3D audio cookbook

Simple Effects

These simple effects are quick to render and easy to use. The advanced effects, listed in the next section, take longer to cook and more effort to setup. All of these effects are optional. If all effects are off, the Spatial Audio chop behaves like a regular mixer.

Distance Delay and the Doppler Effect

Sound does not travel instantly, so distant sounds have a noticeable delay. This delay also causes the Doppler effect, which is the shifting of pitch as a sound source rapidly passes a listener (e.g. the pitch of an ambulance’s siren lowers as it passes).

The amount of delay is determined by the speed that sound travels. Usually you will be using the speed of 331 m/s, which is the speed of sound in air (at zero degrees Celsius, but room temperature is very close to this also). Another useful speed to know is 1452 m/s - the speed of sound in water (at 15 degrees Celsius). The speed of sound is defined in the Spatial Audio CHOP's 'Speed of Sound' parameter in the Environment page.

If you want to exaggerate the delay or the Doppler effect, lower the speed of sound from its realistic value. A speed of 166 m/s will double the delay and the Doppler shift in air.

Volume Loss over Distance

Sound also loses intensity as it travels. The Spatial Audio CHOP supports two methods for determining the volume loss at any distance.

Using Realistic Drop-off

This method is the easiest to set up, and produces a realistic result. It is based on a physics equation, power = 1/[distance^2] .

To change how fast this drop-off occurs, you can modify the '10m Distance Loss' parameter on the Spatial Audio CHOP's Effects page. This parameter specifies the amount of sound intensity loss after traveling 10 meters. A value of 0.4 means that the sound will loss 40% of its intensity after 10 meters (because of the drop-off curve, after 20 meters the sound loss will be at 85%).

Using a Drop-off Table

This method requires you to create a curve that represents the drop-off of the sound as it travels farther. This curve can be created with another CHOP and passed to Spatial Audio CHOP by connecting it to its first input.

The Volume Lookup Range parameter determines the range of distances that this curve describes. If this parameter is set to (5, 100), then the first value of the curve will be the volume level at 5 meters, and the last value of the curve will be the volume at 100 meters. Anything outside the lookup range will be sampled in the curve’s extend regions.

This method is a little more work to setup, but allows you to create any kind of drop-off, even completely unrealistic ones.

Echoes

Echoes and environmental reverb can be produced with very little effort. The Spatial Audio CHOP supports two types of echoes; static environment echoes and dynamic echoes. Static environment echoes are a good choice for enclosed areas, while dynamic echoes simulate open areas well.

Both echo methods allow you to specify the number of echoes, the echo delay (or the minimum echo delay), and the echo volume.

The echo drop-off is determined by the same drop-off method as normal sounds. If the sound delay is 0.1 seconds, and the speed of sound is 331m/s, the echo travels 33 meters farther than the original sound, so the volume drops according. Each echo has the same delay from the previous echo (so the 4th echo arrives 0.4 seconds after the original sound if the echo delay is 0.1 seconds).

If you find the echoes are too subtle, increase the Echo Volume parameter.

Using Static Environment Echoes

Static echoes always have the same delay between echoes, regardless of the distance between the source and the listener.

Using Dynamic Echoes

Dynamic echoes base their echo delay on how far the original sound traveled. Distant sounds will have longer pauses between the echoes and close sounds will have almost no echo delay.

The 'Dynamic Echo Effect' parameter determines the portion of the traveling delay that is used as the echo delay. So, if the Dynamic Echo Effect is set to 0.3, and the sound traveled 4 seconds to reach the listener, the echo delay will be 1.2 seconds (0.3×4).

To ensure a minimum echo delay, set the 'Echo Delay' parameter to a non-zero value. This will add a static echo delay to the dynamic echo delay. So an Echo Delay of 0.1, Dynamic Echo Effect of 0.3 and a traveling time of 4 seconds would produce an echo delay of 1.3 seconds (0.3 * 4 + 0.1).

Advanced Effects

These effects require filters . While most of the simple effects are done by the Spatial Audio CHOP, these effects require other objects and CHOPs, so they take slightly longer to set up.

Obstacle Occlusion

This effect allows you to create geometry object and use them to block sound. “Blocked” sound can be completely blocked, partially blocked or even amplified. A filter is used to describe which frequencies are blocked by the object, and by how much.

Setting up an Object for 3D Audio

Any geometry can be used as an obstacle. Most obstacles should be fairly low resolution geometry, since increasing the resolution has little effect on the overall audible result.

Each obstacle needs a Sound Material associated with it, to describe how it affects the sound passing through it. If the Sound Material includes a transmission filter, the filter is used to filter the sound directly. Otherwise, the absorption or reflection filter is used (but inverted to determine the transmission filter). See the next section, Designing a Filter, for details on filters.

To add the geometry as an obstacle to the Spatial Audio CHOP, turn on Check For Obstacles on the Effects tab. Then add the geometry object to the parameter 'Obstacles' (using the pull-down menu). Objects without Sound Materials will be ignored.

Tweaking the Occlusion Effect

The parameter 'Obstacle Softness' determines how soft the obstacle is. Soft obstacles allow quite a lot of sound around them, while hard obstacles do not (softness of zero). Two of the main factors that affect how much sound get by the obstacle are:

how far both the sound source and the microphone are from the obstacle (more sound gets around the obstacle at longer distances)

the size of the obstacle (larger objects allow less sound around them)

So, an object between a sound source and the microphone that are close together will block most of the sound, while the same object roughly in the middle of a widely separated source source and microphone will have little effect. A larger object will always have more of an effect than a smaller one in the same situation.

If you don’t want any sound to get around the obstacles, set the obstacle softness to zero.

Microphone Filters

Each microphone listens to the full spectrum by default. However, you can setup a microphone to only listen to a certain parts of the spectrum. This allows you to create microphones which create sub-woofer tracks, high frequency tracks, or mid-range tracks. This effect is created by adding a filter to a Microphone Object.

The Microphone object has a Filter parameter on its Microphone page which references a filter. The filter should be created by an Acoustic CHOP, and it should be an Absorption filter (“absorb”).

The Echo Filter

The echo filter is an optional filter that is applied to each echo. This allows you to change the sound characteristics of the echo.

The Spatial Audio CHOP must be generating echoes for this filter to have any effect. To add an echo filter, connect the CHOP containing the filter to the third input (the 'Echo Filter' input).

The filter is applied more than once to higher numbered echoes. The first echo has the filter applied to it once, the second has the filter applied to it twice, the third three times, and so on. Because of this, you should not make the filter block sound too drastically (with values close to 0).

The Environment Filter

The environment filter is an optional filter applied to all sounds. The effect of the filter varies with distance, exactly like 'Volume Loss over Distance'. It uses the same method to calculate the volume loss. The filter is multiplied by the calculated volume loss and applied to the sound.

When designing an environment filter, you can use either a transmission or absorption filter to describe the volume loss. You should not make the losses too large (not more than 0.5), otherwise sounds will lose most of their intensity over a very short distance.

To add an environment filter, connect the CHOP containing the filter to the Spatial Audio CHOP's second input (the “Environment Filter” input).