The four normal breath sounds

Where do the four normal sounds come from?

In the previous module we saw that breath sound starts in the trachea and large bronchi. Air flows fast and irregularly there. This turbulent flow produces a broadband rustle, which means a sound that contains many pitches at once. The small airways and alveoli add almost nothing to this sound.

You hear different sounds at different points on the chest because the path the sound travels to your ear changes. On the neck, the stethoscope lies right over the source. Over the periphery, several centimetres of air-filled lung lie in between. Air-filled lung lets low-pitched sound through, but it filters out much of the high-pitched sound.

So the names of the four normal sounds are not the names of four separate sources. Each name tells you how the sound reaches the ear and where it is usually heard:

• Tracheal breath sound: neck, suprasternal notch.
• Bronchial breath sound: over the manubrium.
• Bronchovesicular breath sound: upper parasternal area at the front, between the scapulae at the back.
• Vesicular breath sound: most of the peripheral lung fields.

How do you tell the four sounds apart by ear?

Listen for four features: the intensity of the sound, its pitch, how long each of the two phases is heard and whether there is a silence between the phases. These are the intensity, pitch and timing we covered in the Three measures of sound lesson.

Why do these sounds differ?

The first reason is distance. Over the periphery, sound passes through air-filled lung and loses more of its high-pitched components on the way. So a soft, low-pitched rustle reaches the chest wall. Over the trachea, no lung lies in between. So the sound stays loud, harsh and broadband (How does sound reach the chest wall?).

The second reason is that the sound arises in different places in the two phases. In inspiration, the regions that produce sound are spread through the lung. In expiration, the sound comes mainly from the large central airways and reaches the peripheral chest wall only weakly. This is why expiration is short and faint in the vesicular sound.

Closer to the centre, expiration becomes clearer as well. On the neck, the two phases are heard at almost equal strength.

Finally, consider the silence between the phases. This silence follows from the first rule: no flow, no sound. When breathing turns from inspiration to expiration, flow stops for a moment and the sound stops too.

Over the trachea and manubrium, both phases are loud, so this short break is clearly heard. Over the periphery, expiration already starts faintly, so you do not notice the gap.

How do you train your ear?

Listen to the four sounds in order, from the source outwards. At each step, focus on what has changed from the previous sound.

1. Tracheal breath sound. It is loud, harsh and broadband. Both phases are strong, with a short silence between them. Listen: tracheal breath sound
2. Bronchial breath sound. It is still loud and high-pitched, but a little softer. Notice how long and loud expiration stays. Listen: bronchial breath sound
3. Bronchovesicular breath sound. The high-pitched components have faded. The two phases are about equal in length, and the silence between them is now indistinct. Listen: bronchovesicular breath sound
4. Vesicular breath sound. It is a soft, low-pitched rustle. Inspiration dominates. Expiration is short and fades at once. Listen: vesicular breath sound

Then reverse the order and look for the same differences, this time starting from the vesicular sound. You can do the same exercise on a colleague. Start with the stethoscope on the neck. Then move it in turn to the manubrium, between the scapulae and finally to the lung bases.

What does a bronchial sound over the periphery mean?

First, keep two terms apart. Over the manubrium you hear a loud, high-pitched sound with a long expiration. We call it bronchial breath sound. This sound is normal there.

If you hear a sound of the same character over the lung periphery, for example over the right lower zone posteriorly, we call it bronchial breathing. Bronchial breathing is an abnormal finding. The character of the sound is similar. What makes it abnormal is where you hear it.

If you hear bronchial breathing over the periphery, the path of the sound has changed. The most common cause is consolidation. In consolidation, fluid and cells have replaced the air in the alveoli. If the bronchus stays open, the central sound reaches the chest wall without being filtered. This follows from the second rule: solid lung carries sound.

The interpretation is stronger if the voice transmission tests also show increased transmission over the same area. These tests are bronchophony, whispered pectoriloquy and egophony (Voice transmission tests). The upper border of an effusion and compressive atelectasis can also give bronchial breathing over the periphery. A bronchovesicular sound over a peripheral area points the same way, but it is a weaker clue.

Listen: bronchial breathing and crackles over consolidation

You can read how consolidation carries sound in the How does sound reach the chest wall? lesson. The picture in pneumonia is on the Pneumonia and consolidation page.