Which linguistic concept was deliberately incorporated into the logo design for the International Phonetic Association's 2015 rebrand?

In the intersection of linguistics and branding, logos that incorporate language elements can become powerful brand identifiers. Some companies cleverly use linguistic features in their logos to create distinctive brand recognition across global markets. This poll tests your knowledge about a famous language-related logo design that revolutionized how we think about visual brand representation in the linguistics industry. Can you identify the correct linguistic principle behind this innovative design?

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The logo features overlapping speech waveforms that form the letters 'IPA', representing sound visualization in phonetics
The logo uses negative space to create both the Latin alphabet and International Phonetic Alphabet symbols simultaneously
The logo incorporates a stylized universal tongue position diagram that morphs into the organization's initials
The logo features a spectrogram of the words 'International Phonetic Association' spoken in six different languages