Since sound waves are fluctuations in air pressure, it follows that the down side of the pressure wave, if the sound is loud enough, will reach 0 PSI and at that point the wave breaks down and releases its energy in another way, heat for example, or even light.
So what is the loudest sound? Well, you have to first know how sound is measured. Our perception of sound pressure is logarithmic, which means our ears hear sound in a non-linear fashion. The louder the sound, the more of it is required to cause a perceptible difference.
For that reason, sound pressure level (SPL) is reported using a log scale and the unit is the Decibel (dB). In this scale, and this is very important, every doubling in sound pressure equates to a 6 dB increase in measured SPL.
Ok, now. Here are some examples of everyday sounds and their equivalent Decibel rating:
Soft whisper: 30 dB
Refrigerator: 40 dB
Normal conversation: 50 dB
Noisy restaurant: 70 dB
Vacuum cleaner: 75 dB
Blow dryer: 80 dB
Roar of crowd at sporting event: 90 dB
Leaf blower: 102 dB
Rock concert: 120 dB
Thunderclap, very close: 120 dB
Jet engine at take-off, measured near the exhaust nozzle: 140 dB
As you can see, a leaf blower is 2 x 6 dB louder than the crowd at a sporting event, which means it's two doublings, or FOUR TIMES louder. Keep that in mind when I show you how high it goes.
As you know, a jet engine is very loud up close, loud enough to cause permanent hearing loss. You are never exposed to that level of sound because the airports are arranged such that the plane is far from people and oriented to minimize sound levels when the engines are spooled up for takeoff.
There are much louder sounds than a jet engine.
The first (lift-off) stage of the Saturn V rocket that went to the moon was powered by five huge F-1 engines burning kerosene and liquid oxygen at the rate of 15 tons per second. The engines generated an astounding 180 million horsepower. So what does 180 million horsepower sound like? The thermal reactions created SPLs that measured 180 dB at the closest distance possible without damaging the specialized microphones, about 100 feet away. Since SPL drops dramatically with distance, the actual sound at the nozzle was much louder and probably exceeded the wave breakdown point. An SPL of 180 dB is crushing...almost 7 times as loud as a jet engine at take-off. Sound at this level can be fatal. Your hearing apparatus would be instantly and permanently destroyed. In fact, this sound can cause serious structural damage to concrete and steel. The thrust exit guides in the launch pad had to be carefully designed to channel the sound energy away from the structure because a direct hit with 180 dB sound would have destroyed the launch pad. The viewing platform was three miles away and the sound at that distance was still loud enough to make ears bleed.
Now, we're ready to talk about the loudest sound. If you assume the sound wave is a perfect sine wave, the loudest sound would correspond to a low-side pressure drop of 14.7 PSI (our average atmospheric pressure) which amounts to an SPL of 194 dB. The loudest theoretical sound.
But since we had to make two assumptions above (sinusoidal waveform and 14.7 PSI atmospheric pressure), we can see that there are cases this maximum can be exceeded. A higher pressure environment for example, or sound waves that are formed differently can give a higher number.
For that reason sounds have been measured up to slightly north of 200 dB. Remember that a 200 dB sound is twice as loud as a 194 dB sound (+6 dB).
Examples of natural phenomena that can create 200 dB sounds are volcanic eruptions and atomic explosions. The eruption of Krakatoa in 1883 was heard 3,000 miles away. Just imagine how loud it was on Java or Sumatra. And measuring equipment at the site of the first A-bomb test actually recorded the moment of silence after the blast caused by sound too loud to form coherent waves. This soon resolved into 200+ dB sounds as the levels dropped into the wave-formation-friendly zone.
No comments:
Post a Comment