Getting Back to Subwoofer Basics
Our story to date… Some months ago I got a letter from Gunnar Kullenberg asking me to write about subwoofers, and I promised to do so.
In one sense, subwoofers are utterly simple. However, their performance and effectiveness can be highly variable for a range of not-so-obvious reasons. The engineering of subwoofers calls for some interesting compromises in terms of size, price and performance. Finally, in our broadcast world, we have come to use subwoofers in some ways that turn out to be a little confusing.
So, these concerns about performance, engineering and production practices really do need some discussion. Let's get to it.
ABOUT LOW FREQUENCIES
Low frequencies are the range of audible frequencies whose wavelengths are equal to or larger than the dimensions of our listening rooms, which is to say, wavelengths ranging from about 7 feet to 60 feet (ca. 150 Hz. to 20 Hz.). They are commonly called "bass." They have several interesting characteristics:
- • Because their wavelengths are as large as the rooms in which we listen, their performance in such rooms varies as a function of our position in the room, and bass frequency response at any given location is more related to the room than the loudspeaker;
- • For some interesting evolutionary reasons, our auditory system responds to the presence of those frequencies with the generation of adrenalin, among other things, stimulating the so-called "fight or flight" response. This means that low frequencies are strong emotional, physical and dramatic medicine for us;
- • For some other interesting evolutionary reasons, our perception of low frequencies by our two ears enhances our perception of room size and wall distance as well as a sense of envelopment in the perceived space. Such envelopment is a sensation we really enjoy, even though us couch potatoes generally don't consciously notice it (which makes it a sort of "magic sauce" for audio engineers);
- • Finally, because of the physical size of their wavelengths, the generation of low frequencies requires the movement of a large volume of air, which in turn requires loudspeakers that are very large and/or very efficient. Very efficient small loudspeakers usually have their low-frequency efficiency in fairly narrow frequency bands (using ported enclosures), which leads to somewhat compromised frequency response. Very large loudspeakers, on the other hand, require lots of space, lots of amplifier power and lots of cost.
THE SUBWOOFER IN PRINCIPLE
Genelec PD7270A Subwoofer
The subwoofer came into being as a cost-saving device. The idea is based on the assumption that the bass of all audio channels is essentially the same and we can mix all that multichannel bass information into a single channel and send it to a single dedicated loudspeaker that specializes in bass, by being big, efficient or both.
This means we don't need to have big and/or efficient speakers (which are the most expensive kind) for each of our audio channels. Equally important, we don't have to provide big and expensive amplifiers for those speakers. For stereo, we can cut our cost in half for generating the low frequencies. For surround sound we can cut it to one-fifth!
Finally, we can comparatively easily place that single subwoofer at a reasonably optimum point in the room for both bass response and level.
THE SUBWOOFER IN PRACTICE
The same urge to reduce costs that led to the adoption of a subwoofer, unfortunately, also carries over into the manufacture of subwoofers. As a result, the performance of speakers described as subwoofers varies widely. Many are too small to cover the entire frequency band from 20 to 150 Hz. Many have quite erratic frequency response because of ported designs aimed more at greater power output than smooth response.
Finally, many so-called "computer monitor speakers" use so-called "subwoofers" that simply are too small and/or under-powered to even get into that low-frequency range, much less support it.
Another problem with subwoofers is that the single subwoofer sacrifices the "bass differences" between channels that support the sensation of envelopment. For me, that's a big loss. You can get a convincing explosion, for example, but you can't get the sensation of being in the same acoustic space as the explosion.
THE LFE CHANNEL
This leads us to the LFE (Low Frequency Effect) Channel. This is a dedicated channel that can be used to feed a subwoofer, usually carrying special low-frequency effects for film and video. You know, explosions, foreboding LF tension via tympanis, double basses and small-block Chevy V8s in unmuffled race tune, and the end of the universe. Stuff like that. You knew all this, right? For a detailed consideration of the LFE possibilities, check out Dolby's paper "What is the LFE channel?" at www.dolby.com
But what happens when you have main loudspeakers that can also play back low frequencies? And what happens when you would like to have stereo or surround low-frequency materials in your main speakers? (This latter is a common wish for music program material—music often doesn't do very well with subwoofers.)
And what about when you'd like to mix low-frequency information in the primary audio channels with LFE materials for the subwoofer?
These are all questions, conditions and needs that show up for the end-user, and are not under the control of those of us producing or transmitting the broadcast. Further, we have no metadata for controlling the behavior of the LFE channel or low-frequency material in the main channels. We cannot vary or even control the results of these conditions, except through the unfortunate imposition of "loudspeaker police," a practice we have not yet embraced.
WHAT DOES IT ALL MEAN?
The result of this is that subwoofers mostly don't work all that well, except in really carefully engineered commercial or home theatre installations. Worse, their performance varies as a function of the various production techniques used in the program material our beloved viewers wish to play back, as well as the sensations they seek while listening/viewing.
Envelopment vs. defibrillation? Deep foreboding vs. drag strip strippers? It's a puzzler.
Finally, a single, lonely subwoofer just can't
work very well in any normal room without a lot of room correction. Loudspeaker guru Floyd Toole has even suggested that what we really need is up to four subwoofers, playing a single mono LFE signal, positioned along various walls in the playback room just to smooth out the frequency response bumps and dips that are there. And that still doesn't get us all the envelopment that we crave.
So, subwoofers remain both somewhat under-engineered and over-hyped in their daily middle-class usage.
My suggestion to you is to use them, and their LFE channel, very conservatively, for key effects and dramatic moments in your production. Don't beat your viewers up with a constant barrage of infrasound designed to put them into a state of quivering shock, but instead, remind them at key moments that the world is a little tougher, meaner, scarier and more dramatic than they really thought. They may even thank you for it.
Thanks for listening.
Dave Moulton is pretty subdued these days, and just devoted to basics. You can complain to him about anything at his website, www.moultonlabs.com. Thanks, Gunnar!