Aspeaker fixed to a moving platform moves toward a wall, emitting a steady sound with a frequency of 225 hz . a person on the platform right next to the speaker detects the sound waves reflected off the wall and those emitted by the speaker. part a how fast should the platform move, vp, for the person to detect a beat frequency of 7.00 hz ? take the speed of sound to be 344 m/s. express your answer