Because the old model will be a sound problem, later modified products have no relevant issues. Current noise test project should be different tests but developers neglect does not modify the standard book.
Because the old models have problems too loud, and later improved after the product has no related issues. At present, the actual noise test tone test project should be different, but researchers neglected to modify standard operation.
Because of the old models will be the voice of big problem, then product after improvement has no related problems. While the actual noise test items should be test different sound but R & D personnel negligence does not modify the standard operation procedure.