Audiophile 101, Part 3: Subwoofers
Updated February 15, 2022
In the third part of our series Audiophile 101 we take a look at subwoofers. If you haven’t read parts 1 & 2, we recommend you do so before proceeding. (Part 1 & Part 2)
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 active 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 such as Dirac. The other choice is to send the left and right outputs to the subwoofer.
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 are sometimes required. 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 inexpensive LFE units in musicality.
Set the phase control to zero, the crossover to 80 Hz, and the gain to zero.
Begin by placing the subwoofer in the listening position. (Yes, the place where you sit to listen to music.) Play some music that has deep bass (Diana Krall’s The Girl in the Other Room: Track 3 – Temptation). Turn up the volume on the sub to where it is a little over emphasized in relation to your mains. Now, walk slowly around the walls of the room and listen. You will discover that in certain places along the wall the bass is far more pronounced. Mark these with a piece of masking tape.
You should now have at least one place along your wall where the sound from the sub is most present. This is where to place the sub in the room.
(The next part is much easier if you have a helper.)
Subwoofer Phase Adjustment
Phase adjustment is best performed with a helper. Play the test track and have your helper slowly turn the phase control. You will find that somewhere between 0 and 180 degrees of phase the bass is most intense. You may have to repeat this process a couple of times to get it right.
Subwoofer Volume & Crossover Adjustment
This is best performed with a SPL meter such as the Radio Shack 33-2055. Play a test track of pink noise through your main speakers at 75 dB with the sub powered off. Disconnect or switch off your main speakers. Play the pink noise track again with only the sub powered on. Raise the volume on the sub until the meter reads 75 dB.
If you have loudspeakers with large woofers you may want to lower the crossover frequency of the sub. Never raise it beyond 80 Hz unless you are using stereo subs placed near your mains. If there is too much bass try lowering the crossover frequency in 10 Hz increments until the bass is neutral and even sounding.
Your subwoofer should now be sounding like it’s a blended part of the system.
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).