Note 1 to entry: The absorption measurement is carried out at the following two wavenumbers:

1. at the reference wavenumber (3,800 × 10⁵ m^–1 or 3,979 × 10⁵ m^–1), where the infrared absorption is very small,
2. at the absorption wavenumber (3,500 × 10⁵ m^–1 or 3,585 × 10⁵ m^–1), where the infrared absorption due to hydroxyl (OH) impurities in the crystal lattice becomes large.

Note 2 to entry: Hydroxyl (OH) impurities create mechanical loss in resonators and their presence is correlated with the presence of other loss-inducing impurities. The infrared absorption coefficient is a measure of hydroxyl (OH) concentration and is correlated with expected mechanical losses due to material impurities.

Note 3 to entry: The infrared absorption coefficient is expressed by the following equation using the logarithm base 10:

${\displaystyle \alpha =\frac{1}{t}{\log }_{10}\frac{T_1}{T_2}}$

where

α is the infrared absorption coefficient;

t is the thickness of the Y-cut sample, in centimetres;

T₁ is the transmittance at a wavenumber of 3,800 × 10⁵ m^–1 or 3,979 × 10⁵ m^–1;

T₂ is the transmittance at a wavenumber of 3,500 × 10⁵ m^–1 or 3,585 × 10⁵ m^–1.