Very few loudspeakers are full range. Yes, nearly all extend higher than we humans can hear, but rarely as low.
Audio setup guru Jim Smith, in his continuing series on Subwoofery, is covering a lot the specific details on why you need a subwoofer and what’s involved in its setup. For the rest of us it’s helpful to start at the beginning, keeping it simple.
Why speakers measure one way and sound another
Though full range loudspeakers may advertise their low frequency response as going down as low as 30Hz or even 20Hz, it is rare they achieve this in your room.
When speaker manufacturers measure the response of their products a microphone is placed a few feet from the speaker (1 meter, to be exact). This close, speakers can produce very low frequencies, but those low notes don’t necessarily reach your listening position—where it counts. So, nearfield measurements of the kind speaker manufacturers rely upon serve marketing and brochures more than listeners. This is one reason most speakers fail to deliver low bass in a typical room.
There are two main reasons for this: too small a woofer to effectively couple the air in your room, and placement. It is to the latter issue we will focus our subwoofer basics column.
Bass reproduces differently in your room
Bass is not consistent within your room. Depending on where your speakers are, compared to where you are, the bass may be very different. Here’s an experiment you can easily make. Find a track with good bass on it. I like Brian Bromberg’s Wood. It’s an excellent recording and should have deep bass with great presence. As Bromberg is playing, walk around your room and note just how different the strength of bass notes are. Make sure you walk into the corners of the room too. Notice anything? Sure, bass collects up in the corners. You might also notice that in the middle of your room, there’s likely very little bass. Yet, stand near your rear wall, the front wall, the corners, and the bass will be stronger.
Everything we hear is actually sound pressure changes: air pressure that comes in waves. High frequencies have short waves (often measured in increments of an inch) while low frequencies have long waves (often measured in many feet). A 20Hz bass note is approximately 50 feet long! Air pressure waves of this length bunch up at the boundaries of your room, like walls and corners.
Do you need a subwoofer?
Unless your loudspeaker has a built in subwoofer, chances are excellent your system would benefit from the addition of an aftermarket sub.
Where do you place it?
Our first inclination would be to place the new subwoofer in the same place our existing full range speaker is. That’s typically exactly where you don’t want it to be. And here’s why.
The best place for imaging and mid to high frequency reproduction from a loudspeaker is almost never the best place to make bass. To make matters worse, it’s almost never where you’re sitting, either.
Now that we understand the subwoofer is likely not going to be placed where our speakers are now, the obvious question is, where to place it?
Simple way to place it
Since we know the best bass reproduction in your room is likely not where the speakers are, nor where your listening position is, we have to find the right spot to place our new subwoofer so we get perfect bass in the listening position.
Here’s the trick. Place the subwoofer where your listening chair is, crank up the music, and then walk behind the main loudspeakers until you find where the bass sounds perfect. Bingo!
Now, move the sub into that exact spot, behind the loudspeaker, and when you play music again, bass at your listening position will sound identical.
It’s that easy.