This is a very detailed document covering nearly everything to know about subwoofers. Here is brief summary to get an overview of what is covered in this document:
- What is a subwoofer? A speaker that produces low bass frequencies from 200 Hz down to at least 30-35 Hz. A "true subwoofer" extends down to 20 Hz or lower.
Key bass frequency ranges:
- Mid-bass (50-80 Hz) - Provides impact and punch
- Low bass (30-50 Hz) - Provides low-end rumble and bass fullness/body
- Ultra low bass (25 Hz and below) - Creates a pulsating vibration you feel more than hear
- Room size matters. Larger rooms need larger subs to pressurize the space.
- Subwoofer placement and listening position greatly affect bass response. Try the "subwoofer crawl" to find the optimal sub location.
Look for subs with:
- Good low frequency extension
- Flat/smooth frequency response
- High clean output capability (see CEA/CTA2010 specs)
Enclosure types: sealed vs ported
- Sealed: often more compact, less low bass output capability
- Ported: more efficient, greater output, especially in deep bass
- Consider getting 2 subs instead of 1 large sub to smooth out room acoustic issues
- Powered sub vs unpowered sub (needs separate amp)
- Power cable if powered sub
- RCA or balanced XLR cable from A/V receiver sub out
- More complex stereo connections may be needed
The Most Comprehensive Guide to Choosing a Subwoofer
Looking for a subwoofer to help augment your speaker setup for improved bass performance? We’ve created this comprehensive guide to help you on your search to find the best sub for your speaker system.
First, what’s a subwoofer?
A subwoofer is a loudspeaker that produces lower frequencies ranging from 200 Hz down to at least 30-35 Hz. Most subs will play down to the 30-35 Hz. However, a good subwoofer designed for home usage can provide significantly further extension. We call a “true subwoofer” one that is able to extend to 20 Hz or lower.
What do these frequencies mean?
With all this talk of frequencies, let’s dive a bit deeper into the different kinds of bass and what they mean to you as the listener. The main range of frequencies a subwoofer produces is 80 to 20 Hz and below.
The 50-80 Hz region is known as mid-bass. But what does that feel like?
Mid-bass is responsible for providing impact and punch. It’s the in-your-chest type punch you feel from the movie explosion or kick drum. The mid-bass region also is where a lot of the perceived detail of the bass sits as well.
Low bass runs from 30-50 Hz, which is responsible for the low-end rumble of the subwoofer. If mid-bass is for details, low bass gives you the fullness and body of the bass. Fun fact: if you stand next to a subwoofer playing low-bass, your pant legs will shake.
Ultra Low Bass:
Ultra low bass runs from 25 Hz and below and is a relatively unique feeling to experience. Rather than being a tone you can hear, it’s a pulsating vibration that you feel in the air around you.
Most music doesn’t go lower than 30 Hz. The exceptions are pipe organ music or electronic dance music (EDM), which can reach frequencies lower than 20 Hz. The majority of movies play down to 25-30 Hz, but a handful of major releases will reach sub 20 Hz. Most mixes don’t go that low because subwoofers in commercial movie theaters aren’t designed to. They are designed to play loud and clean but at the cost of extension, so most of those subs really only play down to 30-35 Hz or so.
What should you know about the room it’ll be used for?
1) What application is the sub being used for?
Is this strictly for two channel music, home theater, or both? If you’re listening to home theater material, then you may want to look for a sub or multiple subs that will provide you with adequate ultra-deep bass output for your room and listening preferences. If you listen to strictly two channel music but like to listen to material that digs very low such as pipe organ or electronic music, then you may also want to pay attention to ultra-deep bass capability as well. That being said the majority of music may not contain material that extends much lower than 30 Hz.
2) What is the size of your room?
One of the most important things to consider is the size of the room that it’s going in. The room is going to have a tremendous impact over how the bass is perceived at your main listening position. It will not only affect how even the response is at your listening position, but also how loud the sub is perceived where you are. Generally, the larger the room, the larger the sub you will likely need to get the output you’re looking for. You also need to consider not just the size of the immediate room, but also the rooms that it opens up to, especially if they are large openings. This is largely due to the long wavelengths that the sub is producing. With your main speakers, those have shorter wavelengths so your distance from the physical speaker is one of the primary drivers of perceived output and response. With subs, people liken it to filling a pool with water. The larger the area the more water you will need to more adequately “pressurize” the room. That’s not exactly 100% of how it works, but it does give you a general idea. If you are in a smaller enclosed bedroom, for example, then going with an 8-12” subwoofer may be suitable. If you’re in a medium to large sized room then going with a 12” or larger would probably be a better bet to go with. Other than driver size, there are other things to factor in when looking for a sub that may better pressurize your room. See the efficiency and output section for more details
3) How do you decide on your subwoofer placement and listening position?
Physical placement of the subwoofer and where you’re seated make a world of difference in achieving a smooth, accurate bass response. Improper placement of the subwoofer will give you a noticeably uneven response that will make your bass sound thin, boomy, or non-existent.
Unlike speakers, which generally have limited placement options, subwoofers have almost no restrictions on where you place them. This is because sub-bass is not localizable––you can’t pinpoint where the bass is coming from.
Most people place their sub in the front corners of the room for aesthetics, but in many cases, those may be one of the worst places for the sub to be and will cause major dips in frequency response and make the sub boomy and lack punch and definition. Don’t be afraid to move that sub around the room! Even nearfield placement (next to your listening position) or placing the sub along the back wall may actually provide you a smoother, more detailed bass at your listening position than in the front. Similarly, your seating position has a major impact on the bass response you perceive. But practically speaking, most won’t have too much flexibility to move their seating position around.
The subwoofer crawl has become one of the most popular methods of figuring out subwoofer placement in a practical way. Instead of moving a large sub around the room and testing with bass material, place the sub at your listening position, play some bass material as your crouch and move around the room. Find the spot where the bass sounds best. That’s where you should put your sub. If you have any specific questions in regards to placement within your room, please don’t hesitate to contact us.
What should I be looking for in a good subwoofer?
We’ll dive a little bit deeper into subwoofers and teach you what to look out for when evaluating what sub would work best for you. In general, you want a sub that will 1) give you suitable low end extension to cover the vast majority / all of your listening material 2) produce bass accurately and 3) play loud and clean enough to meet your personal preferences in levels.
Low-end Bass Extension
Extension refers to how low a frequency the subwoofer can play down to. So how much extension should you aim for? We personally found 16 Hz to be a good tradeoff, but you can say that it is relatively arbitrary.
Some get laser focused into maximizing the extension they get out of their subwoofer setup. While having a system that can extend that low has merit, the law of diminishing returns applies. Importantly, you should be aware of tradeoffs.
- There is usually a tradeoff between lower frequencies and output (volume). The lower frequency you go, the more output you will need to get perceivably the same amount of output as higher frequencies. Human hearing exponentially degrades when listening to low frequencies. If one were to be looking to get max extension down to say 10 Hz, and hit that frequency with authority, they will need multiple large subwoofers in order to achieve that.
- Size matters. The lower the frequency, the larger the sub you will need to get that low, which generally coincides with a larger physical size of the sub and a higher cost.
- The lower the frequency the less likely you will find material that extends that low.
Let’s say you’ve figured out how low you want your subwoofer to go. How do you know whether the subwoofer you’re looking to buy reaches it?
Most reputable brands will post either a frequency response chart and/or state the extension in their specifications.
The frequency response chart provides you the natural output of the subwoofer across a frequency spectrum. It’s a great way to determine how a subwoofer naturally outputs materials when played at clean levels. From a graphical perspective the x-axis refers to the frequency range and the y-axis refers to the output (volume) in decibels or dB. In our case the frequency range (x-axis) is usually from 200 Hz to 10 Hz. If the line is relatively linear, that means the sub is producing the same amount of output across that range which means that it is fairly accurate. Generally, the “flatter” the response, the better.
Below is an example of a frequency response graph for the ULS-15 Mk2:
Note: this does not mean that the sub is only capable of producing that dB level at maximum output. This is a relatively arbitrary reference level in which the sub is measured.
Additionally, the frequency response doesn’t always give the full picture. If you continue to crank up the output on the subwoofer, at some point, the frequency response will change or compress as the sub starts to hit its limits. When measuring frequency response, the standard is to measure it at a level where it is playing cleanly with no compression.
If the subwoofer maker does not provide a frequency response chart, they may provide the response in their specifications. One thing to note is that once you put the subwoofer in your room, you’re going to deal with changes in the response due to reflections in the room, so your actual frequency response in the room may not be as flat. That's why we recommend trying out different placements in your room in order for you to achieve the smoothest bass possible at your main listening position.
Producing Bass Accurately
You may see something like 200 - 20 Hz +/- 1 dB.
The +/- number refers to the deviation of the output within that range so you can see how accurate the measurements are. You want to aim for a smooth response; a 1 dB variation is pretty flat and wouldn't be very perceptible in real world situations so try not to get too caught up on small deviations from the reference frequency.
Efficiency and Output
The efficiency and output of a subwoofer is determined by three factors:
- Enclosure design
- Amplifier power
- Speaker or driver efficiency
Designing speakers and subwoofers is all about balance. You can’t change one thing without affecting another. So let’s go over these three factors with some of those tradeoffs in mind.
There are three most common enclosure designs on the market: 1) sealed (acoustic suspension) 2) ported (bass reflex), and 3) passive radiator. We’ll focus on the first two, which make up ~75% of the subwoofers on the market. Choosing your enclosure design really comes down to what you’re listening to and your size constraints.
1) Sealed subwoofer
A sealed subwoofer, the most classic design, has a completely enclosed box around one or more active speakers. The active drivers are the main transducers producing the sound.
Because there is only one active driver producing the audio, they tend to be more compact than their ported counterparts in many cases.
It is the most inefficient of designs over the other two, particularly in the deeper bass. So it won’t give as much clean output in the deeper bass. If you’re watching a movie at a high output level, the sub will bottom out quicker.
If designed well, sealed subs can be quicker than a ported sub in the deeper bass in terms of measurements. However, this is often not noticeable to the human ear.
There is less roll-off when you get to lower frequencies so you can potentially get a bit more bass extension, but as mentioned earlier, you need tremendous output in the ultra low bass for it to be meaningful and a sealed sub cannot achieve much output in this range.
Sealed subwoofers are the design choice of many stereo channel audiophiles. Their thought is that ported subwoofers or passive radiators are not quick enough. However, we believe that, perceptively, a well designed ported subwoofer can be perceived as fast and accurate as a sealed subwoofer, and things like room acoustics, etc are usually the main culprit behind perceived boominess.
2) Ported subwoofer
A ported speaker produces output from both the active driver and from the port. A port is an opening in the enclosure that extends into the enclosure. The most common port shapes are round and rectangular. The ports function as a transducer for the sub as it moves air in and out. It doesn’t do this across all frequencies evenly, but more closely to the frequency that the port is tuned for. For example, if the port is tuned to 18 Hz, the port won’t be contributing much to the sub in the mid-bass but once you start moving down to the ultra deep bass, the port will begin to move more and more air until you get to the port tuning frequency (18 Hz) where it will move the most amount of air. The active driver will contribute less to the output as it starts nearing the port tuning as well.
Ports add significantly more clean output capability than the sealed counterpart in the deep and ultra deep bass.
Because it’s moving a massive amount of air in and out of the port, at higher levels near the tuning you may end up hearing the turbulent air move through the port, called port chuffing.
Ports make the subwoofer much more efficient and is recommended if you’re in a moderately sized room and playing movies/music with ultra deep bass content.
Generally a ported subwoofer would have a steeper roll off in the low bass after the port tuning, than a sealed subwoofer. This means it may not be able to take as much advantage of room gain versus a sealed subwoofer with the same roll off point
How can you get the best of both worlds? Variable Tuning Frequency.
Variable tuning disrupted the audio industry because it gave subwoofers flexibility to be sealed or ported. Dr. Hsu, from Hsu Research, was the original inventor of this concept. Variable tuning references the port configuration on the subwoofer. You may have noticed that all ported subs with variable tuning options have multiple ports (note: just because there are multiple ports doesn’t mean it has variable tuning frequency).
Variable tuning, as the name suggests, allows you to modify the port tuning of the subwoofer by closing or opening one or more of the ports. When a port plug is inserted into the port, it effectively disables that port. Changing the port tuning will give the user more flexibility in modifying not only the response of the subwoofer but also the output capability of it.
Let’s run through an example of a subwoofer with two ports.
- If you run with both ports open, the subwoofer will be tuned higher, say to 25 Hz. But its output capability will be higher.
- If you run with one port open and one sealed, the subwoofer will tuned lower, say to 18 Hz. But its output capability will be lower.
Some of you may be asking – why would you want to run with two ports open if you lose bass extension?
One reason is that you may be experiencing room gain––when the room acoustics cause an increased output in the ultra deep bass. This happens commonly in smaller rooms. When you run with two ports open, it will naturally reduce the output in the ultra deep bass, due to the higher tuning, and that can help compensate for that boost in output. What may end up happening is that even though the sub is naturally rolling off earlier, you can still get a smooth linear response to 18 Hz or even more.
The second reason is efficiency. Running two ports open may gain you a few dB of increased output through much of the subwoofer range which may allow the sub to play a bit louder and cleaner. On top of that, you are now using two ports to move the same volume of air so that can significantly reduce the possibility of port noise or port compression.
We’ll cover the two important reported metrics in looking at amp power, which is measured in watts (W).
- RMS (root mean square) / continuous power: average continuous power that can be applied over a certain longer period of time.
- Peak power: power that can be applied over a very shortened period of time.
In general, the higher the number, the more output capability you get from the amp. However, that can be limited by the other two things such as driver efficiency and enclosure design.
Driver efficiency is determined by several factors such as size, weight, etc. which influence the driver’s efficiency to move a volume of air. As a general rule of thumb:
Larger driver → More efficient at moving the same amount of air → Higher output (volume)
Now that you understand enclosure design, amp power and driver efficiency, how do you determine the overall efficiency and clean output of a subwoofer?
Clean output is the ability of a subwoofer to produce output with relatively low distortion. Distortion is when the playback audio deviates from the original waveforms being sent from the source material. Truthfully, there will always be distortion, but those levels are usually so low they aren’t perceptible to the human ear. We always tell people that distortion only becomes an issue when you can hear it.
How do you determine how much clean output the subwoofer is capable of?
The CEA2010 (now CTA2010) is one of the most popular industry measurement standards. It measures the max peak (quick burst) output from a subwoofer outside, on the ground, from a fixed distance away. This usually spans across a set of ⅓ octave frequencies ranging to as low at 10 Hz to upwards of 125 Hz. The numbers recorded need to be under a set of distortion levels in order to be considered a passing result. As such, they give you a general idea of the clean output capability of the subwoofer.
Note: we don’t recommend getting too analytical in comparing various sets of numbers because equipment, environments, etc. all affect it. But it’s still a useful proxy.
How CEA/CTA2010 measurements are done has changed over the years.
Original, still used by many manufacturers
More commonly used at present
Less common but still exists
At Hsu Research, we use one meter, peak, to report our measurements. If you need to compare that with another set that was taken using two meters, RMS, you subtract 9 dB from our numbers. With two meters, peak, you subtract 6 dB from our numbers instead of 9 dB. Don’t be afraid to ask the subwoofer company which method they used.
The CEA/CTA2010 measurements allow you to more specifically assess which sub would work best for you based on your output preferences. Here are practical examples:
- If you listen to primarily music that doesn’t go really low and you want something that can provide dynamic bass, you’ll want to focus mainly on the mid-bass frequencies and a bit on the low bass frequencies, and ignore the ultra low bass measurements for the most part.
- If you want a sub that will reach the ultra low bass and rumble with authority, then you want to pay attention to the 40 Hz and below numbers. That being said, don’t neglect the mid-bass as those are very important in movie theater and music material.
Lastly, there is no such thing as “too much output” from a performative standpoint. Having a sub that will provide more than the output you need is ideal in that it will help ensure you have the most dynamic range possible. Also because the subwoofer doesn’t have to work as hard as a smaller sub to produce the same output, that would be beneficial from a longevity standpoint.
Now you’ve found your dream subwoofer. Should you consider getting multiple subwoofers over one larger one?
More recently, multiple subwoofers have gained a lot of popularity with audio enthusiasts. Multiple subs bring a lot of potential benefits, but there are also some potential cons, especially if this is your first foray into subwoofers.
- If integrated well, multiple subs gives you more output than a single one. In theory, room acoustics aside, doubling the number of subs can gain you upwards to 6 dB. In common cases, it’s around 3-4 dB on average. You can look to the CEA/CTA2010 numbers to see what you might expect.
- Room acoustics is the main reason why people recommend going with multiple subs over a larger single subwoofer. When using only one sub, you may encounter major dips or peaks in the response no matter where you place the subwoofer. They may change at what frequencies they occur but these dips are always present. If properly placed, adding a second sub can help fill in those nulls or help tame peaks which may provide you a more even and accurate response overall.
- It takes up increased area so if you are in a space-constrained room, it might be less ideal from an aesthetics standpoint.
- Adding subs adds significantly more complexity to integration. You have two subs basically reproducing the same material, so you will not only need to worry about making sure they integrate well with the speakers, but also with each other. If not well placed or the settings are not set correctly, the subs could end up canceling out their outputs. This will make your bass sound uneven and/or have significantly less output.
If you need help in deciding whether you may need to consider two or whether one may be sufficient, please don’t hesitate to reach out to us and we would be happy to assist.
What kind of connections do I need for a subwoofer?
What kind of connection you need depends on what components you’re running and what type of sub you’re purchasing.
1) Source of Power
Subwoofers can either be powered (active) or unpowered (passive). Powered subwoofers have built-in plate amplifiers with various controls, like volume, so you can adjust the sub output to your needs. If a subwoofer is powered, you will need a power cable to connect to the sub. If the subwoofer is unpowered, that means you will need to purchase an external amplifier to power your subwoofer.
The tldr? If powered, get a power cable. Most powered subs would come with a power cable. If unpowered, get an external amplifier.
2) Connection to Audio Equipment
Most people use A/V receivers for home theater systems, which have a subwoofer output built in. This is also known as sub pre-out, LFE output, etc. The most common type of connection for that output is a single RCA connector. There may be anywhere from one to four RCA outputs if your receiver can handle multiple subs.
Basically all you would need to connect the sub to the A/V receiver is a single (mono) RCA cable to connect from the sub output on the receiver to the sub input on the sub.
Our subs are labeled low level inputs for the RCA connections and you can connect to either the left or right low level input. Some higher end A/V receivers also have something called balanced outputs. This is often used in professional settings because it has noise interference rejection capabilities. In that case, a single XLR or microphone cable is used to send the audio signal. Not all subwoofers have balanced inputs available, so make sure you check with the manufacturer or look at the plate amp to see if that is an option. If not, RCA is still a great way to go for the vast majority of homes.
For two channel stereo, some, albeit few, stereo receivers have subwoofer outputs. If they do, you connect it the same way as outlined for A/V receivers. If they do not, the receiver may have a set of preamp output that you can send to the subwoofer. You will need a stereo RCA cable to send the left and right preamp output to the sub’s left and right preamp input. You also want to check with the sub manufacturer or check the amp because not all subs have a left and right preamp input. They only have one for the sub in. If that’s the case, you will either need to only run one channel to the sub or get an RCA summing cable (not just a simple y cable) to sum both the left and right into one signal. If you’re using stereo separates, meaning separate preamp and power amp, I recommend taking the signal from the preamp either by splitting the L/R output or using a secondary set of preamp outputs if that is built in on the preamp.
If you do not have any preamp outputs on your stereo receiver, you may need to connect the amplifier outputs, by either doubling up on the amplifier output posts or maybe on the speaker input posts, to send the signal to the subwoofer amplifier inputs. We’d highly recommend contacting the manufacturer at this point to make sure you can do this. At Hsu Research, we call these high level inputs.
The tldr? Single (mono) RCA cable does the trick for most. If you have a more higher end receiver, you you may be able to use a single XLR or microphone cable. If you’re listening to a two channel stereo with a receiver that has no subwoofer outputs, it’s more complicated.
We hope you found this guide useful in helping you navigate through the various aspects of subwoofers and help you to make a more informed decision on which subwoofer would work best for you. As always, if you have any questions and need assistance in choosing a sub for your setup, please don’t hesitate to contact us at firstname.lastname@example.org