... crystal to shake any hard surface, like wood, glass, wall, stone and so on, and make hard ... Direction to usage: STEP 1 Connection Firstly, insert one side of the audio cable into the signal input jack ... , MP3, MP4, MP5, PDA etc. Then, insert one side of the power plug into the power interface of vibration ... crystal to shake any hard surface, like wood, glass, wall, stone and so on, and make hard ...