There is a lot of misunderstanding around subwoofers. However, a sub is an essential part of a surround setup and can also play a role in stereo. Time to dive deeper into the topic of ‘subwoofer’! What is a subwoofer and why do you need one?
Introduction of subwoofers
Subwoofers have a less positive image for many music lovers. There are a number of reasons for this. Anyone who wants to keep their living room tidy and free of technology will stumble – sometimes literally – over the size of this device. Placing a subwoofer correctly is also said to be ‘difficult’. This is not entirely unfair, because the interaction between the subwoofer and your room often produces unpleasant results. And then you still have to nicely ‘integrate’ a subwoofer (read: let it play together) with your other speakers, which may require you to use all kinds of settings on an AV receiver has to get started. In short, there are many reasons why a subwoofer is not a favorite device for many. But much of this dislike has to do with misunderstanding. A well-tuned subwoofer takes an entire surround setup to a higher level. And even if you are completely convinced that you only want to listen to stereo, then a step up to a 2.1 setup can sometimes yield surprising improvements. In this article, we will therefore take a closer look at what a subwoofer is and how you can use it properly.
What are basses again?
There is often talk of ‘bass’, as if it were one thing. You can indeed think that listening to bad headphones or cheap sound bars. But it is actually a frequency range that is relatively wide. A good subwoofer is able to display the details in that range well. That sounds obvious, but subs that come with a cheaper soundbar rarely succeed. They do produce a lot of volume, but there is mainly some woolly rumbling in the low.
Usually the word ‘bass’ is used for frequencies from 60 to 250 Hz. Below 60 Hz it is referred to as sub-bass. These are very low frequencies. Few natural instruments can dive this deep. Electronic music and film effects do seek out the sub-bass region relatively often. When you talk about bass, you are talking about more than just percussion or beats. A considerable part of a piano keyboard is in the bass range, for example. Male voices can even start from 100 Hz. Just to say: basses – and therefore a subwoofer – are quite important. If you go above 250 Hz, you are (already) in a range where a subwoofer is not active. However, this lower midrange (250 – 500 Hz) is also called the bass presence region because it does match those deeper tones.
What you should keep in mind here is that music and sounds are in reality more complex than one frequency. When a guitarist strikes a string, you don’t hear a single frequency. You hear a lot more, including so-called harmonic tones (plurals of the root). In order not to make this article too complex, we will not go into this further. That is something for a later article on sound and acoustics.
What you must remember from this is that integration of speakers and subwoofer is very important. If you have a 2.1 setup (stereo plus sub), then the sound that the subwoofer outputs should match the sound coming from the other speakers seamlessly. For example, the sub must produce everything below 80 Hz, while the two stereo speakers take over from 80 Hz. The starting point may not be 100 Hz, for example, because then you miss a bit of frequency range. Certain notes of a piano would disappear as a result.
What about our ears?
Theoretically, our hearing ranges from 20Hz to 20kHz. Unfortunately, that reach naturally declines after childhood. In addition, you also have forms of hearing damage that can occur. Most of the purchase takes place in the high, where you can already be at 50-60 years with an upper limit of 15 kHz. When it comes to bass, there is also a gradual decline, but it is less strong. In addition, you will experience very low sub-basses through your body, not just through your ears.
There is a lot to say about our hearing, in the context of this article it is especially important to look at an interesting phenomenon. We are set to hear certain sounds better. In fact, our brains will do their utmost to locate certain sounds as quickly as possible. After all, you never know whether the rustling in the tall grass is a prowling tiger, the primeval human in you thinks. However, it is difficult to locate low tones, also because the long bass waves reach both ears at the same time. This phenomenon is useful, because it allows you (theoretically) to place a subwoofer unfocused and freely in a room. A single subwoofer can therefore also ‘support’ multiple speakers, which is very useful for a surround setup. You can safely work with small speakers for your surround and height channels, because they should not be able to produce low notes. The subwoofer does that.
The point at which we hear more directional is around 80Hz, although that varies from person to person. Some argue that this point is rather around 60Hz. This is important, because you have to take this into account when placing and setting up your subwoofer. It is no coincidence, for example, that 80Hz is the upper limit for a sub in the THX specification. But you may be wondering: what is a sub anyway?
What is a subwoofer?
Basically, a subwoofer is a speaker like any other. You could even say that it is a very simple speaker, as it usually only contains one speaker or driver. There are models with two drivers, but they work together to sound more powerful. A subwoofer only produces bass, so low frequencies. The exact frequency range of a subwoofer varies from model to model.
Most subwoofers are active. This means that they have their own built-in amplifier. You do not connect them to your amplifier with speaker cable like the other speakers in the room, but with an audio cable. Passive subwoofers also exist, but are rather rare. You come across them, for example, at low-end sound bars, but sometimes also with more exotic high-end setups. There is a good reason why most subwoofers are active: controlling them is difficult and requires quite a bit of power. Subwoofers with power ratings of 500, 1,000 or 2,000 watts are not that exceptional. If your AV receiver had to provide that power, you should buy a much more powerful model.
Why do subs require so much power? Very simple: they have to move a lot of air. Low tones or basses are large sound waves and you can only produce them with a larger driver. 8, 10, 12 or more inches is not uncommon. It is also a rule that basses require more volume to be audible. Moving a large driver not only requires more power, but also more control. This works best with an amplifier specifically designed for that speaker – another reason to make subwoofers active.
There are several ways to build subwoofers, despite their simple construction. The most common are single driver subs. But that driver can be placed in the front or face down. You see this, for example, with narrow subwoofers that are bundled with sound bars. A subwoofer with a woofer facing the floor has the advantage that it is less sensitive to placement. A disadvantage is that the result depends on the floor type.
There are many more expensive subs that have two drivers. Different setups are possible here, but usually back to back is chosen. A double setup can provide more volume (because you move more air), but also more control (if one driver is connected out of phase with the other). An advantage of a dual driver approach is that the unwanted “kickback” of each driver is counteracted by the other. After all, every driver moves in two directions and therefore also displaces air (= sound) at the rear. But that air displacement is undesirable.
Often a subwoofer will have an output: a reflex or bass port. This serves to release the air pressure in the sub, but also to produce sound. Since air is compressible, the air in the box acts like a spring. This also produces sound (called a Helmholtz resonator). Engineers can determine at what frequency that happens, and thus have a sub produce even lower frequencies. Reflexes are very often used to allow small subwoofers to produce deeper bass. A point with subwoofers with bass ports is that you have to take the distance to the wall into account. Too close and the airflow exiting the port will produce additional noises. The designer of the subwoofer also needs to know what he is doing, because a poorly designed reflex can produce background noises that are disturbing.
A useful extension for stereo
Precisely because producing bass is so difficult, there is much to be said for assigning this task to a specialized loudspeaker. Yes, even if you own floorstanders that turn out with a lower limit of 40-50 Hz. There are indeed floorstanders that can produce very low frequencies. But the lower a floorstander dives, the greater the chance of cabinet staining. This is the phenomenon in which the loudspeaker housing vibrates and ‘pollutes’ other frequencies. Lower tones also move more air, which can cause problems inside the speaker cabinet. In reality, most speakers will not really perform well in the low end, despite the frequency range published by the manufacturer. There are exceptions to the rule, such as very large speakers (think of the 800 Series from Bowers &
A 2.1 setup is a very good idea if you want to work with monitors or stand speakers. After all, smaller speakers will not be good in low reproduction, but can be spectacularly good in the midrange and higher. Complete a pair of high-quality bookshelf speakers with a well-tuned sub and you get an unobtrusive stereo system that plays at a high level.
In short, by working with a subwoofer you supplement speakers that cannot reproduce low frequencies properly. A bookshelf speaker or monitor, for example. A second use is to relieve the load on larger speakers that can handle bass on paper, but perhaps shouldn’t. Because they no longer have to produce bass, these speakers can perform better at higher frequencies.
Necessary for surround
In a surround setup, a subwoofer is nothing short of indispensable. Surround codecs support a separate LFE channel (Low Frequency Energy), which contains specific layer information. Movie soundtracks also do a lot more in the low frequencies than pure music, as five minutes to a typical Marvel movie will demonstrate. Besides the film music itself, it is about spectacular sound effects. Very deep basses are also more tangible than audible, something that filmmakers like to exploit to make a dramatic moment even more impressive. Low tones also often sound threatening. Do you remember how the wolf was announced in Prokofiev’s “Peter and the Wolf”?
Another plus: thanks to a subwoofer you can also go for smaller surround speakers, which is an asset in many living rooms. After all, the subwoofer relieves all speakers in the surround setup. Although we would still recommend choosing somewhat larger speakers for the front channels (left-center-right).
What can go wrong?
Placing a subwoofer can be difficult because low tones contain a lot of energy. As a result, there is a greater chance of an interaction with the room properties, which therefore has an impact on the sound you experience. Several things can go wrong. For example, loud basses can make a cabinet vibrate, making the furniture itself an unwanted sound source. A common problem is room modes, where a certain low frequency in the room is amplified and therefore sounds louder than the rest. You can experience this problem if, for example, a subwoofer is close to a room corner. You also have a reverse phenomenon: bass traps. This just dampens a low frequency, making the basses sound very thin.
To understand these problems you can work with an analogy. Imagine your room is a pond. If you throw a stone into the water, you will get expanding circles around the point where the stone made an impact. Those circles move more or less like (low) sound waves. Every stone’s throw is a bass sound coming from your subwoofer. So note that a sub outputs sound in all directions, while a speaker outputs the mid and high tones in a specific direction. That is why you have to point a floorstander or bookshelf speaker towards your listening position, while in theory a subwoofer can be placed somewhere in the room.
Back to our analogy. The aim is to keep the circles in the pond as clean as possible until they reach you. But if you throw a rock close to a bank of the pond, you will see the circles that move towards the bank bounce back there and affect the following waves. Sometimes waves are opposed, sometimes you get even bigger waves. It can get very complex in rooms with a lot of surfaces, because you can get an accumulation of reflections that all interact with each other. That’s the problem with subwoofers that are close to a corner. The basses that end up in the corner are amplified and pushed back into the room. In addition, that tone continues to reverberate longer, giving you a woolly, less tight sound. You also have standing waves,
You also have something similar if you have to deal with so-called ‘boundary gain’. You may have to deal with this if, for example, you place the sofa against the back wall in the living room. Low frequencies that come straight at you are immediately reflected behind your head, so you experience a louder bass. But the reverse is also possible: that the reflected sound wave counteracts the next wave. Then you hear slightly less bass.
How do you fix this?
The difficult thing about problems like room modes and boundary effects is that they are not easy to identify and solve. One rule of thumb is that a larger space will have fewer problems than a smaller space. You will also experience more problems the higher the volume. A good AV receiver with a calibration system can solve certain issues via software, but only very advanced models can tackle really tough issues. You can’t do everything with software. Yes, you can use software to play just that one frequency that activates a room mode much quieter. But even an advanced system like Dirac can only be used to compensate 4-5 dB.
Because room problems mainly affect basses, you see more and more subwoofers appear with their own measurement and equalization system. At Bowers & Wilkins we saw the new DB series so recently, at Monitor Audio the Silver W12 subwoofer, and at ELAC you will find EQ systems in the high-end Sub 2070 and 2090, and in the highest Debut subwoofers. . They work with measurements via the smartphone’s microphone or via an included measuring microphone.