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

• A playback device for the test tone.

• A loudspeaker system to play the test tone in the space.

• A stereo pair of microphones to record the test tone.

• A recording device for capturing the microphones’ input.

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

• A playback device for the test tone that connects to the inputs of the device.

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

The quality of the impulse will be directly affected by the quality of your playback and recording system. The flatter the response of your system, the less coloration that will be added to the impulse response.

1. Transfer the test tone you want to use to your playback device.

   There are test tones in Test Tones folder on the Sound Forge application disc. The 24-second tone will work well for most applications. As a general rule, the longer the tone, the greater the signal-to-noise ratio. However, the 48-second tone included on the application disc should be used in noisy environments or when the decay time of the space you are recording in is over six seconds.

There are spikes at the beginning and end of the test tone. These should be included in the recording for easier correlation of the impulse in the later stages of the process

2. Set up your playback system and recording system.

• If you are recording an impulse in an acoustic space, you’ll need to set up your playback system, speakers, microphones and recording system in the space. Microphone placement is crucial to the outcome. The distance between the speakers and the microphone will be the perceived distance of a sound processed with the impulse you create. In other words, if you record the test tone with the speakers at a distance of 100 feet from the microphones, any sound you process with the resulting impulse will sound like it is at a distance of 100 feet.

• If you are recording an impulse from an electronic device, simply hook up the playback system outputs to the electronic device’s inputs. Then, hook up the electronic device’s outputs to the recording system’s inputs.

3. Start playback of the test tone and set the levels appropriately on the recording device. The test tone should be played as loud as possible (or practical) to gain the best signal-to-noise ratio.

4. Start recording on the recording device and then begin playback of the test tone. Remember to include the spikes at the beginning and end of the test tone, and be sure to save the file as a .wav.

Record the impulse more than once in each location, and then move the microphones to different locations.

Once you have recorded a test tone through a system, it must be processed and converted into an impulse response.

1. Open the recording of the test tone you made (not the pure, generated test tone but the room-processed output test tone) in Sound Forge.

2. From the View menu, choose Zoom in Full and delete all audio before the first timing spike (do not delete the spike itself).

3. Delete all data from the start of the second spike to the end of the file.

4. Save the file. You should now have a sound file with a spike at the beginning, a test tone, and then silence (the second spike will have been deleted).

5. From the Effects menu, choose Acoustic Mirror and select the Recover tab.

6. In the Recorded file box, select the file you saved in Step 4.

7. In the Test file used box, select the original test file used to create the file in the Recorded file box. This should be a file in the Test Tones folder on the Sound Forge application disc.

8. In the Impulse output file box, enter the name of the impulse response file you want to create. You can specify a folder by using the Browse button.

In most cases, you will want to leave the Remove very low frequencies check box selected.

9. Select the Use start and end of the recorded file as timing spikes radio button.

10. Click the Recover Impulse button.

11. To modify the impulse’s summary information or embed a picture, use the Sound Forge Summary dialog.

If you plan on overdubbing dialog recorded in the field or adding sound effects, you could create an impulse in each of the locations you are filming in and process the new effects and dialog with those impulses during post production.

This will add more realism to the production and alleviate much of the tedium in trying to recreate the correct ambiance using standard reverb effects.

There are few things to keep in mind, however:

• Distance information is determined by how far the microphone is from the speaker when creating the impulse. Therefore, you
  should create multiple impulses at different distances for each location to create the necessary realism.

• Because the human ear’s frequency response changes with the overall level of a sound, impulses that were created at great distances may sound strange when listened to at loud levels.

• Placing the speakers left or right of the microphones when recording can create more directional information in the recovered impulse. For example, you can make the impulse sound as if it is coming from left if you place the speakers to the left of the microphones.

Impulses do not recover correctly

If you are experiencing problems when recovering your own impulse recordings, make sure that you have done the following:

1. Trim the test tone file and choose an appropriate Impulse recovery mode on the Recover tab. In general, the test tone should be 1.5 seconds or less.

2. The second spike must exist in the recorded file if you have the Auto-detect timing spikes radio button selected.

3. The file specified in the Test file used box must be the same file you used to when making the recording. Do not change its length or data.

4. If the impulse still is not recovered correctly and the Auto-detect timing spikes radio button is selected, try normalizing the timing spikes. This can help the auto-detect algorithm detect the spikes and recover the impulse correctly.

5. The output impulse file should be trimmed so that the peak orccurs within the first few samples. To prevent phase problems when mixing the dry and wet signals, check the phase of the impulse file to ensure that it goes positive (above the centerline) before it goes negative. A simple invert will fix this problem.

Impulse recordings derived from electronic devices producing nonlinear effects such as overdrives, distortion boxes, pitch shifters, harmonic enhancers, choruses, and flangers cannot be modeled with the Acoustic Mirror plug-in. The effects are usually interesting, but the desired effect will not be correctly replicated.

Excessive noise in the recovered impulse

To maximize the impulse’s signal-to-noise ratio, ensure the field recording’s noise floor is not too high. When recording in noisy environments, you must increase the test tone’s amplitude so that the test tone is at least 25 dB louder than the noise floor. At least 40 dB of signal to noise is recommended. For very noisy recordings, our Noise Reduction plug-in can salvage a session.

Nonlinear distortion can cause impulses to be noisy. The most common nonlinear distortion is loudspeaker harmonics. All speakers have rather large amount of harmonic distortion at low frequencies: when you play a 60-Hz tone, the speaker not only vibrates at 60 Hz, it also outputs low-level sounds at 120 Hz, 180 Hz, and so on. Impulse recovery minimizes these low-frequency distortions. In general, do not overdrive the speaker and do not use low-quality components.

Sony Extended Shock Protection (ESP) technology compresses audio so that portable CD players can store audio in memory to prevent skipping. The audio compression can also produce distortion in high frequencies. If you’re recording your test tone from a portable CD player, turn off ESP.