High linearity (low THD)
Sound in the near field
Compared to earphones and headsets, which are positioned in the ear canal or directly onto the ear, the loudspeakers in smart glasses are situated in a so called near field related to the ear entrance. Therefore the tuning of the acoustic parameters are quite different from ear- and headphones, both in repects of desired frequency response and loudness. The acoustic impedance “seen” by a near field driver is very different from the in-ear impedance, which is why it takes much larger driver to produce a satisfying bass response compared to sealed earphones.
Choosing an appropriate driver
When designing a pair of smart glasses it is tempting to choose as small a driver as possible to keep the overall thickness of the temples down. However, the laws of physics still apply no matter how smart the smart glasses are. In order to get as much “bass” frequencies as possible, a certain volume of air must be moved which, in general, correlates with both the diameter and the thickness of the chosen driver. Other important parameters to consider are