Creating impulse files

You can obtain impulse responses from anything that accepts test tone input and supports recording the output. This includes physical spaces as well as electronic audio equipment. Creating custom impulse files requires planning, work, and additional audio equipment.

Note: Impulses derived from electronic devices that produce nonlinear effects such as overdrives, distortion pedals, pitch shifters, harmonic enhancers, chorus pedals, or flange pedals cannot be modeled using the Acoustic Mirror tool. While they produce interesting effects, the acoustic signature cannot be correctly replicated.

What you need to create custom impulses

The equipment required to create custom impulses depends upon whether you want to create the impulse from a physical acoustic space or from a piece of equipment. Regardless of the method, you need a playback device that reproduces test tones and a recording device that has microphone or line-level inputs. Be aware that the quality of the impulse is directly affected by the quality of your playback and recording devices. The flatter your system’s response, the more accurate the impulse response.

Recovering an impulse from an acoustic space

To recover an impulse from an acoustic space, you need the following equipment:

•A playback device and speakers

•A stereo pair of microphones to record the test tone

•A recording device for recording the signal captured by the microphones

Recovering an impulse from an electronic device

To recover an impulse from an electronic device, you need the following equipment:

•A playback device that connects to the device’s inputs

•A recording device that connects to the device’s outputs

Recording the impulse in an acoustic space

When you have assembled the required equipment, you are ready to begin recording the impulse. The following sections describe the typical impulse recording procedure.

Transferring the test tone

The first step in recording the impulse is to transfer the desired test tone to your playback device. The Sound Forge application disc contains two test tones: a 24-second test tone and a 48-second test tone. We typically recommend that you use the 24-second tone because longer tones result in greater signal-to-noise ratios. The 48-second tone should be used in particularly noisy environments or when the decay time of the acoustic space is greater than six seconds.

Tip: There are spikes at the beginning and end of each test tone. You should include the spikes in the recording to simplify the recovery of the impulse in the later stages of the process.

Placing equipment

When recording the test tone in an acoustic space, you must determine where to place your playback system, speakers, microphones, and recording system to produce optimal results. Microphone placement is crucial to the quality of the impulse. The distance between the speakers and the microphone is the perceived distance of audio processed with the impulse you create. For example, if you record the test tone with the speakers positioned 100 feet from the microphones, all sounds processed with the resulting impulse sound as if they are originating 100 feet from the listener.

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