Sound localisation and Achieving a sense of distance and depth within a film soundtrack

Using a selection of mixing tools it is possible to forge a sense of distance and depth within a Post-Production sound mix.

EQ

Positioning of an object emitting sound on screen in relation to the camera needs to be paid attention to. In real life the position of a sound source will directly effect the frequency spectrum of the sound which will reach our ears. Wyatt and Amyes in the book Audio Post Production for Television  and Film  state that “As a performer walks away from a camera his voice will reduce in bass content.” (2005 : 222). To try and emulate this phenomenon in post-production Equalisation is employed to create apparent distance and perspective.

Volume

Quite simply, as we all have experienced, sounds created by sources which appear far away are quieter than those created by a source of similar nature which is nearby our particular location, assuming there are no obstacles present to cause diffraction and absorption which significantly alters the characteristics of the sound created by the nearby source .

Panning

Only effective on horizontal plane, amplitude panning can be used to suggest the location of a sound source in terms of distance from the camera. Extreme panning values can be used to suggest a far away sound source coming from “out-of-view” to the far left or far right. For more information on how panning was utilised in the making of the stereo soundtrack for Big Buck Bunny click here.

Reverb

“Natural reverberation helps to tell the ears the direction that sound is coming from, the type of environment that the sound was recorded in, and approximately how far away it is.” (Wyatt and Aymes, 2005: 229).

Sounds which have a more reverberant character are perceived to be situated further away than those which are relatively dry sounds. Additionally the tail of a particular reverb can effect the distance at which a sound is approximated to emanate from, with longer tails appearing to increase the perceived distance, as opposed to shorter tails which by comparison make a sound source seem more upfront.

In the real world, early reflections communicate the position and distance of a sound source, such as the geometry of the room. Scientific audio

Kruszielski, Kamekawa and Marui for their paper The Influence of Camera Focal Length
in the Direct to Reverb Ratio Suitability and Its Effects in the Perception of Distance for a Motion Picture (Kruszielshi et al, 2011) asked subjects to select what they perceived to be suitable the amount of reverberated sound for a presented motion picture. As visual stimuli a video of a saxophone player at two different distances and two different lenses focal length was used. The video was split into 3 key scenes: for Scene (“Frame”) A the player was placed 3 meters from the camera and the lenses focal length set to 18 mm, producing a field of view of 76 degrees. For Scene B the camera was moved 1.5 meters towards the player with the same focal length. The camera was positioned in the same
location as Scene A when creating Scene C, however the focal length was changed to 55 mm producing a field of view of 28.8 degrees. The area occupied by the saxophonist in the foreground of Scene C was the same as in Scene B but with much less visible space in the background area.

From analysing results from two different tests (a direct to reverberated sound ratio (D/R) test and a pairwise comparison made between the three different frames and using three reverb patterns) they found that visual stimuli processed with more distant recorded sounds were perceived to be further than the ones processed with closer recorded sound. They also found that the perceived distance in the visual increases, sounds recorded further away tend to be more suitable. They concluded that “Perceived distance caused by the image plays a major role in the desired amount of reverberation”.

In another study investigating auditory-visual perception of a room width, Larsson et al found that compared to individual senses alone a combined auditory-visual delivered stimulus increased the accuracy of the room width perception. (Larsson et al, 2002)

MSc Project examples

Bunny Snoring Clip

EQ

EQ bunny snore 2

A high-pass filter was added for the duration of this clip to remove unwanted sub/ low frequency noise such as rumble from the Mic stand.

EQ bunny snore copy

At the start of the clip, where the bunny is seen sleeping in the distance, further low frequency information was removed along with some top end. This technique helped to create the impression of the bunny snoring from quite far away in relation to the camera. In the Pro Tools project the above EQ is bypassed as the camera shifts to a closer shot of the bunny in order to provide a contrast between the different viewpoints. Close-up the bunny snore sounds richer in comparison, mimicing what might occur in the real-world (not that youwould ever find a giant rabbit snoring under a tree… a human maybe) as there are less obstacles to alter the sound.

Volume

The following automation curve was created to synchronise volume with camera position whilst the bunny is snoring:

volume bunny snore

As you can see, as the camera moves closer to Big Buck Bunny the sound of his snore is designed to become louder.

Reverb

Listening to the video clip above you will notice that when Big Buck Bunny is seen from a far his snore is much more reverberant than when he is close up. Two different reverbs were used to further imply the distance of the character from camera. For the shots from further away I used the “Field, Wide, Small Hills and Forest” stereo convolution reverb with a tail of 2.7. The level of the reverb send on the ‘snore’ track was automated to decrease slightly as the camera slowly zooms in. For the bunny close-ups the level of this reverb was significantly decreased to give an overall drier feel.

At this point the another reverb was employed to capture the reflections that the sound of the snoring would create within the cave the bunny is sleeping in. This was kept as a mono reverb because the camera is set outside the cave/burrow, therefore an immersive stereo reverb would not have been suitable. As the rabbit ventures out of the cave the level of this reverb is decreased to zero for dramatic effect.  Click here for pictures of the reverb settings as well as more information on how reverb was used during the making of the ‘BBB’ soundtrack.

Flying Gimera

Simliar techniques were used in the creation of distance this section of the film. Lots of the “Field, Wide, Small Hills and Forest” (5.3 sec tail) was applied as Gimera is flung to his demise in the distance. This gave the impression of his yell echoing across the landscape, getting more reverberant and less loud as he disappeared into the distance.

The need for panning was greater in this clip in order to synchronise Gimera’s yelling in the stereo-field to the perceived position of Gimera’s body on-screen. As the visual cuts back to Big Buck Bunny can still hear Gimera to the very far left of the stereo-field, yelling and crashing into a tree (presumably). Below is a screen shot of the panning automation for Gimera’s voice during this sequence.

Pan gimera fly

References

Kruszielski, L,F, Kamekawa, T, and Marui, A. (2011) The Influence of Camera Focal Length in the Direct to Reverb Ratio Suitability and Its Effects in the Perception of Distance for a Motion Picture. Tokyo University of the Arts, Japan. 131th AES
Convention, Convention Paper 8580.

Larsson, P, Västfjäll, D, and Kleiner, M. (2002) Auditory-visual interaction in real and virtual rooms. Proceedings of Forum Acusticum, Seville, Spain, PSY05-004-IP.

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

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