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During a conversation with a friend visiting from Japan, I noticed that the “th” sound was being replaced with other sounds. Thank was heard as “sank” and think as “shink.” After a while, my brain recognized what was happening and compensated for it. It could recover the words despite sounds being different from what is expected. All the same, it made me wonder why pronunciation differed.

The “th” sound as in the words thin, thought, and path is made with a continuous flow of air passing through a narrow gap between the tongue and the upper teeth as the vocal cords are folded back. The “th” sound as in the words the, that, and those is made with a continuous flow of air passing through a narrow gap between the tongue and the upper teeth as the vocal cords are vibrating.

There are several issues involved, including the status of the “th” sound in different languages, how language learners acquire another language, and the manner in which it is produced. We’ll examine them in this article.

The Challenges Involved

The “th” sounds is relatively rare. It is found as a distinctive speech sound in less than five percent of the world’s languages. So, those who don’t speak a language that features it as a distinctive speech sound do not listen for it in daily speech and have not developed the process for producing it.

A further challenge associated with the “th” sound is that in English, the letter sequence “th” refers to two different and distinct sounds. And, those sounds are not different versions of the same sound. The following word pairs—thigh & thy, teeth & teethe, and ether & either—are not different pronunciations of the same word. They are different words with different meanings. This provides evidence that the two sounds associated with the letters “th” are distinctive speech sounds in English.

There is also another challenge associated with the “th” sounds. They are also relatively quiet sounds, which make them difficult to hear and distinguish. For language learners, it will be difficult to perceive and recognize distinctions based on these sounds until awareness is automated in the brain.

The production of these sounds, however, is relatively easy.

Friction in the Fricatives

The two sounds associated with the letters “th” belong to the fricative category of consonants. Fricatives are produced when a continuous flow of air passes through a narrow gap between two articulators, or parts of the mouth.

The “th” sound found in words like thin, thistle, and tooth is defined as a voiceless lingua-dental fricative. Sometimes, it is also called the voiceless interdental fricative.

Each term in an IPA (International Phonetic Association) descriptor defines a specific part of the sound-making process. For a fricative, the second term in the description refers to the parts of the mouth that form the narrowing. The first term refers to the lower part and the second the upper part. In this case, “lingua” refers to the tongue and “dental” to the teeth. Since the second term refers to the upper part, dental in this case refers specifically to the upper teeth.

To make the “th” sound, the blade (or front part) of the tongue is placed slightly beneath the upper teeth. Depending on the individual, the the tip of the tongue might stick out past the teeth; thus, the alternative term interdental. The tongue is between the upper and lower teeth.

When air passes through this narrowing, it creates friction, or turbulence. This turbulence is not a by-product of the sound, but the sound itself. The mouth is being positioned to create this turbulence in a specific part of the mouth.

Notice the position of the parts of the mouth when you make the “f” sound, then shift to the “s” sound. These are also fricatives, but the locations of the narrow gaps are different. Position is important, but the size of that gap is also critical. We can experience this with a simple demonstration.

Place your index fingers horizontally and parallel to each other a few inches (or centimeters) in front of your mouth. Create a small gap between them. Now, blow air toward that opening.

You should hear a turbulent sound.

Now, slowly move the fingers apart. At some distance from each other, that turbulent sound will disappear. The two fingers are too far apart to create friction.

Similarly, the parts of the mouth have to be in the correct position and at the appropriate distance from each other to create turbulence that is produced and recognized as a particular speech sound.

Young children spend much of their time tuning themselves to produce the sounds of the language spoken to them. Older language learners have to do the same to help their brains recognize unfamiliar sounds and to program the appropriate parts of the body to produce it.

Using the Next Best Sound

When someone who is learning or not proficient in a second language speaks that language, they will often use a sound from the first language when they encounter a sound not found in the first. They make do with the best sound available. This is natural if they are conversing at speed. Their speech “operating system” is doing the best it can with the resources available to it.

These sound replacement aren’t random. They are systematic. For example, Japanese doesn’t have the “th” sound. So, Japanese speakers speaking English usually replace it with the “s” sound or the “sh” sound. So, thank may sound like “sank” and thin like “seen” or “sheen.”

Japanese does have the “s” and “sh” sounds.

Notice the position of your tongue when you make the “th” sound as in thin. Now, say the word sin. Is the tongue in the same place at the beginning of the word? Now, try it with shin.

You will notice that tongue moves back as you move from the “th” sound to the “s” to the “sh.” As discussed earlier, fricatives are made by passing air through narrow gaps created by different parts of the mouth.

Japanese speakers have tuned for the “s” and “sh” sounds, but not for the “th” sounds. So, their tongues will automatically move to those positions when they try to pronounce “th” in English speech. But, you ask, why don’t they consistently use the “s” sound.

In Japanese, the “s” sound precedes the vowels a, u, e. and o. However, the “sh” sound precedes the i vowel, which is pronounced “ee,” as in shimashe ma” (island) and shirushe roo” (to know). That is why some Japanese will pronounce think as “shink.”

So far, we’ve examined how the sound is made and why Japanese speakers might mistake the sound when speaking English, but we still have to look at why there are two different sounds associated with the letters “th.”

Voicing the Differences

IPA descriptors for consonant sounds provide three pieces of information: the way in which the flow of air is manipulated, the parts of the mouth involved in that manipulation, and what the vocal cords are doing. Voicing refers to this last element.

At the most general level, the vocal cords can be in two different configurations. The term voiceless refers to the condition where the vocal cords are folded back, which means they do not obstruct the flow of air in any way. Air is free to pass toward the mouth.

The term voiced refers to the condition where the vocal cords open and close rapidly. Often you will see this referred to as the vocal cords are vibrating. Now, a word of clarification. This is not caused by the vocal cords simply vibrating in place. The rapid opening and closing of the vocal cords cause a sensation of vibration.

You can feel the difference by placing two fingers to the side of your neck. 

Make the “s” sound and hiss like a snake. Nothing. No vibration. This is a voiceless sound.

Now, you’ve become a bee; so, start buzzing by making the “z” sound. Do you feel the sensation of vibrating? The “z” sound is a voiced sound.

Try it with the “f” sound. Voiceless.

Now, the “v” sound. Voiced.

The two different sounds associated with the letters “th” are made in exactly the same except that one is voiced and the other voiceless.

Contrast thigh with thy. Ether with either. Teeth with teethe.

With awareness and repeated practice, these sounds can become part of the sound palette of anyone learning English.

Related Questions

Is the “th” sound a combination of the “t” and the “h” sounds? No, there isn’t a one-to-one correspondence between the letters of the Roman alphabet and the sounds of a language that uses that alphabet; so, some sounds are represented by two-letter combinations. Besides “th,” “sh” refers to a single sound. Also, particular letters can refer to different sounds and particular sounds can be represented by different letter combinations.

What is the purpose of the IPA alphabet? The IPA (International Phonetic Association) alphabet was developed to offer a consistent framework for representing speech sounds. As much as possible, the effort is made to specify a particular speech sound with a unique symbol.

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