The color of noise

Kevin Farrell, Principal Engineer, Computational Simulation & Validation

The arrival of spring in many areas ushers in a welcome variation of color to the brown and gray shades of winter. Colors can lift our spirits from any winter doldrums. Surprisingly, one medium that also offers a spectrum of color—even year round—is noise.

Both music and noise are airborne vibrations, but while listening to our favorite music is pleasing to our ears, we generally perceive noise as unpleasant and disruptive. Noise color refers to the intensity of sound energy across the frequency spectrum, shown in Figure 1.

The color of noise refers to the relationship of sound intensity to frequency
Figure 1. The color of noise refers to the relationship of sound intensity to frequency

Several noise colors offer some actual benefits while others can simply irritate.

  • White noise refers to sound energy that is equally distributed across all audible frequencies. White noise is that steady hum emanating from a fan or window air conditioner. Because it masks the effect of loud sounds that can interrupt sleep, white noise is often recommended to alleviate sleep disorders like insomnia.
  • Pink noise may also improve your sleep. However, pink noise is more intense at lower frequencies, adding a dominant bass rumble. Natural sources of pink noise are a steady rain or wind. Smartphone apps advertised as noise sleep aids allow the user to choose different white or pink noise sources to foster relaxation.
  • Red or brown noise has an even greater proportion of energy at lower frequencies than pink noise. Examples are roaring sounds like strong waterfalls or thunder. Although research is not definitive, brown noise is also thought to help with sleep and relaxation [1].
  • Blue noise generates significant energy at the high-frequency end. It can sound rather harsh, so any sleep sound app likely does not include it. However, sound engineers often use the high frequencies of blue noise in a process called dithering to smooth out the sound of a recording [2].
  • Black noise may be your favorite if you live with really active children or teenagers in your house. Black noise refers to the absence of sound or to silence rarely interrupted by random noise. For many people, silence is the preferred environment for deep sleep.

So what color is acoustic vibration, the noise that may occur in your heat exchanger? Actually, this sound vibration is not very colorful at all—and frequently it is quite unpleasant. No plant operator has ever said that they enjoyed listening to their large gas heat exchanger belting out Beethoven’s Fifth, or that the noise was so relaxing that they would fall asleep just walking by a vibrating exchanger.

Unfortunately, heat exchanger noise is rich in tones and loud, which makes it physiologically annoying. The intensity spectra looks like a picket fence, as shown in Figure 2, because the tones occur at harmonics or integer multiples of the fundamental frequency.

A typical noise intensity spectrum radiating from a heat exchanger plagued with acoustic vibration
Figure 2. A typical noise intensity spectrum radiating from a heat exchanger plagued with acoustic vibration

So if I had to pick a color to represent this noise, I would pick neon orange, the universal warning color!

References

  1. K. Nunez, What is pink noise and how does it compare with other sonic hues?, Internet, https://www.healthline.com/health/pink-noise-sleep (19 April 2020).
  2. A. Capritto, The secret to better sleep: Pink, blue and brown noise, Internet, https://www.cnet.com/news/white-noise-pink-noise-blue-noise-brown-noise/ (19 April 2020).