Bass management Confusion Reduction

If you’re reading this magazine, then you know that the world of multichannel sound brings with it great joys: increased immersion in the listening experience, ability to hear more detail, less need for equalization required simply in order to hear all the lines in the music, and a corresponding lesser need for audio compression. But it also has areas of confusion, perhaps primary among which is, “What do I do with the bass?”

Playback Bass Management
Bass management on the playback side made the whole thing possible. The reason is that very few people have room for five 20 Hz speakers in their listening room. Re-directing the lowest bass to a single subwoofer, and making satellite loudspeakers do the rest of the work per channel, greatly improve the WAF (wife acceptance factor) to the point where home theater took off and delivered a platform for the multichannel experience for both movies and music. This is fitting due to its invention in 1939 for music accompanying movies by Walt Disney for Fantasia; the stimulus was The Flight of the Bumblebee (which didn’t wind up in the picture!).

The reason that the satellites can be limited in size in a sub-sat system is the frequency range they cover, both for a simple reason, and for a more subtle one that is less widely understood. The simple reason is that limiting the frequency range to something about an octave higher than normal in stereo speakers, moving from say 40 Hz to 80 reduces the box size. But that box size reduction is usually also accompanied by a sensitivity increase, if the parameters are well chosen. Why is sensitivity important? Because higher sensitivity speakers require less power for equal SPL output, and this makes practical a single chassis receiver that is the basis for most home systems today. While we often use powered speakers in the studio, homes still lump all the power amps in one receiver chassis, and so limiting the size and weight of the receiver (although it’s a two-man job to lift a good one) makes multichannel practical.
Figure 1

An example from one manufacturer is in order. The Snell AIIIi speakers are large, floor-standing, and 3-way passively crossed over, with anti-diffraction baffling and a remarkably flat response, among the best ever tested at the NRC in Canada, and with high rankings from audiophile magazines. Its sensitivity is 83 dB/m/W. The Snell LCR500 is an 80 Hz satellite, also having very flat response in its operating range, and has a much smaller box. But its sensitivity is 89 dB/m/W, meaning it needs 1/4 the power of the larger model to reach the same SPL. This is actually a remarkable “improvement.” Improvement is in quotes because, all other things being equal, with sufficient power (and not operating at the extremes of dynamic range), these two could perform level-matched in an A/B comparison. However, the larger model, which does go very low, sacrifices sensitivity to do so, and makes a requirement for large external power amplifiers, something that would preclude multichannel for most people.

Professional Environment
So bass management in the home made home theater practical. What about professional use? Many believe that their 12-inch woofer in a 3-way monitor constitutes a “wide-range” system. However, professional monitors have to make the tradeoff of bandwidth vs. sensitivity just as home speakers do — it’s the same math everybody is dealing with after all, and guess what — they’re often made so they play loud and don’t go very low. Typical performance of a professional monitor varies among the types of course, but it is not at all unusual to see 40 or even 50 Hz, –3 dB corner frequencies among monitor speakers, with very significant rolloff in the bottom octave.

The problem with using such speakers and believing them is that the home user may have a “better” system, one with a lower –3 dB corner frequency, formed from the subwoofer-satellite system. And that means the home listener hears vocal mic pops, thumping feet on music stands, sets moving backstage in live shows, and so forth, that the professional couldn’t hear. (These examples are all taken from real released recordings!)

Listening to mixes made on NS-10’s in the stereo era over a wide-range monitor reveals rumbles and thumps that were unheard at the time. One Band-Aid would be to simply high-pass filter (low-cut) all of the tracks for a multichannel mix, but this is overkill, and wrong. You might find that you selectively need a high-pass filter, but applying one all of the time does miss some fundamentals and potential subharmonics that may be useful in music.

I was frankly surprised to hear the difference between a 22 Hz –3 dB subwoofer and a 15 Hz one on classical music, not pipe organ: the very lowest frequencies between 15 and 25 Hz actually contributed something to the feeling of “weight” of the music. And it wasn’t just room rumble coming from there, although that, of course, could happen, it was actually musical sounding content. I turned off the main channels and listened to the bass-managed subwoofer playing 15–25 Hz material (the main channels were that good that they could be crossed over at 25 Hz), and learned to hear what it sounded like, then turned the main system back on, and I believe that I could then tell the difference with the 15–25 Hz A/B’d on and off.

Perhaps the foregoing has at least convinced you of the need to monitor well down into the bass, below what typical studio speakers can do. Then why not put a separate subwoofer on each channel, you ask? That’s a good question. It will work to monitor everything, but there are two problems with this approach: [1] each channel will exhibit a different frequency response at your ears due to the differing transfer functions between the subwoofer location and the listening location, and [2] electrical summation is phasor addition, that is, out-of-phase components will subtract, and two in-phase channels will add by 6 dB, but this is not necessarily true of acoustical addition.

The difference is that acoustical addition includes the phase effects of the room, which could do anything from causing a 6 dB addition of two sources, to completely canceling them out at one frequency. So the results are uncertain. A DVD was made with five really full-range monitors and without bass management, and it was found that when the channels were subsequently bass managed in home products, that the bass canceled. Out-of-phase bass in the control room might even be nice, but it will cause cancellations on the majority of listener’s systems!

Another objection people seem to have to bass management is “phase coherence.” They think that there’s no way to splice each of the channels to one subwoofer and get a smooth transition. But they haven’t tried a good receiver recently, because they include time delay on each channel to solve this problem, as well as the problem of getting good phantoms between the loudspeakers. If your speakers LCR are in a straight line, and you sit at the center, you hear center first. That makes it predominant due to the Law of the First Wavefront, meaning as soon as you pan something off left or right it soon seems to snap to center. Either put your speakers on an arc, or delay the center to do the equivalent thing, to get the phantoms between left and center and center and right working properly.

The 0.1 channel, or Low Frequency Enhancement, refers to a separate channel with its level set so that it has 10 dB greater headroom than any one of the main channels. Its use at its origin for movies was in added headroom for low-frequency effects, which were separately made for it.

There is a debate as to whether it is needed at all for music. Richard Elen writing recently in these pages said just don’t use it. I’m a little more sanguine on its use, and see some utility in it for music. Of course, there are the canon blasts in the “1812,” that are obvious, but then there’s that rap music, see. No, I too don’t see that it’s being used for much classical music or jazz. But heavy rock, maybe, and rap, almost certainly. So it depends on the bass content of the material. Anyone mastering a broad range of material has probably run into the problem that the bass seems to overload the CD medium first, so to get more level on, the natural thing to do is cut the amount of bass. This is what LFE was meant to solve — by following the equal loudness contours of hearing to low frequencies, it makes up for the fact that it takes more stimulus at these frequencies to sound equally as loud as midrange frequencies.

I have written in a previous article about just what level the subwoofer should measure playing LFE band-limited pink noise, and it ain’t 10 dB greater than one main channel, because the bandwidth is narrower than a main channel. Overall, band limited pink on LFE should measure about 4 dB, C weighted, greater than a main channel, not 10. (See article in the August 2000 issue).

Equal loudness contours of hearing show that it takes higher physical sound pressure level at lower frequencies to sound as loud as midrange frequencies (see fig. 1). The “real world” is the square grid of the graph, and the lines of phons show the needed sound pressure to sound equally as loud, vs. frequency, at various levels.

Surround Professional Magazine