Difference in levels (D)

Professional Definition

The difference in levels (D) is the difference between the time- and space-averaged sound pressure level in the source room (L1) and the level in the receiving room (L2), produced by the same sound source. Formula: D = L1 - L2.

This value is expressed in decibels [dB]. It should be noted that the raw difference in levels (D) depends not only on the quality of the partition (wall) but also on the acoustic absorption of the receiving room. Therefore, in comprehensive engineering analyses, it is often transformed into the specific sound insulation (R'), which is a parameter more independent of environmental conditions.

Acoustics in Simple Words

Imagine a dam. On one side, the water stands high (loud sound in the source room, e.g., 90 dB), and on the other side, behind the dam, the water level is low (the sound that has penetrated to the second room, e.g., 40 dB). The difference in levels is simply the difference in water height in front of and behind the dam (in this case, 50 dB). The higher this difference, the tighter and better the dam (wall), as it has retained more energy on the other side.

Summary

The difference in levels (D) is a key in situ parameter indicating how much the wall reduces noise passing from one room to another. Although it is a raw measure, it gives an immediate picture of the actual noise protection in a given building.