A customer call reflects the problem. After understanding, the original is the guests to buy, the requirements of the test stack using the effect, so in the store to the user test, the results of the phenomenon of sound off, this is because the test method is wrong. When stacking is used, all the microphones are required to be 5 meters above the receiver, and each two microphones should be spaced apart by 30cm or more, which is consistent with the actual use case. But the customer tested, all 10 microphones were stacked together, and the microphone was less than 2 meters from the receiver. Technically, when the open microphone is more, their signals enter the receiver at the same time, will interact with each other, resulting in some new signals, the formation of interference. The closer the microphone, as well as the distance between the microphone and the receiver, the more serious the impact. Some customers in the test machine, single-handedly hold two microphones, which in the use of a single product will not be a problem, but many sets of products may cause problems. So the stack test machine, should be in strict accordance with the above requirements of the distance, it will not go wrong.