This may be remembered as a low point in science reporting. The original article is solid science and fascinating in its own right, but somewhere between the author’s pen and the mainstream press, something went badly off the rails. Let’s restrict ourselves to just one issue: frequency and period of waves.
There are essentially two types of waves: longitudinal (or compression) and transverse. Sound and mechanical shock waves travel as the former, whereas electromagnetic (EM) waves, such as light, x-rays, microwaves, radio, etc, travel at light speed as two intersecting transverse waves, one magnetic, the other electric. There are two main types of seismic (earthquake) waves, one of them longitudinal and the other transverse. Everything that follows applies to all waves.
The frequency of a wave tells us how many times per second something is occurring. For example, if you strike the “A” key above Middle-C on the piano, you hear a dominant tone of 440 Hz (Hertz). That means that the air in your ear is vibrating (compressing) 440 times per second. Lower frequencies have a lower pitch, and visa-versa. FM radio waves – which are not sound waves – are vibrating or oscillating at tens of millions of times per second.
The word “Period” has a specific meaning here, which was lost on the science reporters. For those with a little physics, recall that T=1/f; that is, the period of a wave is the reciprocal of its frequency. In simple language, the period of a wave is the time between wave vibrations. In the above example with the “A” note on the piano, the frequency (f) is 440 Hz. The period (T) is 1/440 = 0.0023 seconds. That does not mean the wave lasts that long, it means that a wave compression hits your ear every 0.0023 seconds.
Now that we have covered that, look at the reporting of the article. The reporters’ claim was that seismic waves “last from 15 to 300 seconds”. That is not what the authors said – they said that the period of the waves varied from 15 to 300 seconds. For those who have a little math background, you’ll notice that we can rearrange the formula to be f=1/T. So in the case of these seismic waves, the frequency of these waves vary from 0.0033 Hz up to 0.066 Hz. The human auditory system begins recognizing vibrations as tones or hums at around 20 Hz.
The Hum has not been solved. But we are getting closer, and I intend to get to the bottom of it despite the bad reporting.