What is a Filter?
A filter is a series of volume controls per frequency. There are several types of filters.
|All pass and all stop filters|
The simplest filters. An all pass filter passes all frequencies equally, and acts like a normal volume control. An all stop filter is an all pass filter with a value of 0... it does not pass any frequencies.
|Low pass filters|
Pass low frequencies, but not higher frequencies. They usually have a transition period where increasing frequencies are partially passed. These filters are used to remove noise and mute sound. They normally make audio sound muffled. Low pass filters are excellent for modeling obstacles (like walls).
|High pass filters|
The opposite of low pass filters. They pass high frequencies but not low frequencies. They tend to make audio sound tinny. High pass filters are an excellent choice for modeling atmospheric sound loss and reflections.
|Band pass filters|
Pass mid-range frequencies, and not high or low frequencies. They are sometimes referred to as notch filters.
|Band stop filters|
Suppress mid-range frequencies and pass both high and low frequencies. Both Band Pass and Band Stop filters aren’t quite as useful for Spatial Audio as the previous filters, but they can be used for special cases.
How to create a Filter
The Acoustic CHOP lets you create a full or partial spectrum filter. You can create several filters can be created at once, with the added benefit that you can constrain multiple filters to preserve total signal power. You can then manipulate the filter with any other CHOP to modify the filter.
|Transmit, Absorb, Reflect|
Toggle the transmission, absorption and reflection filters on or off. Normally, you will want to create one filter of a specific type.
The next parameter, Frequency Range, determines the range that the filter will cover. The beginning of the channel corresponds to the start of the range, and the end of the channel corresponds to the end of the range.
|Power and Lock|
The Power and Lock parameters are for conserving power when creating several different filters for the same object. If an object reflects some sound and transmits the rest, you may only want to design the reflection filter and have the transmission filter automatically adjusted so that the total signal power is conserved.
The Power parameter can be turned off for unconstrained modeling, set to “Conserve Total Power” so that the total signal power is conserved (but not the individual frequency power), or “Conserve Power Per Frequency” which ensures that each frequency has its own power conserved. The Lock parameter allows you to lock one of the filters so that it is not affected by the automatic power conservation algorithm (mainly for use with 3 filters). If you increase one filter beyond 1, then power will no longer be conserved.
The No Resonance parameter constrains the filter so that filter values cannot be increased beyond 1. This prevents the filter from adding power to the signal.
The Handles page contains all the parameters for modeling. The Handles parameter specifies the number of interactive handles that are created. This can be changed on the fly, but may result in some data loss.
Since audio filters are logarithmic in nature, it is easier to shape filters with logarithmically spaced handles. These will give you more control in the lower frequencies, and less control on higher frequencies. They are enabled by default, but you can use evenly spaced handles by toggling Logarithmic Handles off.
The Samples parameter determines how many samples are in the actual filter. You can increase the filter accuracy by increasing the number of samples if you notice your filter is lacking resolution in areas of high detail.
The Interpolation parameter specifies how the samples of the filter are interpolated from the handles (none, linear or cubic). Cubic produces smooth filters and None produces notched filters.
You can reset all the waveforms to constant values by clicking Reset All Waveforms. If you want to clear a single filter, but not the rest, toggle that filter off, then on using the toggles on the Acoustic page.