Creating Impulse Responses With Sweeps
Author: Guido Helbling, Avosound - last updated June 20, 2017
Based on the following principle, impulse responses are created with sweeps:
1. The Sweep
A sweep is a sinusoidal signal that starts at 20Hz, increasing continuously through the entire frequency range up to 20kHz.
Sweeps differ depending on the system or on their use: while the sweep of Apple Impulse Response Utilities is straight, the sweep of Altiverb is optimized for the speaker and for speech reproduction - therefore it is a different signal. It also contains specific information about playback speed and pitch, which the Altiverb impulse response processor later recognises and analyses.
2. Playing back and recording the sweep
This sinusoidal signal is played back through a speaker in a room and then recorded. This way, the sine signal sweeps through the whole frequency range and produces reflections. A mixture of direct sound and reflections is recorded that will later generate the properties of the space.
Figure 7 shows the frequency spectrum of a recorded sweep. Compared to the original sine sweep in Figure 6, significant colourations of the frequency spectrum in the overtones and in the low frequencies can be seen. More information about the range and what you can read from it will follow later in this tutorial (see analiyse sweeps).
3. Converting the sweep into an impulse response
These recorded sweeps with reflections of the room can now be transformed into impulse responses based on the principle of deconvolution.
Sweeps of different systems might produce a virtually identical impulse response, but they are not compatible with each other. Apple's Spacedesigner creates an Apple-proprietary '.SDIR' impulse response format that can be read by Space Designer only. However, SDIR files can be cracked, as they contain conventional .aif files. Converted into wave files, these impulse responses can be used on other systems too.
Altiverb on the other hand, goes one step further, encrypting the converted impulse responses completely, so that they are only available in Altiverb. Altiverb is thus a closed system.
Detailed information on cracked '.SDIR' files will follow soon in the second part of the impulse responses tutorials.
Pros and Cons of sweeps / Bang Method
In contrast to the bang method, a sweep can be much better controlled. Assuming you don't blow your speakers at first attempt, a sweep can be accurately reproduced. This is crucial when recording stereo and multi-channel impulse responses. Since the output signal from the speaker in each case is identical, impulse responses with the same level can be recorded on different positions, allowing accurate stereo impulse responses. Sweeps can also be used in different lengths, which gives much better results in large and reverberant rooms.
However, the disadvantage of sweeps is obvious: they require a lot more equipment since a speaker is used for playback. Compact active speakers generally won't do the job. Depending on the room size, quite a bit of power might be needed to offset a room so that it vibrates and generates sufficient reflections. In addition, the workload is shifted to post-production, where one has to deal with deconvolution and the problems that can arise when things don't work out the way they should. It's easy to end up sitting on your sweeps without them working.
Sweeps can not be created 'on the fly', e.g. on a film set or in a similar ad hoc environment. Even with sweeps it is possible to damage your hearing - a common danger with most activities involving loudspeakers. More probable than losing your hearing, however, is the risk of damage to the box when playing the sweep. Test signals such as pink noise, sine wave or sine sweeps make extremely high demands on the material, particularly when reproduced at high sound levels. Therefore, a high-quality speaker is required - one that can play a sine sweep at high volume and without distortion. In other words: your kitchen radio won't do.
Conclusion
Objectively speaking, the sweep method comes with lots of disadvantages. Ultimately, though, it also results in a much higher quality impulse response. For this reason, we have decided to use sweeps and leave the nail gun at home. One advantage of sweeps: you won't have to explain the holes in the historic cathedral floor.
Author: Guido Helbling, Avosound