The resulting files can be imported into Space Designer, Altiverb, and other apps as well. In Apple’s Impulse Response Utility, you can generate sweeps right in the application. In this article, I will explain some of the main principles behind the methodology, list some popular paid & freeware applications that use this technology, identify 3rd party sources of Play Video Impulse Response or IR files are used in audio processing for convolution reverbs and amp/cabinet emulations. This information includes sample rate, length of the sweep and the silences around the sweep. These specialized sweep files include header and footer information that Audio Ease needs to implement the IR.
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The video below from Audio Ease describes how to use the sine sweeps available from their site to create IR files for Altiverb. Making great IR files is a highly specialized art and there are several companies that excel in this area.
If fidelity is less of a concern, creative approaches to mic placement can yield unusual and idiosyncratic results not otherwise possible or envisioned. Strategic placement of devices in the space is dependent on the type of IR to be produced (mono, stereo, B-format surround, etc.) and the nature of the space itself. ( Source)įor both methods, the placement of the impulse source and microphone(s) is crucial for accuracy, if that is the goal. With this method, the resulting recording must then be deconvolved, a process that uses division (as opposed to multiplication of signals) in which all recorded reflections that are present over the entire recorded sine sweep are time and level aligned into the very beginning of the file. Sine Wave Sweep and Deconvolution – The second method uses a sine wave sweep that spans the audible range as the response is measured across the spectrum.
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But on the other hand, the resulting IR requires no further processing in order to import it into convolution software that will accept the file format. The challenges of recording an undistorted pistol shot and the lack of full-spectrum content make this method less desirable if fidelity is a concern. Transient Method – This involves the use of a transient like a pistol shot. There are two distinct methods for creating an impulse response: While some amp/cab simulation applications use algorithmic approaches with filtering and distortion, others use IR files produced by the running impulse signals through the actual equipment. IR files are also particularly useful for replicating the sound of a miked speaker cabinet. You can also impose the acoustic properties of an existing environment on any input signal, which is precisely what convolution reverbs are designed to do (Creasey 310).
With an IR file, you can identify the acoustic properties of a space and investigate ways to optimize its acoustics.
Processes like reverberation, modulation, cross-synthesis and filtering all use convolution (Roads 419).įilter slopes ( Source ) What Is an Impulse Response (IR) FileĪn impulse response file is a sort of snapshot that reflects how a physical space or audio system responds to and combines with an input signal to produce some output. Convolution lies at the heart of FFT or fast Fourier transform processes as well, which include time expansion and compression independent of pitch and vice versa. The method is used in IIR filters (infinite impulse response) which are capable of filter slopes of 12 dB per octave & above, and FIR (finite impulse response) filters that can produce a 6 dB per octave slope. Put simply, this process involves multiplication (or division in the case of deconvolution) of two audio signals to produce a third (Collins 137).
Underlying the use of IR files is the concept of convolution. In this article, I will explain some of the main principles behind the methodology, list some popular paid & freeware applications that use this technology, identify 3rd party sources of IR files and offer some creative approaches to making and using your own IRs.
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