Features October 1, 1999 Issue

Soundown and Hough Put a Lid on Engine Noise

Tests of 10 sound-attenuation products find the best are also the most expensive, with a few notable exceptions.

Engines make noise. Crew—especially when they’re trying to hold a conversation, sleep, or just relax—don’t like noise. The solution? Isolate the crew from the engine noise and vibration. In this update of our May 1, 1993 report, 10 sound-attenuating products are tested and ranked.

Noise and vibration are inseparable. Noise exists because vibration causes variations in the air pressure that reaches the ear. This is perceived as sound. An effective control deals with both the vibration and the airborne noise.

Noise levels are measured in decibel units, usually referred to as dBA (the A denoting that the measurement is adjusted for the frequency response of the human ear, rather than the total sound level energy). Everyday sounds fall into a range from roughly 25 dBA (a suburban bedroom at night) to 100 dBA (a chain saw at a distance of 3’).

A sensitive ear can distinguish differences of 1 dBA, but it takes a 5-dBA difference to be noticeable. On a boat, an unshielded diesel genset will produce sound levels of 100 dBA at 3’; an inboard typically raises the noise in the engine room to about 110 dBA. The object is to drop those levels to approximately 75 dBA for more-or-less normal conversation, and another 20 dBA for comfortable sleeping with the engine running.

There are three basic approaches to making your boat quieter. The first step is to use flexible mounts to isolate the vibrating machinery from the hull. These help prevent the transmission of vibration through the solid structure of the boat, and the consequent reverberation of hull sections that can act like amplifiers. Correcting any engine-shaft misalignment will certainly help.

The next step is to surround the noise-producing machinery in a tight, insulated enclosure to reduce air-transmitted noise.

The final step is to line enclosed living quarters, such as cabins, with sound-absorbent materials.

Once you’ve reduced vibration—and the associated structure-transmitted noise—you’re ready to deal with airborne noise. The first step is to enclose the noise-generating components—the engine—in some sort of box or compartment. Almost any sort of enclosure will reduce noise levels by 5 to 10 dBA, but improving on that takes an understanding of the nature of noise.

There are only three things that can be done with the air vibrations that we regard as noise. They can be reflected, transmitted or absorbed. Transmission doesn’t do us any good in terms of noise reduction. The 5-10 dBA reduction that comes from putting the engine in a box is due almost entirely to the sound energy absorbed by the box; most of the noise is transmitted right through the box’s walls. Reflection may have some value in some situations, but reflecting sound waves back into an enclosure simply makes it noisier inside the enclosure. A reflective material works when there’s somewhere to reflect the sound.

Sound, after all, is a form of energy, and you can’t just make it vanish. You can, however, convert sound energy into another form of energy—heat. The energy dissipates when the absorbent material becomes displaced or compressed.

The amount of energy absorbed depends upon the mass (or weight) of the material, how far it’s displaced or compressed, and the material’s damping capability. Materials like lightweight foams and fiberglass wool have good damping but not enough mass to be effective by themselves. They have a role in dealing with noise, insofar as they can reduce reflection. More specialized sound-absorbers are more effective. These are composite materials with a high-mass layer, one or more damping layers, and (usually) a thin plastic film at each face to protect the damping layer(s) from mechanical damage and moisture.

The high-mass layer should be heavy, limp, and nonporous. It should also be as thin as possible, simply because space aboard is usually at a premium. A sheet of lead works best, but a lead-filled sheet of plastic can be used where a physically tougher material is required. Mass layers of lead typically weigh 1-2 lbs. per square foot; lead-filled plastic weighs about half that for an equivalent thickness.

On either side of the mass layer is a layer of foam or fiberglass mat. The layer facing the noise source is called the absorption layer; its function is to damp out the vibrations caused by the movement of the mass layer. On the other side of the mass layer is an outer layer of foam or fiberglass: the decoupling layer. It isolates the heavy layer from the engine’s enclosure.

The range of frequencies that a sound-isolation material can deal with effectively varies with the product of the square root of the weight of the mass layer and the thickness of the decoupling layer. Thus, a 1-lb. per sq. ft. barrier on a 1/4" decoupler might only be effective in dealing with high-pitched sounds (500 Hz and higher); doubling both the weight and the thickness extends the useful frequency range to 125 Hz.

Reductions in dB are additive. Let’s say an unshielded engine produces a noise level of 110 dBA. If we build a box from 5/8" plywood around the engine, we’d lower the sound level (measured at 1') to roughly 100 dBA, assuming that the box doesn’t leak and that all openings are taped. If the vent openings to the box lead overboard rather than into the cabin, you can reduce noise by another 15 dBA or so. Make sure that your vent ducts are baffled, or make up a labyrinth with several 90-degree turns.

If you line the box and the vent ducting with a simple absorption layer of fiberglass or foam, you’d only lower the noise level outside the box by 5 dBA or so, even if you used a thick layer of sound-absorber. If you lined the box with a composite material (1-lb./sq. ft. mass layer, 2" thick overall), you’d get the noise level 2' from the box down to a bearable 76-78 dB, or 61-63 dBA if the engine’s air vents are ducted overboard.

The actual noise level in the cabin will depend on the distance between the engine box and the cabin; sound levels drop by 6 dBA each time you double the distance from the source. If the berths are close to the engine, you can also apply absorbent material to the cabin ceiling and walls (1/2" to 1” foam-backed fabric and perforated vinyl headliners are popular choices). If you find that noise is coming up through the floor, try noise-absorbing carpet. As a bonus, these materials are also good thermal insulators.

The Materials
We asked four major manufacturers of sound-control products for their recommendations. While there are other products available, we believe the selections discussed in this report nicely cover the range of products.

In all, 10 different products were tested, ranging from 1/4"-thick fiberglass sheets to plywood with a built-in sound-deadening layer to 2"-thick foam/lead/foam composites. For comparison purposes, we also included exterior grade plywood, which is typically used for engine enclosures.

The Tests
Not all noise is equal. It’s much easier to block high-pitched (high frequency) sounds than low-pitched ones. So it’s not too meaningful to attach a single number to a sound-deadener’s effectiveness without specifying the nature of the sound. A common method of evaluating a soundproofing material is to conduct a series of tests at different sound frequencies and plot on a graph the percentage of sound reduction at each frequency.

We took a more direct, if less sophisticated approach. The noise produced by an engine is a non-uniform mix of frequencies covering the entire audible spectrum, from very low to very high frequencies. Rather than trying to analyze the composition of that noise, we used high-fidelity sound equipment to record the noises produced by an eight-cylinder gasoline engine and a four-cylinder diesel. For the purposes of our tests, our “engine” was a three-element loudspeaker system (woofer, mid-range, and tweeter) in a 14” x 14” x 24” enclosure. This allowed us to dial in any desired noise level.

Our “engine room” consisted of a plywood box, 2' x 2' x 3', with all openings sealed or gasketed to avoid sound leaks (venting wasn’t a consideration). We used a Simpson dB meter to measure sound levels. The amplifier’s controls were set to produce a sound level inside the “engine room” that corresponded to the sound levels produced by the engines themselves: 115 dBA for the gas engine and 110 dBA for the smaller diesel. Sound outside the enclosure was measured at a distance of 2’ from the speaker.

We tested the sound-deadening plywood by replacing the “engine room” with a new box made of the plywood under test. All of the other products were evaluated for noise-reduction by installing them, one at a time, as a liner for the plywood box.

In addition to our tests on sound-deadening effectiveness, we evaluated each product’s ease of cutting and installation, and its resistance to gasoline, diesel fuel, motor oil, antifreeze, and water.

The Findings
Our first tests confirmed what we had known all along: Simply enclosing the engine in a plywood box reduced the noise level outside the box but raised it considerably inside the enclosure. Our “gas engine,” with no enclosure, produced sound of 90 dBA. With the speaker placed in the box, we measured a sound level of 72 dBA 2' away. At the same time, the noise level inside the box increased to 115 dBA. We found similar results with the diesel recording.

The effective reduction in dBA we observed with the products tested is shown on the chart above. Not surprisingly, we found that the thicker and heavier the sound-control product was, the more effectively it worked. Generally, the effectiveness of the products was comparable regardless of which “engine” was used, though we did find somewhat more effective sound-reduction from the diesel than the gas. This was due to the fact that the diesel engine’s noise consisted of more high-frequency components. A few products, such as Soundcoat Soundmat, were noticeably better at dealing with the diesel engine’s noise; the chart lists both numbers.

The products fell into three groups, based on their effectiveness. The three groups generally carried prices that corresponded with their performance. The most effective products tended to be the most expensive, although there were some notable exceptions. The most effective products gave a 12-15 dBA noise reduction, the least effective about 3-5 dBA.

None of the products showed any signs of degradation after several days of exposure to any of the liquids we applied to them. All were easy to work with and apply; the foam products were physically stronger than the fiberglass ones and could be glued, which gave them an advantage. On the other hand, the fiberglass products can withstand much higher temperatures than the foams and are all flame-resistant (although the facing material on some may support combustion). All the foams burned when exposed to a flame.

Here's our product-by-product evaluation:

Greenwood dB-Ply
Greenwood dB-Ply is a sound-absorbing plywood. It’s not intended to be installed in an existing enclosure. The sample tested consisted of two layers of three-ply fir plywood making a 1/2" sandwich ; the core or filling was a resilient plastic material. Compared to conventional plywood of the same thickness, dB-Ply was about 5 oz. heavier per square foot. It can be sawed, drilled, nailed, or glued just like conventional plywood.

Bottom Line: Greenwood dB-Ply, compared to conventional plywood, offers a modest but real reduction in transmitted sound (4 dBA decrease in sound level is equal to a 25% reduction in perceived noise). The 1/2" thickness lists for $3.03 per sq. ft., which makes it pricier than exterior grade fir plywood. If you’re building a new engine enclosure, or replacing a deck that happens to be noisy, dB-Ply is a good choice.

Greenwood FGSA-SR
FGSA-SR is a rigid self-supporting sound absorber with a fiberglass core covered on both faces with reinforced aluminum foil. A 1/4" panel weighs only about 4 oz. per sq. ft. In terms of pure sound-absorption, it’s one of the least-effective products tested, even when mounted (as per instructions) on spacers that supported it 1" in from the enclosure wall. FGSA-SR is easy to cut with a sharp knife.

Bottom Line: At $3.75 per square foot, FGSA-SR is moderately priced and moderately successful as a sound barrier. It can be useful when demands are light, and its light weight and rigidity are desirable features.

Hough Marine: Acoustical Fiberglass
This product is 1"-thick fiberglass covered with a scrim-reinforced, white-painted aluminum foil on one side. It’s a moderately effective barrier, and, at $1.30 per sq. ft., it’s the lowest-priced product we tested. Like the other fiberglass products, it’s flame-retardant. It weighs 4-1/2 oz. per sq. ft.

Bottom Line: Hough Acoustical Fiberglass is hard to beat when your requirements aren’t severe. It’s inexpensive, lightweight and easy to use. It’s fastened to the interior of an enclosure with nails or screws.

Hough Marine: Acoustical Foam 100
Hough’s Marine Acoustical Foam can be thought of as the foam equivalent of their Acoustical Fiberglass. It’s 1" thick and has a metalized plastic facing on one surface. It’s extremely lightweight—less than 4 ounces per sq. ft. It cuts easily, and can be fastened with glue, screws, or nails. Sound attenuation was the same as for Hough Acoustical Fiberglass—moderately good. It’s not fire retardant.

Bottom Line: Hough Marine Acoustical Foam 100 is a bit more versatile than Acoustical Fiberglass in that you can glue it with a structural adhesive. There’s a price for this convenience, however. At $2.61 per sq. ft., it’s about twice the cost of the Acoustical Fiberglass but provides only comparable noise reduction. This is one instance in which high cost isn’t best.

Soundcoat Soundfoam w/ Tedlar
Soundfoam with Tedlar consists of a sheet of foam with a plastic film on one face. It’s available in thicknesses ranging from 1/2" to 2". We tested the 1" thickness. This is the lightest product tested—only 2-3/4 oz. per sq. ft. It’s also among the most expensive of the group of moderately effective products ($3.48 per sq. ft.). It cuts and mounts easily, using glue, nails, or screws. It’s not fire retardant.

Bottom Line: Soundfoam with Tedlar performed comparably (though a trifle less effectively) than Hough Acoustical Foam 100, at a 33% higher cost. We don’t see any reason to pick it over either Hough’s Acoustical Foam or Acoustical Fiberglass.

Soundcoat Soundmat LFM
Soundmat LFM was the least effective and most expensive of the three-layer composites tested, falling into last place in the moderately effective group. It was outperformed by three single-layer products with prices ranging from $1.30 to $3.48 per square foot. Soundmat LFM lists for a staggering $8.89.

Bottom Line: Don’t bother.

Hough Marine Barrier 104
While it headed the group of moderately effective products, Hough Marine Barrier 104 was only slightly more effective than its Acoustical Fiberglass or Acoustical Foam. This was surprising, because Marine Barrier 104 is a three-layer composite, featuring a filled-vinyl layer between two foam layers.

We suspect that its relatively weak performance is due to its very thin (1/4") decoupling layer.

Bottom Line: Marine Barrier doesn’t offer enough improvement over the same company’s Acoustical Fiberglass to justify the additional weight (1.21 vs. 0.28 pounds per sq. ft.) or price ($5 vs. $1.30 per sq. ft.).

Soundown 2" Lead/Fiberglass
Soundown’s 2" Lead/Fiberglass is one of the three products that made up the top performance group. It’s a three-layer composite, with a layer of lead between two 1" fiberglass bats with a scrim-reinforced metalized-plastic layer on the inner facing. We observed a 12-dBA reduction in noise with the gas engine and 14 dBA with our diesel. Like the other fiberglass products, it can’t be glued in place, but cutting and installation are easy. It’s fire retardant.

Bottom Line: An effective sound barrier product. Recommended.

Hough Sonovinyl Supreme
Sonovinyl Supreme had test numbers identical to Soundown’s 2" Lead/ Fiberglass composite. It’s marginally heavier (1.52 vs. 1.38 lbs. per sq. ft.) and slightly less expensive ($6.44 vs. $6.77 per sq. ft.). Construction is also similar to the Soundown product, except that Sonovinyl Supreme uses a filled-vinyl layer instead of a lead layer.

Bottom Line: An effective sound barrier product. Recommended.

Soundown 2" Lead Foam
This product is essentially the same as Soundown’s 2" Lead/Fiberglass, with foam replacing the fiberglass. It was the most effective (by a hair), producing the same 12-dBA reduction in noise from the gas engine as Soundown’s 2" Lead/Fiberglass and Hough’s Sonovinyl Supreme, and a 1-dBA greater reduction (15 dBA) for the diesel noise. Its advantages are slightly easier installation (it can be glued) and better performance. Its comparative disadvantages are lack of fire retardance, increased weight—at 1.61 lbs. per sq. ft., it’s the heaviest product tested—and higher cost ($7.19 per sq. ft.).

Bottom Line: An effective sound barrier product. Recommended.

Conclusions/Recommendations
The most effective sound barriers were three 2"-thick composites—Soundown 2" Lead/Foam, Soundown 2" Lead/Fiberglass, and Hough Marine Sonovinyl Supreme. Any one of these products should do a good job of suppressing noise at its source.

If all you’re looking for is the maximum in noise control, Soundown 1" Lead/Foam is your choice, though not by much. Hough Marine’s Sonovinyl Supreme performed as well for a gas engine, almost as well for diesel, and costs less. If you’re concerned about fire protection, Soundown 2" Lead/Fiberglass will provide comparable performance combined with fire retardance at a comparable price. You won’t be able to glue it, however.

If all you need is a sound barrier for quieting a cabin or the like, Hough Marine Acoustical Fiberglass is effective and inexpensive. And if you’re starting from scratch, Greenwood’s dB-Ply is a good choice.

Many older boats may not have fire retardant engine box insulation. Installing a quality product is neither terribly difficult nor expensive. It’s a good upgrade project.

Contacts- Greenwood Forest Products, 25895 S.E. 72 Ave. Suite 200, Portland, OR 97224; 800/333-3898. Hough Marine and Machine Co., Inc., 1111 NW Ballard Way, Seattle, WA 98107; 800/423-3509. Soundcoat Co., Inc., 1 Burt Dr., Deer Park, NY 11729; 516/242-2200. Soundown Corp., 17 Lime St., Marblehead, MA 01945; 800/359-1036.

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