An English friend used to use a few quietly spoken words to restore order in a noisy meeting. “Let’s have a little bit of hush,” he would say. This phrase had an amazing ability to get everyone to quiet down. There are many times when I wish I had a similar means for quieting machinery noise on a boat.
The first step in getting noise under control is to understand that what we perceive as noise is actually vibration, the back and forth movement of air molecules acting on our eardrums. To control the noise we need to find and minimize the source of the vibration. We also need to understand that whatever is vibrating is changing the form of, or transporting, energy.
The typical marine engine or genset presents us with a wealth of vibration sources and therefore noise-makers. The rapid rise in cylinder pressure during the combustion cycle vibrates the entire engine. The meshing of gears creates vibrations. The exhaust gas flow (and in water cooled exhaust systems, the flow of cooling water) create pressure pulses and therefore noise. An engine’s air intake is a prime source of noise, a fact easily confirmed by removing the intake silencer, the din may astound you. (One of the reasons the Onan QD series of diesel gensets are unusually quiet is their use of tuned helmholtz resonators in the air intake system, a technique that works especially well for engines that run at constant speed). Fact is, there will be things vibrating and therefore creating noise so long as we have a mechanical power source on our boats. The challenge is to obtain that “little bit of hush.”
Resilient engine mounts
To begin we need to deal with the vibration-to-sound-to-vibration cycle, to reduce the magnitude of the vibrations transferred to the vessel’s structure. Installing an engine on resilient mounts will minimize the transfer of energy to the engine beds. Some engine manufacturer’s specify each engine mount individually, providing a different frequency response and stiffness at each location. Where location-specific mount characteristics are not specified, a general purpose resilient mount such as the Metalastik “Cushyfloat” from The Evolution Company (www.evolutionmarine.com) should be used.
However, to obtain the maximum value from the use of resilient engine mounts, vibration transfer through the prop shaft should also be addressed, either with the use of a flexible shaft coupling or for the best results, a shaft system that incorporates a thrust bearing at the shaft log. The combination of flex coupling and thrust bearing removes the propeller thrust load from the engine mounts, increasing their effectiveness in isolating engine vibration from the vessel structure. The use of a shaft system that combines the flex coupling, thrust bearing and a prop shaft enclosure that eliminates the Magnus effect loss created by an open, rotating prop shaft can be a very attractive choice. Such systems are available from companies such as The Evolution Company and Seatorque Control Systems (www.seatorque.com).
Once all practical steps have been taken to minimize vibration energy transfer to the vessel’s structure we can begin to deal with trapping the sound energy created by the vibrating machinery. Sound energy behaves like water; it will leak though even the smallest hole or crack. There is not much to be gained by lining an engine enclosure with sound absorbing materials if the sound is leaking out through overlooked apertures. The objective is to trap all of the sound energy and convert it to something we can’t hear, heat!
If you doubt the effectiveness of a reasonably airtight enclosure in reducing sound level you might try placing a portable radio, tuned between stations to obtain a wide spectrum noise, in the bottom of an open cardboard box and noting the sound level outside and above the box. Then close the box and note the change in noise level. Since judging sound level differences with the unaided ear is quite imprecise, we used a sound level meter, set on the “A” scale. The sound level from the radio in the open top box was 71 dBA, with the cover of the box closed, 60 dBA, a major reduction in sound level. Admittedly, the sound from the radio does not have a frequency spectrum anything like that of a marine engine, however the test does prove the point, trapping the sound in the most air tight enclosure you can achieve does a great deal toward achieving that little bit of hush we all want.
Sound absorbing material
Once we have created an effective sound isolation barrier we may find it useful to cover the interior surface with a sound absorbing material. This may range from a special paint-like coating to a multi-layer construction that includes a limp, high density barrier constrained within the covering. The high density material resists being deflected or vibrated by the sound energy, converting that mechanical energy into heat.
If the sound shielding enclosure is to be successful, the intake and exhaust air ducts must include a sound blocking mechanism, typically a labyrinth that forces the incoming air to go around corners and simultaneously prevents sound energy from flowing freely out from the port or duct. The interior surfaces and the partitions that make up the labyrinth in the duct are usually lined with sound-absorbing materials.
Allowing an engine to obtain the required air volume from the bilge of a boat or through improperly designed air intake ports or ducts will make it very difficult to achieve the desired level of engine noise reduction.
The surfaces of an enclosure intended to reduce the sound level in a vessel must not be allowed to resonate, to vibrate in sympathy with the impinging sound energy. Since the spectrum of sound frequencies from an engine can cover a very wide spectrum — from a few hertz to the high-pitched whine of the engine’s alternator cooling fan — assuring that the panels of the enclosure are not contributing to the din can be challenging. Fortunately, there are numerous sound treatment products, ranging from special elastometric paints to multi layer passive sound absorbing mats that can keep the enclosure from singing its own song. It’s important that any hatches, ports or doors in a sound enclosure are as carefully gasketed as the door on the freezer.
An ideal engine enclosure would be totally sealed. Unfortunately, the engine in this ideal enclosure would run only until the oxygen in the enclosure was consumed. Even if given just sufficient oxygen for the combustion of fuel, the engine would not continue to run for very long since it would soon overheat. A practical enclosure must be equipped with an air intake port that allows the unimpeded entry of a very large amount of air and a second port through which an even larger volume of heated air can be extracted.
Totally closing the intake and exhaust air ducts in a properly built enclosure or engine room would shut down the engine in a very short time. You may not be able to equal this level of air-tightness, however it is a worthwhile goal. There is little point in devoting time and money to trying to quiet an engine whose bottom end is exposed to the entire length of the boat’s area.
Active noise reduction
In addition to the passive sound reduction techniques listed above, MTU, the manufacturer of large diesel engines, is working to bring active noise reduction technology to the marine world. MTU is now testing active vibration engine mounts for use on its 2000 and 4000 series engines. The operating principal of the MTU system parallels that used in noise canceling headphones. Interfering noise is sampled, amplified and phase inverted to produce an “anti-noise” within the headphone’s earcup that will ideally cancel the unwanted noise.
While the sound canceling headset deals with interfering sound energy, quieting a massive diesel engine must begin at the vibration level. To that end each of the four engine mounts used in the system are built with the typical three-axis set of elastometric vibration-absorbing supports. The frequency response of the mount system, 6 to 7 Hz vertical, 4 to 5 Hz lateral, is tuned to the optimum for each type of engine and for each axis. Three vibration sensors are built into each mount along with three collinear force actuators. The signal output from the 12 sensors in the engine mounts, plus eight additional sensors located elsewhere on the engine and engine bed are fed to a computer where a proprietary algorithm is used to create 12 audio frequency signals, each uniquely configured to power a particular force actuator in an engine mount. (The developmental control unit can accept up to 32 analog inputs and provide an equal number of analog outputs).
The results achieved by MTU in their testing show substantial promise. Single peak vibration reductions of up to approximately 30 dB have been measured with an overall reduction of about 8 to 10 dB. MTU demonstrated the operation of a single active engine mount at the recent Miami Boat Show, using a low frequency vibrator to excite the water in a container fastened to its top and a second container, connected to the vibrator through an active vibration canceling mount. The demonstration was convincing, the water in the lower basin was entirely free of the vibration excited waves seen in the upper water container.
The application of active vibration cancellation systems will initially be directed to very large engines and vessels where the considerable cost of the equipment and especially the development of the control algorithms can most easily be cost justified. However history teaches us that when electronic technology is involved, today’s high-tech/high-cost equipment and products evolve quite rapidly into equipment suitable for the mass market.
If you decide to undertake a significant sound elimination project keep in mind that success depends on careful attention to all of the details.
Contributing editor Chuck Husick is a sailor, pilot, photographer and engineer who lives in Tierra Verde, Fla., and rides a surprisingly quiet bicycle.