The idea of Worldizing is not a new concept and for anyone who is not familiar with the term I’ll happily let Walter Murch eloquently describe it:
Inevitably, we can’t always take a playback system and all of our prepared sound elements out into quiet exterior acoustic spaces that match the exterior locations of a film to re-record them, so necessity being the mother of all inventions I thought it would be interesting to have a discussion of what approaches have worked best for you, when you need to make sound elements appear to be outdoors? Recording sounds with perspective is of course one option, but for my current project I want to create ambiences composed of layers of worldized elements, hence my research… (And bear in mind my maths may be wonky, so if there is an error please comment so I can learn & also will correct any mistakes)
I do appreciate that ‘outdoors’ is only a loose term, since as with any acoustic space, the specifics matter intensely: just as clapping your hands in a small carpeted church sounds different to doing the same in the foyer of Tate Modern gallery, the same applies when you are in an alleyway compared with an open park, compared with a clearing in a forest, compared with a desert, compared with a tennis court, compared with a lawn bowls green…. So for my own sake as much as anyone elses I am going to try & quantify how the physical environment contributes to how sound is affected when it is heard outdoors, so I can attempt to emulate the effect…
The caveat to all of this is, of course, that film re-recording mixers deal with this process on almost every project, and the best solution is whatever sounds right in context. How a sound ‘reads’ in context is what matters, since a physically accurate example of a distant sound may be completely inapproriate in a film, when combined with all of the other elements of the soundtrack. Undoubtedly every re-recording mixer has their favourite custom presets available, as a starting point if nothing else, but nevertheless I am interested to explore this subject to also get a real reference and a basis for experimenting…
While doing some searches about this subject I came across this pdf which is aimed at the “outdoor noise consultant” as part of the ME 458 Engineering Noise Control course at the Pennsylvania State University Engineering Department and thanks to the author Daniel A. Russell, Ph.D. for making it available. The forumulas in that course paper PDF almost made me go into a trance, but it goes into great detail as to the contributing factors of how sound propagates in exterior environments:
a) Source geometry and type (point, line, coherent, incoherent)
b) Meteorological conditions (wind and temperature variations, atmospheric turbulence)
c) Atmospheric absorption of sound
d) Terrain type and contour (ground absorption of sound, reflections)
e) Obstructions (buildings, barriers, vegetation, etc)
Direct Sound – High Frequency Dissipation in Free Air
As a starting point lets say we are standing in an open space, eg a desert or a paddock or a playing field. Other than the ground, which we’ll consider seperately, there are no reflective surfaces anywhere near us. So someone is standing a distance X metres from us and making a sound. All that we hear is direct sound, from the person to us, as that sound travels X metres. Most people are aware that sound pressure decreases with distance, following the inverse square law and most people are also aware that outdoors high frequencies decrease with distance – sounds become more ‘muffled’, but its the details of how that occurs thats interesting & very relevant to recreating the effect. There is a handy calculator here for calculating the fall off with distance of frequencies, but distance is not the only input: temperature and humidity are important factors too.
With regards to humidity, from that PDF: “It is interesting to note that absorption generally decreases with increasing humidity. The exception is totally dry air, which has the least absorption.”
I’d quite like an EQ plugin with this functionality built in, but failing that a collection of EQ plugin settings for variations would be useful eg:
distance: 10m, 20m, 35m 50m, 75m, 100m
temperature: 5, 10, 15, 20, 25, 30, 35
humidity: 20% 40% 60% 80% 100%
That would equate to 5 x 7 x 5 = 175 presets, but it is likely that in use you would choose temperature and humidity, and then vary the distance factor to layer elements in an ambient space. So say its summer and 30 degrees, with 60% humidity… or its a cold winters morning 5 degrees
So the first element of artifically distancing a sound is level, and the second is EQ via a low pass filter – using that calculator here are some example values (which would need to be added to the overall level decrease with distance)
20 degrees C 50% humidity
100Hz = 0dB/m
250Hz = 0.001dB/m so an object 100m away = -0.1dB
500Hz = 0.003dB/m so an object 100m away = -0.3dB
750Hz = 0.004dB/m so an object 100m away = -0.4dB
1000Hz = 0.005dB/m so an object 100m away = -0.5dB
1250Hz = 0.006dB/m so an object 100m away = -0.6dB
1500Hz = 0.007dB/m so an object 100m away = -0.7dB
1750Hz = 0.008dB/m so an object 100m away = -0.8dB
2000Hz = 0.01dB/m so an object 100m away = -1dB
2500Hz = 0.014dB/m so an object 100m away = -1.4dB
3000Hz = 0.018dB/m so an object 100m away = -1.8dB
3500Hz = 0.024dB/m so an object 100m away = -2.4dB
4000Hz = 0.03dB/m so an object 100m away = -3dB
4500Hz = 0.037dB/m so an object 100m away = -3.7dB
5000Hz = 0.044dB/m so an object 100m away = -4.4dB
6000Hz = 0.062dB/m so an object 100m away = -6.2dB
7000Hz = 0.082dB/m so an object 100m away = -8.2dB
8000Hz = 0.105dB/m so an object 100m away = -10.5dB
9000Hz = 0.131dB/m so an object 100m away = -13.1dB
10000Hz = 0.159dB/m so an object 100m away = -15.9dB
15000Hz = 0.327dB/m so an object 100m away = -32.7dB
30 degrees C 80% humidity
100Hz = 0dB/m
250Hz = .001dB/m so an object 100m away = -0.1dB
500Hz = .003dB/m so an object 100m away = -0.3dB
750Hz = .005dB/m so an object 100m away = -0.5dB
1000Hz = .007dB/m so an object 100m away = -0.7dB
1250Hz = .009dB/m so an object 100m away = -0.9dB
1500Hz = .011dB/m so an object 100m away = -1.1dB
1750Hz = .012dB/m so an object 100m away = -1.2dB
2000Hz = .013dB/m so an object 100m away = -1.3dB
2500Hz = .016dB/m so an object 100m away = -1.6dB
3000Hz = .018dB/m so an object 100m away = -1.8dB
3500Hz = .021dB/m so an object 100m away = -2.1dB
4000Hz = .023dB/m so an object 100m away = -2.3dB
4500Hz = .026dB/m so an object 100m away = -2.6dB
5000Hz = .030dB/m so an object 100m away = -3dB
6000Hz = .0327dB/m so an object 100m away = -3.27dB
7000Hz = .046dB/m so an object 100m away = -4.6dB
8000Hz = .056dB/m so an object 100m away = -5.6dB
9000Hz = .068dB/m so an object 100m away = -6.8dB
10000Hz = .081dB/m so an object 100m away = -8.1dB
15000Hz = .164dB/m so an object 100m away = -16.4dB
(Note: if anyone uses Mathematica or similar and has a means of automating/calculating the variables I would love to see a series of frequency response graphs for a number of temperatures & humidities… anyone keen?)
Other meteorlogical factors discussed in that course paper are the effects of wind, which causes sound to refract – for an explanation of refraction (with animations of sound waves) have a look here
The first surface to consider in terms of non-direct/reflected sound reaching the listener is the ground. And of course what the ground is covered in alters the frequency response of that early reflection. A grassy or dirt paddock is going to absorb sound & reflect less than a smooth flat tennis court. We experience these most noticeably when we change locations and for example are near a very reflective surface such as snow or an icy lake.
Other surfaces that may reflect sound outdoors in a more pronounced way are buildings, vehicles, trees etc And bear in mind that these late reflected or delayed sounds also go through their own distance filtering. So an example of a physical echo off a building: If I am 100m from the sound source and a building is 50m from the sound source but 300m to my right, the sound travels directly to me at 343 metres/s, so the direct sound arrives at 100/343 seconds = 0.291 seconds or 291 milliseconds. The reflection travels to the building, and bounces off (the frequency response of the reflection is dependent on the surface of the building) and the echo travels to me. So the distance that echo’d sound has travelled by the time it reaches me is calculated as 304.14m x 2 = 608.28m. Travelling at 343m/s the echo of the building reaches me at 1.77seconds, or 1.48 seconds after the direct sound. Having travelled six times further than the direct sound, the echo has proportionally lost level and high frequencies.
In a real, complex outdoor environment each echo would need to be calculated and added to the direct sound, and it is only in rare circumstances such as an enclosed canyon that these multiple echoes start to even vaguely resemble an interior acoustic, so it is not hard to see why attempting to use reverb plugins that are modelled to recreate interior spaces are not so useful when emulating exterior acoustics.
Another factor that is deeply influential on how we perceive sound outdoors is of course the character of the sound itself. Every kid knows that if you clap your hands near a flat wall outside you get an echo back, whereas a sound with a soft attack will produce a less pronounced echo…..
Level: inverse square law (double the distance halve the level)
EQ: dependent on distance, temperature & humidity
Early reflections: minimal/singular depending on ground surface or proximity of listener to walls
Reflections: multiple complex delays dependent on number & proximity of surrounding reflective surfaces
This subject has been discussed a few times in various forums, so for some further reading:
Please comment with any other related articles etc online?
Considering how common the need is, how come no one has created a purpose built EXTERIOR DISTANCE plugin?
What devices and plugins do you primarily use to create these effects?
Whats a good EQ plugin for dialling in complex presets?
What films are good examples of beautifully treated exterior sound?
Also what films have made use of the time delay as sound travels to the listener?