Saved by the Bel
You have probably heard the word decibel used before you ever got into train horns. Most people know that decibels are a measurement of sound, but do you know how it’s measured and why?
The term decibel actually consists of two words: “Deci”- meaning ten and “Bel” named after the inventor of the telephone. Alexander Graham Bell (decibels are a relatively new unit of measurement, originally used to calculate the signal loss in telegraphs and telephone circuits). So, the base measurement of volume is a bel and decibel is a bel to the 10th power. This is key to understanding how decibels are measured because each decibel is ten times louder than a previous decibel. This means that the relationship between two decibels will increase as the decibel level goes up. For example, 10 decibels is 10 times louder than zero, but 20 decibels are 100 times louder than zero. Scientists and mathematicians call this a logarithmic scale.
Decibels can be measured by a device called a Decibel Meter or a Sound Level Meter. Sound waves create pressure in the atmosphere that can be measured. The louder the sound, the more pressure that it will create. The Decibel Meter is a small device with a microphone on the end that picks up the changes in pressure and reads off on a display. There are even some phone apps out there that claim to be Decibel Meters, but the accuracy for these apps are heavily debated.
It’s no secret that distance from the source of the sound with change the volume that you are hearing it at. One of the problems with measuring the decibel level is there is no standard distance to test at. Say you have two companies selling a “150dB” horn. If the first company test their horn at 3 feet away and the other test at 100 feet away, they are going to have different readings. When you see a company advertise their decibel levels, do they tell you what the distance was? The only truly accurate way to test is by testing all the horns at the same distance.