Normalized longitudinal impact level

Professional Definition

The normalized longitudinal impact sound level (usually denoted as $L_{n,f}$ or $L'_{n,f}$) is the level of impact sound pressure in the receiving room, originating solely from sound transmission through a specific side path (e.g., a continuous floor or exterior wall), normalized to an acoustic absorption reference of $A_0 = 10 m^2$.

This quantity is determined based on the formulas contained in the standards of the series PN-EN ISO 12354 (part 2). It is a component parameter necessary for calculating the total acoustic insulation between rooms, taking into account that impact energy (e.g., footsteps on the floor) travels through the building material and radiates into the receiving room not only through the separating partition but also through adjacent elements.

Acoustics in Simple Words

Imagine two rooms separated by a thick brick wall. Theoretically, nothing should pass through it. However, both rooms share a common concrete floor that runs the entire length of the building. When someone in the first room hits the floor (e.g., walking in hard shoes), these vibrations enter the concrete and travel through it like water in a pipe, bypassing the partition wall. Then, the floor in the second room starts to vibrate and emits noise.

The normalized longitudinal impact sound level is a measure of this noise that came to you “sideways.” It’s somewhat like sealing your front door tightly to not hear the noise from the stairwell, but the sound still gets inside through the connected ventilation system. In building acoustics, the “ventilation” for noise are the party walls and floors connecting neighbors.

Summary

This parameter is crucial when designing multi-family buildings with enhanced acoustic standards. It allows engineers to accurately predict whether simply thickening the wall between apartments will be enough, or whether it will be necessary to use expansion joints or special floating floors to cut off the side path for impact sounds.