Impulse-Response technique

The Impulse is a short, strong sound of a sweep of sine waves of all pitches, sending a large acoustic energy with a wide frequency spectrum into the church in a short time.

The Response are the reflections of the sounds through walls, pillars and ceiling. The closest walls give their reflections first, the objects that are further away take longer. Its volume gradually decreases and therefore the sounds die out. It's a lot of reverberations, all with their own reverberation times that are sent via microphones to a powerful computer. The computer calculates the complex process of all reflections and records it in a digital memory as an exact copy of the acoustics of the room.

Convolution is created by modulating sounds on the acoustic processes in the memory. Any sound sent through that memory will sound as if the sound originated in that acoustic space.

The organist playing at home on a Hauptwerk organ with two channels with DRY samples plus reverberation from an Impulse-Response recording hears the same sounds in the same acoustics as when playing the organ in the church. He hears the sounds from the pipes at a short distance and experiences the reflections in the acoustics of the church. The first reflection at close range is quickly followed by multiple reflections from increasingly distant objects, causing the reverberation to bloom and eventually die out. This is only possible with DRY samples.

WET samples are recorded at a great distance from the organ and reproduce the reverberation as it sounds at that location. It is a completely different effect from the reproduction of the growing reverberation, such as the organist undergoes at the console. Surround is a collection of WET samples, which do show a lot of reverberating sounds, but it remains a poor representation of the acoustics of the church.

Reflections in acoustics

In a church, the organ is the source of the sounds. The tones from the pipes reflect against the walls, the pillars and the ceiling. These objects are at increasing distances from the source and therefore the reflections come from increasing distances, the volume decreases until the tones die out. The reverberation moves away from the source.

This can also be heard in the mountains, when the mountaineer's call is echoed off increasingly distant rocks, with the volume decreasing evenly. The characteristic of the reverberation is that it moves away from the source and dies out in the distance. Just like the stone in the pond, the waves around it get weaker until they die out.

When I press a key in church, I hear the tone immediately, even before the first reflection occurs. It sounds without delay so that I can use the articulation and phrasing in the way I want to express it. A person who plays from a sheet can play the indicated notes and is less dependent on what he hears, but I rarely play from a sheet. In order to improvise, I need to be able to move from tone to tone without delay. I hear how the tones form close by and how they fade away in the distance.

If I want to experience that in the living room on the Hauptwerk organ, I have to use DRY samples and convolution reverberation. The tones from the DRY samples correspond to the tones from the pipes and the convolution reverberation is a copy of the acoustics of the church. The situation thus corresponds to playing the organ in the church.

How to select the appropriate IR

Impulse Response (IR) is a footprint of the acoustic characteristics of the given space (church). IR can be used to reproduce these characteristics and add them to an input signal with the help of dedicated software (such as Hauptwerk‘s convolution reverb module). The most distinctive acoustic characteristic of a space is the reverberation time (i.e. how long it takes for the sound to die away). Therefore, the IRs described in this guide are ordered according to increasing reverberation time. However, there are other features according to which you might choose your favorite IR.

RT, T20, T30 - Reverberation time (RT) is defined as the time needed for the sound to decay 60dB below the original value.

The reverb time depends on the frequency
Each frequency band will die differently. Usually very high frequencies die very quickly, the maximum RT is somewhere between 500-2000 Hz. Each IR description contains a graph showing the frequency decay for different frequency bands. The colors indicate the intensity of the sound: yellow = high energy, red = less energy, violet = low energy, black = no energy (background noise). See pictures Martini and Michaels

Sonus Paradisi - Impulse Response (IR) User Guide

To install Impulse Responses, follow the process described in chapter "Installing/un-installing/upgrading organs, temperaments, or reverbs (the component installer)" found in the Hauptwerk User Guide. In Hauptwerk, installing an IR is similar to installing an organ or temperament. In short:

1. Start Hauptwerk.
2. Select the File menu, and then select "Install organ ...".
3. Hauptwerk opens a dialog box. Navigate through the directory tree to the installation package, and select the     installation package you have downloaded.
4. Hauptwerk unpacks and installs the package. Confirm installation when prompted by Hauptwerk.
5. If there are more installation packages, repeat steps 2-4 for each remaining installation package. If you have     purchased many IRs in one order, there will be many small installation packages. It is necessary to install     each of them, one by one, to Hauptwerk.               
                                                   Impulse Response Sonus Paradisi

To install the IR reverb

To be able to use the reverb, the control slider must first be visible
1. Go to View - Large floating control panels and then to:
2. Audio mixer, routing and voicing / panning settings set the control slider to 100
3. Select at General settings: Audio mixer for adding reverb
4. The Audio mixer screen is now visible. Choose Select this can take up to 10 seconds.
5. Choose the desired Impulse Response Reverb
6. The pre-delay, wet-mix and reverb level can be adjusted a
     little if desired, but the best result is achieved by making little changes.