A sound trap is a special acoustical

Acoustics

Acoustics is the interdisciplinary science that deals with the study of all mechanical waves in gases, liquids, and solids including vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician while someone working in the field of acoustics...

 treatment of Heating Ventilating and Air-Conditioning (HVAC)

HVAC

HVAC refers to technology of indoor or automotive environmental comfort. HVAC system design is a major subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer...

 duct

Duct (HVAC)

Ducts are used in heating, ventilation, and air conditioning to deliver and remove air. These needed airflows include, for example, supply air, return air, and exhaust air. Ducts also deliver, most commonly as part of the supply air, ventilation air...

work designed to reduce transmission of noise

Noise

In common use, the word noise means any unwanted sound. In both analog and digital electronics, noise is random unwanted perturbation to a wanted signal; it is called noise as a generalisation of the acoustic noise heard when listening to a weak radio transmission with significant electrical noise...

 through the ductwork, either from equipment into occupied spaces in a building, or between occupied spaces.

In its simplest form, a sound trap consists of an offset in the ductwork to reflect the sound back to its source. This configuration is often combined with the use of sound-absorbing

Absorption (acoustics)

Acoustic absorption is that property of any material that changes the acoustic energy of sound waves into another form, often heat, which it to some extent retains, as opposed to that sound energy that material reflects or conducts. Acoustic absorption is represented by the symbol A in calculations...

 material inside the trap. The physical dimensions of the sound trap may be selected to tune

Tune

Tune may refer to:* A melody* A tune-family* A tune , a short piece of instrumental music, usually with repeating sections, and often played a number of times* British slang term, often said when referring to a piece of music that is enjoyed...

 the trap to specific frequencies

Frequency

Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

 of sound. As such, it is then essentially a Helmholtz resonator used as a passive noise-control device.