The windpipe (trachea), voicebox (larynx), pharynx, and nasal passages make up the upper airway. Respiratory noise happens when there is a change in the anatomy or function of any part of the upper airway, and which interferes with normal smooth airflow. Turbulent airflow causes noise.

Diseases of the lower respiratory tract (lungs) are less likely to cause obvious sounds.

Some respiratory noise is normal. A common puffing, blowing sound is heard at each stride of the canter in some horses. This is known in the horse world as "High Blowing" and usually refers to vibration of the nostril or false nostril during exhalation. It is generally not considered a problem.

But there are a variety of conditions affecting the upper airway of performance horses that cause respiratory noise that only exist or become apparent during exercise. The negative pressures during inspiration cause collapse of various structures of the upper airway into the path of the air resulting in audible, turbulent sounds.

The most familiar abnormal condition causing audible respiratory noise is laryngeal hemiplegia, or left laryngeal paralysis. This causes a sound known in the horse world as "roaring". But there are many other less common conditions.

Not all conditions causing respiratory noise negatively affect performance. It can be hard to tell whether a noise is associate with a reduction in performance. Conditions change and progress, so a problem that might be causing little trouble now (and is easily treated) could progress later into something that is both damaging to the horse and difficult to treat.