**8. External noise**

The SNR in decibel is defined as

$$R\_{\rm SNR} = L\_{\rm REF} - L\_{\rm N} \tag{38}$$

using reference SPL *L*REF from Eq. (37) and a noise SPL *L*N. The stationary noise caused by the microphone and other electronic parts can be measured with a muted stimulus in a single test at any point **r**. The instantaneous SNR can be used to validate the distortion ratios TDR in Eq. (30) and IDR in Eq. (36) to remove invalid data.
