The SPL Ladder

SPL means sound pressure level, how hard the air is being pushed, measured in decibels (dB). The scale is not like a ruler. Every 10 dB up takes ten times the acoustic power, and every 20 dB is ten times the air pressure. So 150 dB is not a little louder than 140 dB: it takes ten times the power to get there. The Box Size, Loudness and Decibels page shows what each extra dB costs you in amp power.

A competition reading is taken at one spot, wherever the judge places the sensor. That is a very different place from the driver’s seat. Pressure builds at the glass and in the corners of the cabin, so the seat can read quite a bit lower, or in some cars higher, at the same note.

Most vehicles are not sealed up when the reading is taken. With openings in the cabin, the vehicle itself becomes part of the enclosure: the box and cabin act like the chambers of a 4th or 6th order bandpass box, and the openings act like its ports. Change the openings and you change the note the vehicle is loudest at.

It also means a score is about the whole vehicle, not just the sub and box. The same system in a different car can read very differently. That’s why scores are compared within classes and builds.

Hearing damage adds up and does not come back. A common rule from hearing safety research is that every 3 dB louder halves the time you can safely spend there. Around 85 dB is fine for a working day. A few steps up the ladder, safe time is down to minutes, and at competition levels it is effectively zero.

Wear proper hearing protection at shows, around the lanes and anywhere near a car being metered, and never sit in a car during a burp with your ears open. Earplugs under earmuffs gives the most protection. Protect the kids and anyone watching too: bass you feel in your chest is still reaching your ears.