Acoustic channel node

Design audio filters and sound materials for the spatial audio system.

The Acoustic CHOP allows you to design audio filters and sound materials for the SpatialAudio 3D audio system.

Three different types of filters are available:

Transmission	Describes which frequencies are transmitted (values close to 1) and which are absorbed by the material (values close to 0).
Reflection	Describes which frequencies are reflected off the material. Currently reflections are not supported.
Absorption	Describes which frequencies are absorbed by the filter (inverse of the transmission filter).

A sound material for a geometry object or a transmission filter requires either a transmission or absorption filter. A microphone filter requires an absorption filter.

For more information on spatial audio, see the Spatial Audio CHOP, Microphone object, Geometry object and the Sound object.

When designing the filter, the CHOP graph should be in Frames mode, so that 1 frame on the graph equals 1KHz and frame 0 corresponds to 0Hz (DC). This means that frame 5.5 corresponds to 5.5kHz (5500Hz).

Filters cannot have negative values. If a filter has values of more than one, it will add power to that frequency (turn on No Resonance to avoid this). A value of 0.4 in a filter will reduce that frequency by 60%.

Parameters

Acoustic

Transmit Sound

If checked, a transmission filter is created.

Reflect Sound

If checked, a reflection filter is created.

Absorb Sound

If checked, an absorption filter is created.

Frequency Range

The frequency range of the filter(s).

Power

Selects a power conservation method. Applicable when more than 1 filter is being designed.

No Power Conservation	Power can be reduced or amplified.
Conserve Total Power	Total power is conserved but the individual frequencies' power is not.
Conserve Power Per Frequency	Each frequency has its power conserved.

Lock

If power conservation is on, moving a handle on a filter will adjust the other filters. This parameter allows you to lock one of the filters.

No Resonance

Disallows filter values greater than 1.

Handles

Handles	The number of manipulation handles to create.
Logarithmic Handles	If on, the handles are logarithmically spaced, which is more useful than linear spacing (off) for designing audio filters.
Samples	The number of samples in each filter.
Interpolation	How the samples are interpolated from the handles; either nearest neighbor, linearly or cubically.
Reset All Waveforms	Resets all filters to a constant value.

Common

Some of these parameters may not be avaiable on all CHOP nodes.

Scope

To determine which channels get affected, some CHOPs have a scope string. Patterns can be used in the scope, for example * (match all), and ? (match single character).

The following are examples of possible channel name matching options:

`chan2`	Matches a single channel name.
`chan3 tx ty tz`	Matches four channel names, separated by spaces.
`chan*`	Matches each channel that starts with `chan`.
`foot`	Matches each channel that has `foot` in it.
`t?`	The `?` matches a single character. `t?` matches two-character channels starting with t.
`r[xyz]`	Matches channels `rx`, `ry` and `rz`.
`blend[3-7:2]`	Matches number ranges giving `blend3`, `blend5`, and `blend7`.
`blend[2-3,5,13]`	Matches channels `blend2`, `blend3`, `blend5`, `blend13`.
`t[xyz]`	`[xyz]`matches three characters, giving channels `tx`, `ty` and `tz`.

Sample Rate Match

The Sample Rate Match Options handle cases where multiple input CHOPs’ sample rates are different.

Resample At First Input’s Rate	Use rate of first input to resample others.
Resample At Maximum Rate	Resample to highest sample rate.
Resample At Minimum Rate	Resample to the lowest sample rate.
Error if Rates Differ	Does not accept conflicting sample rates.

Units

The units for which time parameters are specified.

For example, you can specify the amount of time a lag should last for in seconds (default), frames (at the Houdini FPS), or samples (in the CHOP’s sample rate).

Note

When you change the Units parameter, it does not convert the existing parameters to the new units.

Time Slice

Time Slicing is a feature which boosts cooking performance and reduces memory usage. Traditionally, CHOPs calculate the channel over its entire frame range. If the channel does need to be evaluated every frame, then cooking the entire range of the channel is unnecessary. It is more efficient to calculate only the fraction of the channel that is needed. This fraction is known as a Time Slice.

Unload

Causes the memory consumed by a CHOP to be released after it is cooked and the data passed to the next CHOP.

Export Prefix

The Export prefix is prepended to CHOP channel names to determine where to export to.

For example, if the CHOP channel was named geo1:tx, and the prefix was /obj, the channel would be exported to /obj/geo1/tx.

Note

You can leave the Export Prefix blank, but then your CHOP track names need to be absolute paths, such as obj:geo1:tx.

Graph Color

Every CHOP has this option. Each CHOP gets a default color assigned for display in the Graph port, but you can override the color in the Common page under Graph Color. There are 36 RGB color combinations in the Palette.

Graph Color Step

When the graph displays the animation curves and a CHOP has two or more channels, this defines the difference in color from one channel to the next, giving a rainbow spectrum of colors.

Usages in other examples

Example name	Example for