What Is An Engine Noise Test Cell?

Hemi-Anechoic Chambers

With modern equipment it is easier to accurately measure and to realistically predict outdoor noise characteristics indoors. Typically, the chambers used are hemi-anechoic and have many features in common with the Vehicle Semi-Anechoic Chambers (VSACs) used for the measurement of noise and vibrations in cars. Hemi-anechoic design means that rather than the engine being suspended in a spherical chamber baffled in all directions, they can stand above a solid floor. This is not only easier and more affordable to achieve but is also more appropriate for modelling most of the relevant real-world operating conditions.

A jet engine test cell will feature a variety of powerful ventilation and cooling systems. Appropriate dynamometers and other test equipment are also supplied. Designing, setting up and operating such chambers is understandably more straightforward when a single manufacturer and contractor supplies their own tried, tested and compatible acoustic doors, acoustic windows, wedges, support services and expertise.

Design Challenges

While sound absorption materials themselves are now a highly advanced art, test chambers for engines of any kind must overcome a variety of additional critical challenges. Jet engines, rockets, generators, internal combustion engines and so on don’t just make noise – they also consume fuel and oxygen, generate waste gases and heat and exert thrust. The chamber itself must also often contain and protect human beings and delicate measuring equipment. The systems that cope with these side issues all penetrate the engine noise test cell in one way or another and pose noise control issues of their own which require isolating from the performance of the engine on test.

Instrumentation

Properly designed, an engine noise test cell is capable of much more than measuring gross frequency spectrums and decibels. They can be focused upon specific vibrations in component parts and across their mounting systems and study the effects of a variety of deflection shapes – either to model environmental conditions or to suggest design and development enhancements. Structure-borne transfer path analysis can be related to engine power characteristics over a range of conditions impossible to control in any other way.
The range of measuring technologies is wide. Your designer will advise you on the kind that best suits your particular needs. They include transducers, laser vibrometers, acoustic holography and binaural head instruments.
Anechoic chambers of any kind can be designed in whatever size and configuration is required by their test subject, but typical dimensions for a hemi-anechoic chamber are in the range of 6.5 x 6.5 x 4.0 metres, so they are substantial in scale as well as sophistication. Ensure your jet engine test cell is designed by an equally substantial and sophisticated company like IAC.