How Speakers Work
A speaker cone at 40 Hz travels 16× farther than at 160 Hz. That's why subwoofers are big.
The Electrodynamic Loudspeaker
A loudspeaker converts an electrical signal into sound. A coil of wire (the voice coil) sits inside the gap of a permanent magnet. When current flows through the coil, the magnetic field pushes the coil back and forth. The coil is glued to a lightweight cone, so the cone moves with it, pushing air molecules to create pressure waves we hear as sound. It is essentially a dynamic microphone in reverse. A microphone turns sound into electricity; a speaker turns electricity into sound.
Cone Displacement vs Frequency
At low frequencies the cone must travel a long way to move enough air for a given loudness. The physics is simple: for the same sound pressure level, cone displacement scales as 1/f². At 40 Hz the cone moves (160/40)² = 16× farther than at 160 Hz. This is why subwoofers need large, heavy cones. They must physically move a lot of air. A small tweeter cone would tear itself apart trying to reproduce deep bass at any useful volume.
Crossover Networks
Real speaker systems split the signal so each driver handles the frequencies it is built for. A woofer handles the lows, a tweeter handles the highs. The circuit that does this is called a crossover network, and it is simply a pair of complementary filters: a lowpass filter feeding the woofer and a highpass filter feeding the tweeter, both set to the same crossover frequency. Below the crossover frequency, most energy goes to the woofer; above it, most goes to the tweeter. In between, both drivers contribute.
Try it: watch the cone displacement shrink as you raise the frequency, then split the signal through the crossover.