2-channel Sound Insulation

When a Single Channel is not Enough
Façade sound insulation is measured like sound insulation between rooms, except that one "room" is open space. The sound level is measured inside the receiving room and outside the façade of the building. The level difference is then corrected for the influence of the reverberation time and background noise level in the receiving room. Finally a single-number index is calculated by averaging over all the frequency bands. A loudspeaker source can be used for the measurement, giving a choice of sound incidence angle. But in practice, placing the loudspeaker and getting a high enough sound level can be difficult.

Two Channels for Façade Sound Insulation
Instead of using a loudspeaker source, it is sometimes better to use existing traffic noise. The measurement then relates to actual conditions for source type and sound incidence. But since the sound level varies over time, the levels outside and inside the room must be measured at the same time.
2-channel measurement is essential to achieve this. For several measurement positions, the average of the outdoor-indoor level differences is taken rather than the difference of the average levels in each room.

When Two Channels are Better for the Job
As well as being useful for façade sound insulation, 2-channel measurement can also be used for normal airborne sound insulation tasks. By setting up a microphone in each room (transmitting and receiving), both levels can be measured simultaneously. Both microphones can then be moved from position to position between measurements, or a rotating microphone boom can be used for spatial averaging in one (or both) rooms.