Submarine Noise

The following is an excerpt on acoustic quieting from Matt's excellent article February 3, 2015: Sea Denial: Analysis of the CSBA's Proposal The Case for Taiwanese Midget Submarines - Part I in Matt's website American Innovation.

Pete has added comments in [...] brackets.

Acoustic signatures for US and Russian submarines. [The Russian "Lead Ship Keel - 4th Generation SSN became the Yasen class. Of US subs "688" is Los Angeles SSN, 688I Improved Los Angeles, SSN-21 Seawolf class, NSSN Virginia class (Image Credit: Federation of American Scientists.)]

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Stealth, first and foremost, is the most critical aspect of an attack submarine as it determines the submarine's relative survivability and is a prerequisite to the submarine acting as an effective area denial, power projection, or ISR platform. 

Quieting technology has progressed steadily since the end of the Second World War with the advent of improved anechoic tiles, integrated electric drive systems (over mechanical), X-shaped stern control surfaces, pump jet propulsion systems (over conventional propellers), AIP technology, [elastic or dampner mounts for engines and other moving parts] etc. The acoustic signature of a submarine, which is measured in decibels, is an indicator of a submarine's relative detectability to passive sonar systems. The following is from the article "China's Anti Access Strategy: Submarine Force - Part I":

A decibel is "a unit used to measure the intensity of a sound or the power level of an electrical signal by comparing it with a given level on a logarithmic scale" (American English in Oxford dictionary, 2013). Decibels do not scale linearly. A 3db change is signifies a doubling power and a change of 10 db signifies the power increasing by a factor of ten. Therefore, the 636 Kilo class with an acoustic signature of 105 decibels is 10 times as loud as the 95 decibel acoustic signature of the Virginia class submarine.

The United States is widely recognized as fielding the most capable and stealthiest submarines with the Virginia and Seawolf class submarines. The following is from  Chinese Evaluations of the U.S. Navy Submarine Force by Andrew S. Erickson, William Murray, and Lyle Goldstein:

"Chinese observers are intensely interested in and closely follow other modern U.S. nuclear submarines, including the [Seawolf class SSN-21 USS Jimmy Carter, Seawolf}, and [Virginia class USS Hawaii (SSN-776)]. Highly detailed, full-page color photos of Seawolf- and Virginia-class submarines appear in China’s most prominent naval journals. These photos are usually accompanied by articles that imply an advanced state of technology and advanced acoustic quieting...

Chinese authors believe Seawolf possesses 'beyond-first-class performance' and is regarded as the most sophisticated and lethal submarine yet to go to sea, despite its 'tortuous development history'. The Chinese also respect Virginia-class submarines for their advanced technology and quietness...'Its acoustic signature is lower than that of the improved model of Russia’s Akula-class attack submarine and Russia’s fourth-generation attack submarine that will hereafter be in active service'. 

Another analyst, in discussing the Virginia class’s acoustic achievements, reports, 'The Virginia-class has been called the world’s quietest submarine,’ with a cruising sound level that is only '1/10 that emitted by a Los Angeles class boat pierside'. The construction techniques used to build Virginia and its sister ships also evoke respect'" 

The following acoustic signature figures are from Chinese Evaluations of the U.S. Navy Submarine Force and China’s Future Nuclear Submarine Force: [made into a Table by Pete]

Submarine Source	Decibels
Ocean background noise [1]	90 decibels
SSN-21 Seawolf class SSN	95 decibels
Virginia class SSN	95 decibels
Russian Improved 636 Kilo class SSK	105 decibels
Akula class SSN	110 decibels
Improved Los Angeles SSN	105 - 110 decibels
Type 093 Shang SSN	110 decibels
Type 094 Jin SSBN [2]	120 decibels

[1]. Regarding "Ocean background noise" or ambient noise - see Figure 1.       at http://www.nature.com/articles/srep00437 for noise levels (at sea) from natural/biological and Shipping Noise - yielding at least 90 decibels in 2001 and 2007. The 90 decibels appears more an average than a constant noise level everywhere. So submarines that can be very quiet in quiet ambient sea conditions can use their Passive Sonars to hear much more enemy activity over longer distances.

[2]. China’s Type 094 Jin SSBN is the loudest, too loud to go on regular patrol. In 2009, USN ONI listed the Type 094 as being slightly noisier than Project 667BDR (NATO reporting name Delta III) from the late-1970s, some of which were in service through 2015 with the Russian navy.]

 Relative detectability of Russian and Chinese diesel and nuclear powered submarines. Image Credit: US Office of Naval Intelligence, 2009. 

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Also see:

Source: Worldwide Submarine Challenges 1996, Office of US Naval Intelligence via Jeffrey Lewis's "China's Noisy New Boomer"  at Arms Control Wonk, November 24, 2009  https://www.armscontrolwonk.com/archive/202544/chinas-noisy-new-boomer/