On this page I want to show the differences between mono, stereo and 'true stereo', and why it is necessary to use four-channel impulse responses for an optimal result for stereo. But let's start from the beginning: the mono signals.
Mono to Mono
When a single impulse response is recorded, a mono impulse response is produced. When added to a mono signal, the result is a reverberated mono signal.
Mono to Stereo
When recording a mono impulse response in stereo, a mono impulse response each is created for the left and the right channel.
Sending a mono signal through the stereo impulse response creates a true stereo signal.
Stereo to Stereo / TrueStereo
Of course, a stereo signal with a two-channel 'mono to stereo' impulse response can be used for reverb. The 'correct' way of adding reverb to stereo signals uses four impulse responses (also called 'True Stereo'). When comparing the two versions, the differences should immediately be clear:
For a true stereo sweep, a sweep for the left and right side of the room is created, producing two stereo impulse responses, one for each side. Each stereo impulse response contains the specific acoustic information of the respective side of the room, while the 'mono to stereo' impulse response contains only the information from the acoustic centre of the room.
This comes into play when audio material needs to be placed in the room. If you want to place a sound source far left, 'True Stereo' has the acoustic information, and may also bounce back its reflection on the right side. Figures 13 and 14 clearly illustrate the different impulse responses and their intensity.
In contrast, there is simply less room for information included in the 'mono-to-stereo' impulse response. 'Mono-to-stereo' does create a space, but 'True Stereo' allows for the playback of a much more accurate and broader reverb with four impulse responses.
Figure 15 shows the spectrum of the rendered reverb signal from a signal panned hard to the left with a mono-to-stereo impulse response. In comparison, Figure 16 shows that the reverb in true stereo is much stronger because the impulse responses in the 'corners' are much stronger compared to mono-to-stereo.
If you have the opportunity, 'true stereo' is the preferable alternative to two-channel mono-to-stereo. When you create impulse responses, it is appropriate to include both versions, given time and opportunity.