Low-frequency effects

The low-frequency effects (LFE) channel is the name of an audio track specifically intended for deep, low-pitched sounds ranging from 3-120 Hz. This track is normally sent to a speaker that is specially designed for low-pitched sounds called the subwoofer. While LFE channels originated in Dolby Stereo 70 mm film prints, they became commonplace in the 1990s and 2000s in home theater systems used to reproduce film soundtracks for DVDs and Blu-ray discs.

LFEs include both low-pitched musical notes and low-pitched sound effects. The musical soundtrack for many films includes bass instruments that produce very low notes. Until the 1970s, most of the low-pitched instruments were natural, acoustic instruments, such as the double bass or the pipe organ's pedal keyboard. After the 1980s, film scores increasingly used synthesized instruments, including synth bass keyboards, which included very low-pitched notes.

The most challenging sounds to reproduce from a sound engineering soundpoint are usually the extremely low-pitched sound effects in the 20 Hz range, such as those used to simulate the sound of an earthquake, a rocket launch, or submarine depth charges. The human ear is not very sensitive to sounds at these low frequencies, so it takes a tremendous amount of amplification for the human ear to hear them. As well, since they are sound effects, they may have a longer duration or sustain than many low-pitched musical notes, which makes them harder to reproduce accurately.

It is a formidable challenge for an amplifier, subwoofer speakers, and cabinet to reproduce these sound effects at a high volume without problems such as power amplifier clipping (distortion), unwanted rattle or resonance in the wooden cabinet, or excessive "chuffing" sounds from the bass reflex vent (if a vent or port is used in the cabinet). Sound recording magazines sometimes use the loud, rumbling sound effects simulating the sound of the submarine depth charges which were used in the World War II film U-571 (2000) to test the accuracy of subwoofer systems.

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