Introduction
Have you ever been in a noisy environment that has left you uneasy and grimacing? Have you covered your ears while walking past a noisy factory? If you have you must have wondered what makes some sound louder than others. If noise is a problem for you then you must take steps to measure the noise levels so that you could reduce them. Measuring noise levels was not an easy affair earlier, but now with sophisticated and automated meters, one can measure sound intensity and volume easily and with accuracy.
In this article, we will dive into understanding the unit of sound, which is the decibel, and get to know how a decibel scale (dB scale) works but before that let us understand how sound travels and what makes one sound louder than the other.
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Sound Waves and What Makes one Sound Louder than the Other
Sound is produced in a medium and caused by vibration. A vibrating guitar forces the air particles around it to compress and expand. This series of alternating compressions and rarefactions create a pressure difference and the disturbance travels through the particles of the medium transporting energy. The vibrating air particles then reach the listener and vibrate the small parts within the listener’s ears which produces sound.
The amount of energy being pumped by the source of the sound to the listener determines how loud the sound is. This energy is determined by the amplitude of the vibrating source (in this case the amplitude of the guitar). If you put more energy into plucking the guitar wires (i.e. the strings are displaced more), greater will be the amplitude of vibration.
How is Sound Measured?
The sound intensity or sound power is measured in decibels (dB) with the use of a device called a decibel meter. The decibel meter gives out readout by sampling and measuring the pressure of sound waves emanating from a source of the noise. A decibel meter is also called a sound-level meter and one can even access the sound meter app on smartphones. The name decibels is in honor of Alexander Graham Bell who invented the audiometer and the telephone and also devised the decibel scale first.
How Does the Decibel Scale Work?
A human ear is more versatile than you can think. With rising sound levels, the built-in mechanism of our ears reduces its own sensitivity. Our ears also have an exceptional range of handling different levels of sound power. A human ear can discern the dropping of a pin in close range as well as the roaring of a jet engine far away. Although we can make out if there is a rise in the number of pins dropping, we can not really distinguish if there are 1000 or 1001 pins dropping since our ears are not a linear device.
A decibel scale is a logarithmic scale and works differently than a ruler (which is a linear scale). On a ruler 10 cm is twice as long as 5 cm or 30 cm is thrice as long as 10 cm but on a decibel, scale levels go up in powers of 10. This means that if there is an increase of 10 decibels on the dB scale, it translates to a 10-fold increase in sound intensity and it corresponds to doubling in sound intensity as well.
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The lowest audible sound on a decibel scale is 0 DB. Here is a quick look at some of the everyday sounds we hear and how the noise level dB looks like for each of these sounds.
As per research, if one is exposed to noise level dB of 85 decibels for 8 continuous hours or 100 decibels for even 15 minutes, it could cause hearing damage.
Relationship of Decibel to Hertz
Hertz measures the amount or frequency of air pressure change by the vibration. Decibels measure the resulting change in air pressure which the vibrating object creates. This means that decibels measure the loudness or the power of sound while hertz measures sound’s frequency. We cannot convert from decibel to Hz and vice versa as they measure different components of sound waves.
Sound Level Meters
A sound level meter is a simple-looking device that has a pointy stick at the top which has a microphone that samples and measures sound. The body of the instrument is cut off from the microphone by the stick so that reflections do not perturb the readings and we can get more accurate results. The bottom of the meter has a square box that contains electronic circuits to measure the sound which the microphone detects. The detected sound is modified and filtered by this electronic circuit before the final readout is displayed in the LCD display.
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FAQs on Decibel Scale - Sound
1. What is the decibel (dB) scale used for in Physics?
The decibel (dB) scale is a logarithmic scale used to measure the intensity or loudness of a sound. It is designed to handle the vast range of sound pressures the human ear can perceive, from the faintest whisper to a loud jet engine. Instead of a linear scale, it compares the intensity of a sound to a reference level, which is the threshold of human hearing. This makes it a more manageable and intuitive way to represent sound loudness.
2. What are some examples of common sounds and their decibel levels?
The decibel scale helps us understand the loudness of everyday sounds. Here are some common examples:
0 dB: The threshold of hearing.
20 dB: A quiet whisper.
60 dB: Normal conversation.
85 dB: Heavy city traffic or a lawnmower.
110 dB: A rock concert or a power saw.
130 dB: A jet engine at 100 metres (pain threshold).
3. What are considered safe and unsafe decibel levels for human hearing?
Sounds below 85 decibels (dB) are generally considered safe for human ears without time limits. However, prolonged or repeated exposure to sounds at or above 85 dB can cause permanent hearing damage. Sounds above 120 dB can cause immediate harm and pain to your ears. Understanding these levels is crucial for preventing the negative effects of noise pollution on health.
4. How is the sound level in decibels calculated using the logarithmic formula?
The sound intensity level (β) in decibels is calculated using the formula: β = 10 log₁₀(I / I₀). In this formula:
β is the sound intensity level in decibels (dB).
I is the intensity of the sound wave being measured (in watts per square metre, W/m²).
I₀ is the reference intensity, which is the threshold of hearing (1 × 10⁻¹² W/m²). The sound intensity formula is based on this ratio.
5. Why is the decibel scale logarithmic and not linear?
The decibel scale is logarithmic because the human ear perceives loudness on a logarithmic basis. The range of sound intensities we can hear is enormous—from the quietest sound to the loudest is a factor of over a trillion. A linear scale would result in extremely large and impractical numbers. Logarithmic functions compress this vast range into a more manageable scale, typically from 0 to 140 dB, which aligns better with our perception of loudness.
6. What is the difference between loudness in decibels (dB) and pitch in hertz (Hz)?
This is a common point of confusion. Loudness and pitch are two different characteristics of sound.
Decibels (dB) measure loudness, which is related to the amplitude or intensity of a sound wave.
Hertz (Hz) measures pitch, which is determined by the frequency of the sound wave (how many vibrations per second).
A sound can be high-pitched but quiet, or low-pitched but loud. One cannot be converted into the other. For a detailed explanation, you can refer to loudness, pitch, and quality of sound.
7. How does doubling the sound intensity affect the decibel level?
Due to its logarithmic nature, doubling the sound intensity does not double the decibel level. When the sound intensity (I) is doubled, the decibel level increases by approximately 3 dB. This is because the change is calculated as 10 log₁₀(2), and the logarithm of 2 to the base 10 is approximately 0.3. This highlights how a small change in decibels can represent a significant change in sound energy. This is a key application of the properties of logarithms in physics.
8. What is the "threshold of hearing" and what does it correspond to on the decibel scale?
The threshold of hearing is the minimum sound intensity that a young, healthy human ear can detect in a very quiet environment. This intensity is internationally agreed to be 1 x 10⁻¹² watts per square metre (W/m²). On the decibel scale, this faintest audible sound serves as the reference point and is defined as 0 dB.
