Box Size, Loudness and Decibels

There is an old rule in speaker design. For a given sub you can have a small box, bass that reaches low, or loudness for the power you put in. You get to choose two, and the third one pays for it.

• Small and deep works, but it takes a lot of power to get loud.
• Small and loud works, but only higher up. The low notes fall away.
• Deep and loud works, but it needs a big box.

Competitors fight this every day, because a vehicle only has so much room. That is why port tuning, the vehicle’s own resonance and plenty of amplifier power all matter so much.

The bigger box lets the sub reach lower before it starts to fall off. The small box is a touch louder up top, but loses the low end fast. A port changes the shape, as How a Port Works shows, but the same trade-off is still there.

Bigger only helps up to a point. Past a certain size a sealed box stops reaching any lower, which is why 4 and 8 cubic feet would look almost the same here. And this chart is outside a vehicle. In a car the cabin lifts the lowest notes a lot, which is why small sealed boxes work so well there.

A decibel compares two levels. It isn’t a fixed amount like an inch. Because our ears work across a huge range, the scale goes up by ratios, not steps, so each dB is harder to earn than the last.

So doubling your amp power, a big and expensive step, gains about 3 dB. Ten times the power gains 10 dB. Every extra dB at the top end of a build is hard won.

Adding a second identical sub, with twice the box space so each one has the same room, gains about 3 dB on the same total power. The two cones work together and push the air more efficiently. Give the pair twice the power as well, and it is about 6 dB over the single sub.

That is why big builds stack cones as well as amps. More cone area moves more air with less work from each sub. The cost is space, weight and the electrical system to feed it. Where those dB end up on the scale is on The SPL Ladder.