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Studio Monitors Part 4: Subwoofers
By Glen Stephan on October 18 2008 08:00 AM | Permalink | Author Info
When independent recordists are faced with monitor selection, the question usually comes up, “Should I buy a subwoofer, and if so, which one?” In this final part of our look at nearfield monitor selection and installation, we’ll take a simple look at the oft-controversial subject of subwoofers in the studio.

TO WOOF OR NOT TO WOOF

Should we mix with a subwoofer or not? The short answer: The way I see it, a subwoofer is necessary under three separate conditions:

- If we are mixing for 5.1 or 7.1 surround sound or for computer gaming, and we need that .1 channel to specifically mix to.

- If we are mixing for genres such as techno, dance, etc. where the sub is really where it's all at.

- If our stereo main monitors are fairly lacking in low-frequency response to begin with.

For any other situation, mixing with a subwoofer is strictly optional at best. I’d much prefer spending the money on better main monitors or better acoustic treatment or, having fulfilled those, a nice weekend at Sybaris with my gal.

It’s often said that subwoofers are good in general because by taking much of the bass they take a load off of the main speakers and amplifiers, allowing them to work easier, more efficiently and with less overall distortion. While there may be some degree of truth to this, my personal reaction to that is that if your mains are overworked, inefficient or producing too much distortion under a normal or even slightly above normal workload, you have the wrong mains. The best solution to which is to replace or upgrade the mains, not keep them and try to band-aid them with the addition of another loudspeaker.

But for the three instances bulleted above, subwoofers play a very important role, and are more than just optional. Deciding which ones to get for your situation is where it gets fun.



ISN’T THAT SPEC-IAL

It's very important to note that there are almost as many different flavors of subwoofers as there are standard monitors.

Some are made to act best in that third situation; design is more that of a quality woofer than as a true subwoofer.

Others kick more power, and delever more powerful china-endangering sub-bass, but assuming your mains can handle themselves pretty well. These are usually best used in larger rooms and/or with main monitors whose woofers don't really need any help delivering “normal” bass on their own.

And, as always in a consumer-driven market, there are plenty of models that fall somewhere in the middle.

Choosing the right subwoofer is a bit different than choosing the right monitor; much of what makes a subwoofer the right choice for us can’t necessarily be demonstrated in a listening test they way it should be when choosing main monitors.

This means that the specifications for a subwoofer play a rather more significant role in the selection process than they do for the mains. While there are other specifications that are important, the three main specifications we’ll look at today are the frequency response, the crossover frequency and the roll-off slope, as these have the most bearing on how to match up a subwoofer to our main monitors.



FREQUENCY RESPONSE

As with all loudspeakers, the frequency response indicates the range of frequencies that the subwoofer is designed to reproduce. Obviously subwoofers will reproduce their energy mainly in the bass frequencies. Here we would want to look at a) how much overlap there is in the flat response of the subwoofer and the flat response of our mains, and b) how much lower the flat response of the subwoofer will go than the flat response of the mains (i.e. how much further the subwoofer will cleanly extend the overall frequency response of our monitoring system.)

Overlap in the flat frequency response specs of our subwoofer and our mains is important because we don’t want to find a gap in the clean response between them, as this will give us a gap in the overall frequency response of the system. If our mains only go down to 75 Hz cleanly (i.e. with an accuracy of +/- 3dB or less) but the subwoofer only holds it accuracy up to 70Hz before it’s frequency response starts wavering, we could have an important gap in our ability to hear what’s going on in the bass. How much of an overlap do we want? We’ll talk more about that in a minute, but first we need to explain the next two specifications to get the full picture there.

And as far as how low the subwoofers can go, this is important to make sure we’re getting out money’s worth and actually getting good frequency response extension. If we have main monitors rated down to 40Hz +/- 3dB (just for example), a subwoofer that only goes down to 35Hz +/-3dB may not be the best value of the choices available to us.



CROSSOVER FREQUENCY

The crossover frequency on a subwoofer is the cutoff frequency at which the subwoofer is set to take over from the mains. Everything below this frequency is sent to the subwoofer, everything above it is sent to the mains. This is (usually) an adjustable setting, allowing us to dial in just the right spot in the frequency response overlap between the sub and the mains so we can get a smooth transition in frequency response overall.

Different subwoofers have different crossover options and ranges; obviously the one that meets the overlap sweet spot the best is one we should find attractive for our needs.

Many folks often quote a “standard” crossover frequency of 80Hz. And in many cases this is not a bad generic default setting to go with. There are, however, a couple of real-life situations where this is not an optimal crossover setting.

The “standard” that people are referring to is the THX standard for multimedia and entertainment center playback systems. The 80Hz crossover works great when used with subwoofers and main monitors that also meet THX standards for frequency response. But the sad truth is that not all gear designed or marked for the home recording market meet these specifications.

There are plenty of loudspeakers marketed as “studio monitors” or as “high fidelity” bookshelf speakers that actually only have a clean bass response down to 80Hz to 100Hz or so. In such cases, an 80Hz crossover will leave a huge hole in our overall frequency response.

Or, if we have mains that are pretty flat down to 50Hz, but a sub that starts getting sloppy above 60Hz, we might want to dial the crossover down to the 55-60Hz range instead of 80Hz to get the best of both speakers.



ROLL-OFF SLOPE

No crossover circuit is a brick wall. If we have our crossover set to 80Hz, that doesn’t mean that right at 81Hz the subwoofer is silent and the mains at full power. There is instead a gradual crossover from one two the other where after that frequency, less and less of one speaker is juiced and more and more of the other gets the juice.

The roll-off slope is the specification that tells us how fast or how gradual the crossover transition is. This is usually specified in decibels (dBs) per octave. The higher the number, the faster and sharper the transition, and the less frequency response overlap required to cover the transition. A 12dB per octave crossover would require more of an overlap to ensure smooth transition than a 24dB per octave crossover would.



A TALE OF THREE SUBBIES

Let’s look at three wildly different, but real-life and popular examples of subwoofers, and how their different designs suit different needs and rules. (This is neither an endorsement nor a criticism of any of these models, just an examination and analysis of the different character of features.)

One of the most popular subs in small-budget home studios today is the M-Audio BX-10s. This is where one probably should relax their definition of "subwoofer" the way some manufacturers have, and look at a "subwoofer" that is actually serving best as a separate powered woofer. The  BX10s, which, with a frequency range and a variable crossover that extends all the way to 200Hz, but with a response that gets a little shaky down below 40Hz, is a likely choice for augmenting smaller nearfields that may not have a strong or even bass response of their own. There’s plenty of overlap available in the mid bass region above 80Hz and a crossover designed to be set as high as you need. In fact, M-Audio often advertises them for use with bass-hungry computer speakers and smaller nearfields.

On the other side of the coin you have something like the Mackie HRS120, which IS THX-centric and certified. Its crossover setting only goes up to 110Hz, and its frequency response only goes up to 150Hz, so it is not the most ideal for covering anemic computer speakers or bookshelves, but rather for co-existing with quality mains that can go down into the double-digit frequencies well on their own. On the other side of the frequency scale, this sub is a rock-solid weapon of flatware destruction, with almost twice the power of the BX-10s and an ultra-tight response (+/- 1.5dB) all the way down to 21Hz. This is a true subwoofer designed to handle the chest-pounding stuff and for letting the mains fend for themselves with the rest.

Finally, we have choices such as the Tannoy TS10, designed by one of the higher-end manufacturers of studio monitors, and designed to make assumptions that you just need a nice plug-and-play subwoofer solution that smoothly and gradually hands over the bass to the sub.. Both the M-Audio and the Mackie have tunable crossovers, and both have admirable 24dB/octave slopes, but the Tannoy is a different animal with a different design. Its “crossover” is a fixed low pass filter at 6dB/octave below 150Hz. The THX standard 80Hz crossover isn’t even applicable to this subwoofer.

So choose your nearfield monitors wisely, and then choose your subwoofers just as wisely, getting a custom fit between the two. Put them in a well-designed room, and you should be a happy fader jockey.



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