The creation of BBB background ambience part 2

As touched upon in the previous post, the ambience track needed several adjustments to make sure it functioned correctly.

The main issues to contend with were:

  1. Unwanted noises, including clips and pops
  2. Relatively high noise floor

Here I will demonstrate some methods I used to reduce the effect of undesirable artefacts.

  1. Unwanted noises, including clips and pops 

    This was done by using fades in Pro Tools. Each time an undesirable noise emerged I split the audio clip, cut back some of the audio either side of the noise and then dragged and joined the resultant clips together until no trace of it remained. Crossfades were then employed to prevent any potential popping and to ensure smooth transitions. Both “equal-power” and “equal-gain” crossfades were used at various points, although “equal-power” more often than not provided the most level-consistent fades. The most probable reason for this is due to a higher presence of clips which when put together are not phase-coherent. “Equal power” fades ensure the overall volume is maintained through a crossfade, avoiding any sudden dips in overall volume. “Equal-gain” crossfades seemingly work best when employed to link phase-coherent clips together, such as recordings of the same sound source. “Equal-power” fades in this instance would cause sudden increases in volume at the centre of the crossfade. More insightful information on how best to use crossfades in Pro Tools can be found in this SoundOnSound article.

    This method was sufficient for removing most unwanted noises of short duration. For removing low frequency rumble caused by wind another technique could also be utilised however. Unfortunately on the days I visited the forest were fairly windy and so even with windshields fitted to the microphones, wind was still able to make its way on to some recordings, tarnishing them for periods of up to 5 minutes. To save removing large sections of audio which otherwise would have been usable, I instead turned to high pass filters to help me improve them. Through careful listening I found that rolling off frequencies after 90-150Hz adequately removed rumbling and other low frequency sounds without harming the natural characteristic of the recordings. Fabfilter Pro-Q was my go to EQ plug-in for this project because of its built-in spectrum analyzer with Pre-EQ, Post-EQ and SC modes. These graphical aids enabled me to accurately determine problematic frequency bands and thus treat them effectively.

  2. Relatively high noise floor

As mentioned previously, hiss was a little too audible on these location recordings, to the point where it became a distraction during first attempts to integrate the ambience tracks with the other sound effects. EQ alone did not do enough to reduce the sound due to hiss containing energy across the whole audible frequency spectrum and so it was decided that “heavier duty” measures were needed in the form of tools found within Adobe Audition 1.5.

Although over 10 years old, Adobe Audition 1.5 contains some excellent tools for removing unwanted audio artefacts. After experimenting with a few these, it was the “Hiss Reduction” tool which produced the most convincing and natural sounding results.

nr-hissreductionThe following procedure, which required a significant amount if time spent tweaking, eventually proved a successful:

  1. Capture Noise Floor – To create a graph that most accurately reflected the noise floor of a particular clip, I selected audio containing only hiss/ the least amount of desirable audio, and then clicked Get Noise Floor.
  2. Reduce hiss – Moving the Noise Floor Adjust slider further to the right removed more hiss, however care had to be taken to find a point at which distortions and odd bubbling effects were not also created. The Hiss reduction tool works to lower the amplitude of a frequency range if it falls below the captured the noise floor. Audio in frequency ranges that are louder than the threshold remain untouched. It works on higher frequencies, where hiss generally occurs, therefore removing more hiss culminates in a reduction of frequencies from other sounds, causing almost metallic sounding artefacts. Care had to be taken as the most common sounds within the location recordings were bird calls which naturally contain a lot of high frequency information. The Reduce By function allowed me to set the level of hiss reduction for audio below the noise floor. Higher values (above 20 dB) resulted in more dramatic hiss reduction can be achieved. The level at which Reduce By was set varied quite a lot between different clips as some did not contain as much his as others.

I always erred on the side of caution whilst removing hiss or noise. Overall I am happy with the results.



This entry was posted in Audio, MSc Audio Production, Post Production, Sound Design, University of Salford and tagged , , , . Bookmark the permalink.

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