That thin thief thoughtlessly threw those things through the thick thorns.
Each of these words starts with an interdental fricative. By definition, interdental sounds are produced between the teeth. Interdental consonants are relatively rare: they don't appear as phonemes in many languages, and there are very few examples of interdental sounds with different manners of articulation. However, interdental sounds are still an important aspect of human speech.
Definition of Interdental
The literal definition of interdental is between the teeth. Interdental consonants are produced by putting your tongue between your upper and lower teeth.
Interdental sounds are sounds that are produced with a constriction between the tongue and the upper and/or lower teeth.
Interdental sounds are similar in articulation and sound to both labiodental and dental sounds.
Labiodental sounds are sounds that are produced with a constriction between the lower lip and upper teeth.
Dental sounds are sounds produced with a constriction between the tongue and the back of the upper teeth.
These three places of articulation are similar enough that many languages use them interchangeably.
Fig. 1 - Interdental sounds are produced by bringing the tongue between the upper and lower teeth.
Interdental Sounds
Unlike sounds at other places of articulation, like bilabial and alveolar, interdental sounds are relatively unvaried. The only unique interdental sounds included in the International Phonetic Alphabet (IPA) are the interdental fricatives. Interdental fricatives are usually written as th in English (as in that and whether).
Fricatives are consonants produced by forcing air quickly through a narrow constriction in the vocal tract.
When you produce an interdental fricative, you bring the blade of your tongue to the edges of the upper teeth, leaving a narrow gap. You then force air through the gap, creating a stream of turbulent airflow. Interdental fricatives can be voiced or voiceless.
| IPA Symbol | Articulatory Description | Examples |
| θ | voiceless interdental fricative | math [mæθ], thatch [θætʃ] |
| ð | voiced interdental fricative | bathe [beɪð], that [ðæt] |
The phonetic symbol for the voiceless interdental fricative is the Greek theta symbol (θ). The symbol for the voiced interdental fricative is the Old English (and Icelandic) letter eth (ð).
Other interdental sounds are written as alveolar sounds marked with the advanced diacritic [ ̟ ].
Alveolar sounds are sounds produced with a constriction between the tongue and the alveolar ridge behind the upper teeth.
Diacritics are extra symbols written above and below IPA symbols to show an altered pronunciation.
This combination of an alveolar consonant and advanced diacritic represents an alveolar sound that has moved forward in the mouth to the point of becoming interdental.
| IPA Symbol | Articulatory Description |
| t̟ | voiceless interdental plosive |
| d̟ | voiced interdental plosive |
| n̟ | interdental nasal |
Interdental Fricatives
As mentioned before, an interdental fricative is a turbulent stream of airflow forced through the narrow opening between the tongue and teeth. The result is a random (or aperiodic) pressure wave, a bit like TV static.
You can see this random fricative noise by looking at a spectrogram.
A spectrogram is a graph of a sound wave's component frequencies over time. Component frequencies are the range of frequencies present in the sound.
A spectrogram provides clues about the nature of different speech sounds. Fricatives appear on the spectrogram as "fuzzy" strips of noise.
Produce the sounds [f] as in father, [θ] as in throw, and [s] as in sat to yourself. You might notice that [f] and [θ] sound similar to each other, while [s] sounds very different from both [f] and [θ]. You can see this difference on the spectrogram.
Fig. 2 - The interdental fricative looks similar to other fricatives on a spectrogram, with slight differences in amplitude.1
On the spectrogram, the voiceless labiodental fricative [f] and the voiceless interdental fricative [θ] both look like fairly consistent fuzzy stripes. The voiceless alveolar fricative [s] looks similar, the major difference being a much darker area at the top of the spectrogram. This represents a very high, loud frequency range characteristic of fricatives like [s].
Interdental Consonants
Interdental consonants can appear in languages as phonemes or as allophones.
A phoneme is a single unit of sound that is meaningful and capable of distinguishing words from one another in a language.
Allophones are different articulatory realizations of the same phoneme. For example, the [t] sounds can be produced with or without an exhalation of air.
As you've seen, the voiced and voiceless interdental fricatives are phonemes in English. These are the only interdental phonemes in English. Even then, English speakers sometimes replace interdental consonants with allophones. Let's look a little closer at allophones now.
Allophones
Not all English speakers produce interdental consonants in the same way. For example, many American English speakers produce them as truly interdental, with the tongue protruding from between the teeth and touching the edges of the upper teeth. Many British English speakers, though, pronounce these consonants with the tip of the tongue touching the back of the upper teeth, producing a dental fricative.2
Different articulations of the same phoneme, as in this example, are called allophones.
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