EMI - ElectroMagnetic Interference is a fact of modern life. RF energy from nearby cellular phone, magnetic fields from large motors, even spurious emissions from computer systems can and do induce significant errors into Sound Level Meters, Analyzers, Data Recorders and other measurement equipment. 
US and International Standards are being amended to reflect this concern and the need to confirm possible EMI errors.
The ACOustiCap™ acoustical isolator permits easy testing for stray EMI, AC electrical and even Ultrasonic interference. The patented multilayer design, transparent to EMI, provides 30 dB (typical) acoustic isolation over the audible spectrum.
It's as Simple as 1, 2, 3
1.) Measure the surrounding SPL
2.) Place the ACOustiCap™ over the microphone
3.) The measured SPL should drop 20 - 30 dB
