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Marine Silencing Technology
Noise Reduction Measurements
Noise levels are measured in decibels on a logarithmic scale. On a linear scale twice the power would give twice the reading. On the decibel scale twice the power adds 3 db to the reading. If one engine has a reading of 92db then when you start up a second engine the reading will increase to 95 db. If you then start two more the reading goes to 98 db. Now if you quiet your engine with a silencer by 3 db then only half the sound power remains but if it is quieted by 6 db then only a quarter remains. 9 db is 1/8th, and 12 db is 1/16th the sound power. Until 2002 a marine silencer would typically provide only 6 db noise reduction. Today some APL silencers are getting as high as 11 db.
Sound is measured on at least two different decibel scales each being 'weighted' for a specific purpose. The 'C' scale gives full 'weight' to all frequencies recorded and is not normally employed in noise measurements however a sound technician at a recording studio finds it useful. The 'A' scale is 'weighted' to respond like the human ear, very low and very high frequencies are ignored since they are not heard anyway. So the 'A' scale is used in noise measurement and values are recorded in units of dba. On this website wherever db is used dba is implied.
Common Noise Reduction Techniques
In today's world of performance boating, new laws make it very difficult to maintain a boats performance without creating too much noise. These laws made restrictions so tight that some boats couldn't be run at all with the technology that existed before 2002. This caused manufacturers to become very innovative, constantly searching for new ways to limit excess noise without sacrificing performance.
Sound reduction is always accomplished by reducing intensity levels. Sound intensity is acoustic energy distributed over the surface area of a sound wave. Several methods are employed to reducing unwanted noise such as baffling, restriction, absorption, and frequency cancellation.
Baffling essentially breaks up the wave into a number of less intense waves having the same total energy as the original wave. Baffling can also create additional frequencies as a result of the way waves are reflected.
Restriction is a commonly used method that forces sound waves to flow through narrow passages . Depending on the passage design certain frequencies will not be able to pass. Also the losses involved tend to reduce acoustic energy as the flow scrubs the passage walls.
Frequency cancellation splits the waves into two paths then rejoins them 180 degrees out of phase with the result that specific frequencies and their multiples can be completely removed.
Absorption is accomplished by using energy absorbing materials, such as heavy foams and glass or steel wool, in the path of the sound. Acoustic energy is converted into first mechanical then heat energy by vibrating materials which heat up thus reducing the total amount of acoustic energy present.
Another energy absorbing material is the propagation media itself. Air does not absorb sound energy to a significant degree but water droplets do. When minute droplets are suspended in air and a sound wave passes, the droplets are accelerated by the approaching wave then decelerated to a stop and accelerated in the opposite direction as the wave passes then stops again. The forces that accelerate and decelerate the droplets are derived from the wave energy and, by this means, wave energy is reduced (being converted to heat).
When the sound source involves fluid flow such as engines, compressors, or heating ducts, an added complication is introduced. That is, flow must not be overly restricted or the device will lose efficiency. Given enough volume almost any method can be successfully employed to reduce sound without flow restriction but as available volume is decreased the problem becomes much more difficult.
A silencer used in a "wet" marine system always employs water droplet energy absorption as part of the sound reduction technology. Within a wet system only flow-through silencers have the potential to provide sound reduction without excessive restriction. Frequency cancellation silencers cannot be compacted below a certain practical limit, and absorption type silencers have the possibility of deterioring with use. As a result we chose to use only flow-through designs employing baffles. The problem is to design baffling to be sufficiently effective for the application while keeping the flow passages large enough to avoid excessive restriction. Aqua Powers' patented technology has solved this problem in a way that provides essentially no exhaust flow restriction while improving noise reduction.
Click here to see typical noise levels from some common sources.
Click here to hear comparative noise levels.
Click here to see the internal components of APL silencers.
Click here for state to state noise regulations.
Click here for estimates of boat noise levels.