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Cover photo for the two is better than one home audio subwoofer blog.

Two Is Better Than One: Home Theater Subwoofers

By Matt Pecharich | January 23rd, 2021/Home Audio and Video


(No, not the fish, the rumbly kind.)

The kind that either impresses or annoys the neighbors. The kind that makes you FEEL the movie or music. The kind a home theater or surround sound needs in order to sound full and complete.

What produces these low end frequencies? A subwoofer, of course!

BUT.. there is more to it! Especially if you want to maximize your audio performance (and the money you spent). If you are building, designing, remodeling, or have an existing home theater or media room then read on if you dare..

No, seriously, this is going to be a shorter writeup than a lot of our others but it is also going to go from simple explanations to very technical. You may also recognize the pictures used from our (much longer) surround sound writeup. If you have come directly here from there, we suggest stepping away for a few minutes before reading this.

With that being said, let’s begin!

Bass - Low Frequency Sound Wave Basics

So we all know what “bass” is. We have been to a movie theater, heard a car next to us, seen the adjustment on the radio in our own car, etc.. It is the lower end frequencies (the foundation) of audio produced by movies and music.

For this writeup, we want you to keep the frequency range of 20-300Hz in mind (Hz = Hertz).

Everything we discuss here focuses on that range, because it is what is universally considered the “bass” range. Frequencies are measured in hertz, which can simply be explained as the number of vibrations a frequency produces per second. So, a 50Hz frequency is vibrating 50 times per second.

Have you ever wondered why you can hear the bass from a car on the road, but not the vocals? Maybe the house down the block is throwing a party and you can hear the bass, but not any of the actual music. This is because the sound waves (more specifically, the wavelengths) produced in the 20-300Hz range are longer than those in the higher ranges.

The image below and the one after it show you what we mean about them being longer and also explain a few terms you need to know that we use throughout the rest of the writeup.


Picture of a 1Hz soundwave and a 10Hz soundwave.

A picture of a soundwave with the peaks, troughs, wave length, and amplitude.

This is one of the biggest reasons why we can hear bass from so far away, along with the fact that it is not as easily absorbed as higher frequencies are. Because a 5000Hz sound wave is vibrating so many times a second and is shorter, it has to spend a lot more energy to travel the same distance a 30Hz wave can with less energy. This, in turn, also makes higher frequencies easier for objects and materials (even air) to absorb. They are also much more susceptible to reflecting off surfaces instead of passing through them.

Since lower end wavelengths are longer and can travel without expending as much energy, they are harder to absorb and tend to either pass through or reflect off materials. This is why we hear bass from farther away.

This is also why two subs are better than one in an enclosed space. It has to do with the wavelength. Let’s take a look at an example.

Longer Than the Room? Bass Wavelengths

So we know that lower end frequencies have a longer wavelength, but exactly how long are the waves they produce? Well that is easy, you just have to know the frequency of the wave and the speed of sound. (We’re getting to why two subwoofers are better than one, trust us here for a moment)

The speed of sound is generally between 330-345m (meters) a second. This is through the air with no obstructions. It can change depending on atmospheric pressure and the temperature, as well as if it is traveling through a specific substance.

For instance, sound travels around 1450-1550m/s through water and close to 6000m/s through steel.

Note: Now, in our surround sound writeup we specifically mentioned that the room in the pictures had no specific size. We were only using the pictures to reference speaker positions. In this one, we want you to assume the room is 12ft x 20ft and a dedicated media room.

The formula to figure out the length of a soundwave is easy. You just divide the speed of sound by the frequency of the wavelength. Our lovely friend Google says 343m/s specifically for the speed, so we will use that. So if the speed of sound is 343m per second and we are dealing with a 300Hz frequency, the length of a 300Hz sound wave is 1.14m. That is from one peak to the next in the wave.

Now, do the math on a 40Hz frequency.. yea buddy..

That is a 8.58 meter long wavelength, from peak to peak. For us Americans, that is approximately 28.15 feet long and 8.15ft longer than our media room example. Now may we all understand why we hear bass from so far away and why it is very hard to control in an enclosed space. Especially with reflections off ceilings and walls.

Reflections and Standing Waves: The Single Sub Dilemma

Let’s go ahead and rip the bandaid off right quick.

It is impossible to experience a smooth bass response with only one subwoofer in a room.

This is not a sales ploy for us to sell you an extra subwoofer, it is a scientifically backed fact. Look at the picture below. It demonstrates placing a subwoofer in the back left corner of the room and the sound wave that is leaving it. Coming off the front wall (parallel to the rear) the SAME sound wave is being reflected back towards the source (the subwoofer).

Picture of how one subwoofer reflects off the opposite wall.

You see where we marked “Bad” on the picture is also where the reflected soundwave overlaps with the original.

Contrary to popular belief or myth, two soundwaves of the same frequency that smack into one another do not cancel each other out. It’s a lot more complicated than that. Remember, we’re dealing with a wave. So a reflected wave hitting the original could hit it at a LOT of different points of amplitude (the ups and downs).

However, when it hits the original and overlaps with a peak or trough, then we have a problem.

Let’s use a 112hz note and plot it out in this theater room. Remember, speed of sound (343m/s) divided by frequency. So a 112Hz note is 3.06m long, or 10.04ft. Our media room is 20ft deep and we have the subwoofer in the back, firing to the front.

For simplicity, we will assume the wave starts at the back wall and round down to exactly 10ft on this graphic.

Example of the reflections in a media room creating standing waves.

When the peaks and troughs of a sound wave overlap, they produce what are called “standing waves”. At the overlap of the peaks there is a surge of energy that causes the note to be much louder than the original. The opposite happens when the troughs line up. This causes a noticeable reduction where the note becomes very quiet.

You can see how that would be a problem for the first row of listeners in the above example. Even the second row will probably get a standing trough wave. Of course we're technically assuming a lot here, but you get the just of it.

In the animation below, the blue wave is the original produced by your subwoofer and the red wave is the reflection. The black is the produced standing wave when they line up. This is why bass is so hard to control in an enclosed space. Our brain and ears are much better at suppressing higher frequency standing waves than lower ones because there are so many of them when listening to music or watching movies.

Animation of a sound wave and its reflection creating a standing wave.

The room controls the sound, especially in the lower range, because the wavelengths are within the range of the room’s dimensions. Reflections become part of the sound almost instantly and what we hear ends up being dominated by the room and not the speaker anymore.

For a quick example, have you ever been talking, singing, or made a noise in a bathroom or locker room and all of a sudden you hit a frequency that got a LOT louder than the rest? You’ve found the peak standing wave for the frequency and where you are.

Two Subwoofers: The Solution + Symmetry is Bad

Sorry if there are any OCD people reading this, but it is what it is.

We always suggest putting subwoofers in corners of the room. They perform better most of the time. So what if we added two subs to the back corners of the room firing forward?

Two symmetrical home audio subwoofers creating standing waves in a home theater room.

You’d just double your problem, not your pleasure.

So what’s the point? You may be asking that right about now. We said two subwoofers would solve the issue. Well, this is true, but even two can run into the same problem one does. We’re dealing with soundwaves.

This is why some people buy a second subwoofer for their home theater and they are still disappointed. We have a habit of setting up home audio in a symmetrical format, like all of the other speakers. Even diagonally opposite corners wouldn’t work in this situation though, let alone a symmetrical setup.

What you want is two subwoofers set up but NOT aimed at a parallel or perpendicular position to each other or the walls (if possible) or at least set up in a very asymmetrical manner if they are. We are also talking about three dimensional asymmetry, so even going vertical counts (one subwoofer a foot higher than the other).

Picture of two different asymmetrical subwoofer setups for a home theater room.

Nomatter what, in an enclosed space, you’re going to run into standing wave issues. So playing around with the subwoofer placement if possible is key. However, adding a second subwoofer to your theater room “smooths” out the bass by providing a second source of low frequencies.

And making sure this second source is asymmetrical to the first adds a “second layer” of low end frequencies coming from a dramatically different direction to fill in/smooth out the gaps.

That's Enough Science for Today

Hopefully we explained things simply enough for most of you to understand. To us, it’s not enough to just say “take our word for it” a lot of the time. So our writeups end up being a bit more technical than others. We may have even helped you with a current issue in your existing theater room.

You may also better understand why it is important to come get an expert opinion when you are designing things to do with home audio and theater rooms.

We suggest two subwoofers not because it’s louder, but because it solves an issue 100% of media and listening rooms have with just one. If you don’t believe us and have just one subwoofer, then turn up the volume with a bass heavier song and slowly walk around your room. You’ll find the standing wave.

Of course, this whole issue may not matter to some customers. But our stance is if you’re going to spend the money on a custom home theater you may as well do it right. So we are going to suggest how to do it the 100% way, not the 90% way.

We aren’t a big box store stepping stone. Our consultants on both the home audio and car audio sides of the store understand the science behind what we sell. We have training and certifications from some of the biggest companies that set our industry standards. There is also well over 100 years of combined experience on either side FOR each side (home or car audio).

If you have questions, or need help with an existing or future design, come in and see us! Our certified consultants would love to help. You can set up an appointment or ask questions in person, over the phone, or you can email us here.

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