Much like full anechoic facilities, hemi-anechoic chambers are lined with absorbent materials, however the floor is left as a flat finish with no absorption. Hemi-anechoic chambers are more common and still allow very precise measurements to be taken, but the free-field area is only half a sphere (or hemisphere, hence the name). Hemi-anechoic chambers offer an ideal solution to testing large or heavy items as there is no need for a specialised floor to cover any wedges found in full anechoic laboratories.
Although some design issues are overcome with hemi-anechoic chambers, facilities are usually large in size and have complex requirements to suit the product being examined. In the case of taking acoustic measurements for a large generator as an example, it is not simply a case of placing the item in the middle of the lab and taking the necessary measurements. The following are just some of the elements which enable such a facility to operate effectively:
- Silent ventilation system capable of passing large volumes of air into the hemi-anechoic chamber when an engine is running
- Fuel system for powering the generator whilst on test
- A silenced extraction system for removing all of the noxious fumes from the enclosure
- Heating / cooling system to ensure that a constant temperature and humidity are kept inside the chamber to get the most consistent results
- A control and monitoring system for engineers to use to effectively run the hemi-anechoic chamber
- All the above (and many more possible options) need to be carefully considered at the planning stage of the hemi-anechoic enclosure in order to get the best possible results for the end-user.