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Metal and Sound

Perforated metal can perform as a sound suppression surface, reducing noise and echo. These surfaces can be sound-transparent, allowing sound frequencies to pass through them and into another sound-absorbing material. Or perforated metal can act as a "tuned acoustic absorber." Depending on the hole size and spacing, various frequencies pass through the holes into the surface behind without reflecting back.

In the first instance, when perforated metal acts as a transparent acoustic cover, the amount of free area needs to be maximized. Sound-absorbing insulation material must be placed directly behind the surface to capture the sound frequencies. One inch (2.5 centimeters) of glass-fiber insulation, for example, will absorb high-frequency sounds effectively, but other frequencies are not so readily absorbed. Increasing the glass fiber to six inches (15 centimeters) makes the absorption very efficient, approaching 99 percent of all incident noise energy.

When the metal surface is being used as a tuned resonance absorber, sound is absorbed in a narrow range of frequencies. The frequency to be absorbed can be determined by considering the thickness of the perforated metal, the openness of the perforated sheet, and the distance back to the absorbing material.

Additionally, perforated metals can be designed to suppress other wavelengths and frequencies. Perforated metals can inhibit microwaves, radio waves, and other electromagnetic waves.

Metal and Light

Perforated metals allow light to pass through and reflect on other surfaces, creating shadows and patterns. The light passing through the perforations creates patterns that match those cut into the metal but elongated by the angle of light and altered by light-bending effects.

The interaction of the perforated metal surface with light can be one of the more intriguing and beautiful behaviors observed with metal. The effects are multiplied when the interplay of light is not simply with a single surface but with multiple surfaces. Moiré pattern effects develop when one perforated surface is positioned over another.

The image formed on the retina at one instant is superimposed over the image formed the moment before. Your eyes constantly scan the surface, and the overlapping images give the illusion of movement. With round holes, the patterns are round and radiate as you walk about the surface.

Another intriguing effect occurs with light passing through perforated metal surfaces onto another surface. Light interference causes light and dark fringes to appear. At the dark regions, the perforated holes appear as a light contrast, while on the light bands, the perforated holes are fuzzy and less defined. This is due to interference from one opening to the next. Different effects are generated from different patterns.

Manipulating Transparency

Perforated metal surfaces can look solid during daylight and be internally illuminated beacons at night. Depending on the pattern and hole spacing, the surface can give the impression of solid illumination. Or, if the perforations are more irregular, the surface can take on a motif as the light creates strong contrasts. When backlit, the metal becomes dark, regardless of the color and reflectivity.

Apparent transparency through perforated surfaces is affected by the size of the perforations and their spacing. If the surface is pierced with many regular small holes, and the open area is about 40 percent range or greater, the surface acts more like a screen. Your eyes focus on the brighter regions, which are on the other side of the perforated metal. If the perforations are fewer and the open area is less than 40 percent, your eyes tend to focus on the metal.

If the holes are large, 1 inch (25 millimeters) in diameter or greater, transparency depends on how close you stand to the surface. At close proximity, say 6 feet (2 meters) from the wall or closer, your eyes tend to focus on a single hole or a few closely spaced large holes. As you move away, the perforated wall takes on the appearance of a large diaphanous screen.

Mixing small holes and large holes on the same surface can produce disturbing and uncomfortable effects as you attempt to look through the surface. When you stand close to it, your eyes have difficulty focusing. As your eyes scan the surface, they go from the distant view through the large holes back to the surface at the small perforations.

There are a multitude of mechanical processes that can be used on metal. Because they are currently limited only by the physical restraints of the machines or the artistic ability of the finish applicator, you can expect the number and variety of finishes to increase in the future.

The mechanical finishes sit well between the chemical processes and deposition process used to enhance the surface appearance of metals. Many of the finishes can be combined to produce stunningly beautiful surfaces unlike any seen before.

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L. William Zahner is president and CEO of A. Zahner Company and Zahner Architectural Metal Consultants. He has worked with many of the world's leading architects and contributed to a number of high-profile projects using metal as a major building material.

This article is excerpted from Architectural Metal Surfaces, copyright © 2004, available from John Wiley & Sons and at Amazon.com.

SUBSCRIPTION SAMPLE Custom-embossed and perforated panels for the de Young Museum of Art, designed by Herzog and de Meuron. Photo: Courtesy of L. William Zahner Custom-perforated stainless steel screen designed by Gould Evans Goodman Architects for the Cerner Corporation, North Kansas City, Missouri. Photo: Courtesy of L. William Zahner Two standard patterns, staggered grid and straight grid, both with round holes. Image: Courtesy of L. William Zahner Custom perforation over a glass curtain wall. Photo: Courtesy of L. William Zahner Custom-perforated stainless steel wall. Photo: Courtesy of L. William Zahner Custom waterjet-cut perforated sconce. Photo: Courtesy of L. William Zahner Light passing through perforated metal creates light bands of varying intensities. Image: Courtesy of L. William Zahner Architectural Metal Surfaces by L. William Zahner. Photo: Courtesy of L. William Zahner   Click on thumbnail images to view full-size pictures.