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Subwoofers and Bass Management
Managing the low end in your studio
By Giles Reaves

When talking about speaker systems, once upon a time we would use words like “mono” or “stereo” or even “quad” to refer to how many individual “channels” or speakers were involved. Now instead, we hear numbers like 2.1, 5.1, 7.1, and even 10.2 and beyond

For now, we are only interested in the number after the decimal point. That’s the part that refers to the subwoofer. Later we’ll discuss why it’s called that.

The subwoofer: what is it?

A subwoofer (or ‘sub’) is any speaker (or cabinet containing a speaker) that is designed to handle roughly the lowest two octaves of the audible spectrum, typically from around 20–80 Hz. Sometimes the subwoofer is contained in the same cabinet as the speaker(s) for midrange (called the woofer) and for high frequencies (called the tweeter), but more often it’s in a separate cabinet.

In the music world, the primary function of a subwoofer is to seamlessly extend the frequency range of smaller monitors, creating the effect of a single, full-range cabinet. In home theater and post production for film and TV, its primary function is for effects like explosions, earthquakes, aliens and the like. In this discussion we’ll focus primarily on musical applications—setting up a sub and a specialized Low Frequency Effects (LFE) channel in a mix for post is another story.

A brief history of the subwoofer

In the early days of sound reproduction, the frequency response of most recordings didn’t include much (if any) information in the bottom-most octaves of the audible range, called LF as in ‘low frequencies’. This meant that speakers didn’t have to reproduce this range. But as recording technology advanced, speakers had to keep up. The hi-fi era introduced woofers and tweeters, but we were still not able to easily reproduce the entire audible spectrum.

Eventually, woofers got bigger, cabinets got bigger, and power amps got bigger (actually, they got smaller, but more powerful). Sometime around the early ’80s, the subwoofer was born as a separate entity. Shortly thereafter, amplifiers were getting powerful enough and small enough to be assimilated by the sub’s cabinet, and active subs were born. Finally, it was possible to hear all the music without having to mortgage your house.

Why use a sub in the studio?

Many of today’s home and car systems (and a few PC speaker packages) include subwoofers as a part of the sound system, whether for stereo or for surround. So if the client hears the LF information in that way, a modern studio, even a small one, needs to have an awareness of this frequency range, and can’t afford to be the only link in the chain that doesn’t hear LF information accurately—or even at all.

Additionally, there are all sorts of potential problems caused by very low-frequency information (called infrasonic or subsonic, commonly assumed to be below 25 Hz), such as footsteps or foot tapping, air conditioner vibrations, and other sources of low-frequency ‘trash’. With a sub as a part of your audio monitor system, you can bring out this information if any is present, so these issues are easy to spot and fix.

Choosing a sub

Which sub is right for your setup? First, let’s cover what your options are.

Subs can come in many different forms. They can be powered, also called active (incorporating a power amp in the enclosure and accepting audio signals at line level) or passive (driven by a speaker-level signal from an external power amp); front-firing or down-firing; sealed or ported/vented; include a passive radiator or not.

Then there are the distinctions of how the sub receives its LF information, which has to do with filtering and signal routing, covered in a moment.

Where to begin? One obvious place to start is a listening test, at your local hi-fi dealer, music store, and at any friend’s studios where a sub is a part of the monitor system. Listening to many different systems can be overwhelming, so stay focused and take your time. Your monitor environment is an important part of your studio, and worth the effort to get it right.

These days, it’s almost impossible to find a sub that’s not powered, so that decision is probably already made for most of you. From there it’s a matter of taste and of budget, as there is not one style that will work in all circumstances.

There are two different scenarios for those shopping for a new sub: those who wish to add a sub to an existing system, and those looking for a new monitor package including a sub. You can buy an integrated system from one vendor, or mix and match components from different makers.

To those who are looking for a new 2.1 or 5.1 system, my advice is that it’s much easier to set up an integrated system, i.e. a sub and mains that were designed to work together. Most of these packages will include some sort of setup system, including a test CD with all the tones you need, and certain models now offer DSP correction built-in and even come with a measurement mic to listen to your room. There are many companies offering 2.1 and 5.1 packages, including ADAM Audio, Alesis, Blue Sky, Genelec, JBL, M-Audio, and Samson.

If you choose to mix and match your system, I would advise you to have some patience in selecting and setting up your sub, since you will be the one matching the components. For those looking to add a sub to their existing system, seek out any other folks you can find who have already walked that path and try to learn from their mistakes. There’s nothing like experience, especially someone else’s. Again, talk to your local hi-fi or music store, go online and search for folks who have the same main monitors as you, and see if they found a good match in a subwoofer. Note that the L+R speakers are often called “satellites” when linked to a subwoofer.

One major factor to consider is to try to match the sub to your existing monitors and your listening room. Bigger monitors and bigger rooms typically need bigger subs to keep up. Also look for a sub that has the most flexibility with its features, including a polarity switch, variable crossover frequency, variable input level and sensitivity, as well as choices in input and output connections that match your needs.

Where does it go?

It’s big, it’s loud, where does it go? Anywhere it wants. At least that’s part of the story.

Low bass is pretty much omnidirectional below 80 Hz, which means that it shouldn’t matter where you put the darn thing—in theory. However, there are other factors that do affect the placement of the sub. These are mainly related to room modes, accentuated frequencies which are created by the size and shape of the room. Every room has them, and no one likes them. They can cause all sorts of problems, mainly dips and bumps in the frequency response of any system in the room. But you can deal with most of these issues simply by careful placement and tweaking of your sub.

Most manufacturers recommend experimenting with the placement of your sub, as there are no hard and fast rules regarding placement. As you consider where to place your sub, be aware of a few common issues. First and foremost is the boundary effect. A boundary is a floor, wall, or ceiling, and the closer you place a speaker to a boundary the more the bass frequencies will be exaggerated. This can work for you or against you, depending on whether there’s already enough bass in your room or not. Keep in mind that you can control the amount of bass at your listening position by moving your sub towards or away from a boundary.

The most common placement of a sub is on the floor, which means that you will be getting a little boost from the start. From there, moving the sub toward a wall will allow two boundaries to contribute to the exaggerated bass, and a corner will further increase this effect. This may result in a muddy or out-of-control low end, or it may be just what you need. I’ve seen subs in corners that worked great, but most of the time they are on the floor away from any walls. As always, experiment and choose the best position for your situation.

One interesting technique for finding an optimum position for your sub uses what I call “acoustic role reversal”. This technique involves trading places with your sub—the sub gets to sit in your comfy chair, while you crawl around on your hands and knees searching out the best sounding bass while blasting your favorite rafter-rattling tune. It may be easier to judge the sub’s effect if you turn off the main monitors off at this point.

If you have a sound pressure level (SPL) meter and a source of pink noise, you can be a bit more scientific and actually measure the average SPL at different locations around your room. Make a note of the best positions (two or three is plenty) and then assume your original roles. Listen to the sub in each of your choice positions and see which one sounds best. Now you’re ready for tweaking.

There are other techniques requiring more sophisticated measurement gear and a good bit of training, if you are so inclined. For the rest of us, I’ve got a few basic pointers..the most basic one being to read and follow any advice provided by the manufacturer of your sub! But first, a word about bass management.

How to manage all that low end

Bass management (sometimes called bass redirection) is any process that controls which frequencies are sent to which monitors, specifically on systems including a subwoofer. This is accomplished by the use of a crossover network and other controls for sending the correct signal to the correct monitor. Without some form of bass management, you would end up with a range of frequencies being produced by two drivers, causing a bump in the frequency response. It would be similar to building a two-way speaker without using a crossover.

In a 2.1 system, bass management usually takes place in the subwoofer, and consists mainly of a crossover network (more on that later). Bass management in a 5.1 system exists to deal with one additional issue, which is whether to simply send the LFE channel to the sub, or do as your 2.1 system does and send all frequencies below the crossover point (usually around 80 Hz) from all channels to the sub. In practice, you usually do both, but you need to have all options available. There are systems out there that can handle bass management for you if you mix to 5.1, including systems like the Blue Sky BMC, Martinsound ManagerMAX , and M&K’s LFE-4.

Bass management is a function of your monitor system, and therefore does not effect your final mix directly. But it can have a profound effect on the decisions you make, so in this way, it can very much affect your mix.

To sum up: most 2.1 systems require that the full-bandwidth audio go first to the sub, which extracts the LF info. From the sub, the remaining filtered audio gets passed through to the satellites. A few systems require that the full-bandwidth audio go first to the satellites, which then filter out the LF info and pass it through to the sub.

For 5.1 production setups, the routing system can be way more complex, involving bus outputs and discrete amplification and other complexities that you’ll be well aware of if you are the proud owner of such a system.

The crossover

The idea of calling the LFE channel “point one” refers to the fact that it is not a full audible-spectrum channel, but only represents the bottom ‘one tenth’ of the audible range. On a finished 5.1 product this filtering was done by the mix engineer during the mix process and involved a lowpass filter at 125 Hz. But what if you’re running a 2.1 system, how do you get the “point one”, when it’s not on a separate channel as with 5.1? The answer is inside the sub—most every sub sold today has built-in crossovers (a lowpass for the sub and a complimentary highpass for the mains).

This is basically the same concept as the crossover in a two- or three-way monitor, directing the appropriate frequency range to its respective driver. On a two-way system, the high end (material above the crossover point) goes to the tweeter, and the low end (below the crossover) to the woofer. Adding a subwoofer to your system will further this concept, adding a second crossover and creating a third band below the woofer (which is now a midrange driver, technically speaking). This essentially turns your two-way system into a three-way system, with the left and right monitors sharing a common sub.

The crossover in the sub can typically be bypassed and the sub muted for A/B’ing between mains only and mains plus sub, which is almost like having two different sets of monitors to compare between. Many subs allow a simple foot switch to be used to control this feature. This is most helpful during mixdown, where you need to hear as many different scenarios as possible.

If your sub has an adjustable crossover, there are guidelines on how to set it and whether or not to run your main monitors through the crossover (which is the most common setup) or to leave them flat. The main issues involved are trying to create a smooth transition between the two cabinets. Ideally, you should be able to sweep a sine wave across the entire audible spectrum and get the same level at the listening position at every frequency. Issues to look out for are either doubling up on a certain range, or leaving a “hole” or gap between the sub and the mains’ frequency ranges.

The first can only occur when you don’t filter the mains, leaving the possibility that there are frequencies that are being produced by both the mains and the sub. The other situation can take place in any setup involving a sub, and occurs when you lower the crossover enough to leave a gap between the lowest frequency produced by your mains and the highest frequency produced by your sub. If, for example, your mains only go down to 100 Hz, and you set your crossover at 75 Hz, there is a “missing” gap between 75–100 Hz that neither speaker is fully reproducing.

Remember that filtering is not done by harshly chopping off responses right at the crossover point. If a lowpass filter is set to, say, 80 Hz, that really means that frequencies at 80 Hz are already weaker by 3 dB than the lower part of the spectrum (called “the 3-dB-down point”), and because the filter acts on a slope, you’ll still hear frequencies in the 80s and 90s and higher, gradually weakening acording to the slope of the filter (usually indicated by dB per octave). The problem that arises when crossover frequencies aren’t set properly is rarely an entirely silent gap in the frequency response, but a dip of several dB between the crossing slopes—still something that can ruin your mixes! Again, refer to the setup guide that came with your sub for further guidance.

Further tweaks

You’ll need a buddy to help with this next section. Start by having a good listen to the full system, using music that (a) you are most familiar with, and (b) actually has some good solid low frequency information. If you like the way it sounds, great—congratulations, you’re finished. If not, or if you’d just like to experiment for fun or educational reasons, there’s always more you can do.

The first thing to try is switching the polarity on the sub, best done with a friend at the sub and you in the listening position. Listening for proper polarity is not always easy, and sometimes both positions sound good (or bad). You may need to try moving the sub around some more to hear a more obvious change. You can optionally try facing the sub away from you (if it’s not a “down firing” design, i.e. with the speaker facing the floor). Finding the correct polarity is important, and since it’s not always easy by ear, you and your buddy can try trading places for a second opinion.

Next, adjust the crossover slowly up and down (again with the buddy system), while listening to a repeated section of music. This will help you be sure that you can hear the difference between the different settings. Try a few different music selections to be sure you like it.

Finally, there’s the overall level. Most home theater systems I hear have the sub cranked up above what would be considered anywhere near flat. I can’t tell you where to set this control—flat (if you can measure it) is a good place to start. Many engineers (myself included) love to hear lots of LF information when listening/working, and boosting the sub by around 3–6 dB can actually help you mix better (if you’re like me!) by letting you have a bit of fun without actually sending stuff out that has too much low end.

Subs can create enormous amounts of energy, and physically move things around a bit. The sonic results can range from slights buzzes and rattles to the sound of lamps falling and hitting the floor. You can sweep a sine wave to find and eliminate the bothersome noises.

Wrap up

By now you have learned about basic subwoofer concepts, placement, and bass management. I would encourage you to continue to experiment with the placement and settings of your sub and bass management as you get familiar with your new system. It may take a few attempts to get everything working smoothly for you.

Be patient, don’t be afraid to experiment, and be sure to document your settings if you decide to back up and return to a previous setup. Most of all, enjoy listening to some of your favorite music, and enjoy hearing all the music.

Giles Reaves is a recording engineer, producer, and musician currently dividing his clientele between Nashville and Salt Lake City.

 




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