The Decibel Scale

The decibel scale is perhaps the most misunderstood technical term in audio. Although the concept is actually very simple, the math can get quite complicated. The decibel allows us to quantify level changes with audio and communicate those changes to others.

It is not necessary to understand all of the math involved with the decibel. It is important, however, to understand some of the basic underlying principles of the decibel and how it relates to our real world experience with audio.

The following audio program explains in simple easy to understand terms for the decibel can be used in your music production work. The text below, outlines this audio program.

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What Can Be Measured By The Decibel Scale

The decibel scale can be used to measure audio in a variety of different ways. The decibel can be used to measure sound pressure level, sound intensity level, voltage level and power, just to name a few.

This provides a valuable reference tool for the audio engineer, technical engineer, and the audio equipment designer. Without a reference system, working with audio would become entirely a matter of individual perception and guesswork. The ability to quantify and communicate level changes of any kind would be very limited.

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Audio Is Logarithmic

The purpose of the decibel scale is to make a nonlinear a system linear. From a technical point of view the measurement of audio is logarithmic. What this means is that the numbers used to describe those measurements will vary greatly based on the starting point.

For example: if you define an audio voltage level as being the number 10, twice as loud as him would be defined as 100. If you start at 100, however, twice as loud would then become 1000. As you can see in the 1st example twice as loud is a difference of 90 whereas in the 2nd example twice as loud would be a difference of 900.

For practical purposes, logarithms are used to make these numbers linear so that they are not relative to the starting point. Thus, a linear way of describing audio becomes more practical to the audio engineer and equipment designer. This greatly simplifies our ability to describe our experience of levels and audio.

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dB What? (What Is The Reference)

In order to make the decibel system more practical for describing audio in its many forms, ratios are used to help simplify all of the math. The term dB is meaningless unless the reference is defined. The reference relates to what technical aspect you are actually measuring. Thus, a descriptive term will follow the dB in order to define that reference.

The term dBv, for example is used to describe voltage level with a reference of .775 volts. The term dBspl, is used to describe the sound pressure level with a reference of 20 micro pascals. Unless the person you are communicating with understands the reference implied, the number associated with the term dB is meaningless.

Sound pressure level (dBspl) for example, implies that twice as loud is defined as a 6 DB difference. By contrast, power (dBm) implies that 3 dB is considered twice as loud. Understanding this distinction is important.

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It's All About Perception

In practical reality the decibel system is not perfect. Because our auditory system is non-linear with respect to frequency response and listening level, our perception of audio does not equate exactly with the terms used to describe those differences.

Our perception of loudness is also colored by other content that may be occurring at the same time. For example, raising the level of a guitar track by 3 dBm in a mix may not yield the perception of twice as loud. The other audio tracks within that mix may serve to mask our ability to hear the track we are adjusting clearly.

For practical purposes, our perception of twice as loud is the study of many different listeners using selected listening volume and frequency content. The results of these studies show that 1 dBm is considered the “just noticeable difference” by the average listener and 3 dBm as twice the difference.

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The Practical Use Of The Decibel Scale

For practical purposes with audio engineering work, the reference scale most commonly used is dBm. The “m” uses 1 mV as a reference of power. Since most audio equipment settings uses this as the reference, a 3 dB difference is considered twice or half as loud.

If you are measuring sound pressure level or voltage level however, a 6 dB difference is twice or half as loud. Keep this in mind when when the discussion of audio levels is referred to. Understanding this simple rule of thumb should help you through most of your audio engineering work.

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Conclusion:

Working with the decibel scale is a necessary part of all your work with audio. Understanding what the decibel scale is and what it means in your practical experience with audio will help to make your communication and work much easier.

If you are not clear about what reference is being used, ask. If they do not know, most of the time they are referring to the dBm scale. Whether they know it or not…

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Anatomy of the Ear

Protecting Your Hearing

Temporary Threshold Shift

Critical Listening

Audio Ear Training

Physics of Sound

The Decibel Scale

Fletcher and Munson

Selective Hearing

Speed of Sound and Wavelength

Acoustical Phase

The Sound Envelope

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