No Such Thing as Too Quiet

May 22, 2008
Sometimes proper selection and placement of insulating materials is part art, part science.

Jonathan Alexander
Product Development Specialist
3M Acoustic Solutions
St. Paul, Minn.

Imagine a dishwasher so quiet you have to open the door to see if it’s on. Or how about an automobile as quiet as a Rolex, a laser printer whose flashing LED is the only indication it’s printing, or a vacuum cleaner that needs an indicator light to signal it’s operating. Consumers are demanding this level of noise reduction, and OEMs are scrambling to provide it.

Quiet operation effectively gives products a higher perceived value. But sound insulation can also add weight and assembly cost. A deeper understanding of noise reduction principles can keep the cost-benefit analysis in the black.

Silent strengths
Noise reduction is more than just a creature comfort; it has practical applications. Reduction of structure-borne noise in computer disk drives, for example, speeds access and boosts storage capacity. In the workplace, quiet improves productivity, minimizes distractions, protects hearing, and permits better communication.

In some products, such as luxury automobiles, the goal is more sophisticated than simply reducing noise. Instead, acousticians apply the field of psychoacoustics to deliver a particular sound. They combine sounds from several areas of the vehicle to deliver a distinctive audio signature associated with a particular vehicle brand.

A filtering out of all noise frequencies may be counterproductive. Often it results in a sharp or irritating pitch. In these cases, engineers rebalance the various noise frequencies and actually add some back in. The resulting product may be noisier but more acceptable to the consumer.

What Is Noise?
In its most basic sense, noise is wasted energy. Noise, or any sound, starts with a vibration pushing into surrounding molecules and creating a wave of sound that eventually reaches our ears.

Engineers address two types of noise. They refer to vibration energy traveling through the air as airborne noise. Structure-borne noises are vibrations that pass through structures to be released as airborne noise elsewhere.

Most products generate both types of noise. A cooling fan’s motion, for example, creates structureborne noise. The turbulent air flow the fan generates must be ducted to reduce airborne noise.

We experience acoustic energy as noise when its frequency is audible to us, 20 Hz to 20 kHz. Automotive designers worry about sounds ranging from 800 Hz to 3 kHz. Consumer appliances like washers and driers generate noise in lower frequency ranges.

As air molecules excited by a sound wave encounter the fibers of an acoustic insulator, some of their energy converts into friction and heat. The more sound energy that changes to heat, the greater the sound reduction.

The absorption coefficient measures a material’s ability to convert sound energy. It can range from zero when sound is completely reflected or transmitted to one when the energy is completely absorbed. For example, a coefficient of 0.8 means the insulation absorbed 80% of the noise.

Savvy selection
But there’s more to selecting acoustic insulation than absorption coefficient alone. Putting the absorbing material in the right place can make a big difference. Insulators work best if placed perpendicular to prevailing air flow. As the angle strays from the perpendicular position, more sound gets through.

The surface area of the insulator also affects performance. More surface area means less reflection and refraction of sound waves, resulting in a more efficient absorber. At the same time, your goal is the thinnest insulation that reduces noise to the desired level.

Thinner insulation gives more leeway for aesthetic considerations. Plus, it can save weight, leaving room for innovative design, especially in automotive uses. Too much weight can make the vehicle slow, harm its fuel efficiency, and degrade its handling.

Of course, ease of installation helps keeps costs low in any application. Material that can be compressed to conform to a given space without losing its insulating capabilities means fewer forming operations and part numbers. Insulation that can be trimmed by die cutters, utility knives, rotary cutters, or common scissors takes less time to install. Installers should be able to attach the insulation with such common methods as spray adhesive, pressure-sensitive adhesive, sonic welding, thermal bonding, or push pins.

Finally, consider the environment where you’re installing the absorber. Extreme temperatures, industrial chemicals and lubricants, and moisture can all reduce the life of the insulation. Hydrophobic insulation that won’t absorb water resists mold and mildew, lasts longer, and maintains its installation weight regardless of the environment.

Glass wool
The most commonly used nonwoven acoustic insulator is fiberglass or glass wool. A random mat of 5-μm-diameter vitreous fibers may be sandwiched between layers of fabric, paper, or polymer, or it may be sprayed loose into a cavity. Its efficiency, especially at damping sounds above 2.5 kHz, and low cost account for its wide use.

Fiberglass can irritate the skin, eyes, and respiratory tract. The North American Insulation Manufacturers Association has published guidelines for exposure limits and personal protective equipment, including Niosh N95-Series dust masks, for fiberglass installers.

Polymer foams
High-density polymer foam is also a widely used insulator. Foam owes its acoustic absorption efficiency to its ability to fill the cavity into which sound waves propagate. The frequencies absorbed depend on the polymer material and the foam pore size, so installers can specify a foam that meets application needs. In addition, foam is often preferred for aesthetic reasons.

Forming, molding, and trimming the foam to fit into a given cavity drive up the cost of producing and installing the material. Each piece of insulation is a separate assembly, so inventory costs could be a factor.

Fabric filler
Shoddy insulation, shredded textile waste fibers blended with other select fibers to make a filling material, has a low cost/pound. However, the amount needed to absorb a given amount of noise is greater than the equivalent fiberglass. Its higher density lets it operate better at lower frequencies.

While the fibers are sometimes treated with flame retardant, shoddy may not work well in applications where water is present because damp shoddy transmits more sound energy. The wet material can also breed fungus and bacteria.

Polymer fibers
Synthetic acoustic insulation materials, like 3M’s Thinsulate, can have high ratios of noise reduction to weight and thickness. The resin-free, nonwoven mats of polymer fibers are also naturally hydrophobic.

The material’s efficiency comes from the high surface area of the fine melt-blown polyolefin fibers. Thicker polyester fibers provide structure and let the mat fill the cavity through which the noise propagates. It is highly conformable, making it easier for both the designer and installer to use in areas and applications previously prohibited.

The amount and size of the fibers can be varied to address sounds in various frequency ranges. A combustion engine, for example, might emit frequencies ranging from 100 Hz to 8 kHz. No single insulating material can block all of those frequencies, but a combination of acoustic insulation materials can effectively control them.

Thinsulate can be die cut to custom shapes and sizes. Thickness ranges from 8 to 44 mm depending on the noise absorption needed.

Testing, testing
Before specifying a particular insulator, it’s a good idea to run a few tests to make sure of its performance. Some methods, such as ASTM E-1050 and ASTM C-423, have been standardized, but others are being developed and refined for particular situations.

ASTM E-1050 measures impedance and absorption of acoustical materials using an impedance tube, two microphones, and a digital frequency- analysis system. A minimal test setup, small sample sizes, and quick turnaround make this test useful for comparing how well candidate materials absorb sound over a range of frequencies.

ASTM C-423 measures the change in sound absorption and sound absorption coefficients between an empty room and the same room with insulation material covering the floor. This delta, the decay rate, may be more representative of the performance of the absorber in an open space, but the start-up cost and sample-size requirements are higher.

The Standard Test Method for Measurement of Normal Incidence Sound Transmission of Acoustical Materials Based on the Transfer Function Method is a fast, compact, four-microphone impedance system. The test precisely measures the absorption and transmission loss of both fibrous and nonfibrous absorbers and highlights the effects of completely filling a cavity with acoustical absorbers. Energy dissipation at various levels of absorber compression within the sample chamber can also be evaluated.

Make Contact
3M Acoustic Solutions,

Fiberglass insulation is lightweight and absorbs sound efficiently, especially at higher frequencies. In addition to its availability as rolls of batting, the glass wool can be sprayed into place.

Thinsulate acoustic insulation has a mix of fine and coarse fibers that provide sound damping and structural integrity. The insulation can be die cut to spec or trimmed in place (left).

Noise can be divided into airborne and structure-borne forms. Insulation controls noise by dissipating the sound or vibration energy as heat. Barriers and isolators prevent noise transmission by deflecting or decoupling the vibration or sound energy.

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