With more consumers looking for the HD experience, an increasing number of subwoofers are finding their way into home systems. Home systems primarily use the subs for LFE in a 5.1 configuration, but some send stereo information to the subs via bass management for SD broadcasts.
For proper translation, studios increasingly need to hear more deep bass when working on small, close or midfield monitors whose low-end response is limited. And because one cannot always place the monitors in the best position or properly trap in for a smooth low-end response, subwoofers are the answer.
For example, in many small rooms, the proper placement for mains may be bad for imaging or simply not achievable due to room dimensions or lack of symmetry. A subwoofer, on the other hand, can be placed just where it needs to go because you don't have to worry about imaging. This allows you to place the main monitors without a great deal of concern for their low end.
Sub wish list
I believe that a proper subwoofer should reproduce frequencies at least down to 20Hz. I'm a big fan of the self-powered varieties. There should be some sort of phase adjustment with the minimum being a simple 180-degree flip and the best being a continuously variable pot providing up to 270 degrees of shift. This can come in handy when there is limited space in which to move the subwoofer in relation to the mains.
There should be a continuously variable selection for low-pass/high-pass frequencies — a true crossover — not just a low pass for the sub. The amplifier needs to provide enough power so that distortion is low.
For those who want a sub to do the double duty of extension as well as LFE, then there should be a separate LFE input. I love to see some parametric equalizer bands on the sub as well, though this is still a rare commodity.
Set the crossover point so that you take some of the strain off the main system woofers. This point may vary a bit from system to system, depending on speaker location, main system low-end response and room size. For example, by sending everything below 80Hz to the subwoofer, you can relieve the main system from having to produce the deep bass. This often translates into better midbass definition.
These low frequencies are also the ones that can destroy your main's woofers, so the subwoofer addition should create less overall distortion and clipping, and more system power. Also, by selecting a lower subwoofer crossover point, you leave the bass and kick drum punch at 100Hz to 125Hz in the main system so that it feels connected to the upper and midbass.
In an ideal audiophile world, it would be great to simply add the subwoofer into your system. This would mean no crossover on the mains and adding in the sub with only a low-pass filter on it. I've had such luck on a few occasions when the winds blew in a favorable direction. Most of the time, though, whether because of room ergonomics or design oversight, the mains do not end up in an ideal position for good bass reproduction, and the sub needs to crossover at a higher frequency in order to make up for the poor mains response.
Figures 1 and 2 show before and after shots of a speaker in a typical control room. All charts are 48th octave resolution shot with a Meyer SIM 3. In the upper chart of both figures, the red trace along the top shows coherence, while the yellow middle trace is frequency response. The bottom chart shows phase.
The speaker in Figure 1 shows a severe roll-off at about 53Hz and a large hole at 87Hz. Note the bass extension that is capable with the addition of a subwoofer. In Figure 2's phase chart, the subwoofer polarity and position is properly oriented. Phase integration of the subwoofer is important at the crossover.
However, simply adding a subwoofer to a system won't always correct bass issues. Figures 3 and 4 show a room with a mains and sub system. Note that even with the subwoofer, the same room response anomalies exist at 32Hz, 90Hz and 135Hz. You should be able to correct some of these problems if there is enough space to move the subwoofer to an ideal location in the room.
Sub in stereo
I am a firm believer in stereo subwoofers. It is a common misconception that we cannot hear bass directionality. People often say that bass is omnidirectional and subwoofer room position is not important.
I believe this misconception developed because of the way bass is treated when cutting lacquer masters for records. Frequencies below 200Hz are combined to mono in case there are any low frequency phase problems. Out-of-phase bass would make the lathe cutting head jump off the lacquer. So for many years, we never had a chance to hear stereo bass.
In this digital age, this is no longer a problem. You can experiment by placing your subwoofer off to one side and see if you can hear its location. I'm sure you will.
If you only use a mono subwoofer, then you should consider placing the subwoofer symmetrically between your speakers. Placing the subwoofer off to one side may cause a nonsymmetrical response in the left and right speakers at the crossover point. This is based on the uneven distance of the left and right speakers to the subwoofer and could require equalization to balance the system. Note that the lower the crossover point, the less symmetrical the placement needs to be because of the long wavelengths at the crossover point.
For studios with close field monitors, the subwoofer application is often not about power but about the ability to hear the low frequencies that may make recordings sound unclear and muddy. (See Figures 5 and 6.) Some of the more popular, expensive, small monitors extend down to 40Hz, which is sufficient in many cases. But there are quite a few systems on the market with limited bass response. Film and television composers need to hear the low bass frequencies in order to get their music right as well.
Placement of any subwoofer system requires a good analyzer and someone who knows how to use it. Subwoofer manufacturers suggest using tones, but the results are crude. I have yet to see a room with subs properly set up without analysis.
I personally use a Meyer SIM 3, which allows me to see a 48th octave resolution linear display of both phase and frequency in real time. This gives me fast, accurate results.
You want to achieve a linear phase response at the crossover point in order to get the best frequency response. An analyzer that displays phase is a must for this process.
The process can be time consuming and requires trying placement in multiple locations and phase switch adjustments. Moving a subwoofer just 6in to 1ft can make a significant difference. If there is a rule of thumb, I haven't found it yet.
Bob Hodas is an acoustic consultant and owner of Bob Hodas Acoustic Analysis. He tunes studios around the world, from Sony in Tokyo to Abbey Road in London and all parts in between.
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