MEASURING NOISE WITH AN DECIBEL METER
Updated: Jan 23, 2022
A decibel meter, or sound level meter, is an electronic gadget that measures the level of sound or noise. In a previous post, we looked at a device that allows you to measure the energy of electromagnetic waves. In the same way, sound waves moving in the air carry energy, and we can measure them with a decibel meter. Like electromagnetic waves, we are surrounded by sound waves almost everywhere and all the time, unless you are somewhere in a very exotic place. We are already so used to sounds and noise that complete silence can actually scare us. According to the Guinness Book of Records, the quietest room in the world is located in the Orfield Laboratories in Minneapolis, and the longest time spent in that room is 45 minutes. On the other hand, no one probably wants to live in the midst of loud noise, as it can have devastating effects on both physical and mental health. In addition to light pollution, noise pollution is very common in today's world. Just as it is difficult to find a place where you can clearly see the starry sky, it is increasingly difficult to find a place where it would be pleasantly quiet. Before we look at how the decibel meter evaluates the sound level and what is exactly loud and what is quiet in decibels, we first look at what is a sound wave and its main characteristics.
A sound wave is a kind of traveling disturbance in some medium, like in the air, water, or metal. However, unlike an electromagnetic wave, a sound wave cannot occur in a vacuum, because there are no particles in the vacuum that can transmit the oscillations. Any vibrating object can cause air or some other medium to vibrate and create sound waves. The most obvious example is probably the loudspeaker with a vibrating diaphragm that produces sound. When the diaphragm pushes against the air, it creates a region of higher pressure in the air, while when the diaphragm moves back, it creates a region of low pressure in the air.
The two most important characteristics of a sound wave are its frequency and amplitude. The frequency of the sound wave shows how many times the sound wave repeats itself in one second. The unit of measurement for frequency is hertz (Hz). When the diaphragm of our speaker oscillates back and forth exactly once a second, it causes condensations and rarefactions in the air, which causes the air particles to oscillate at a frequency of 1 Hz. The higher the oscillation frequency of the sound source, the higher the pitch. The human ear can hear a fairly large frequency range from 20 Hz to 20 000 Hz, although as we age, we can no longer hear the higher frequencies so well.
The volume or loudness of the sound is directly related to the amplitude of the sound wave. The higher the sound wave pressure, the louder the sound we hear. As the diaphragm of the loudspeaker moves further, it creates high pressure, and we hear a louder sound. However, if the diaphragm moves only slightly, the pressure on the air is lower and we hear a quiet sound.
The top image has two speakers, that produce sound waves with the same frequency, but different amplitudes. The left speaker produces a quieter sound and the right speaker produces a louder sound. The graphs show how the air pressure changes over time as the sound wave propagates. Although people may perceive the volume of sound very differently, depending on how good their hearing is, then the decibel meters objectively evaluate sound levels by measuring the pressure, caused by the sound waves.
The decibel meter or sound level meter is equipped with a microphone, whose diaphragm responds to the air pressure generated by sound waves. Changes in air pressure are converted into electrical signals, and measurement results are displayed in decibels (dB).
Like any other wave, the sound wave carries energy. Obviously, the sound wave, caused by whispering carries significantly less energy than the sound wave generated by a flying aircraft. The energy transmitted by a sound wave per unit of time is measured in watts, and we can call it the power of the sound wave. As you can imagine, a sound wave expands spherically as it leaves the sound source, and the farther it moves from the sound source, the lower its energy, as its power is distributed over a larger area. If we know the power of the sound wave (P), we can find the intensity (I) of the sound wave at a certain distance (r) from the sound source.
Let's say, for example, that the power of a sound wave generated by a loudspeaker is 5 watts.
This means that at a distance of 1 meter from the loudspeaker, the intensity of this sound wave is 0.4 wats per square meter ( the total power of a sound wave is divided by the area of a sphere if the radius of that sphere is 1 m), which is actually a very large number and corresponds to 116 dB, but not to 46.8 dB as can be seen from the illustration above :)
Because the human air is very sensitive and able to detect sound waves with a wide range of intensities, we use a logarithmic scale to assess how loud or soft sound is in decibels. The human ear is still able to detect a sound wave with an intensity of about 0.000000000012 W/m2. We use this intensity as a standard value to estimate in decibels how much louder or softer sound is.
Let's say you're somewhere in a noisy factory, where the sound intensity is about 0.01 W/m2. The sound intensity in decibels can find as follows:
Where I is the intensity of the sound to be measured, Io is the intensity of the sound that the person is still able to hear (0.000000000012 W/m2) and symbol log stands for the logarithm to the base 10 (you can find it on the calculator). So, if the sound intensity 0.000000000012 W/m2 is the lower limit, we can still hear and which corresponds to 0 dB on a logarithmic decibel scale, then the sound intensity 0.01 W/m2 in some noisy factory is already 10000000000 times larger and corresponds to 100 dB on a decibel scale. As we can see, on the decibel scale the difference between near-total silence, which is 0 dB, and a sound 10 times more powerful is 10 dB, the difference between near-total silence and a sound 100 times more powerful is 20 dB, etc.
Most decibel meters generally measure over a range of 30 - 130 dB. It's usually written in dBA instead of dB, which means that the decibel meter is A-weighted. Since human hearing is not very sensitive at low and high frequencies, A-weighting corrects the measured sound pressure level readings.
The image below shows some of the noise sources we are familiar with and the noise levels they produce in decibels. Noise above 70 dB can already be dangerous. The higher the noise level, the less time person should be in this noisy environment. A noise level of 150 dB is already sufficient to burst eardrums and an extremely loud noise of 180 dB to 200 dB can be fatal.