Linguists often think of vowel sonority as equivalent to the frequency of the first formant (F1), or alternatively to vocal tract openness. One problem with equating F1 with sonority is that this approach cannot be extended to consonants. In previous work, I have shown that vowel sonority is in fact well modeled by perceived loudness -- a measure that can also be applied to consonants. In this talk, I examine vowel loudness as quantified by audibility studies and its relationship to F1. Surprisingly, increasing the value of F1 does not uniformly increase audibility.