If a robotic vacuum cleaner works while no one is home, does it still make a sound? That new spin on the tree falling in the forest may not inspire you to a night course in philosophy, but it does underscore an important point in the realm of sound quality: there is a distinction between the objective reality and the subjective experience. The former relates to the physical phenomenon of generating sound pressure waves that can be quantified and characterized by a variety of standard metrics such as intensity level, frequency, harmonics, and so on. This objective aspect is often referred to as noise.

The subjective aspect, by contrast, is fuzzier in nature as it relates to how people perceive and react to what they’re hearing. This aspect is typically referred to as sound. The distinction is important to understanding that improving sound quality is not just about making things quieter. To some, a dripping faucet can be more intrusive than the jet engine noise from an airplane overhead. Sometimes a product’s sound quality can be improved by changing its character without actually lowering its noise level.

For product designers lacking expertise in this area, the whole field can seem like a black art combining physics and psychology, math and magic. But such a designer need not be intimidated; there are many good acoustical consultants and testing laboratories that can lead the designer through that murky territory.

At Material Sciences Corp., a front-loading clothes washer undergoes acoustic testing.

If a robotic vacuum cleaner works while no one is home, does it still make a sound? That new spin on the tree falling in the forest may not inspire you to a night course in philosophy, but it does underscore an important point in the realm of sound quality: there is a distinction between the objective reality and the subjective experience. The former relates to the physical phenomenon of generating sound pressure waves that can be quantified and characterized by a variety of standard metrics such as intensity level, frequency, harmonics, and so on. This objective aspect is often referred to as noise.

The subjective aspect, by contrast, is fuzzier in nature as it relates to how people perceive and react to what they’re hearing. This aspect is typically referred to as sound. The distinction is important to understanding that improving sound quality is not just about making things quieter. To some, a dripping faucet can be more intrusive than the jet engine noise from an airplane overhead. Sometimes a product’s sound quality can be improved by changing its character without actually lowering its noise level.

For product designers lacking expertise in this area, the whole field can seem like a black art combining physics and psychology, math and magic. But such a designer need not be intimidated; there are many good acoustical consultants and testing laboratories that can lead the designer through that murky territory.

A clothes washer undergoes testing in a reverberation chamber at Owens Corning.

For the manufacturers of this appliance, the path toward a winning solution started by finding an acoustic expert — Lyon — to test the product, explain the conundrum, and find a way to make a product that is quiet, but not too quiet. Or, to put it another way, quiet the part of the noise that is annoying, while keeping the sounds perceived as reassuring of the product’s performance.

As it turns out, consumers think the vacuum cleaner does a good job when they hear airflow from the nozzle where it meets the rug, but are annoyed with the noise created from the main suction fan and the motor cooling fan. By redesigning the components, including tweaking the impellers, the annoying sound was reduced and was masked by the less annoying sound from the nozzle.

From an OEM’s point of view, finding and fixing suspected noise problems can be pivotal in a marketplace where products are sold as being “three decibels quieter” than their competitor’s. In some cases, municipalities and other regulatory agencies address the noise issue from a comfort and health perspective. Many cities, for example, have laws regarding the operation and noise levels of leaf blowers.

But, as in the vacuum cleaner example, noise problems are not always easy to find or fix. Appliance manufacturers face a number of potential noise sources including fans, compressors, pumps and motors, some of which have intermittent operation. The vibrations from these components resonate off of nearby support columns and stamped parts, creating the noises that grate on nerves.

Acoustic holography is used by Owens Corning researchers to test where noise is emanating from an automobile.

Acoustic testing facilities offer OEMs the expertise and the state-of-the-art equipment that is used to track down noise and suggest ways to civilize it. But how do designers go about finding the best lab for their needs? Determining what lab to choose is not simply a matter of going to the NIST Web site and finding a list of certified labs, although that is a good resource. (For details, visit http://ts.nist.gov/Standards/scopes/acots.htm) The manufacturers first need to determine their objectives, says Kevin Herreman, program leader for the Owens Corning Acoustic Testing Center, Granville, Ohio.

The objectives may range from simply reducing overall noise levels, to improving sound quality, and may include providing technically accurate marketing materials or meeting governmental and industry standards. The objectives then logically lead to essential testing procedure questions.

Will the product as a whole be tested, or will tests be conducted only on noise-producing components? Will the product be tested in an anechoic chamber or in a simulated end-use environment?

A semi-anechoic chamber at Material Sciences $15 million acoustic research facility.

Do particular tests need to be done such as those from ANSI, OSHA or the European Union, which often require data to be how the human ear will respond? For that matter, what test data is required?

Are sound level measurements desired on the standard dB scale, or are they preferred on the A-weighted dBA scale? (A-weighting refers to a standarized means of measurement adjustments that take into account the fact that our hearing is more sensitive to some frequencies than others. So perceived loudness is not the same as actual sound intensity.)

Once noise sources are identified, further questions must be posed as to how to deal with it? Do parts need to be redesigned or respecified? Or can the situation be handled by either isolation or damping methods? If damping materials are to be employed, which ones are best for targeted frequencies, where are they optimally applied, and in what quantity and thickness? And, most importantly, will they do the job?

Proving out the noise-reduction capability of materials is one reason that in 2006, Material Sciences Corp. of Elk Grove Village, Ill., opened a $15 million, state-of-the art Application Research Center in Canton, Mich., featuring sophisticated acoustic testing chambers and equipment. The facility tests appliances, as well as automobiles and other products, to determine if its sound damping materials such as Quiet Steel, MagnaDamp and SoundTrap can be employed to tune down unwanted noise.

“The OEM can find out the exact benefits they can receive from these materials,” says Mark Gresser, vice president of sales and marketing. “That burden had previously been on the OEM to do the actual testing and proof.”

Gresser adds that OEMs searching for an acoustic laboratory must be confident that the results are fair and not fudged one way or the other. “We have taken great pains to be scientifically accurate and as neutral as possible so that when we say that Quiet Steel will work great for you, it is not just that we are trying to sell a product.”

Dan Eigel, general manager, OEM Products, at Owens Corning, says that being independent is a critical consideration for his lab. He adds, however, that having the ability to determine the cause of noise in the lab and deliver the means to solve the problem is something that many OEMs find appealing. Owens Corning, like MSC, will suggest a material solution involving their materials to dampen sound.

A microphone array at MSC's test facility captures sound levels in an anechoic chamber. Below this room, noise is generated and the microphones measure the decible levels that filter through the component, which is inset into the floor.

Acoustic testing labs can do many different jobs. MSC’s technology center for instance has the capability to analyze individual components and entire units in sizes ranging from the small, such as a disc drive, to the large, such as a truck.

It features acoustic array holography capabilities, and fully and semi-anechoic (quiet) chambers, and reverberation rooms, that have all of the needed hook-ups for repeatable and real world analysis and testing including water, gas, and electric hook-ups for all product types. Using state of the art facilities and computer aided engineering and simulation software, the three main parameters that drive noise and vibration problems — temperature, frequency and part design — are identified and the recommended solutions are developed.

The anechoic chambers — literally rooms that do not echo — feature walls made from wedges of fiberglass foam. The rooms are isolated so that it is incredibly quiet and the wedges will absorb the sound waves and reflect almost no waves back.

“The purpose of this type of room is to simulate the outside world but without any extraneous noise,” says Alan Hufnagel, NVH Technical Manager for Material Sciences Corp. “The room is so quiet it is at the threshold of hearing. If you have a microwave oven in here, any sound that it makes will be recorded without any extraneous sound interfering.”

Hufnagel adds that if testing for noise, rooms such as this give the researchers a repeatable and controlled environment. For additional testing purposes, a reverb chamber is located underneath the anechoic chamber and here noise is generated. A stamping can be fixtured to a “window” in the floor of the anechoic chamber. In the reverb chamber, loud speakers can blare noise and the researchers can measure the noise levels that are coming through the part.

These rooms can also help isolate where noise is coming from on an appliance, says Herreman, whose lab also features anechoic and reverb chambers as well as the acoustic holography technology.

Herreman says that the microphones placed strategically around the test object will measure the magnitude and direction of sound across the plane of the microphone and from there the noise can be tracked back to the surface of the product. “It is like shooting an arrow and knowing the angle of the flight,” he says. “Because I can track the waves back to the surface of the product, I can predict where the noise is coming from.”

This is one of the tests that laboratories such as MSC and Owens Corning can do, but not the only type of test. While not all labs can do every test, some tests that can be undertaken include airflow resistance of acoustical materials (ASTM C522), normal incidence sound impedance and absorption (ASTM E1050), diffuse field sound absorption (ASTM C423), sound transmission loss (ASTM E90), sound power (ISO 3741/ANSI S12.31 or ISO 3745/ANSI S12.35) and sound Intensity (ISO 9614).

Owens Corning Acoustic Research Center can conduct these material and other product tests. Two of the most often run material tests at Owens Corning are the impedance tube testing and the airflow resistance testing.

The impedance tube test is a fast way of generating the absorption characteristics of a material or material system. In this test, researchers use a tube about 4 inches in diameter. A speaker is at one end and a rigid hard wall is on the other end. The sample is mounted against the wall. Sound is generated with a speaker and it flows down the tube. The sound energy goes through the material, hits the rigid wall and is reflected back. Microphones measure the sound that is flowing down the tube and the sound energy reflected back and the difference between the two is what was absorbed by the sample.

The airflow resistance test works on the idea that sound energy can be turned into heat through viscous flow loss as sound waves move through the porous material. The determination of this property measures the ability of the material to dissipate acoustic energy. For example, noise from a dishwasher is often muted with a blanket. The goal is to ensure that the right amount of flow resistance is designed into that blanket material to match the performance requirements of that product.

The test blows air through a material at a velocity that maintains laminar flow and pressure behind the material develops and is measured. The backpressure is directly proportional to the resistance to flow within that material and the flow resistivity can be calculated from that. “Ultimately, from flow resistivity, we can predict the sound absorption from that material,” says Herreman.

In short, a good acoustic lab can provide an OEM with a wealth of information about the noise level and sound quality of their products, but in order to optimize the use of such facilities, OEMs should have some clearly defined objectives. And, as with any testing whose results may lead to a product design change, it is better to find solutions earlier in the design process than later.

For more information, email:
R.H Lyon.: lyoncorp@lyoncorp.com
Material Sciences Corp: Jeffrey.Vellines@matsci.com
Owens Corning Acoustic Research Center: Kevin.herreman@owenscorning.com