The unit for expressing the loudness of sound is called dB (decibel).
How is this dB defined?
Actually, the dB represents the sound pressure level that takes into account the human senses.
Let's take a closer look at sound pressure (dB), the loudness of sound.
There are various units to express the loudness of sound
You may be wondering what I am talking about when I say that dB (decibel) is a unit to express the loudness of sound. There are meters, miles, yards, etc. There is more than one unit of measurement for the volume of sound. There are two types of units used to express the volume of sound: intensity and frequency. You may have heard of frequency before.
Sound travels through air. Therefore, the atmospheric pressure changes when sound is present or absent at a given instant at a point. This change in air pressure is called sound pressure.
The unit of sound pressure is usually called Pa (pascal). Do you ever see news reports such as "This typhoon is strong because the pressure at the center is 970 hPa (hectopascal)"? Do you ever see news reports like that? This is Pascal.
Pa can be converted to \(Pa=\frac{N}{m^2}\).
This means that a force of 1N is applied to 1 square meter. Furthermore, \(1\frac{N}{m^2}\) is equal to 10μbar (microbar). Well, you don't need to know about bar because it is not in the SI unit system. As shown above, there are various units that express sound pressure, which is the loudness of sound, such as Pa, \(\frac{N}{m^2}\), bar, and so on.
Relationship between dB and other sound pressure units
Now we know that Pa and \(\frac{N}{m^2}\) are units for sound pressure.
But what about dB? You may be asking yourself, "What about dB?
I realized while I was writing that I didn't explain about dB. Pa is a value calculated based on physical quantities in nature. On the other hand, dB, which represents sound pressure level, is a physical quantity that takes human sensation into consideration. Human sensory quantities have the property of being proportional to the logarithm of physical quantities. Therefore, dB is the logarithm of sound pressure Pa. The formula is \(L=20\log_{10}\frac{P}{P_0}\)[dB].
In this case, \(P_0\) is called the reference sound pressure value, and 1 kHz is the lowest audible sound \(2×10^{-5}\) Pa. It all gets a bit confusing at once! Just think of the physical unit as dB, and you'll be fine!
Sound loudness is not determined by sound pressure alone
I have explained that dB is a unit to express the loudness of sound.
In fact, however, the loudness of sound is not determined by sound pressure alone. Since it is a sensory value, it is also affected by frequency and waveform. Even if the sound is the same loudness, if the frequency is different, the sound pressure level as a physical quantity will be different. It is said that we humans can perceive sounds from 20 Hz to 20 kHz.
However, not all frequencies can be perceived in the same way. For example, Pascal, when you grow up, it is said that you can't hear the mosquito sound. Physically, the sound is certainly there, and even if it is sounding at, say, 80 dB, you are not aware that it is sounding. Thus, there is a difference between hearing and not hearing a sound as a sensation. It is quite difficult to understand, isn't it?
TV volume is not a unit
We often hear that our grandparents watch TV with the volume set at 30 and that it is too loud. Unfortunately, the TV volume here is not based on any standard. There are many things that are called volume, such as TVs and music players. However, these are values set by the manufacturer, and there is no standardization in terms of how many dB of sound pressure is increased when the volume is increased by 1 dB. Therefore, it may happen that you can hear the sound at a volume of 30 on this TV set, but not at 40 on that TV set. It is important to understand that the volume is only a guide and does not represent the loudness of the sound.
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