How to setup a subwoofer
Each dedicated home cinema room and each living room being different, it would be futile to pretend to give a placement method or to recommend settings that would work systematically. But the following tips will help you avoid some disappointments and optimize the operation of your subwoofer. Many home cinema amplifiers have an automatic speaker calibration which also takes care of adjusting the subwoofer and correcting unoptimized placement or specific room acoustics. Adjustments “by ear” are also important and can be useful to finalize the procedure and make some adjustments. There are many parameters to take into account to optimize the operation of a subwoofer, because your entire installation would suffer from a poorly adjusted subwoofer.
First of all, make sure to decouple the subwoofer from the ground, using rubber feet or decoupling spikes if it is not already provided. The low register will gain clarity. Then, it is necessary to choose carefully the cable which connects the amplifier to the subwoofer to ensure that the signal reaches the subwoofer in the best conditions, without alteration or disturbances.
The position of the subwoofer in the room greatly influences the listener's perception of bass. Poorly placed, a powerful subwoofer can be disappointing when listening, producing too much bass or not enough.
Generally, by positioning the subwoofer in a corner of the room, you get a higher level of bass: the walls then act as a pavilion, amplifying the sound wave towards the listening point. Compared to a front positioning of the subwoofer, between the front left and right speakers, the gain in sound level can easily vary from half to double. However, if you want to place your subwoofer in a corner, you must adjust the cutoff frequency so that it only reproduces frequencies below 80 Hz: otherwise, it would be localized by ear and the sound would seem come from the point where it is located rather than from the center of the sound stage. Such a filtering adjustment requires the use of speakers capable of effectively going down to this frequency of 80 Hz.
In a listening room favorable to low frequencies, it is recommended to place the subwoofer between the main speakers, without sticking it to the wall, to the right or left of the center speaker or electronics.
-The electric phase
The amplifiers built into the subwoofers offer several settings, including the electrical phase of the signal.The electronic components of these filters induce a more or less significant phase rotation, the main consequence of which is to modify the spatialization of the sound scene. Regardless of the phase rotation of the main speakers, it is therefore essential that the subwoofer can align with it. If not, the bass of the main speakers and that of the subwoofer will not operate at the same timing and will interfere. Some of the frequencies produced will be weaker or cancel out.
If this phenomenon may seem obscure, it is easy to experience it by reversing the phase of one of the front speakers (by reversing the cables "+" and "-" on the terminal block). The sound impact of misalignment of the electrical phase is immediately audible.
Some subwoofers only offer a phase inverter (0 ° or 180 °), the most sophisticated of which can offer continuously variable phase adjustment, from 0 ° to 180 ° or even 270 °. To find the right setting, you must listen to music on all the speakers as well as on the subwoofer, preferably a track with a recurring bass line. It is then necessary to vary the phase of the subwoofer in order to obtain the highest level of bass, or the one that you like the most.
It prevents the subwoofer from reproducing sounds above a specific frequency. This parameter is defined with a potentiometer graduated in Hertz, generally from 40 Hz to 150 Hz. It can be called Low-Pass or Crossover depending on the brand.
In a stereo hi-fi system
The setting of the cutoff frequency on the subwoofer is generally only used when the latter is connected to a stereo amplifier via a high level connection (speaker terminals). In this case, the stereo amplifier delivers a wideband signal to the subwoofer, which must be filtered so that the signals reproduced by the subwoofer and the speakers complement each other harmoniously. They should not overlap, but neither should they leave a gap between the highest frequencies reproduced by the subwoofer and the lowest frequencies reproduced by the speakers.
On a stereo amplifier with a low level subwoofer output (mono or stereo RCA), the latter is generally filtered around 80 or 100 Hz. Read your user manual to make sure. If it's not filtered, then adjust the subwoofer filter with the dedicated potentiometer.
In hi-fi, it is advisable to adjust the active filter of the subwoofer so that it reproduces only the frequencies below the passband of the speaker. You can optionally adjust the subwoofer so that the highest frequency it reproduces slightly overlaps the lowest frequency reproduced by the speaker. This allows some speakers to compensate for the gradual attenuation of level when approaching their low limit frequency. This fine adjustment can only be done by ear.
In a home theater system
Home cinema amplifiers incorporate active filtering of the signal sent to the subwoofer. This adjustment can therefore be made via the amplifier's audio settings menu. Note that the filtering by the amplifier has a major advantage compared to the filtering integrated into the subwoofer. The home theater amplifier uses a fully digital filter with an ultra-steep slope (this point will be discussed later in this guide). The risk of overlap between the frequencies reproduced by the speakers and those reproduced by the subwoofer is therefore zero. On this point, the filtering integrated into the subwoofer is less efficient since the attenuation slope of its active filter is progressive and moreover induces a phase rotation.
It is not necessarily recommended to have a heavy hand on the bass: the subwoofer is not intended to "knock" constantly, but rather must provide support and depth to listening. Too high a level will on the contrary remove any relief when listening.
Some high-end subwoofers like these MartinLogans have an automatic microphone calibration system that adjusts the volume, cut-off frequency and sometimes the phase, depending on the acoustic characteristics of the listening room. In the absence of such a setting on the subwoofer, the home theater amplifier's autocalibration system can do this. Recent models thus modify the nature of the signal sent to the subwoofer (equalization, cut-off, phase) in order to restore an optimized signal.
-Adjusting the home theater receiver
In addition to the autocalibration system, setting the speaker type in the home theater amplifier menu has a huge impact on the behavior of the subwoofer. Depending on whether you indicate to the amplifier that the speakers are "Large" or "Small", the signal transmitted to the subwoofer by the LFE output is not at all the same. If the center speaker is declared small, the amplifier redirects the low frequencies of that channel to the subwoofer. Same thing for the front and surround speakers. Unless the speakers are unable to produce bass down to 50 Hz - in the case of ultra-thin speakers or very compact satellite speakers - this is a setting to be avoided because it is preferable to entrust the subwoofer only with reproduction from the single LFE track.
As a reminder, the LFE (Low Frequency Effects) track is the famous channel .1 of the Dolby Digital and DTS soundtracks in their different versions. The LFE track contains information that is not found on the other 5 or 7 channels: generally very low frequencies that only a subwoofer can reproduce effectively. It is therefore recommended to not disturb the operation of the subwoofer's by requiring it to reproduce the bass of the other 5 or 7 channels.