The EM Poll

Tech Basics Without Tears

Mar 14, 2008 5:29 PM, By Jon Chappell With Steve Oppenheimer

           

A PAIN-FREE EXPLANATION OF FIVE CONCEPTS ALL MUSICIANS SHOULD UNDERSTAND

Why Logarithms?

Loudness, which is also known by the nontechy term volume, is a description of how humans experience sound-pressure levels (SPL), or acoustic amplitude. Mathematically describing audio phenomena such as sound-pressure levels (and our ears' response to them) involves a huge range of numbers; for instance, the ratio of the loudest sound we can endure to the softest we can perceive is approximately a trillion to one. If you were to plot changes like this on a graph using a linear scale, it would require a gigantic graph that would be very difficult to understand. Logarithms (which are essentially the opposite of exponents) reduce these large numbers to a manageable scale and make it easier to understand what is going on.

Why Decibels?

Bels are a logarithmic unit of measurement originally developed at Bell Labs to compare two power values. It turned out that Bels were too large for practically measuring audio circuits, so the scientists developed the decibel, a unit of measurement equal to one-tenth of a Bel.

There are several distinct types of decibels, and each uses a different formula. The decibels used to measure electrical values are not quite the same as those used to measure acoustic phenomena, although they are related. For our purposes, we are specifically dealing with the type of decibel used to measure acoustic sound-pressure levels, the symbol for which is dB SPL. Thanks to logarithms and decibels, rather than say that the level of a pianissimo flute line is two one-thousandths of a dyne per centimeter squared, we can say it's 20 dB SPL.

The table “Relative Sound Pressure Levels” shows the SPL of some common sounds. Notice that as SPL increases additively by 10 dB (for example, 10, 20, 30), the amplitude increases by a factor of 10 (10, 100, 1,000). So a signal that increases by 20 dB SPL is actually 100 times more powerful than the original.

Keep in mind that loudness is something else again, responding nonlinearly, depending on the pitch (frequency). Our ears cannot detect when extremely high or low sounds get louder or softer, as they are not sensitized as much to extreme frequencies. For example, if you increase a 5 Hz signal by 10 dB, you would perceive no change in volume because our eardrums are not sensitive to frequencies that low. However, if you raise the frequency to the range of a baby's cry or a tenor's high note in an aria — sounds whose fundamental frequencies fall in the middle of our hearing range — you'll hear the difference because our ears are very good at detecting subtle changes in amplitude in that frequency range. Obviously, that affects how we listen to music.

How can understanding this relationship of loudness help you in action? Well, if you turn up the highs in a system, it won't be perceived as louder, only brighter, to our midrange-oriented ears. And if you turn up the bass in a two-way P.A. speaker too much, you'll distort the signal before you can hear an appreciable boost in level. Even though you're turning up the bass, you'll hear the distortion first in critical information such as the vocals and acoustic guitar — midrange instruments to which our ears are highly sensitive. So though the bass might be boosted in terms of acoustical power, that will not necessarily translate to perceived loudness. For increased perceived loudness, crank those mids!

Now you know that when the tech says to turn the guitar down 20 dB, that means you need to bring it way down. Come on, pal, are you trying to damage my ears?