Examples with distance
Sound Pressure Level (dB re 20 µPa)
1883 Krakatoa eruption ...........................310 (N) 1908 Tunguska comet explosion................... 300 (N) Treshold of irreparable damage; Jet 50m away ......... 140 Treshold of pain ................................130–140 Treshold of discomfort; Rock concert ................. 120 Disco, 1m from speaker; Power lawnmower at 1m ....... 100 Hearing damage from long-term exposure............... 90 Diesel truck, 10m away ................................ 80 Kerbside of busy road, 5m.............................. 80 Office environment ................................... 60 Average home ........................................ 50 Quiet bedroom at night ............................... 30 Whisper at 1m ....................................... 20 Quiet rustling leaves; calm human breathing ............ 10 Treshold of hearing (undamaged human ears) ........... 0
Table 3.1: Examples of sound pressure levels in decibels of sounds in our environment. The pain threshold for humans is 130–140 dB. Any sound above 85 dB can cause hearing loss. The loss is related both to the power of the sound as well as the length of exposure. ‘N’ represents a dB level representative of the object, and not necessarily what the listener would experience at a distance of 1m. The decibel (dB) is the unit used to measure the intensity of a sound. The decibel scale is a little odd because the human ear is incredibly sensitive. Your ears can hear everything from your fingertip brushing lightly over your skin to a loud jet engine. In terms of power, the sound of the jet engine is about 1,000,000,000,000 times more powerful than the smallest audible sound. That’s a big difference! On the decibel scale, the smallest audible sound (near total silence) is 0 dB. A sound 10 times more powerful is 10 dB. A sound 100 times more powerful than near total silence is 20 dB, and so on.
shook in the wake of the blast, which was estimated to be equivalent to 200 megatons of TNT – about four times the yield of the Tsar Bomba, the largest nuclear device ever detonated, in the Novaya Zemlya archipelago in 1961. Shock waves from Krakatoa echoed around the world 36 times and lasted for about a month. Te bang was heard 4,653 km away on Ro- driguez Island in the Indian Ocean. Literature suggests that the sound level was around 180 dB at 161 km (100 miles). Assuming that the sound level falls off as the inverse of distance, the decibel level on Rodriguez Island would still be 150!
3.7.2 Sound in Water
Great care must be taken when comparing sound levels in water with sound levels in air; 100 dB in water is not the same as 100 dB in air. Two complications must be addressed. Firstly, researchers studying sound in water and air use a different reference pressure. In water, the common reference is 1 µPa, instead of 20 µPa as in air. Tese two references are, therefore, 26 dB apart (20 log 20 = 26). Secondly, the impedances (density times velocity) of water
and air differ. Te impedance ratio is 1,540,000/415, or 3,711. As a result, similar sound intensities (measured in watts per square metre) in water and air will give a pressure that in water is the square root of the impedance ratio lower, or 61 times lower, than that in air, corresponding to a difference of 35.6 dB (20 log 61 = 35.6). Terefore, to compare airborne sound to waterborne sound, 26 + 35.6 dB, or approximately 62 dB, must be added to the air
126
1.5
unfiltered precursor
1.0
0.5
0.0
-0.5 10 10 20 Time (ms)
Figure 3.63: Near field measurement of a water gun. The precursor is probably caused by a cavity collapsing inside the water chamber of the gun. The strong signal observed after 32 ms is caused by cavities collapsing in the water.
measurement to obtain the in-water equivalent of the sound pressure level. Tis compares well with studies on human underwater hearing, which show that the hearing threshold is 67 dB for a signal of 1,000 Hz. When measuring sound from a source, it is necessary to
measure the distance from the source as well, since the sound pressure in homogeneous space decreases with distance with a 1/ distance relationship. Terefore, when presenting dB values, they must be related to the reference pressure at a reference distance of one metre. In water, then, the unit is dB re 1 µPa @1m. A simple example: if a jet engine is 140 dB re 20 µPa @ 1m,
then underwater this would be equivalent to 202 dB re 1 µPa. To convert from water to air, simply subtract the 62 dB from the sound pressure level in water. A supertanker generating a 170 dB sound level would be roughly equivalent to a 108 dB sound in air.
3.7.3 The Pistol Shrimp – The First Water Gun
Te pistol shrimp generates a high-frequency signal by creating a high velocity water jet into the water (see Section 3.7.4), which will cause cavities because of significant local velocity variations in the water close to the source. Exactly the same principle is used for the seismic water gun, a source that creates water jets that propagate out of four nozzles. Te cavity is filled with water vapour, which turns into water molecules again during the collapse. Already by 1917 Lord Rayleigh had derived an equation relating the collapse time t to the
Figure 3.64: The S15 water gun.
radius R of the cavity: t ~ R √ρ ⁄ ph
, where ρ is the 30 40
Radiated Pressure (bar-m)
Sercel
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