Every time I'm at the SPL side of an event, someone walks up with an entry form and asks the same thing: "what's the actual difference between T, B, R and W?" It looks like alphabet soup until you realise it's not a power ladder at all — it's an installation freedom ladder. Where you're allowed to put your subs is what separates these classes, not just how much you're allowed to spend.
The ESPL classes, by where the subs can live
- T — Trunk. The most restrained tier. Subwoofers, enclosures and ports have to stay in the trunk area, with the enclosure no higher than the top of the original rear seat. Your doors and seating stay OEM. This is the closest thing SPL has to a "daily driver" class.
- B / R — B-pillar / Rear. One step up in freedom. In the B variant, the enclosure can sit behind the OEM B-pillar, as long as it doesn't clear the original rear seat back without headrests. In the R variant, the build stays trunk-based but the enclosure is allowed to reach all the way up to the roof. Either way, you start getting more latitude for interior damping and panel work from the B-pillar forward.
- W — Wall. Subwoofers and enclosures move behind the OEM B-pillar entirely and can reach the roof, and the rulebook's language here is blunt: equipment can be installed "everywhere" so long as the car still functions as a car. This is where I start seeing full false walls and serious cabin rebuilds.
- Expert. No price limit and effectively no restrictions — bracing on the windscreen and windows is allowed up to 4cm, and the car doesn't even need to be road-registered. This is the pure SPL build, built to one number and nothing else.
Each tier also carries its own equipment value cap underneath that install-freedom rule, rising as you move from T through to W before Expert removes the cap altogether. I'd rather send you to the current rulebook and the official EMMA value database for the exact euro figures than quote numbers here that can shift between rulebook editions — the install-freedom boundary above is the part that actually decides which class your build belongs in, more often than the budget does.
Why this matters more than it looks like it should
I see competitors pick a class by looking at what they can afford, then get caught out at scrutineering because their enclosure placement doesn't match what that class actually allows. A perfectly legal, well-built T-class sub box becomes an instant problem if it's mounted behind the B-pillar — it's not a budget violation, it's a class violation, and it's picked up before a single dB is measured. Know which letter you're building to before you start cutting trim, not after.
How this sits next to everything else on the SPL side
This is purely about where the equipment can go — it says nothing about how loud you need to be or how that's measured. For the actual peak and average readings taken on the day, see my breakdown of how EMMA measures SPL. And if you're still weighing up SPL against a Sound Quality build entirely, EMMA SQ vs SPL covers the bigger-picture decision between the two formats.
My honest advice: decide your class by what you're willing to do to the car, not by what number you want to hit. The install-freedom boundary is the one that actually gets enforced on the day.
If you're planning an SPL build and want a second opinion on which class your enclosure plan actually fits before you commit to cutting anything, get in touch. And if you're earlier in the process and still deciding what to build at all, our car sound upgrade range is a reasonable place to start thinking about the gear itself.
Written by Pete — Integrated Audio
Pete is the founder of Integrated Audio and a Certified International EMMA Head Judge, judging EMMA Sound Quality and Installation competitions in Australia and internationally. He writes from the judging side of the table, drawing on hands-on experience building and installing HELIX, MATCH and BRAX car audio systems.