. The sound is created by two distinct sounds: heartbeat sound lub dub
The "lub-dub" sound of a heartbeat is the sound of the heart's valves snapping shut to ensure blood flows in one direction. It is not the sound of the heart muscle contracting, but rather the vibration caused by the closure of these valves. The Two Core Sounds Longer, lower, louder, at the start of systole (contraction)
The heartbeat sound "lub-dub" is a familiar and reassuring sound that indicates a healthy heart rhythm. Here's some helpful content related to this topic: heartbeat sound lub dub