"What most people think of as boominess in the midbass is really the sound of distortion harmonics from the bass speakers operating a couple of octaves below." -- Edgar Villchur
December 19, 2011, by Jeff Berryman
These days, concert loudspeaker systems can sound very good over most of the frequency range. Things have improved a lot in the last five or ten years.
But to my ears, bass sound hasn’t made nearly as much progress.
How often have do you go to a show, particularly a fairly loud one, and feel that you’re listening to a war between the upper and lower halves of the spectrum?
It all sounds pretty good at low levels, but when things get cooking, up comes the roar from below.
To preserve some semblance of transparency, the poor sound man has no choice but to produce an offsetting screech from the high mid, and the fight is on. The music (remember the music?) is lost somewhere in the middle.
At the end of the show, you might hear people making excuses: saying that the room acoustics were bad, or that the band was too loud on stage, or that there wasn’t enough PA, or who knows what else.
Although there are lots of bad rooms and other issues out there, I think that a lot of the problem simply comes from bad bass in the sound system. In what follows, I’ll describe what makes bad bass, and how NOT to have it.
One of the first people to think hard about bass was a man named Edgar Villchur. Villchur founded the famous home hi-fi loudspeaker company Acoustic Research (AR) in 1954.
AR built the world’s first compact low-distortion woofer, the AR-1, which had a clean, tight, musical bass sound, in contrast to the big boom boxes that were popular at the time.
About the sound, Villchur said something that’s still the biggest single key to good concert bass: “What most people think of as boominess in the midbass is really the sound of distortion harmonics from the bass speakers operating a couple of octaves below.”
Back in 1991, I was developing a new woofer for Jasonaudio’s predecessor, Jason Sound of Vancouver. The new woofer replaced an earlier model that had the same output level and same frequency response.
The only difference, other than a more convenient box shape, was that the new box had about 6 dB less distortion than the old box. The old box had about 4 percent distortion at high power, while the new one had about 1 percent.
When we compared the two models, the main difference we noticed wasn’t in the bass. The bass was about the same.
But with the new box, the entire sound of the PA below 1 kHz was cleaned up.
The midbass and lower midrange were more transparent and less confused-sounding, and the sense of separation between bass and midrange frequencies was greatly increased.
Why? Because the new woofers were not spraying ugly harmonics across the whole lower half of the musical spectrum.
If you’ve ever hung around with people who tune PA systems using modern measurement tools like Meyer SIM or Rational Acoustics Smaart, you might have noticed (as I have) a curious phenomenon: when you look at the system’s frequency response after it’s tuned, you often observe a broad valley between about 200 and 400 or 500 Hz.
Why have they set the tuning like that?
One of the reasons is that when the system gets loud, the woofers fill up that range with harmonics, so the REAL program has to be reduced to maintain overall musical balance.
That’s a shame, because a lot of very compelling music (piano left hand, cello, saxophone, lower strings of guitar, floor tom…) lives in that range.
Here’s what good bass sounds like:
* Bass impulses do not mask the rest of the music.
* You have a sense that the bass is part of the music, not just some rhythmic sound effect.
* The bass has color and texture that vary from moment to moment—it isn’t all just the same monotonous drone all the time. Unless, of course, that’s the way the music is intended to sound.
* You can hear the pitches of notes, not just vague roars, rumbles, bonks, and thuds.
* Percussive sounds are realistic, and do not sound like giants coughing or rugs being beaten or huge pillows being whacked with sticks.
* Transients are strong and hard, but when a sustained note is called for, it is clearly and musically produced.
* There are no spurious harsh or honky noises added.
* When a really loud, really low note is played, it hits you in the stomach and lower body, not just in the upper chest and head.
Good bass is transparent. This may sound strange - how can bass sound transparent? If you want to hear the answer, go to a pipe organ recital in a fair-sized church or cathedral. No PA system on earth has ever produced the quality of bass that most decent pipe organs put out every Sunday.
Regardless of what kind of music you like, if you care about bass you should seek out a pipe organ concert every couple of years, just to remind your ears what it can be like.
In PA systems, good bass is rare. In fact, many mix engineers have never heard it. Whey they finally do get a chance to mix on a system that does have it, they often need a little time to get used to it.
But after they do, they almost never want to go back to bad bass.
Is A Little Bad Better?
Sometimes people say that a little bass distortion is good, because it makes bass sound louder and adds punch to transients. I think the jury’s still out on the value of that approach.
There’s no doubt that distorted bass does sound louder, heavier, and more oppressive. There’s also no doubt that, if properly mixed, occasional clipping of bass amps can add some crunch to bass transients.
However, I think those effects are hard to handle and can easily get out of hand.
My vote: If you want more bass, get more (or better) woofers. If you want crunch on a bass line, insert an appropriate effect into the mix channel where you want it, or maybe just use a cheaper mic.
I don’t think it makes sense to subject the whole mix to a bunch of nonlinear, intermodulating woofers just to get a few channels tweaked. Bad might be better if you’re on a hot date, but not if you’re a woofer.
We’re talking about bass instincts here, not base instincts.
I have to mention that there’s one exception to all of the above: trance music. By this I mean dance music, rave music, some kinds of techno music, and generally any kind of music that’s designed to produce an altered state of consciousness by physical bombardment of the body and senses.
If you’re into that kind of thing, then you want your bass to have a stunning effect. Such an effect is produced when each bass transient is accompanied by a blast of harmonics that shakes the room, dims the lights, masks the music, and makes people’s eyes go blurry.
If that’s what you’re after, then you’ll probably want high-distortion bass. The easiest way to get that is to use nonlinear woofers. To get more control over the phenomenon, you can use bass-distorting devices, including subharmonic generators and other evil machines.
Lots of Bass
Even outside the trance community, lots of people like lots of bass. Me, for example—I like lots of bass. But if we’re not using distortion to create the illusion of powerful bass, what then do we do?
The answer is: have lots of bass headroom. If you have plenty of bass output capability, then when a real bass note comes along the system can put it out with integrity.
In other words, if you’ve really got it, you don’t have to fake it. But how do you get enough bass headroom and still stay within reasonable space and budget limits?
The answer: use efficient woofers. Most concert woofers are not really efficient enough to provide sufficient headroom for today’s music with today’s budgets.
How low should your bass go?
Here are low-frequency limits of some common musical instruments:
Four-string electric bass—40 Hz
Five-string electric bass—30 Hz
Standup (double) bass —40 Hz
Normal grand piano—27.5 Hz
Bass singer —62 Hz
Ordinary large pipe organ— 16 Hz
Kick drum (approximately)— 60 Hz
What this table doesn’t say is where the most important frequencies of these instruments are.
For a lot of bass instruments, the harmonics are louder and more important than the fundamental frequencies.
Being of higher frequency, the harmonics are more easily and quickly heard by the ear, and often contain the bulk of the melodic and rhythmic information.
What we’ve found for most popular music, the most important part of the musical range is from about 60 Hz up.
The range from 40 to 60 Hz is good for enhancing the experience, but only IF you have the 60-100 Hz range working well, and if you know how to manage the energy in your mix to avoid reverberation problems in big rooms.
Useful musical content below 40 Hz is rare in concert work. It occasionally is needed for acts that feature heavy synthesizer bass. These frequencies don’t work very well with popular music in typical large concert venues.
In their endless quest for impressive specs, loudspeaker manufacturers always seem to be building woofers that put out tons of 35 to 45 Hz at the expense of fidelity and output from 60 to 100 Hz.
Such woofers move a lot of air, but sound ugly.
The Bass Challenge
In big sound systems, it’s not easy to get good bass. Lowering woofer distortion to an acceptable level is really difficult.
There are two main reasons for this:
1. Air is very thin stuff, and bass vibrations are relatively slow. For a slow-moving bass loudspeaker cone to have much effect on the air, it has to be large and it has to travel back and forth a long way. It may seem as though a high-power woofer is a powerful, rugged, macho device, but in reality it’s a piece of paper frantically flapping in the breeze, trying to make a difference. It’s not easy to make that kind of thing linear.
By way of comparison: under water you can get loud bass down to 20 Hz from a 2-inch diameter woofer.
2. It’s surprising how low a level of distortion is actually required. There are two reasons for this: (a) the ear is 10-20 dB more sensitive to bass distortion harmonics than to the actual fundamental frequency, and (b) many people seem to like a lot of bass boost in PA systems.
My general rule of thumb is that 1 percent distortion in the subbass is equivalent to 7-10 percent distortion in most other parts of the frequency range.
You’ll remember from my old Jason Sound story above that when our woofers changed from 4 percent to 1 percent distortion, the effect was very obvious.
Most modern woofers are rated for maximum output at 5-10 percent distortion. That’s a big part of the problem.
There’s another kind of challenge facing good concert bass: Because many, if not most, touring sound professionals haven’t heard good bass, they aren’t asking manufacturers for it.
As a result, there are very few low-distortion concert woofer products available.
In studio monitoring and serious home stereo, the situation is somewhat different—many people know what good bass sounds like. In those markets, many manufacturers are serious about low bass distortion, and there are a number of successful products.
For really good concert bass in the 40 to 100 Hz range, I think horn-loaded woofers are the best solution. I’ll talk about horns and other kinds of bass loudspeakers in the upcoming article about woofers.
Getting It Where You Want It
If you do have a system capable of producing good bass, you’re still faced with the task of designing the woofer arrays for best coverage.
Bass coverage is very predictable, although sometimes the predictions dictate difficult loudspeaker arrangements for production.
This has lead to a lot of denial in the business, in which sound people pretend to have good bass coverage in order to avoid conflicts with lighting, video, and staging colleagues.
Jeff Berryman served as the director of Jasonaudio, a touring sound company based in Canada, and is a senior scientist with Electro-Voice.
Discussion & Analysis Of A Variety Of Bass Coverage Patterns