I remember that when I started to get interested in the world of sound, Subwoofers were a very exclusive audio equipment, reserved for high-fidelity enthusiasts or for Home Cinema systems set up by professionals. However, nowadays we can see them even in low-end sound systems, and it’s rare to see a soundbar or a Home Cinema system that doesn’t already include one from the start.
Nevertheless, the Subwoofer remains the most unknown piece within a sound system. In this article, I want to try to explain what a Subwoofer is, what function it serves within a sound system, and what we should consider regarding its operation and placement. So, if you just bought a soundbar with a Subwoofer or a 5.1 system, but you are new to sound matters, this article is for you.
What is a Subwoofer and what is it for?
The Subwoofer is a speaker responsible for reproducing the lowest frequency sounds. It is a very large transducer, generally not smaller than 6″, because low-frequency sound waves are very large, and therefore a large and powerful transducer is required to generate them.
But exactly what sounds does a Subwoofer reproduce? Generally, the Subwoofer is responsible for reproducing all sound frequencies below 80 Hz, although this frequency cut usually varies up and down, depending on the manufacturer and the sound characteristics of the equipment. In any case, the Subwoofer reproduces low-frequency sounds that the rest of the transducers in a sound system cannot reproduce due to physical reasons (too small). It also reproduces the .1 channel of Home Cinema, which is the LFE (Low Frequency Effects) channel, which are sounds below 50 Hz and whose goal is to make us literally vibrate with sounds of explosions, gunshots, engine roars, and soundtracks with bass tones.
The LFE channel was used almost sporadically in the early days of surround sound with Dolby and DTS, but today it is used quite a bit, not only because action movies are increasingly spectacular, but because sound editing is more sophisticated, capturing many more ambient sounds than before, and soundtracks gain presence in a movie’s sound. But at the same time, in the musical realm, bass sounds have also gained presence, due to the rise of rap, R&B, electronic music, etc… Genres that heavily use bass. This largely explains why Subwoofers have seen increased demand in recent years.
Types of Subwoofers
First, we must differentiate between active and passive Subwoofers. Passive ones do not have their own amplifier and depend on an external amplifier. Therefore, they are not connected to the power supply, but only by audio cable to the A/V receiver. Active Subwoofers, on the other hand, do have an amplifier and their own power stage, so they are plugged into the power supply. These latter ones are the kings of the market right now, as they can connect wirelessly with the receiver or soundbar and are ready to be plugged in and start working without further complications.
Having made this first classification, we can now talk about the types of Subwoofer based on the design of the box. Although there are more, the two main types are these:
- Ported box, Bass Reflex: The box has an air outlet, usually at the back, to let the air generated by the Subwoofer escape. It is the most common, especially in low and mid-range systems, as it is very efficient and requires less power to make the bass sound strong. They also produce very warm and extensive bass. On the downside, if they are not of quality, they can generate distortion, rumble too much, or become uncontrolled, invading the lower areas of the mid frequencies.
- Sealed box: The Subwoofer is inside a box that has no openings and is completely sealed. It requires more power, is less efficient, and does not sound as warm, but it has the advantage of a flatter response and no distortion, and although it offers less extensive bass, it does offer more cohesion, more controlled reverberation, and higher fidelity sound.
Where to place the Subwoofer?
The Subwoofer is the least critical element within a sound system when it comes to finding its location. This is because very low-frequency sounds are omnidirectional, and also because they have a very long wavelength, making them difficult to locate. However, there are a series of drawbacks that can affect the quality of the bass depending on the location of the Subwoofer.
The first of these is that we can cause standing waves that increase certain frequencies, producing undesirable effects. Furniture, chairs, tables, and even the type of wall in the house do not help the task of the Subwoofer, as they can produce reverberations and resonances.
The second is that in most Subwoofers already integrated into a 2.1 or 5.1 system, the crossover cannot be adjusted, and on top of that, if the system is low-end, it is very likely that the frequency cut is much higher than recommended (above 250 Hz), which causes the Subwoofer to invade the mid frequencies, harming sound quality and generating directionality, noticing how some sounds that should be coming from the front speakers actually come from the Subwoofer.
The ideal is for the Subwoofer to be in front of us, just below the soundbar or the central channel of the Home Cinema. This will avoid those possible sound directionality problems with low-end Subwoofers that perform a very high frequency cut. This is not always possible, so the second best option is to place it on the front line but to one side, for example, on either side of the furniture where we have the TV and the soundbar or sound system.
It happens that in many cases these locations do not provide the desired bass, so you have to try different locations. For this, the best way is to perform a Crawl Around Test: place the Subwoofer on the seat of the armchair or sofa from where we usually watch TV, play a bass test like this one, and move around the room until you find the point where the bass sounds best. That point is where we will place the Subwoofer.
One of those locations is usually a corner of the living room, which although not ideal, depending on the acoustics and characteristics of the Subwoofer, it can work. Another place you can try that usually gives good results is on either side of the sofa, or on the same wall as our listening position.
A recommendation in this regard is that if the Subwoofer is Bass Reflex, place the ventilation port at least 30 cm away from the wall to avoid harsh resonances and distortions. At the same time, you should keep in mind that ported Subwoofers are more sensitive to location, so it is not advisable to move them much from the front line of sound, while sealed Subwoofers are more versatile and respond better in other more distant locations.
How to choose a Subwoofer
Most soundbars and 5.1 systems already come with a Subwoofer included, but there are systems that do not come with a Subwoofer and give you the option to purchase one separately and connect it by cable to the bar or receiver.
When choosing a Subwoofer, consider the following:
- Although it is not always the case, there is usually a correlation between the size of the driver and the quality of the bass. Thus, and I say this as a general rule, a Subwoofer with a 12″ cone will be better than one with a 6″ cone. The general recommendation in this regard is to buy Subwoofers with a cone of at least 10″.
- Independent Subwoofers, purchased separately from an already assembled sound system, usually include a Crossover control. This is used to control the frequency cut from which all sounds below that cut will be reproduced only by the Subwoofer. This cut should be between 100 Hz and 80 Hz. If the rest of the system’s speakers are of low quality, and the Subwoofer is of high quality, we can consider raising that cut to 150 Hz, thus freeing the rest of the speakers from having to reproduce frequencies they do not handle well.
- The sound reproduced through a Subwoofer is monaural. Basically, what is done is that all sound below the frequency cut is mixed into a single monaural signal and sent to the Subwoofer for reproduction. Hence the importance of a good and appropriate frequency cut, as if we make it too high, we risk sending mid frequencies to the Subwoofer and harming channel separation. This undesirable effect can be noticed in some low-end Subwoofers, where it is noticeable how voices filter through the Subwoofer.
- Low-end Subwoofers do not usually extend the bass beyond 40 Hz, as they often use small-sized drivers and low-end amplifiers. Even so, adding a low-end Subwoofer to an audio system provides more punch and impact simply by adding a driver with a large enough diameter to move the air generated by the bass.
- The power in watts of a sound system is one of the great myths of the audio world, and the marketing departments of manufacturers are to blame. For a Subwoofer to sound good, 100W of power is enough, as long as the Subwoofer is of decent quality.