Not the signal that is producing the soundwave, but the soundwave itself. A soundwave consists of alternating compressions and rarefactions. We can compress air to several thousand PSIG (at least) which would give a pretty high limit for the positive part of the cycle. But we can only produce rarefactions down to 0 PSIA (about -15 PSIG) for the negative part. So I would imagine that with a high enough level, we would hit the negative limit at least, and start clipping. I am wondering if real life sound levels are even anywhere close to this limit? Or if it has ever actually occured?