Adventitious breath sounds are abnormal sounds heard through a stethoscope in addition to the normal, expected sounds of breathing. They signal that something is interfering with the usual flow of air through the lungs or airways, whether that’s fluid buildup, narrowed passages, inflammation, or obstruction. These sounds fall into two broad categories: continuous sounds (like wheezes and rhonchi) that last longer than 80 milliseconds, and discontinuous sounds (like crackles) that are brief, explosive pops or clicks.
How Adventitious Sounds Differ From Normal Breathing
Normal lung sounds are soft, low-pitched, and rustling. You can think of them as the quiet whoosh of air flowing smoothly through open airways and filling healthy lung tissue. Adventitious sounds layer on top of those normal sounds and stand out because they have distinct musical tones, popping qualities, or grating textures that shouldn’t be there. Their presence tells a clinician where in the airways a problem exists and often what kind of problem it is.
Wheezes: High-Pitched Whistling Sounds
Wheezes are continuous, high-pitched, musical sounds typically heard during exhalation. They occur when airways narrow enough that air passing through them causes the airway walls to vibrate rapidly, fluttering between nearly closed and nearly open. This mechanism follows the same physics principle that makes a garden hose whistle when you partially cover the nozzle: faster airflow through a tighter space drops the pressure inside, causing the walls to oscillate.
Wheezes register above 400 Hz, giving them their characteristic whistling or hissing quality. They’re closely associated with asthma, where the airways constrict, produce excess mucus, and swell, all at once. They also show up commonly in COPD and chronic bronchitis. When wheezing is heard in only one spot rather than throughout both lungs, it can indicate a localized blockage like a tumor or inhaled foreign object. Widespread wheezing across both lungs points more toward a systemic condition like an asthma flare.
Rhonchi: Low-Pitched Snoring Sounds
Rhonchi sound like snoring or low-pitched rumbling. They sit below 200 Hz, which gives them a rougher, coarser quality than wheezes. While both are continuous sounds, rhonchi have a distinctly less musical, more rattling character. The key difference is what’s causing the airway narrowing: rhonchi typically point to mucus or secretions sitting in the larger airways, which is why they often change or disappear after a strong cough clears those secretions.
Rhonchi are most commonly linked to bronchitis and COPD, conditions where thick mucus builds up in the bronchial tree. Even experienced clinicians sometimes struggle to distinguish low-pitched wheezes from rhonchi, and research has documented significant disagreement between listeners when differentiating the two. In practice, whether a sound clears with coughing is one of the most useful clues that it’s rhonchi rather than a true wheeze.
Crackles: Brief Popping or Clicking Sounds
Crackles are short, non-musical, explosive sounds that last just a few milliseconds. They come in two distinct varieties that point to different problems.
Fine Crackles
Fine crackles are high-pitched (around 650 Hz), extremely brief (about 5 milliseconds each), and tend to occur late in inspiration. They’re produced when small airways that have collapsed shut suddenly snap open as the lung expands. The sound is often described as similar to pulling apart Velcro, or the crinkle of cellophane. Fine crackles are associated with pneumonia, congestive heart failure, and pulmonary fibrosis. In fibrosis specifically, “Velcro-like” fine crackles can be one of the earliest detectable signs of the disease.
Coarse Crackles
Coarse crackles are lower-pitched (around 350 Hz), last roughly three times longer than fine crackles (about 15 milliseconds), and tend to appear early in inspiration or during both inspiration and expiration. They’re caused by air bubbling through fluid or mucus in the larger airways and bronchial passages. Chronic bronchitis, bronchiectasis (permanent widening of the airways), and COPD are the most common conditions behind coarse crackles.
The distinction matters because it can narrow down a diagnosis quickly. Fine crackles in someone who’s short of breath might suggest fluid backing up into the lungs from heart failure. Coarse crackles in someone with a chronic productive cough point more toward a secretion-heavy airway disease.
Stridor: A Sign of Upper Airway Narrowing
Stridor is a high-pitched, musical sound heard most prominently during inhalation. Unlike wheezes, which originate in the lower airways and lungs, stridor comes from the upper airway: the larynx, trachea, or throat. It’s produced by turbulent airflow through a significantly narrowed upper passage and registers above 500 Hz, making it audible even without a stethoscope in many cases.
In children, stridor is particularly concerning because their airways are smaller and more vulnerable to complete obstruction. Croup (a viral infection causing swelling below the vocal cords) is one of the most common causes in young children, producing the characteristic “barking” cough alongside stridor. Other causes across all ages include epiglottitis, swelling from allergic reactions, and foreign objects lodged in the airway. Stridor is generally treated as an urgent finding because it signals that the airway may be at risk of closing off entirely.
Pleural Friction Rub: Inflamed Membranes Grinding
The lungs are wrapped in two thin membranes, the visceral pleura (touching the lung) and the parietal pleura (lining the chest wall). A thin layer of fluid normally lets these surfaces glide silently past each other as you breathe. When inflammation roughens one or both surfaces, they grind together and produce a pleural friction rub.
The sound is non-musical, grating, and localized to one area of the chest. It’s been compared to the creak of leather, the squeak of shoes on wet floor, or the crunch of walking on fresh snow. Unlike most other adventitious sounds, a pleural friction rub occurs during both inhalation and exhalation, since the inflamed surfaces rub in both directions. Pleurisy (inflammation of the pleural lining), pleural effusion, and infections that irritate the pleura are common causes. The sound is usually felt as much as heard, and patients often report sharp, localized chest pain that worsens with breathing at the same spot where the rub is detected.
Squawks: A Less Common Finding
Squawks are short inspiratory sounds lasting roughly 200 milliseconds that combine features of both wheezes and crackles. They typically start with a crackle and transition into a brief musical tone in the 200 to 300 Hz range. They’re generated by the oscillation of small peripheral airways and are most characteristic of hypersensitivity pneumonia, an inflammatory lung reaction to inhaled allergens like mold or bird droppings. Squawks have also been noted in some cases of common pneumonia.
What Each Sound Suggests
- Wheezes: asthma, COPD, airway obstruction from a foreign body or tumor
- Rhonchi: bronchitis, COPD, excess mucus in larger airways
- Fine crackles: pneumonia, heart failure, pulmonary fibrosis
- Coarse crackles: chronic bronchitis, bronchiectasis, COPD
- Stridor: croup, epiglottitis, foreign body in the throat, laryngeal swelling
- Pleural friction rub: pleurisy, pleural effusion, chest infections
- Squawks: hypersensitivity pneumonia, pneumonia
How These Sounds Are Detected
Adventitious sounds are identified through auscultation, which simply means listening to the lungs with a stethoscope. The exam works best in a quiet room with the patient sitting upright, breathing deeply through an open mouth. The stethoscope is placed directly on the skin (never over clothing) starting at the top of the chest and moving downward, then repeated on the back. At each spot, the clinician listens for at least one full breath cycle and compares matching locations on the left and right sides to catch asymmetry.
Positioning matters. Some crackles only appear after a patient has been lying down for several minutes, as fluid shifts in the lungs with gravity. Body position can also make certain sounds louder or quieter. In patients who can’t sit up, rolling from side to side allows access to the back of the chest, where many abnormal sounds are easiest to hear. In children, the same principles apply, though the exam requires more patience and the thinner chest wall can actually make abnormal sounds easier to detect.
Adventitious Sounds in Children
Several conditions that produce adventitious sounds are far more common in pediatric patients. Bronchiolitis, a viral infection of the smallest airways typically caused by respiratory syncytial virus (RSV), is a leading cause of wheezing in infants and toddlers. In children with bronchiolitis, wheezing can occupy a significant portion of the breathing cycle. Crackles in children are frequently tied to atelectasis, where small portions of the lung collapse, a finding that’s been reported in up to 30% of cases involving middle lobe collapse. Croup remains one of the most recognizable causes of stridor in young children, producing the distinctive seal-like barking cough alongside the high-pitched inhalation sound.