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Dynamics

How Loud Is It?

By Cary B. Cornett
Jan. 29, 2010

The ability to know, understand, describe, measure, and control how loud things are is so fundamental to recording that without it we might as well not bother even to try. The term "Dynamics" refers to how loud thing are. It has both musical and technical aspects, and a good recording engineer is not only able to understand both, but also to relate the musical understanding to the technical description.


When musicians talk about dynamics, they start speaking Italian, using words like "crescendo" for getting gradually louder, "forte" for "loud" and "piano" for "soft". One important part of musical history was the effort to develop musical instruments that could be played either loudly or softly. In the early history of keyboard instruments, if you wanted to play for just a few people, in a drawing room for example, you used either a harpsichord or a clavichord. Neither of these instruments could be very loud. In fact, the player had no control at all of how loud the instrument was. If you wanted to play for hundreds of people, your choice was a pipe organ. All sorts of organ pipes can be made, some louder than others, but any particular organ pipe can only play at one volume, usually rather loud. In order to vary the volume of a particular group (called a "rank") of pipes, you have to put then inside a box, then put shutters on the box that can be opened or closed by moving a pedal.


For many years keyboard players wanted something smaller than a pipe organ that could play at a range of levels, and that could be loud enough to be heard by many people at once. The instrument that could either be played "forte" (loud) or "piano" (soft) would at first be called the "pianoforte", a name that eventually was shortened to "piano", the instrument that we still know and use today. It took many years to perfect this new instrument, and in fact its development required a major technological advance in steelmaking. What we now call "piano wire" had to be made much stronger than any wire before it in order to stand the high tension needed to enable it to produce loud sounds. Have you ever noticed the steel frame of a piano and how heavy it is? It has to withstand about 20 tons of tension to support the piano strings!


When, in the early 20th century, technicians wanted to describe and measure the loudness of sounds, they had to invent new words. Early telephone engineers developed a unit of loudness called the "Bel" (so named in honor of a certain inventor). If one sound was twice as loud as another, it was one "Bel" louder. A sound that was half as loud was one "Bel" softer. This unit turned out to be a bit large for most uses, so the engineers settled on the smaller "decibel", or one tenth of a Bel, as the standard unit for measuring and comparing volumes. We use the abbreviation "dB" for this unit.


One of the things that makes the decibel a bit confusing at first is the fact that it is not a linear unit. Decibels are, in fact, on what the math experts would call a "logarithmic" scale. I'm not going to explain logarithms here (worth knowing, but I'm trying to stay "on topic"), but I can give you a rough idea. A 10 dB increase in loudness makes something "twice as loud". A 10 dB increase in power means multiplying by a factor of 10. Put another way, making something twice as loud takes ten times the power. If you are using 10 watts of power, and need to make the sound twice as loud, you will need 100 watts of power. For 4 times as loud, you would then need 1000 watts (1 kW). A loud home stereo may have hundreds of watts of power, but for a big rock concert the sound systems easily run to tens of kilowatts.


The first time that I could actually attach numbers to what happens in a musical performance was when I was running sound for a local stage musical production. There were several singers, each wearing a wireless microphone. As I looked at the metering on my recording/PA rig, I discovered that it was not unusual for a singer to have a "dynamic range" of 30 dB during a performance. In other words, the loudest parts were about 30 dB louder than the softest parts of the performance. I would routinely see this amount of level change in a single song.


A recording engineer has to use an electronic system to capture a musical performance in a physical environment. Just as the performance has a dynamic range, so does the electronic system, and so, too, does the physical environment.


Any electronic sound system has limits to how loud or soft a level it can handle. As things get louder, there is a level where it simply runs out of power. When this happens, the system is overloaded. Peaks in level above the upper power limit are "clipped". Guitar players routinely overdrive their instrument amplifiers as part of the way they shape the sound of their instruments. In our recording and PA systems, though, we usually want to avoid this distortion. At the other end of its dynamic range, the system has a certain amount of internal noise, usually heard as a hiss, although sometimes hum and other interfering sounds may be present. For the most part, the dynamic range of a sound system is the range from "hiss" to "splat".


The good news here is that most decent recording and PA systems these days, when properly operated, have a wider dynamic range than either the performance or the environment.


The least understood kind of dynamic range is the limits of the environment. Actually, we have to include human hearing as part of this, because that is what sets the upper limit of dynamic range in the physical world. Sound in air is a series of pressure waves. Every physical object has a limit to how much pressure it can survive. The most dramatic example of this is when a bomb explodes. The resulting pressure waves can destroy buildings and kill people. So, there is an upper limit to the pressure that you can tolerate. There is also an upper limit to the pressure that your ears can tolerate. This upper limit is sometimes called either the "threshold of feeling" or the "threshold of pain" depending on whose sound level chart you look at.


There are charts that show the "sound pressure level" of various common sounds. For our purposes, the upper limit is the "threshold of pain" at about 130 dB spl. Sound levels in a rock concert may go above 120 dB. On the same scale, the "threshold of hearing" is 0 dB. So, the maximum dynamic range of human hearing, in theory, is about 130 dB.


We may sometimes record sounds that are louder than that, such as when we put a microphone within inches of a drum (where we would never actually put our ears except maybe when tuning the drum). At levels like that, we are usually not much worried about the "noise floor" of either the electronics or of the room. We often, though, record things that are much less loud, where noise levels may become important.


Most of us have never been in a room that is so quiet that we cannot hear any noise in it. Even an empty theater is never truly silent. One chart shows a expect sound level of just under 20 dB for a "quiet theater", and just over 20 dB for a "quiet whisper". Add an audience, or even a bunch of performers sitting still, and the noise level goes up. Just for fun, I took a look at a recent choir concert recording I had made, comparing the highest peak level with the lowest average background noise level, and found the difference to be about 65 dB. If I looked only at the range of average levels (more the way we hear), the difference was only about 55 dB.


The dynamic range of most popular music recordings is far less (not counting pauses between songs). Some of this is due to the nature of the songs and their musical arrangements, but the amount of dynamics processing, particularly compression and limiting, that is commonly used during recording, mixing, and finally mastering, often almost completely removes any remaining dynamics from the music. There has been a lot of discussion about this in the professional recording and mastering community, and I will not cover that discussion here, except to say that there are many who would like to see somewhat less dynamics processing used so that the music can have more dynamic impact.

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