One Subwoofer Isn’t Enough (and What You’re Missing)

If you are into high-end home theater, check out our Display and Audio Calibration Guides to maximize your experience.

Intro

It is always fascinating to see ongoing conversations about the role of subwoofers, both single and multi-sub setups, in audio systems despite their well-established benefits. While it is common in professional and commercial setups, there are still debates on whether more than one subwoofer is necessary for home applications, with some insisting that a single sub is plenty.

In this article, let us explore why just one subwoofer is not sufficient in most cases, especially in home theaters, and discover what you have been missing out on!

Understanding Subwoofer Basics

For anyone that is new to the audio world, let us start from the beginning.

What is a Subwoofer?

A subwoofer is a loudspeaker that specializes in reproducing low-frequency sounds, typically ranging from 20 to 200 Hz, filling in the bass region that conventional full-range speakers may struggle with – although they typically operate from 10Hz to 120Hz in dedicated home cinemas. They work similarly to regular loudspeakers, but are optimized for low frequencies. Subwoofers use large, high-excursion drivers, also known as woofers, ranging in sizes such as 8, 10, 12, 15, or 16+ inches in diameter. These drivers convert electrical signals into sound waves, emphasizing the production of low-frequency sounds. 

So, why do you need a subwoofer? Simply put, it is tough for one speaker cone to handle all frequencies accurately. High notes, mid-range vocals, and bass demands require specialized drivers, and that is where subwoofers step in.

How Low Frequencies (Bass) Behave In a Small Room

Conventional wisdom once suggested that typical house-sized rooms behave acoustically like large auditorium-sized rooms. However, ongoing research has discovered fundamental differences in how room size influences the behavior of sound waves. This is particularly notable when considering bass frequencies.

In big rooms (commercial cinema), it is important to focus on spreading out the sound evenly. Meanwhile, in small rooms (home theater size), the main concerns are dealing with early reflections and handling low-frequency issues caused by resonances and standing waves. As the wavelength of specific frequencies approaches the dimensions of the room, the sound behavior undergoes a noticeable change. This change is influenced by the relative wavelength of the frequency in comparison to the size of the room.

In smaller rooms, around 300Hz, the transition occurs from wave (omnidirectional) to ray (directional) behavior, known as the room transition frequency. At frequencies lower than the transition frequency the room tends to shape the sound the most and at higher frequencies, it is more about the performance of the loudspeaker. If not controlled, these early reflections, resonances, and room modes greatly reduced the audio quality and clarity.

When it comes to getting good bass in small rooms, controlling standing waves through bass traps is a common practice. The problem with bass traps is they often have to be really large to be effective and they can not fix all acoustical problems. Therefore, an alternative approach involving multiple subwoofers can often be an extremely effective tool to help smooth out the frequency response issues caused by standing waves. Strategically placed multiple subs can augment traditional bass traps, providing consistent bass across seats.

While this is a broad topic that would necessitate a detailed exploration to fully comprehend, which is beyond the scope of this article, the key takeaway is that the most optimal solution involves utilizing a combination of passive bass traps and multiple subwoofers to achieve the best results.

Single Subwoofer Approach

While a single subwoofer is better than no subwoofer and may suffice for basic audio setups, it often struggles to provide even bass distribution across an entire listening area. As stated above, low frequencies can be highly influenced by the room’s acoustics, resulting in uneven bass response. This can lead to areas with overwhelming bass and other areas where the bass is weak or virtually non-existent.

Unfortunately, in most cases, no amount of EQ can effectively eliminate these issues. Additionally, excessive use of EQ can compromise the time domain (group delay and decay time), which can be just as important than achieving a perfect frequency response curve.

Additionally, a single subwoofer may struggle to fill larger spaces with immersive low-frequency effects, such as those found in movies or music with deep bass elements. The limitations become more evident when trying to achieve reference sound pressure levels or when dealing with demanding content, and can lead to distortion during playback or damage to the subwoofer from overdriving.

Benefits of Using Multiple Subwoofers

Improved Room Modes/Frequency Reponses

One of the main reasons to have multiple subwoofers, is to help control the room’s effect on the sound. Room modes, or standing waves, are resonances that occur between reflective surfaces in a room. These modes can create peaks and dips in the frequency response, leading to uneven bass distribution. A single subwoofer may inadvertently excite specific room modes, resulting in exaggerated bass at certain frequencies and nulls at others.

Example of one sub versus two subs (Property of Audioholics).

By employing multiple subwoofers and strategically placing them within the room, it becomes possible to mitigate the impact of room modes. In the image above the null that is at about 48hz is all but eliminated when the second sub was introduced (green line). The interaction between the subwoofers helps distribute energy more evenly, reducing the likelihood of pronounced peaks and nulls in the bass response. This results in a smoother and more accurate representation of low-frequency content throughout the listening space before the equalization. Subsequently, this also makes the process of equalizing much simpler and easier to do.

Multi-Seat Optimization

The second most important improvement is the ability to have great bass in more than one spot. In a typical home audio setup, listeners are often seated in different areas of the room. With a single subwoofer, achieving consistent and satisfying bass for everyone can be very challenging, if not impossible. The interaction between the subwoofer’s output and the room’s acoustics varies depending on the listener’s location.

Results of Roland’s MSO Calibration: Showing how bass response can be dramatically improved over a larger space—in this case, 3 seats—with multiple subs.

Multiple subwoofers, strategically placed in the room, can address this issue by providing a more uniform bass experience across multiple seating positions. This multi-sub configuration minimizes the impact of room modes and standing waves, ensuring that each listener enjoys a well-balanced and immersive audio experience, regardless of their location within the listening area.

Increased Output

One might assume that acquiring an additional subwoofer primarily aims to amplify the bass output, given the common belief that more speakers translate to increased volume. While this is indeed true, adding more subs equals more bass, the reality is not as straightforward, as doubling the amount of subwoofers will not equal a doubling in bass output.

Without delving too deeply into the technical intricacies, which warrant a dedicated article in the future, introducing a second subwoofer typically yields a 3-6 decibel (db) increase in Sound Pressure Level (SPL) depending on the frequency and the room. This increment corresponds to only 30-60% more volume at lower frequencies. This phenomenon aligns with the principle that doubling power merely results in a 3db SPL increase.

Simple take away is that adding a second sub does increase output but doubling the subwoofer count will not mean doubling the bass.

Reduced distortion

Distortion is an inherent challenge when reproducing low-frequency sounds, especially at higher volumes. This is because the lower the frequency the more the cone of the speaker must move, which could result in high distortion if running near the excursion limits of the driver. Single subwoofer setups may face increased distortion as they approach their limits to meet the demands of deep bass content at higher volumes. This distortion can result in a less accurate and enjoyable listening experience.

Introducing multiple subwoofers allows each unit to operate more within its optimal range, reducing the load on the individual drivers. As a result, the overall distortion is minimized, contributing to a cleaner and more articulate bass response. This is particularly important for audiophiles and home theater enthusiasts who seek a high-fidelity experience without compromising on clarity.

Tips for Optimizing Any Subwoofer Setup

People often dedicate hours obsessing over optimizing the bass response and subwoofer setup—I find myself falling into that category. However, there are simple tasks you can immediately undertake to begin improving the bass, gradually progressing towards full calibration for both frequency response and decay time.

Placement

First and foremost, the placement of subs has the most significant impact on how the bass sounds, especially considering that the room itself plays the largest role in shaping the bass. Many people might instinctively position subs on either side of the screen, assuming it is the ideal spot. However, I must emphasize that despite theoretical suggestions like corner loading, diagonal placement, and mid-wall placement, there is no one-size-fits-all solution.

The reason for this lies in various factors such as room dimensions, seating location, construction materials, seating type, other furniture, and even the shape and size of adjacent rooms—all of which can influence the response, group delay, and decay time. Consequently, the optimal location for a subwoofer is where it sounds best in that particular room.

So, how do you determine this ideal location? One approach is to conduct a version of the sub crawl. This involves moving the sub around to different acceptable locations in the room and discerning what sounds best. While you can do this by ear, a more advanced method involves using REW to measure the response.

Just by determining the best placement for each subwoofer, this can have a dramatic impact on the bass.

To learn more about subwoofer placement, explore our audio guides for a more in-depth understanding of how to position your sub.

Calibration and Integration

Once you have gotten the best placement for your subs, the next step is to calibrate them. The objective of audio calibration is to ensure the faithful reproduction of audio through your speakers and subs to further improve the performance. Especially once you get into multi-sub setups, if not configured correctly, it can hurt performance instead of helping. To ensure that is not the case for you we have created various compressive audio guides and other articles, which are listed in the cards below:

At a minimum you should be allowing your AVR or audio processor to calibrate the bass frequencies to the transition frequency, as these systems can measure and then compensate for the room characteristics. You can improve your performance by then getting into manual calibration, with REW and then more multi-seat calibration with MSO.

Accurate calibration and effective integration of multiple subwoofers enable you to extract the optimal performance from your system. This is particularly crucial when integrating multiple subs to ensure their cohesive operation, delivering a seamless and precise bass response.

To Wrap Up

Adding additional subwoofers to your home theater setup is not just a bells-and-whistles upgrade; it improves so much more than just louder bass. It has been one of the most impactful upgrades I have made to my system. And even if you are not chasing audiophile nirvana or cranking the bass to eleven, multiple subs elevate your entire system’s performance in surprising ways.

Here is the thing: bass is the foundation of a truly immersive home theater experience. It is the rumble that shakes your bones during an action sequence, the low, brooding tones that set the mood in a thriller, and the subtle nuances that add depth and realism to every scene. Watch a movie with and without a subwoofer, and the difference is night and day. Suddenly, without bass the content is just flat and lifeless and has no impact on you both physically and emotionally.

With multiple subwoofers, you are not just adding more bass, you are distributing it evenly throughout your space, smoothing out those annoying room resonances and ensuring everyone in the sweet spot gets the full sonic picture. Your system breathes easier, delivering cleaner, more accurate bass even at high volumes. So my advice is, if you have the space for a second sub, add it.

Thank you for reading. If you are into high-end home theater, do not forget to check out our Display and Audio Calibration Guides to maximize your experience.