Written by 5:00 am Room Acoustics

What You Might Not Know — But Really Need to Know — About Bass

Brent Butterworth explains the critical fact that many audio enthusiasts don’t realize about bass.

When reading the responses to my recent article “What Audiophiles Are Getting Wrong About Subwoofers,” I was reminded again of something I discovered a long time ago: Most audio enthusiasts don’t know the single most important fact about bass. Or maybe they know it, but don’t fully understand it.

Schroeder_graph.jpgThat fact was discovered by Dr. Manfred Schroeder, who first described it in 1954. What Schroeder said was that above a certain frequency, a room responds to sound completely differently. In most residential rooms, this frequency is usually between 200 and 250 Hz. We now call it the Schroeder frequency, or the transition frequency.

Below the Schroeder frequency, a room acts like a resonator. It the wavelength of the sound being played matches the dimensions of the room, or some even-numbered fraction (i.e., one half, one quarter, etc.) of one or more of the dimensions, that sound will be reinforced. If it doesn’t, it won’t be. Depending on where the speaker is placed in the room, and where you’re listening from, some of these resonances will reinforce each other, and some will cancel each other out. This is why, if you’re playing low-frequency tones, the level of the bass will change quite a bit as you walk about the room. Sometimes it’ll be very loud. Sometimes it’ll almost disappear. Change the frequency of the tone and you’ll likely get very different results. If you take frequency response sweeps of your system in different places in the room, as I did for the graphic above, you’ll see huge variances in the bass, but above the Schroeder frequency, the response won’t vary so much.

Above the Schroeder frequency, the room acts like a diffuser. Sound bounces around until it’s eventually absorbed by the couch, the drapes, the carpet, or your Havanese. Play tones above the Schroder frequency, and you won’t hear a lot of variance in level as you walk around the room.

You may have figured out by now what this means for your audio system. Each of your speakers is dealing with, in essence, two completely different acoustical environments. Above 200 Hz or so, the room doesn’t have that much of an effect on the sound. Below 200 Hz, the room has a colossal effect on the sound.

The sounds above 200 Hz — i.e., the midrange and treble — are what create stereo imaging and soundstaging. For these sounds, the speakers should be placed in whatever positions deliver the best mix, to your ears, of a strong center image and a big, enveloping soundstage. Usually that’ll mean the speakers are a couple of feet out from the wall, and roughly as far apart from each other as each one is from your listening chair.

Sadly, though, this position isn’t going to deliver the smoothest bass response in your room. You’d get more consistent bass response throughout the room by placing the two speakers in the corners — which would of course completely mess up your stereo soundstaging (unless maybe you get speakers such as Klipschorns that are designed to work in the corners).

The answer is to use a subwoofer. Or even better, two … or four. Unfortunately, you can’t use a subwoofer to cover all the sounds up to 200 Hz, because your ears can locate the subwoofer at frequencies above about 80 Hz. So you run everything below 80 Hz to the subwoofer(s), and everything above that to the main speakers.

Obviously, this plan makes it unnecessary to use tower speakers, because you’ll be throwing away the tower speakers’ bass below 80 Hz.

One possible solution is to use tower speakers that have a separate input for the woofer section, and EQ that section to deliver flat bass response in your listening chair. But there are two downsides here.

First, in most speakers that offer biwiring/biamping capability, usually the top set of connectors feeds the tweeter and the bottom set feeds everything else. That’s a shame; it’d be much more useful to drive the woofer section separately from the tweeter and midrange than drive the tweeter separately from the midrange and woofer. Second, if you’re running two-way speakers, you may not want to EQ the woofer because it probably covers the range up to 2 or 3 kHz, and you don’t want to risk mucking up the midrange with an equalizer. I don’t blame you one bit.

Even if you don’t want to bother with subwoofers, by understanding Schroeder’s 60-year-old discovery, you can do a better job of understanding your system’s flaws — and a better job of fine-tuning your speaker positioning for the best possible sound.

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