How To Understand The Unit Of Decibels (Simple explanation)

The decibel unit is fundamental to understand for anyone working in audio, physics or acoustics. 

A decibel (dB) is a unit used to measure the intensity or power level of sound or signal. It represents the logarithmic ratio between the measured value and a reference value, typically the threshold of human hearing. 

The decibel scale allows for convenient comparison of different sound levels, with each increase of 10 dB representing a tenfold increase in intensity.

Understanding decibels, and more specifically, the decibel scale, can be tricky at first as the scale is logarithmic, which can be difficult for many to understand. 

However, once unravelled, understanding decibels will help you understand how sound works and how it subtly influences our everyday lives. 

In this article, I will explain the unit of decibel in a practical way so you can fully comprehend the unit of sound. 

What Are Decibels?

Decibels, often abbreviated as dB, are a logarithmic unit of measurement used to express the ratio of two values of a physical quantity, most commonly in acoustics for sound. 

The term ‘decibel’ is derived from ‘bel,’ named in honour of Alexander Graham Bell, a prominent inventor and scientist known for his work in telecommunication. 

The prefix ‘deci-‘ denotes a tenth part, making a decibel one-tenth of a bel. Being a relative unit of measure, decibels compare the level of an observed quantity to a reference level. 

In the realm of sound, this reference level is typically the standard threshold of hearing for the average human ear, often denoted as 0 dB.

What Is A Logarithmic Scale?

Before answering what the decibel scale is, it is important to understand what a logarithmic scale is. 

A logarithmic scale, often just referred to as a log scale, is a way of displaying numerical data over a very wide range of values in a compact way. Unlike the linear scale, where equal distances reflect equal changes in quantity, a logarithmic scale shows that equal distances reflect equal ratios of change rather than equal differences in quantities. 

This type of scale is extremely useful when the values to be plotted cover a large range. 

In the case of decibels, the logarithmic scale allows us to represent the vast range of human hearing, from the quietest whisper to the roar of a jet engine, in a compact and manageable way.

What Is The Decibel Scale?

The decibel scale is a logarithmic scale used to quantify the intensity of a sound or the power level of an electrical signal by comparing it to a given level on a logarithmic scale. This scale is a convenient tool because it can represent very large or small values in a manageable format. 

For instance, our human ears can perceive sounds that range over twelve orders of magnitude in pressure changes – a task daunting to comprehend if we didn’t have the decibel scale. In essence, the decibel scale allows us to comprehend and compare the vast range of sound intensities our ears can detect, from the quietest whisper to the roar of a jet engine.

How Do You Understand The Decibel Scale?

Understanding the decibel scale involves acknowledging the logarithmic nature of the scale. Each increment of 10 dB on the scale represents a tenfold increase in the intensity of the sound. 

To illustrate, a 20 dB sound is 10 times more intense than a 10 dB sound, and a 30 dB sound is 100 times more intense than a 10 dB sound. 

Furthermore, an increase of about 3 dB is typically perceived by the human ear as a doubling of the sound intensity, while a 10 dB increase is perceived as a doubling of the loudness. 

In essence, the decibel scale enables us to measure a vast range of sound intensities in a way that reflects how we actually perceive sound.

How Are Decibels Measured?

Decibels are measured using sound level meters, devices that capture the intensity of a sound at a given moment. These devices consist of a microphone to capture sound, a converter to transform the sound into electrical signals, and a display to show the measurement in decibels. 

The measurement process involves capturing the sound with the microphone, converting the sound waves into electrical signals, and then using a logarithmic formula to convert this into a reading on the decibel scale. 

The measurement is often taken several times and averaged to ensure accuracy. It’s worth noting that as decibels are a relative measure, the readings are typically compared to a reference level, which is usually the quietest sound that the average human ear can hear (0 dB).

What Does 20 Decibels Sound Like?

To give you a practical idea, a sound that measures 20 decibels (dB) on the decibel scale is relatively quiet, akin to the rustling of leaves in a slight breeze or the gentle whisper of someone nearby. It’s more audible than the softest sounds our ears can detect (0 dB) but still quite mild. It’s important to remember that these comparisons can vary from person to person due to individual differences in hearing ability.

Examples Of Sounds From 10dB To 140dB

To better understand the decibel scale, here are some common sounds and their typical decibel levels:

  • 10 dB: A light breath or rustling leaves. This level of sound is barely audible to most people.
  • 20 dB: A whisper or quiet library. These are still very quiet sounds that would not interfere with concentration.
  • 40 dB: A quiet residential area or a quiet office. This is considered a comfortable sound level for most people.
  • 60 dB: Normal conversation or a dishwasher. This is a moderate level of noise.
  • 80 dB: City traffic or a garbage disposal. Long exposure to this level of sound can damage hearing.
  • 100 dB: Subway train or a motorcycle. This level of noise can cause discomfort and potential hearing damage.
  • 120 dB: A loud rock concert or a thunderclap. Prolonged exposure can cause immediate harm to your hearing.
  • 140 dB: Jet engine at takeoff or a firearm shot. This level is usually painful and can result in permanent hearing damage. 

These are average levels and can vary based on distance from the source and the specific conditions. Remember, prolonged exposure to sounds above 85 dB can cause hearing damage, and sounds above 120 dB can cause immediate harm. It’s always a good idea to protect your ears when exposed to loud noises.

How Many dB Is Considered Loud?

The perception of loudness can vary from person to person, but generally, sounds that measure 85 dB or higher are considered loud. 

Prolonged or repeated exposure to noises at or above this decibel level can lead to hearing damage. For instance, a city street at rush hour might register at 85 dB, while a chainsaw or loud music concert could reach levels of 100-120 dB. 

Sounds above 120 dB, such as a jet engine at takeoff or a firearm shot, are not just loud but potentially harmful, causing immediate pain and possibly permanent hearing damage. 

As a rule of thumb, if you need to raise your voice to be heard by someone at arm’s length, the noise is probably in the region that could cause damage with prolonged exposure.

Final Thoughts

The decibel scale is an indispensable tool for understanding and quantifying the vast range of sound intensities we encounter in our daily lives. It enables us to fathom and appreciate the remarkable capabilities of our auditory system, which can discern variations from the soft rustling of leaves to the thunderous roar of a jet engine. 

Importantly, understanding decibels empowers us to be vigilant about our aural environment and protect our precious hearing from potentially damaging noise levels. 

For more information on what sounds are considered dangerous in the working environment, as the length of exposure to loud sound is just as damaging as a single loud sound, I recommend checking out the Control Of Noise At Work Regulations. [source]

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