In this study conducted by Julien Meyer, Fanny Meunier, and Laure Dentel, the perception of differentiation of Spanish-based whistled vowels by French speakers is tested, revealing that spoken language speakers (whose language has similar vowel categorizations as the whistled language in question) can differentiate vowels without prior knowledge of the whistled language repeatedly enough to prove that this is not chance. This is a fascinating study! The vowel identification in this study was using the language of Silbo Gomero, and forty French-speaking students that had never come into contact with a whistled language before. Half of the subjects were given the vowels alone; the other half received the vowel in a sentence. The first group correctly identified vowels 55% of the time; the second group did even better, at about 60%. For a control a native speaker of Silbo also tested, correctly answering each vowel 87.5 percent of the time.
Any confusion between vowels, in both the experimental groups and the native speaker, originated from the vowel’s ‘neighbors’ – the vowels that occurred directly below or above the vowel on a scale of Hertz. The vowel /o/ was most often confused with /a/, and so on – which meant that the vowels /a/ and /e/ were the most frequently confused, as they are in the midrange for Hertz value.
The most fascinating part of this study for me personally was the fact that non-whistlers with no prior knowledge of whistled languages could correctly identify the vowels in question more than half of the time out of context. This suggests that some part of a whistled language is intuitive enough, or has enough similarities with, a spoken, root-sharing language as to be partially understandable to the untrained ear. Meyer suggests that this is merely recognition of similar frequency distributions, but either way this is really interesting stuff.
Further topic to look into: formant convergence in vowel classification.
Meyer, Julien, Fanny Meunier, and Laure Dentel. Identification of Natural Whistled Vowels by Non-Whistlers. Interspeech 2007, Aug 2007, Belgium. pp.1593-1596, 2007. Web. Jan 2 2016.<https://hal.archives-ouvertes.fr/hal-00184810/>.