What Is Noise and How Is It Measured?
Noise is a term that is used to describe unwanted sounds. Noise can disrupt the environment, impact wildlife, make working and sleeping difficult or even damage the human ear. Noise pollution is a significant problem, especially in urbanized settings.
What sounds constitute noise may be different for everybody. Different people have different perceptions of what sounds they like and what sounds they don’t like. For instance, some people enjoy the distinct sounds of a bagpipe while others wish the bag would pop. However, some things like a sound from garbage trucks, freeway traffic, or dogs barking are almost universally considered to be noise.
What is Sound?
Noise, like all other sounds, is a form of acoustic energy. This energy is manifested in the form of vibrations that oscillate through the air (or any other medium), traveling from the source of the sound to your ear. Sounds change as the amplitude, frequency, and wavelength of the energy wave changes. Here are some definitions to help you understand these terms:
- Frequency - The number of wave cycles that occur in a given period of time.
- Amplitude - The maximum extent of a wave, measured from the position of equilibrium.
- Wavelength - The distance between successive crests of a wave.
How are Sound/Noise Levels Measured?
The loudness of a sound also changes when the magnitude of the acoustic energy wave changes. The magnitude of the energy wave is basically the amount of energy that the sound has. Think about the magnitude of sound or noise like a volume control button on your smartphone. The greater the volume, the higher the magnitude, and the louder the sound. Sound magnitude is measured in a unit called the decibel which is abbreviated dB.
However, since the human ear cannot hear all frequencies equally, the decibel measurement system has to be "scaled" or "weighted" to account for the fact that human ears are less sensitive to low sound frequencies. If a linear scale (like a ruler) was used to measure all sounds that could be heard by the human ear, most sounds would only occur within the lower 1% of the total range of possible human hearing. With a non-weighted form of measurement, it would be very difficult to show differences in sound levels between various noises.
Instead of a linear scale, a logarithmic scale is used to represent sound levels instead. In this case, the standard decibel measurement is "A" weighted to account for human hearing. "A" weighted measurements are abbreviated as dBA instead of just dB. One of the interesting things about the logarithmic scale is that a 10 dBA increase in the sound level is actually equivalent to a doubling of the magnitude. In other words, a 10 dBA increase means that the sound is twice as loud.
Furthermore, if you combine sounds of the same loudness, the total dBA increase in magnitude is only 3. For example, two speakers that each output 50 dBA of sound would only output a combined total of 53 dBA if placed side-by-side.
Sound Magnitude and Human Hearing
In terms of sounds magnitude, the most agreed upon range of the human ear's ability to hear ranges from 0dBA to 140dBA. The low volume threshold of human hearing is set at 0dBA. For this value, 0dBA technically means that there is no perceptible sound level. Note that this is not the same as having no sound at all. Perhaps ironically, it's almost impossible to be in a location where there is 0dBA being output. A quiet room may have sounds or noises that measure in the 20-50 dBA range. Even in a recording studio, known for being a very quiet place, the "soundproof" rooms would still measure about 10-20 dBA of sound.
As a general rule of thumb, humans can only distinguish between two sound levels that are at least 3 dBA different in loudness. Sounds levels that have less than 3 dBA difference cannot usually be discerned.
Normal everyday sounds may be expected to occur in the 50 to 80 dBA range. For instance, a typical active home or office may have sound levels between 50 and 60 dBA. Someone watching TV comfortably would be expected to have sound levels around 60 dBA but perhaps no louder than 70 dBA. Once sound levels reach 80 dBA, hearing impairment can result if the ears are exposed to these loud sounds for long periods of time.
On the other end of the spectrum, you have particularly loud sounds to contend with. For instance, a jackhammer may output around 120 dBA of sound energy. This is enough to damage the ear and induce pain. At 130 dBA, this amount of sound is often cited as the "pain threshold." And finally, at 140 dBA, serious hearing damage can occur that could result in permanent hearing loss.
The Quietest Place on Earth
We live in a world full of noises and sounds. This is just a part of our lives. At the end of a hard day of work, a lot of people come home hoping to relax while getting some "peace and quiet." However, as I mentioned earlier, most "quiet" places still have some ambient background sounds or noise. If you are really looking for a quiet place, I invite you to check out the Anechoic Chamber at Orfield Labs in Minneapolis.
This specially designed room is so quiet that it actually drives some people crazy because you may start to hear the inner workings of your body in your ear (among other things). In addition, it's been said that the lack of sound in the room confuses the human brain as well. Some people who spend too much time in the room start having auditory hallucinations.
The sound level in the room has been measure at a record -9.4 dBA which is quieter than what the human ear can percieve. Check out this brief video about the effects of total silence as well as the Orfield Labs' Anechoic Chamber.
What is the Range of Human Hearing?
In addition to magnitude, the frequency of sounds that we can hear also lies within a specific range. A person with typical hearing ability can hear sound frequencies between 20 Hz and 20,000 Hz. This is known as 20-20 hearing. However, people tend to lose their hearing ability as they age. An older person may not be able to hear frequencies above about 5,000 Hz.
Other Ways to Measure Sounds
In addition to the decibel and the "A" weighted decibel scale, another useful measurement for assessing noise or sound levels is the Equivalent Sound Level or Leq. The Leq is the energy-based average of the sound energy over a specified time period. What this measurement does is that it represents all of the sounds that occur in a specific location over a period of time with a singule, average value.
In some ways, the Leq measurement is a more effective way to assess noise pollution. For instance, we have long understood that long exposure to high sound levels can do more damage to the human ear than a single very loud noise that occurs over a very short period of time would. This is the case even if the dBA value of the short noise is significantly louder than the long-term sound levels.
Another reason that Leq is a good measurement of sound levels is that it allows for a better comparison of noise pollution levels between locations. Since they representation average or typical conditions, Leq measurements can be used to help identify noise pollution problem areas as well.
© 2018 Christopher Wanamaker