phonoscope

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A device that produces a visible display of the mechanical properties of a sounding body, especially of musical instruments.

from The Century Dictionary.

• noun A machine for recording music as it is played or sung, or for testing the quality of strings for musical instruments.
• noun Same as microphone.

from the GNU version of the Collaborative International Dictionary of English.

• noun An instrument for observing or exhibiting the motions or properties of sounding bodies; especially, an apparatus invented by König for testing the quality of musical strings.
• noun An instrument for producing luminous figures by the vibrations of sounding bodies.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun a device that shows an image representing the acoustical properties of a musical instrument etc.