Rales and rhonchi are both abnormal lung sounds heard through a stethoscope, but they differ in pitch, pattern, timing, and what they suggest about your lungs. Rales (also called crackles) are short, popping or bubbling sounds caused by small airways snapping open, while rhonchi are low-pitched, continuous sounds that resemble snoring, caused by mucus or obstruction in the larger airways.
How Each Sound Is Produced
Rales originate deep in the lungs, in the smallest airways and air sacs. When these tiny passages collapse or fill with fluid, they snap open as you breathe in, producing brief popping or crackling noises. Think of the sound you hear when you slowly pull apart a strip of Velcro: each little pop is a separate event. Research on crackle mechanics confirms that inspiratory crackles come from explosive airway reopening, while the less common expiratory crackles result from sudden airway closure.
Rhonchi come from the larger airways, including the bronchial tubes and trachea. When mucus or secretions line these bigger passages, air flowing through them causes the walls to vibrate. The result is a rumbling, snoring-like sound that can sometimes be felt through the chest wall. Because the cause is secretions sitting in large airways, rhonchi often change or disappear after coughing clears the mucus. Rales typically do not clear with a cough.
What They Sound Like
The easiest way to tell them apart is their pattern and pitch. Rales are discontinuous: each crackle is a distinct, separate sound lasting less than 25 milliseconds, similar to dropping marbles on a hard floor. Rhonchi are continuous sounds lasting 250 milliseconds or longer, more like the low drone of snoring or the hum of a fan.
Rales are further divided into two subtypes:
- Fine crackles are high-pitched, very brief sounds (around 650 Hz, lasting about 5 milliseconds). They’re often compared to the sound of hair being rubbed between your fingers near your ear.
- Coarse crackles are lower-pitched and last longer. They sound wetter and more gurgling, closer to bubbling water.
Rhonchi sit in the low-pitch range and sound musical in a rough, droning way. The American Thoracic Society draws the line between wheezes and rhonchi based on pitch alone: wheezes are high-pitched continuous sounds, and rhonchi are low-pitched continuous sounds. Both are produced by narrowed airways, but rhonchi involve the larger passages.
When They Occur in the Breathing Cycle
Rales are predominantly heard during inspiration, the phase when you’re drawing air in. As air rushes into collapsed or fluid-filled small airways, those tiny passages pop open, generating crackles. Expiratory crackles can occur but are far less common and less energetic.
Rhonchi behave differently. They tend to occur during expiration, or during both inspiration and expiration, but they do not occur during inspiration alone. This makes sense given their source: air being pushed out through mucus-narrowed large airways creates the vibration and turbulence responsible for the sound.
Where in the Lungs They Originate
Rales come from the peripheral lung, the outermost tissue where the smallest airways and air sacs sit. This is why they’re associated with conditions that affect the lung tissue itself, such as fluid buildup or scarring. Fine crackles in particular are unrelated to mucus secretions and instead reflect changes in the lung’s structure or fluid content.
Rhonchi come from the central, larger airways. They tend to be heard in specific areas of the chest that correspond to the location of secretions or infection, rather than being spread diffusely across both lungs.
Conditions Associated With Each Sound
The type of abnormal sound a clinician hears provides a strong clue about what’s going on. Rales point toward problems in the lung tissue itself. Fine crackles are a hallmark of interstitial pulmonary fibrosis (scarring of the lung), pneumonia, and congestive heart failure. In fibrosis, the crackles come from stiffened airways that snap open abnormally. In heart failure and pulmonary edema, fluid leaks into the air sacs and creates wet, bubbling crackles at the base of the lungs. Pulmonologists can sometimes detect early-stage fibrosis or pneumonia from inspiratory crackles alone, even when a chest X-ray still looks normal.
Rhonchi point toward problems in the larger airways, particularly excess mucus. They’re commonly heard in chronic bronchitis, COPD, bronchiectasis, and acute respiratory infections where secretions accumulate. Chronic bronchitis, defined as a cough producing mucus for at least three months a year over two consecutive years, is one of the most frequent causes. Because rhonchi reflect mucus in the airways rather than structural lung damage, they often improve temporarily when a patient coughs or when secretions are cleared.
A Quick Side-by-Side Comparison
- Sound quality: Rales are short, popping, discontinuous. Rhonchi are low, rumbling, continuous.
- Pitch: Fine rales are high-pitched; coarse rales are lower. Rhonchi are consistently low-pitched.
- Timing: Rales are heard mainly on breathing in. Rhonchi are heard on breathing out, or on both phases.
- Location: Rales come from small, peripheral airways. Rhonchi come from large, central airways.
- Cause: Rales result from airways snapping open or fluid in air sacs. Rhonchi result from mucus vibrating in large airways.
- Response to coughing: Rales generally persist. Rhonchi often clear or change after a cough.
- Common conditions: Rales suggest pneumonia, heart failure, or lung fibrosis. Rhonchi suggest bronchitis, COPD, or airway infection.
Why the Terminology Gets Confusing
Part of the confusion is that “rales” and “crackles” mean the same thing. The terms are used interchangeably, and which one a clinician uses often depends on where they trained. Older medical texts favor “rales,” while newer guidelines tend to prefer “crackles” for clarity. Some clinicians also avoid the word “rhonchi” entirely, instead calling those sounds “coarse crackles,” which can blur the distinction further. The key difference remains: crackles are brief, interrupted sound events, and rhonchi are sustained, continuous ones. Keeping that single distinction in mind cuts through most of the naming confusion.