The TH Sound in English: A Clinical Guide for Multilingual Doctors, Nurses, and Pilots
For multilingual doctors, nurses, and pilots, the TH sound is one of the most frequent points of friction in spoken English. It appears constantly in clinical and aviation language, in words like the, this, that, think, three, breathe, within, threshold, and other. When it is substituted with a different sound, the listener registers a small mismatch. In a fast-paced handoff, a patient instruction, or an ATC readback, that mismatch adds up.
The good news is that TH is not a complicated sound to understand. It has a specific placement, a clear physical target, and two forms that follow a consistent pattern. Once you know what you are producing, you can refine it directly.
What Makes TH Unusual
English has two TH sounds, and both require the same tongue placement: the tip of the tongue rests lightly between the upper and lower teeth, and air flows over it. This is what makes TH different from nearly every other sound in English. Most sounds are produced inside the mouth, with the tongue at the roof, the teeth, or the back of the throat. TH brings the tongue forward, to the edge of the mouth.
Because this placement does not exist in many other languages, multilingual speakers often substitute a sound that feels more natural. The most common substitutions are S, T, Z, and D.
T and D are stop sounds. The tongue makes full contact inside the mouth, briefly blocks the airflow, and then releases it. That contact is the opposite of what TH requires. TH is a continuant: the tongue stays lightly at the teeth and air flows continuously over it, without ever stopping.
S and Z are closer in feel, because they are also continuants with steady airflow. The difference is where the sound is being shaped. S and Z are formed inside the mouth, with the tongue raised toward the roof. When speakers attempt TH and the tongue does not move far enough forward, the mouth defaults to that more familiar interior position and produces S or Z instead, sometimes without the speaker noticing at all.
Understanding where these substitutions come from makes it easier to correct them. The target is not just a different sound. It is a different location in the mouth.
A substitution does not always cause a misunderstanding. But it does create a moment of extra processing for your listener, and in settings where clarity is tied to patient safety or flight safety, reducing that processing load matters.
The Two TH Sounds
Voiceless TH is produced with air only. There is no voice, no vibration in the throat. You will hear it in words like think, three, thing, thank you, breath, and both. Place your tongue between your teeth and push a steady stream of air over it. That is the full sound.
Voiced TH adds vibration. The tongue placement is identical, but the voice is on while the air flows. You will hear it in words like this, that, them, the, breathe, other, and with. Place your tongue in the same position as voiceless TH, then add voice. If you place two fingers lightly on your throat, you will feel the difference immediately.
The physical placement is the same for both. The only variable is whether the voice is on or off.
Words That Matter in Your Work
These are words that appear regularly in clinical and aviation communication. Practice them with attention to where your tongue is.
Voiceless TH: think, three, therapy, threshold, both, breath, width, health, through
Voiced TH: the, this, that, them, there, other, breathing, within, smooth
A useful practice sentence that includes both: This is the thing I am thinking about.
Say it slowly at first, with your tongue visibly between your teeth. Then bring it up to natural speed without losing the placement.
A Note on Daily Practice
Two minutes of focused practice each day produces more change over time than occasional longer sessions. Choose two or three words from your clinical vocabulary and practice them with full tongue placement until the position feels automatic. Then add more.
Recording yourself is one of the most effective tools available to you. Play it back and listen specifically for the TH words. If you hear S, T, Z, or D in their place, you know exactly what to work on.
Watch the Video
For a step-by-step demonstration of how to produce both TH sounds, watch the video guide below. Seeing the tongue placement in real time is often more useful than a written description alone.
Watch the video: Video Guide →
Take It Further
If you would like targeted feedback on your TH sound or any other clarity pattern, the diagnostic is a free 15-minute Zoom. You leave with your top three clarity patterns identified and a clear sense of where to focus. No pressure, no obligation.
Claire Costello is a licensed Speech-Language Pathologist with 35 years of clinical experience and a specialist certification in accent modification, specializing in speech clarity coaching for multilingual doctors, nurses, and pilots.
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