What is bass redirection
To be short, bass redirection feeds your subwoofer. Without bass redirection basses are always reproduced by main speakers. But main speakers, even good ones may not reproduce low frequencies well. To have a good bass effect and shake walls, it's necessary to take basses away from main speakers and feed it to the subwoofer.
It is very important to know that LFE (Low-frequency effects) channel does not replaces bass redirection. It is a very bad myth that speakers must be fed as-is and only LFE channel should go to the subwoofer. In most cases you'll get no bass at all in this case. Read an excellent article Bass Management and the LFE Channel that explains bass management and related myths in details.
Threre're two main schemes of bass redirection:
- Redirection to the subwoofer. Used when an audio system has a subwoofer. Takes the bass away form main channels and feed it to the subwoofer.
- Redirection to front channels. Used when rear speakers handle bass worse than front ones (simply, rear speakers are smaller than front) and it is no subwoofer. In this case bass is taken from rear (and center) speakers and fed to front ones.
 Do I need it?
If your audio system has a subwoofer, you definately need bass redirection.
In the absense of a separate subwoofer, you may want to redirect basses from small rear speakers to front ones.
You do not need bass redirection in AC3Filter if it is handled by the speaker system itself (or by your audio card).
 How to use
To enable bass redirection, check "Enable" at "Bass redirection" box at Gains page.
If you're redirecting to the subwoofer, you have to select one of ".1" modes at "Output format".
You can monitor bass redirection activity. Level shows the volume of the bass being redirected. But note, that this activity is not synchronized with actual audio playback. You can see the bass you currently hear at output levels monitor.
To adjust the bass volume, use output gains (Subwoofer gain). Noite, that this gain affects both LFE and redirected bass, to adjust the LFE channel separately, use LFE gain at Mixer page.
You can adjust crossover frequency and bass destination.
Crossover frequency (Cutoff frequency option) defines what the bass is. For example a simple multimedia audio system may have small speakers that cannot reproduce frequencies lower than 200Hz. In this case bass is everything below 200Hz. Good speakers may work good down to 80Hz. But even full-range speakers do not eliminate the bass problem completely, because low-frequency speakers must be carefully placed to avoid room resonances, but main speakers must be placed at fixed positions relative to the listener. It's too hard to combine both conditions (see Bass Management and the LFE Channel, Myths and Confusion #2).
Bass destination is where the bass goes to. It may be either the subwoofer or front left and right channels. If you have a subwoofer, select 'Subwoofer' and choose one of ".1" modes at "Output format" box. Otherwise, select 'Front channels' or disable the bass redirection at all.
Only one bass redirection must be done in the chain Movie -> Audio decoder (AC3Filter) -> Sound card -> Receiver -> Speakers. If you have bass redirection enabled at your sound card driver, receiver or it is done by your audio system itself, you should disable it at AC3Filter.
Redirection may be enabled, but inactive due to the following reasons:
- Bass destination is subwoofer, but output format does include it (no bass destination).
- Bass destination are front channels and output format is stereo (there's nothing to filter).
Because bass from all channels is routed to the single channel this may lead to overload at this channel. The solution to this problem is to decrease the subwoofer's gain at AC3Filter by 10-15dB and increase the volume at the subwoofer itself (most audio systems allows this).
Some implementations of bass redirection provide an option not to filter the bass out from main channels, but just copy bass to the subwoofer. It is not a right way! And AC3Filter does not provide this option. Why? Without these filters following bad things happen:
- Bass at main channels may overload speakers.
- Low-pass filter used to extract basses from main channels is not phase-linear. As the result the phase difference appears between the bass from main channels and the same bass, redirected to the subwoofer.
- Phase correction filter applied to the subwoofer leads to phase difference between sound from main channels and the subwoofer. Without the phase correction, it is a phase difference between the bass redirected and the subwoofer.
Filtering done by AC3Filter is designed to avoid all these effects.
 Technical details
See Bass redirection (math) for technical details.