Subwoofers are a common element of modern home entertainment systems. Not so long ago they were confined to the small number of people who knew how to use both them and analogue crossovers. The rise in popularity of the subwoofer is directly related to the growth of the home theater market. Most subwoofers are connected via the LFE output on surround sound receivers and preamp/processors. The LFE (Low Frequency Effects) output isn’t designed for music, as its name implies, it’s an effects output. It can, however, be compelled into service for music through DSP. This may be a good solution for those not interested in critical listening, but for those who are, it’s a poor substitute for a real low frequency crossover.
Most, if not all, consumer speaker systems do not accurately reproduce low frequency information from the source material. Even if your speaker system claims a frequency response going down to 35 Hz, it’s most likely only capable of that in a controlled environment, measured from a distance of 1 meter. If it is capable of such low frequency reproduction, it’s still very unlikely that it could reproduce the low frequency dynamics which are felt more than heard. Subwoofers provide both natural warmth and dynamics to your system. Therefore, a subwoofer is pretty much a required component for your sound reproduction system to provide a sense of musical realism.
Subwoofers come in a wide variety of designs. The most common are ported and sealed enclosures along with passive radiator systems. Unlike the main monitors in a system , subwoofers do not have an absolute phase, and can fire forwards, downwards, or to the side. At frequencies below 80 Hz they are non-directional, meaning that their placement doesn’t impact system imaging.
A low frequency reproduction system is comprised of three distinct components: Low Frequency Crossover, Amplifier, and Subwoofer. Most consumer grade systems come conveniently packaged in one box, but these are not as reliable as separate components. The biggest problem with these all-in-one systems is overheating of the amplifier. Since the amplifier is built-in to the cabinet, it has limited cooling, which results in premature failure of the device.
Low Frequency Crossovers divide the analogue signal so that the lowest frequencies go to the subwoofer and everything else to the mains. Since the low frequency information requires the most power to reproduce they have the added benefit of providing more dynamic power to the monitors, and reducing distortion.
Good subwoofer amplifiers are both high power and high current designs. The minimum recommended power for a subwoofer in an average sized room is 300 watts. (This is not true of all subwoofer designs, but is a good general rule of thumb regarding low frequency amplification.)
Dual matching subwoofers offer the best performance and added flexibly. This is especially true if you are using 2.5 way main monitors where the crossover frequency needs to be above 80 Hz.
For the purpose of our Audiophile 101 system we have chosen the Klipsch SW-12 subwoofer. It was built in the 1990’s and is readily available on the used market for $150-$200. The SW-12 is a passive radiator design with a 150 watt amplifier and built-in crossover with variable phase control. We chose this sub because it’s relatively easy to setup, and blends well with most monitors. It’s not especially good as an LFE subwoofer, but exceeds most LFE units in musicality.
Assuming you have the Emotiva XDA-2 DAC recommended in Part 1 of this series, run a set of leads from the output of the DAC (or preamplifier) to the input on the SW-12. Then run another set from the output of the SW-12 to the input on your amplifier. These leads should be well shielded and as short as possible. In this configuration the SW-12 is wired between the source and the amplifier. Set the phase control to zero, the crossover to 80 Hz, and the gain to zero.
The SW-12 is a forward firing design with a rear passive radiator. Subwoofer placement is more art than engineering in the home environment where adequate room treatment is usually non-existent. Ideally you want to center the subwoofer between the main monitors and place its voice coil in alignment with the woofers of the mains. This may not be possible or even desirable in a home setup. To get started place the sub somewhere between the mains with the rear of its cabinet about 14″ from the back wall. Turn it on and play Diana Krall’s The Girl in the Other Room: Track 3 – Temptation. Raise the gain on the SW-12 until it blends well with the music, and then raise it to the point where it’s a little too powerful (about half a click). Walk around the room in the general listening area. Does the amplitude of the low frequencies seem to be lower in some areas and higher in others? If so, try towing the sub a few degrees, and test again. You can also try tilting the subwoofer up a few degrees, or a combination of the two. The goal is to get the low frequency to sound the same in any listening position. If you can move the sub further left of right it may help. Chances are you will never get it to sound the same throughout the room, but get it as close as you can.
(The next part is much easier if you have a helper.)
For tuning purposes we recommend that you have the following three recordings available. (There is nothing special about these recordings other than that most people already own them, and their equalization is well known.)
- Temptation – Diana Krall
- Against the Wind – Bob Seger
- Whole Lotta Love – Led Zeppelin
Since we’re dealing with low frequencies it is necessary that your source for each piece of music be either CD, WAV, or FLAC.
Play Temptation and have your helper adjust the gain on the SW-12 to where the bass sounds relatively natural. Then have them slowly rotate the crossover frequency downwards. Listen carefully for the natural roll-off of the mains. You will need to repeat lowering and raising the crossover frequency until you have the proper blend. If you’re using the JBL 4412, it will probably be somewhere between 55 Hz and 70 Hz. You may also need to adjust the gain as a part of this process. Keep at it until you have it blended as best you can.
Now, switch to Against the Wind, and carefully listen to the lower mid-range. Is it muddy sounding or dead and flat? If it sounds natural then you did a great job in the previous step. If not, you’ll need to re-do the previous procedure until it does.
It’s time for Led Zeppelin’s Whole Lotta Love. This recording should sound like you need to turn up the bass control a bit. It shouldn’t be especially thin sounding but a little bass would improve it. If this is what you hear then you’re ready for the phase test.
The Klipsch SW-12, when positioned as we recommend, typically requires no phase adjustment. To verify this return to Diana Krall’s Temptation. Have your helper slowly turn the phase control to 180 degrees out of phase while listening to the low frequencies in the recording. Low frequencies should have dropped off. If they increased you will need to find the position between 0 and 180 where to low frequencies are the loudest, and then adjust the sub’s gain again by listening to the three recommended tracks.
Your subwoofer should now be sounding like it’s a blended part of the system, but you will need to do some critical listening to a wide variety of music to confirm this. If after listening to a wide variety of different recordings you discover that the lower mid-range on some tracks sounds a little muddy, then try moving the sub away from the wall in 2″ increments. You may have to readjust the gain and crossover as well. If the sub is lacking in dynamic punch moving the speaker away from the wall may also help with this. If, however, the sub seems too anemic and the bass isn’t warm and pleasant, then you may need to move it closer to the wall in 2″ increments while also adjusting the crossover frequency and gain.
As you can see from the less than explicit procedures outlined above, tuning a sub to your room and system is a time consuming and frustrating task. The SW-12 is one of the easiest to tune, and that is one of the reasons it was chosen for this article.
In Part 4 of this series we will be further refining your system through the use of a calibrated mic and Room Equalization Wizard (REW).