How to choose and calibrate subwoofer(s)
I often get questions about adding subwoofers to a sound system. Having recent experience with this, I decided to write an article on choosing and calibrating them.
One common question is whether you can achieve quality sound by adding a subwoofer to an existing system without replacing everything. The answer is: with a good subwoofer and proper calibration—absolutely.
I don’t have experience with ultra-expensive high-end subwoofers, but in the mid-range ($500–$3,000 per sub), the top five manufacturers are:
PSA (Power Sound Audio, owned by Tom Vodhanel, a former SVS founder)
Monoprice (Monolith line)
JTR Subwoofers have great reviews, but they are considered hi-end and are pricey compared to the previous manufacturers. For example their 18" sealed sub (Captivator RS1) is currently $3700
You can’t go wrong with any of these. SVS is the most popular, and though they’re a bit pricey, their advantage is accessibility. You can order them easily and return them if they don’t meet your expectations. SVS offers a 45-day no-questions-asked return policy on their website, even covering return shipping costs.
I chose PSA for my system and have been very happy with the results.
My first subwoofer was a 12" DIY dual subwoofer using an old USSR-made driver. The sound quality wasn’t great, but I was thrilled anyway.
My first branded subwoofer was a sealed 12" SVS model. I liked its sound, but it didn’t deliver enough bass compared to the 10" woofers in my KRK Rokit 10-3 powered monitors, so I returned it. This led me to mistakenly believe that if your speakers have 10" woofers, you don’t need a subwoofer. I also thought 12" or larger subwoofers didn’t add much bass compared to 10" woofers. I was wrong, and because of this, I spent 10 years without a subwoofer—a decision I now regret.
The issue was that sealed subwoofers are less powerful (the driver is harder to move without a port) than ported subwoofers. That’s why my 12" sealed sub didn’t add much bass compared to the ported 10" woofers in my speakers. Had I tried a 12" ported sub or a 15" or larger model, I’d have realized how much a subwoofer can enhance music or home theater experience.
Sealed vs. Ported Subwoofers
There are two key differences between sealed and ported subwoofers. Ported subs provide higher SPL (sound pressure level, essentially volume) than sealed subs of the same diameter, especially at ultra-low frequencies (<25 Hz), where the SPL difference will be higher. However, ported subs produce a sound from the air moving through the port at these ultra low frequencies. Sealed subs are typically cheaper. If you don’t need the extra SPL of a ported sub, don't plan to listen to bass below 25 Hz, and dislike or know you won’t like the port’s airflow sound, a sealed subwoofer might be a better option for you. Many manufacturers, like SVS and Rythmik, allow you to close the port(s) with dense foam to convert a ported sub into a sealed one. Check with the manufacturer to confirm if this is possible for your model.
My second branded subwoofer was a 12" Klipsch R-120SW for my home theater. At $250, it’s a great value. The sound quality isn’t top-tier, but it’s solid for the price, and it significantly improved my home theater experience compared to my previous 8" sub. I enjoyed the bass so much that I tested it with my KRK speakers. Since it was a ported sub, it added noticeably more bass.
This experience prompted me to try a larger model, so I ordered a 15" Klipsch SPL-150 for $600. At first, I thought there wasn’t much difference between a 12" and a 15" subwoofer. But after a week with the 15" and then switching back to the 12", I realized how much I preferred the deeper, more visceral bass of the 15". A 12" sub adds plenty of bass, but a 15" delivers a fuller sound that you feel in your body.
After calibration (more on that later), the sound improved significantly, but I still wasn’t fully satisfied. The Klipsch felt slightly mismatched with my speakers. I'll explain what I mean. With good speakers, the drivers are designed to work together seamlessly, with smooth transitions between frequency ranges. The Klipsch SPL-150 didn’t blend perfectly—it felt a little like a separate system. It wasn't bad, I was just not completely happy. Also, it felt like its low-pass filter was not very well done: the subwoofer still sounded a little boomy (an increase at 70-80 Hz), even though I set the crossover frequency lower. Since it was also cosmetically damaged during delivery, I had a valid reason to return it. I then ordered two 18" EV18-NEO subwoofers from PSA.
My current music system uses these two 18" PSA EV18-NEOs. I didn’t strictly need 18" subs, but they were on a pre-order sale, costing less than available PSA’s 15" subs at the time ($2,200 for both). Many recommend getting the largest sub you can afford and fit in your space because: 1) you might not realize initially how much a larger sub can improve your experience, and 2) you may later crave more bass and wish you’d chosen a bigger sub. My 32 sq.m. (350 sq.ft.) room accommodates these large subs well. I've decided not to limit myself in ultra-low frequencies with sealed subwoofers and ordered ported ones. The low-pass filter on the PSA subs feels better than on the Klipsch SPL-150: on my PSA subs I can effectively remove the boomy 70-80+ Hz frequencies from the subwoofers by setting the crossover frequency to 60 Hz.
Multiple Subwoofers
There’s a myth that you can’t tell where bass comes from, so you can place a subwoofer anywhere and only need one. This isn’t true. With a single 12" or 15" subwoofer on my right, I could hear and feel the bass coming from that side, which was uncomfortable. Two subwoofers solve this by balancing the sound. Multiple subs also offer more setup flexibility and make it easier to achieve a smooth frequency response. For example, a single sub in a suboptimal spot might create a null at, say, 40 Hz, where a reflected bass wave cancels out a new one, appearing as a deep and narrow V-dip on a frequency response graph. A second sub in a better location can compensate for this null, providing strong output at the frequency where the first sub struggles. With two or more subs, you’re more likely to achieve a balanced frequency response.
Calibration
Opinions vary on the best way to calibrate an audio system. The following reflects my personal experience, which yielded optimal results for me. Your results may differ, and this approach may not align with other recommended calibration methods.
Placement
Usually the best results are achieved when subwoofers are placed in corners, or at least along a wall. Some people say the middle of the front wall is usually the best location, but in my experience the best locations are corners. But ideally you should check this and not just assume the corner or the middle of the front wall is the best place in your room. Measure the low frequency response and see if the graph looks fine (without big changes in SPL, ideally within +-3 db). If it doesn't look fine - try placing the subwoofer(s) to another location and remeasure.
Crossover frequency
The crossover frequency is the point at which the audio signal is divided, directing lower frequencies to the subwoofer and higher frequencies to the main speakers for optimal sound reproduction. To optimize audio performance, set the crossover frequency on your subwoofer and receiver or digital processor to delegate low frequencies to the subwoofer. High-quality speakers often include built-in filters that block very low frequencies, preventing overload and distortion.
I typically set the crossover frequency to 60 Hz, as I find that 70–80 Hz frequencies sound overly "boomy." In my setups, subwoofers handle all frequencies below 60 Hz. Depending on the speaker system, the speakers either reproduce all frequencies alongside the subwoofer or focus on frequencies above 60 Hz. For smaller speakers, I adjust the crossover frequency to 80–120 Hz for optimal performance.
Many advanced AV receivers and DSPs (like miniDSP) support semi-automatic calibration and may include a calibration microphone. You’ll need a specialized calibration microphone—not just any microphone. The UMIK-1 from miniDSP is a popular and excellent choice, which I use myself.
With a calibration microphone, you’ll need a tool to perform the calibration and generate a filter (equalizer) and possibly a delay (phase adjustment) for your sub(s). This is typically done through a calibration procedure in your AV receiver or software on your PC.
I use Room EQ Wizard (REW), a free, open-source, and highly capable program. Compared to Dirac Live, another popular option that costs $200 and has fewer features, REW is much, much more advanced.
Calibration can be complex, but I’ll simplify it. Essentially, you measure your subwoofer’s frequency response with a calibration microphone, and the software generates filters to improve sound quality and achieve a flatter frequency response. For example, if your sub has a dip at 30 Hz, the filter will boost the gain at that frequency.
The basic calibration process is:
Measure the frequency response.
Generate filter(s) and/or change subwoofer/speaker locations
Apply the filter(s) to the output.
Remeasure to make sure there's improvement
If you’re using software for calibration, you’ll need a tool to apply the filters to the audio output. Equalizer APO is a popular, free option that I use.
Calibrating with REW
If you're using Room EQ Wizard (REW) for calibration, follow these steps.
Connect the Microphone and Launch REW Connect your calibration microphone and open REW. The software should detect the microphone and prompt you to use it. Click "Yes."
The miniDSP UMIK-1 provides two calibration files based on your setup:
- For mono or stereo systems, use the 0-degree file and point the UMIK-1 directly at the speakers.
- For multichannel systems (e.g., 5.1 or 7.1) or surround setups with speakers spread around the room, use the 90-degree file and point the UMIK-1 at the ceiling.
In REW, go to Preferences > Cal Files and select the appropriate calibration file for your microphone.
Set Speaker and Subwoofer Volume Levels Place the measurement microphone at your main listening position (MLP). In REW, open the SPL Meter and Generator tools. In the Generator window, select the Noise tab and choose Pink Random.
Use the left speaker as the reference for initial volume measurement, then match all other channels to it. To measure the left speaker’s volume:
- In the Generator window, select Speaker Cal (500 to 2000 Hz) and choose the left channel (L).
- Disable all other speakers and subwoofers, then press Play.
- Note the average SPL value, then stop the noise.
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- Disable all channels except the subwoofer.
- Ensure the microphone is at your main listening position (MLP).
- Name your measurement (this will help later) and set the frequency range to subwoofer frequencies, e.g., 20 to 150 Hz.
- Click Start and wait for the measurement to complete.
If you don't want to deal with REW, there's another simple option: the MathAudio Room EQ plugin. You can use it with the foobar audio player or other software. It's very simple to use:
Select "Room Measurement", click "Start Measurement", measure, then adjust the target level curve with the slider and switch to "Room EQ" to apply the filter.
Here's a short description of what Room EQ does from their home page:
The resultant processed frequency response of your room and speakers will be shown by a white line as it is shown in Fig.5. It is possible to see that the main resonances of the resultant frequency response (white line) became much smaller. Such resonances added unnatural timbre to the sound. The removal of the resonances dramatically improves the sound.
It is also possible to see that MathAudio Room EQ doesn't boost the spectral components lying below the green line. This important feature prevents the large excursions of the speaker diaphragm that can be caused by the ideal linearization of the frequency response. Besides that this feature allows you to avoid the overcompensation of your room. As a result you will not get a muddy sound that is peculiar to conventional linearizing equalizers.
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