1. Electro-acoustic transducers
Headphones
Insert earphones
Bone-conduction vibrators
Loudspeakers

2. Calibration
Important for all equipment used to present sound and measure thresholds.
Making sure that the sound being produced is what it is supposed to be.
If not, add/subtract a correction factor.

3. Calibration Earlier: Psychoacoustic method
Measuring known normal-hearing listeners’ thresholds
Now: Electroacoustic method
Using transducer, artificial ear or mastoid, weight, microphone, and sound level meter (SLM).