Surround Sound: Psychoacoustics - Part 1

Bass Management and Low-Frequency Enhancement Pyschoacoustics
Localization by human listeners is not equally good at all frequencies. It is much worse at low frequencies, leading to practical satellite" subwoofer systems where the low frequencies from the multiple channels are extracted, summed, and supplied to just one subwoofer.

Experimental work sought the most sensitive listener from among a group of professional mixers and then found the most sensitive program material (which proved to be male speech, not music). The experiment varied the crossover frequency from satellite to a displaced sub-woofer. From this work, a selection of crossover frequency could be made as two standard deviations below the mean of the experimental result from the most sensitive listener listening to the program material found to be most sensitive: that number is 80 Hz.

Many systems are based on this crossover frequency, but professionals may choose monitors that go somewhat lower than this, to 50 or 40 Hz commonly. Even in these cases it is important to re-direct the lowest bass from the multiple channels to the subwoofer in order to hear it; otherwise home listeners with bass management could have a more extended bass response than the professional in the studio, and low-frequency problems could be missed.

The LFE (low-frequency enhancement) channel (the 0.1 of 5.1-channel sound), is a separate channel in the medium from producer to listener. The idea for this channel was generated by the psychoacoustic needs of listeners. Systems that have a flat overload level versus frequency perceptually overload first in the bass. This is because at no level is perception flat: it requires more level at low frequencies to sound equally as loud as in the mid-range. Thus the 0.1 channel, with a bandwidth of 1/400 the sample rate of 44.1 or 48 kHz sampled systems (110 or 120 Hz), was added to the 5 main channels of 5.1-channel systems, so that headroom at low frequencies could be maintained at levels that more closely match perception.

The level standards for this channel call for it to have 10dB greater headroom than any one of the main channels in its frequency band. This channel is monaural, meant for special program material that requires large low-frequency headroom. This may include sound effects, and in some rare instances, music and dialogue. An example of the use of LFE in music is the cannon fire in the 1812 Overture, and for dialogue, the voice of the tomb in Aladdin.

The 10dB greater headroom on the LFE channel is obtained by deliberately recording 10dB low on the medium and then boosting by 10dB in the playback electronics after the medium. Obviously with a linear medium the level is reproduced as it went into this pair of offsets, but the headroom is increased by 10dB. Of course, the signal-to-noise ratio is also decreased by 10dB, but this does not matter because we are speaking of frequencies below 120 Hz where hearing is insensitive to noise.

A study by a Dolby Labs engineer of the peak levels on various channels of the 5.1-channel DVD medium found that the recorded level maximum peaks in the LFE channel are about the same as those in the highest of the other 5 channels, which incidentally is the center channel. Since this measurement was made before the introduction of the 10-dB gain after the medium, it showed the utility of the 10-dB offset.

In film sound, the LFE channel drives subwoofers in the theater, and that is the only signal to drive them. In broadcast and packaged video media sound played in the home, LFE is a channel that is usually bass managed by being added together with the low bass from the 5 main channels and supplied to one or more subwoofers.