Ultrasonics: the study of ultrasound.

Ultrasound: acoustic waves with frequencies, which are too high to be heard by human ears.

Uncertainty: an estimate of the degree of variability or dispersion associated with the result of a measurement.

Unweighted sound pressure level: a sound pressure level, which has not been frequency weighted, sometimes known as the linear sound pressure level; symbol Lp(see also Z-weighting).

Upper exposure action value: a noise exposure level defined in the 2005 Control of Noise at Work Regulations requiring action from employers and employees (a personal daily (or weekly) noise exposure level of 85 dBA, or peak sound pressure level of 137 dBC).