By Seth Dotson
Whether you are out tarpon fishing with your companions or just enjoying a pleasure cruise out on the open ocean, you may find it difficult to enjoy these activities in anything resembling peace and quiet. Between the grinding of the boat’s motor, the humming of the propeller as it drives the boat forward, the slapping of the waves against the hull or the breeze hitting your face, there are a variety of sounds you will encounter on your sea voyage that can make finding a quiet place challenging.
Thankfully, there is an equally wide variety of solutions to these noises that may be found on the aftermarket or incorporated into your brand-new vessel as it is being assembled. The many soundproofing solutions that are available may be categorized either by the source of the sound that they seek to mitigate, the materials that compose them, the class of vessel they are designed for or the location on the vessel in which they are typically installed.
The most popular soundproofing outlet among sportfishers in 2022 by far is Salem, Massachusetts’ Soundown Corporation. Soundown Marine Acoustics Specialist Sam Smullin, who also happens to be the son of founder Joseph Smullin, is regarded as one of the foremost experts in boat soundproofing today. His knowledge and experience ranges in scope from the latest techniques and materials for soundproofing modern vessels to possible future developments and experimental innovations in the field.
“At some level it’s just physics. But to a lot of people it seems like black magic,” Smullin says. “They say ‘so-and-so did this and it worked for
them, so I’ll do that.’ Without an understanding of why something worked in one situation, you won’t have a good indication whether the same thing will work in a different situation.”
Common Noise Sources
The sound encountered on any boat can have one of two basic sources: environmental and internal. Environmental sounds originate from outside of the boat, and are encountered more frequently and with greater severity, when the boat is moving than when it is at rest. Environmental sounds typically constitute sound made by water striking the boat and the noise of the wind. Internal sound is sound made by the occupants of the boat and the boat itself. This can be the sound of chattering passengers, the creaking of planks or the boat’s propeller.
More often, however, the most significant source of noise on a boat is made by the engine. This includes airborne noise coming directly from the engine, structure-borne noise that travels through the mounts and down the driveline and exhaust noise. Because the engine is typically running at all times that the boat is not at the dock, the engine noise produced can be prohibitive whether the boat is moving or stationary.
The first step in determining the best approach for reducing noise is to understand the source of the noise and how the noise is getting to the problem area. For example, if you have excessive engine noise in the salon and little to no insulation, the problem is probably airborne noise radiating off the engine and transmitting through the sole and into the salon. However, if you are on a larger boat and have engine noise in the forwardmost cabin, the issue is probably engine noise that is transmitted through the structure (structure-borne) and radiating off the interior panels.
There are a number of ways to determine the source, ranging from critical thinking and applying an understanding of the vessel’s systems to in-depth acoustic analysis. The easiest sources to define are often the ones that are problematic at rest. This is because systems such as AC, pumps, generators and others can be switched on and off to isolate the problem noise.
The precision with which it is critical to understand the specific source and path usually becomes more important as the vessel gets quieter. This can be illustrated using a typical sportfish setup where too much noise in the salon is a common complaint. “When I hear that the salon is too loud for comfortable conversation, my first thought is that the airborne noise from the engine room is a problem and recommend improving the insulation of the salon sole,” Smullin says.
A Simple Q&A
If the same vessel referenced above has good insulation but still has excess noise, a more detailed assessment is needed. In this situation Smullin says the following questions would be appropriate.
“Where does the engine room take fresh air from?”
Air intake trunks often extend above the insulated deck and have only a thin panel separating them from the salon. This would create a flanking path where the noise is going around the insulation.
“Are there any stanchions that connect the engine beds to the deck?”
Stanchions that connect the engine beds to the overhead create a structure-borne path for vibration from the engine to drive the salon sole. That same energy radiates as noise.
“How is the engine mounted and what condition are the isolators in?”
For a vessel with a well-insulated engine room that still has a significant amount of engine noise, the path is most likely structure-borne through the engine mounts.
“Can you describe the noise or send me a video so I can hear what you are hearing?”
This last question speaks to the nature of how sound works and provides clues to what the source is. Loudness is typically measured in dB(A). This is often referred to as the “A-Level” and, as it applies to boats, is often made up of a number of different peaks occurring across the audible spectrum. For example, an acoustic plot of a bell would show that all of the noise is concentrated at one frequency (the natural frequency of the bell), and it would have a single peak that is at the same height as the A-level. On a boat, things are a bit more complicated. That acoustic plot would show a range of peaks that, added together using the proper formula, will arrive at the A-level.
“At Soundown we have a good crew with a strong understanding of acoustic principles. Applying our experience, understanding of boats and best-practice solutions we can typically come up with very effective recommendations,” Smullin says. “However, there are times when having a full analysis conducted by a qualified acoustic engineer is a better option.”
One of the most compelling reasons to use a consultant is when the cost of the treatment is likely to be high enough that it is worth investing in doing the right thing one time.
An acoustic consultant will most likely sea trial the vessel and run through a range of operating scenarios while taking airborne noise measurements as well as measuring the vibration levels. These measurements will be run through an analyzer to create frequency transfer function plots that show the individual peaks that make up the A-level.
These peaks, at different frequencies, are fingerprints of the noise source. What the operating frequency of each component is can be calculated using an understanding of how the piece of equipment works and some basic arithmetic.
“The noise, at its base, has a distinct frequency,” Smullin says, referring to the engine’s cylinders. “It goes ‘Bang!’ once every other rotation, which is multiplied by the number of cylinders the engine contains. A 12-cylinder engine turning at 2,000 revolutions-per-minute is going to have a firing frequency of 200 hertz, for example.”
For those unfamiliar with acoustic terminology, the unit “hertz” refers to cycles-per-second. The more cycles per second the higher the frequency is. This difference can be easily heard when comparing the low frequency thump of a diesel to the higher-frequency noise of a gasoline engine, which is likely to rev two or more times higher at full power.
Once you have an understanding of the noise source and path you can begin to look at the most effective solution. For airborne noise, the solution is most likely to be an insulation treatment. For structure-borne noise, treatments will consist of resilient mounts, vibration damping or a combination of the two. It is also important to note that in new construction or major refits, the use of floated interiors will be the most effective treatment for both airborne and structure-borne noise.
To understand how sound is mitigated, it is important to understand how sound works on a physical level. In essence, a sound wave is a rapidly expanding sphere of compressed air that results from the air being displaced by a vibrating object contained within. A boat’s engine creates sound as its parts move and create vibrations in its various constituents. These, in turn, create vibrations in the air in the form of sound. The engine also creates noise by vibrating surrounding objects, such as the engine mounting and the parts of the boat’s structure immediately surrounding it. These parts may then vibrate in turn, augmenting the source of the noise.
“Take road noise in your car,” Smullin says. “You’re not actually hearing noise made outside by the tire hitting the pavement. What you’re hearing is the vibration made by that action passing up into the car and making the whole car vibrate, like the ringing of a bell. The frequency of the vibration is the same as that of the tire impacting the road’s surface, so that’s the frequency of the sound you hear. Even if you were to eliminate the passage of any noise coming from outside the car, you would still hear the vibration, because that noise is passed through the car’s structure.”
Types of Insulation
According to Smullin, different materials for insulation that can be installed within a boat fall into two main categories, depending on how they manage sound on a physical level. One of these categories is absorption-type insulation. These materials work by preventing sound from being reflected off walls and other surfaces in a boat.
“Think about a time you’ve gone to look at an empty apartment,” Smullin says. “There’s nothing there on the walls and you can barely have a conversation because the sound of your voice bounces around so much that it becomes hard to hear clearly. But later you go in there with some carpet on the walls and some drapes and a couch and suddenly you no longer have all that reverberant noise. That’s what absorption materials do; they absorb that reflective noise in a space.”
Another type of insulation that can be installed is barrier treatments. Barrier treatments increase the sound-blocking ability, known as transmission loss, of a deck or bulkhead. The most common barrier treatment used in the marine industry is a foam composite with a mass loaded vinyl barrier, which has replaced lead. These are available in a range of thicknesses and barrier weights, with the most significant variables for noise control being the weight of the barrier layer and thickness of the decoupling layer. The decoupler layer is the material between the treated surface and the barrier.
To maximize the effectiveness of a given weight of a barrier the decoupling layer should be increased to as much as three inches if space allows. In practical terms, if you take a sheet of ⅝-inch plywood and apply a one-pound-per-square-foot barrier with a ½-inch thick decoupling layer, you will start to see increased transmission loss at 250Hz. At 500Hz, you will see an improvement of 11 dB(A). If you apply that same one pound per square foot on a one-inch decoupler, it will begin to perform 162 Hz. And at 500Hz, it will increase by 16 dB(A), a significant improvement. Changing this second scenario to a two-pound-per-square-foot mass later will increase the transmission loss to 21dB(A) at 500Hz.
Finding a Compromise
This data supports Smullin’s assertion that “When it comes to stopping noise, thicker and heavier is almost always better.” He does concede that more weight is not always practical, especially in high-performance sportfishing boats.
“There are many compromises to be made when building a boat. We work with builders and owners to understand what is the highest priority and then work within an established weight budget to get the best performance. It’s kind of the least bang for your pound as opposed to the most bang for your buck.”
Decouplers vs. Resilient Mounts
In many ways, the decoupler in a barrier treatment works in a similar fashion to the resilient mounts used for isolating machinery vibration to reduce structure-borne noise. The thicker the decoupler the lower the stiffness, which improves performance, especially at low frequency. Vibration isolators from engine mounts to small rubber bushing on pumps and other equipment work much the same way. The softer the rubber, spring or other resilient material, the better the performance, especially at lower frequencies.
“Most equipment is supplied with some type of resilient mounting from the factory,” Smullin says. “The mount that comes from the equipment manufacturer is going to be a functional mount, designed to meet the requirements of a wide range of applications. Whereas, if you have someone specify a mount for your application, it can be tailored to fit your requirements.”
Working with a qualified supplier to select proper mounts for any piece of equipment will pay dividends in quieter noise levels onboard. Structure-borne noise from engines and generators are the biggest contributors in this realm, but improperly isolated pumps, AC units and other small equipment are often the things that wake people up in the night and generate calls to Soundown or others.
Smullin also emphasizes the importance of having a stiff supporting structure below the mount. While the stiffness of the mounts is a critical characteristic, it is essential that the structure below be sufficiently stiff so that the mount performs as expected. “That is why mounting a small piece of equipment in the middle of a bulkhead can cause a large amount of noise. In this scenario, you basically create a speaker with the equipment as a driver and the panel as your cone.”
Noise Mitigation Solutions
According to Smullin, anybody considering possible solutions to mitigating noise on their vessel should first take into consideration the type of vessel they are dealing with. Smullin categorizes sportfishing boats into three different categories: open flybridge yachts, closed flybridge yachts and express boats.
Concerning the latter, Smullin admits that these are the most difficult to deal with due to a lack of closed off spaces. To a certain extent, people using vessels in this category have to accustom themselves to certain noise levels that those with larger vessels could reasonably expect to exterminate.
“You’re driving from an open area that’s open at the back to where the exhaust noise is, and you’re right on top of the engine,” Smullin says. “So you’ve got some engine noise there, plus exhaust, which tends to be the dominant source of noise in these boats. Express boats don’t have a cabin on them. There isn’t typically a carpet, so there’s no carpet underlayment. So in express boats, the noise at the helm is really the biggest piece of it, because that’s where you drive, and that’s typically where people ride while you’re underway. With the appropriate treatments you can reduce the mechanical noise to the same or lower levels than the wind and water noise, but you’re never going to get that down to a 70 or 75 dB(A) at 30 knots in that area.”
Carpet underlayment is a type of insulation that lies beneath a carpet and helps muffle sounds made by the boat’s support systems lying beneath the floor.
“It has a foam layer on the bottom, which serves as a decoupler, and then your mass barrier on top,” Smullin says. “And, while you can’t go as thick on the floor as you can in the engine room, you do get the advantage of stopping some of the structure-borne noise from radiating off the top of the deck as well as the airborne noise coming through. That’s a very common treatment we see in sportfish boats.”
Installing acoustic carpet underlayment is recommended as a “nice” improvement for existing boats, but should not be considered as a primary treatment in a new build or refit where reducing noise is a primary consideration.
For larger vessels, a variety of bulkhead insulation treatments exist, which dissipate sound as it strikes the boat’s walls and doors. A couple of popular options for sound mitigation for boats containing a salon and cabin include hull boards and vinyl/foam composites. These differ in composition and weight, as well as fire-retardant capability. The hull board is a fiberglass material. While slightly heavier than the foam composite, it is rated incombustible by the USCG, while the foam composite is flame retardant.
“The vinyl foam composite is a sandwichlike construction that has two layers of foam and a mass layer between them,” Smullin says, concerning the latter material. “The layer of foam behind the mass layer gives you your sound blocking. That’s your transmission-loss treatment. And then that front layer of foam that faces the noise gives you absorption. So you’re both absorbing and blocking noise in that situation. That would typically go either in the engine room or in a small machinery space where your pumps and generators are. You would typically line it with this composite and oftentimes you would add some kind of finish layer over top.”
When considering the amount of underway noise on deck or on an open flybridge, the biggest noise source is engine exhaust. There are a wide range of products available for reducing exhaust noise from engines or generators. Similar to other treatments, choosing an option that is optimized for the signature frequency of your equipment is the key to keeping things quiet.
Yet another common noise source on boats are pumps mounted too close to bulkheads, paneling and other components of the boat’s structure. Operating pumps that are mounted snugly against the boat can transmit vibrations across the structure, resonating into the cabin and transmitting into the water.
“One of the most common examples of a structure-borne noise source that I’ve encountered are small pumps that are mounted in the middle of a bulkhead,” Smullin says. “If you were to unbolt it and hold it in your hand as it was running, you wouldn’t hear anything in the next cabin, but if you were to reattach it to the bulkhead, then it’ll make significant noise because the energy from its vibration is now being transferred to the bulkhead itself and radiating out onto the other side.”
Finally, anyone considering solutions for boat noise must keep in mind that any solution, whether a silencer for exhaust or insulation for the bulkheads and flooring, will add additional weight to a boat, with the trade-off being a slightly slower but much quieter boat over a speedy noisemaker.
Concerning the future of noise mitigation solutions for motorized sportfishing vessels, Smullin is convinced that insulation technology has reached its mature form, with significant innovations in that field appearing unlikely, barring the eventual introduction of metamaterials into the boat-building industry. “Everyone wants a highly effective insulation that is very thin and weighs almost nothing, but physics gets in the way.”
At least one future solution that Smullin has admitted to exploring is that of active noise cancellation. This is already a common feature on personal listening devices, such as headphones and earbuds, allowing their users, for example, to silence the roar of a jet engine while listening to music on an overseas flight. However, scaling up such a technology to fishing boat sizes may end up proving more of a challenge than it’s worth.
“Basically, you have a receiver measuring noise being emitted from a sound source paired with a processor creating a canceling wave to match it coupled with a speaker that outputs the canceling wave,” Smullin explains. “I’m by no means an expert in this field, but my understanding was that the area that you could treat with that just wasn’t practical for the large areas you’d want to treat in a boat, like a salon or a stateroom or even the bridge. From what I’ve been told you could only get about six to eight square feet per speaker. There may be some way to create an effective system, but I am relatively sure that it would be highly impractical.”
Yet another possible future development coming soon to modern sportfishing yachts is the replacement of noisy petrol engines with electric batteries, an increasingly common feature in consumer automobiles. But once again, this technology faces scaling issues.
“Certainly, the role of electric power will make a big difference in the amount of noise emitted by boats,” Smullin says. “We’re still a very long way from having the kinds of batteries these boats would need to run the kinds of speed and distance they need, but a hybrid system might work.”
A hybrid system involves the use of a scaled internal combustion engine as an electricity generator. The generator operates much like a typical gasoline engine on a mechanical level but is used to charge a battery that itself provides power to the boat’s propeller and internal systems. This allows for a standard-sized battery to be used, since it can be recharged after being drained, while allowing a smaller motor to be used with a smaller fuel supply, thus creating less noise while operating.