lydtryk

Lydtryk, or acoustic sound pressure, is the local deviation of ambient atmospheric pressure caused by a propagating sound wave. It is a mechanical pressure fluctuation in the medium (usually air) accompanying sound and is a fundamental quantity in acoustics and audio engineering.

The SI unit of lydtryk is the pascal (Pa). In practice, sound-pressure levels are usually described using decibels relative to a reference pressure, denoted dB SPL. SPL is defined as 20 times the logarithm base 10 of the rms sound pressure p_rms divided by the reference pressure p0, where p0 is 20 micro pascals (20 μPa) in air.

Measurement of lydtryk is typically performed with microphones or hydrophones. The measured p_rms depends on distance from the source, environmental conditions, and frequency, and requires proper calibration and accounting for the impedance of the medium.

Lydtryk is distinct from sound intensity. Pressure is a local field quantity, while intensity describes the rate of energy transfer and depends on both pressure and particle velocity, as well as the medium’s characteristic impedance. In a given medium, there is a relationship between pressure level and intensity, but two sources with the same SPL can differ in actual energy flow if the acoustic path impedance varies.

Applications of lydtryk include noise assessment, architectural and automotive acoustics, and occupational health. Typical reference levels include quiet indoor environments (approximately 20–40 dB SPL), normal conversation (~60 dB SPL), and loud music or machinery (often ~100 dB SPL), with higher levels posing greater risk of hearing damage.

In Danish usage, lydtryk is the standard term for acoustic pressure, commonly used alongside SPL in technical contexts.