The Subwoofer


“Got God on the drums and the Devil on the bass”

Why a subwoofer? The whole function of that big box is to reproduce low frequencies that your other speakers may not be able to reproduce well. In a digital audio system, the recordings also use that bass-only channel for unique information. In movies, it can be the speaker that reproduces all the rumble that excites and envelopes. In music, it can be the speaker that reproduces the low notes that stir you. It is a vital component in many modern audio systems.

 

Often, in conversations with others about their entertainment systems, a common complaint is the subwoofer volume is always needing adjustment – turned up when the bass is insufficient, and back down when it’s too much. It’s frustrating having to make these volume changes all the time.

 

There are likely two very simple reasons for this: firstly, the subwoofer level has not been set properly (modern receivers employ audio correction software and a microphone that can balance the level of the subwoofer and the other speakers together based on seating positions), or, more importantly, as this can even affect even the microphone correction accuracy, the subwoofer is sitting in the wrong location in the room.

 

The acoustic correction software built-in to most modern A/V receivers can be quite advanced, but not enough to ever compensate for poor subwoofer placement.

 

To understand why this is the case, there are some things to be aware of regarding how low-level frequencies behave in a room. In most cases, the subwoofer is passing frequencies below 100 Hertz (Hz); these frequencies are omnidirectional, meaning you cannot tell where they are coming from (the sub does not need to be seen to do its job). The issue, though, is that these frequency waves are quite large (most bass we hear in movies is around 30-50 Hz, and a 40 Hz wave, for example, is 28 feet long). If you want a visual correlation about how these waves might interact in a room, think about splashing around in a pool or bathtub: where waves combine with one another, we produce bigger peaks, and where they drop off together, we get troughs. These troughs in the audio world are null spots where we don’t get the impact of the bass frequencies. The nodes in the room where bass waves combine makes bass louder. Unlike the randomness of splashing in the tub, audio waves in a room can exhibit fixed trough/peak locations. Based on the sub location and your seating position, you may find yourself sitting in one of those nulls, getting no decent bass, or on a frequency’s peak, emphasizing certain bass frequencies over all others in your system. When you make volume adjustments, you’re not fixing the underlying issue, you’re just increasing or reducing how you hear the effect.

 

So the question now arises: How do I determine where to put the subwoofer to mitigate these issues?

 

There are a couple basic steps to follow to try to improve bass. When in doubt, the best location for a subwoofer is a corner in the room. Low frequencies are enhanced by barriers, and a corner provides three immediate barriers, loading the bass response and often exciting bass interactions in the room (there are sub designs where this might not work – ported subwoofers need to have the port away from the walls to allow proper tuning and air flow). The second key rule is to never have your seating against a wall. Some bass frequencies will peak at the wall, adding to one another, overdriving your perception of the bass, drowning out other sounds. This can be difficult to accomplish in smaller rooms, but moving your couch or seats away from the wall will give you a better chance of sitting in a location where bass frequencies are more evenly distributed. Once you’ve satisfied the above two steps, recalibrate the speaker levels (rerun the mic audio correction in your receiver).

 

My sub just chillin’ on the couch waiting for popcorn and a Coke.

The corner may not be the ideal acoustic location. You may want to refine the sub placement and this next trick is where magic and science meet. You’ll need a long subwoofer cable (thanks to the internet, these can be inexpensive), a test tone generator (most A/V receivers can generate this tone, or you can download handy apps like THX Tune-Up on your phone), and an SPL meter (apps for phones available). Place the subwoofer on the couch, at the primary listening position. Run the test signal to the sub and walk the room with the SPL meter. When you get to the spot in the room where the meter peaks, you have just found two nodes in the room that cooperate. Place a piece of tape to mark that spot in the room. Now move the sub to another listening position and test again. Mark this new spot with tape. Repeat for a third and maybe fourth listening position, and mark those spots. You should be able to gauge an average location from those tape markings for the sub placement. Once the sub is placed, rerun the audio mic correction to balance the full speaker system.

 

The advanced approach is to have audio RTA (real-time analysis) done for the room and speakers, to determine the speaker placement, levels, and sub placement that will produce the most even overall frequency response. Often, this type of analysis can be done in conjunction with DSP equalization to correct more aggressively (certain higher-end receivers offer this type of equalizing in their audio correction scheme).

 

The ideal scenario is to design a room from scratch, where Dimensional Analysis can be used to calculate the dimensions of the room that are best suited for low frequencies, and then employing any of the above steps will yield the best results possible for bass response.

 

Now you’re armed with some tools to help you with that pesky subwoofer. Good luck, and good listening!