About Acoustic and Microphone Levels: Levels Management I

Some of the most difficult engineering problems in recording involve the management of acoustical sound and audio levels. The crux of the problem is that our ears are simply incredibly good at hearing different loudnesses, particularly soft ones. It is extremely difficult to build equipment and create recordings that come close to matching the range of loudnesses our ears are capable of discerning.

Our so-called “threshold of hearing” is an extremely small magnitude: 2/10ths of one billionth of an atmospheric pressure (which is ca. 14 lbs. per square inch) – slightly louder than the sound of one air molecule bouncing into another! The loudest sound we will willingly tolerate (the “threshold of discomfort”, heh, heh) is a million times greater than that – 2 ten-thousandths of an atmospheric pressure. Our threshold of hearing, that first, lower level, is labeled 0 dB Sound Pressure Level (SPL) and the threshold of discomfort occurs at 120 dB SPL. This is the generally accepted dynamic range of our hearing, from too soft to hear up to pain. 120 decibels. A range of a million-to-one.

By way of comparison, music recording audio signals generally end up with a dynamic range of about 50 dB maximum. 300-to-one.

We are trying to fit a quart into a pint pot, it appears.

Happily, it turns out we can fit that quart into that pint pot pretty convincingly, so long as we exercise considerable craft and care.

For example, I made some noise level analyses of several different kinds of recordings. These measurements yield some really interesting information about the recording, including it’s maximum (Lmax) and minimum (Lmin) levels. First, I measured an extremely wide-range classical recording (Ravel’s Daphnis & Chloe, Slatkin, Telarc). This recording is about as dynamic as any recording gets, and yet the observed range was only 60 dB from Lmin to Lmax.

For comparison, I then made a noise level analysis of Michael Jackson’s Billie Jean. The range of level from Lmin to Lmax in the song is only about 10 dB.

Both of these are very well-crafted and engineered recordings. The engineers involved (Jack Renner and Bruce Swedian) both have a really good feel for what is going on with levels – in the air, at the microphone and through the mic preamp into the console. It shows in their results.

In this article, we’ll look at this first part of the recording process, from the air through the mic to the console. In a future article or two, we’ll take a look at level management during recording, mixing and playback.

Post a Comment