No jargon. Just a clear explanation of what’s actually happening inside the box when your sub produces bass.
The Basic Idea: Moving Air
All speakers work by moving air. A speaker cone moves forward and backward rapidly, creating alternating areas of high and low air pressure. Those pressure waves travel outward and your ears interpret them as sound.
Low-frequency sounds (bass) require large, slow-moving pressure waves. To create those, you need to move a lot of air. That’s why subwoofer drivers are large — typically 8 to 18 inches in diameter. A small tweeter can’t move enough air to produce bass. A large sub driver can’t move fast enough to produce treble. Each is designed for its job.
The Key Parts
The Driver
The driver is the speaker itself — the cone, voice coil, and magnet assembly. Here’s how it works:
- An audio signal (alternating electrical current) flows into the voice coil — a coil of wire wound around a cylinder inside the magnet assembly.
- The alternating current creates an alternating magnetic field in the voice coil.
- That field interacts with the permanent magnet’s static field, pushing and pulling the voice coil back and forth thousands of times per second.
- The voice coil is attached to the cone, so the cone moves with it, pushing and pulling the air in front of it.
Simple electromagnetic principle. The cone converts electrical energy into mechanical motion into air pressure waves.
The Enclosure
Without an enclosure, a speaker driver would cancel itself out. The front of the cone pushes air forward while the back of the cone pulls air backward — and at low frequencies, those two waves collide and cancel each other. Almost no bass output.
The enclosure isolates the rear of the driver, preventing this cancellation. In a sealed box, the air inside is trapped and acts as a spring. In a ported box, the port redirects the rear wave through a tuned tube to add output rather than cancel it.
The Amplifier
Powered (active) subwoofers have a built-in amplifier. The amplifier takes the small signal from your receiver’s sub output and amplifies it to the level needed to drive the voice coil properly. Subwoofers need significantly more power than midrange drivers because moving a large cone at low frequencies requires considerable force.
Why You Feel Bass as Much as Hear It
Below about 40Hz, the wavelengths of sound become so long (17 meters at 20Hz) that they interact physically with your body. Your chest cavity, abdominal cavity, and other resonant structures respond to the pressure waves. This is the physical sensation you feel during a movie explosion or a heavy bass drop. It requires actual sub-bass frequency output — not just “more bass” in the 80-100Hz range.
Why Bass Seems to Come from Everywhere
Your ears can’t determine the direction of sounds below about 80Hz. The wavelengths are too long relative to the distance between your ears for the brain to use its normal directional processing. This is why you can put a subwoofer almost anywhere in a room and the bass still seems to come from the screen or speakers. It’s also why sub placement is about room acoustics rather than directionality.
What the Crossover Does
Your receiver contains a filter (the crossover) that sends frequencies below a set point (typically 80Hz) to the subwoofer and frequencies above that point to your main speakers. Without this filter, the sub would receive the full audio signal and try to reproduce it — which it would do poorly above its design range. The crossover keeps each driver doing what it does best.