Have you ever watched an audio speaker’s cone move while listening to music? If you have, you know that the cone moves in and out as the music plays. Have you ever wondered how that movement looks on a microscopic level? Have you ever wondered how the sound waves themselves actually look as they escape the speaker and make their way to your ear?
Of course you know that sound waves are invisible. However, laser measurement tools have become so powerful that it is possible to use them to measure and visualize sound waves. In fact, Bowers & Wilkins does just that with laser tools (1). In a February 2013 blog post, the company’s senior product manager, Mike Gough, described how the company uses laser measurement tools to understand how the various components of a speaker system behave.
According to Gough, laser measurement tools are used in several ways. The first, and most basic, use involves using a laser to create a three dimensional hologram of an object. Another method known as speckle interferometry involves the use of a video sensor rather than a hologram. The sensor is illuminated by laser light reflected by the object and superimposed over the laser light. These two methods are among the older methods used, and they tend to be both difficult to interpret and prone to “noise.”
To overcome these problems, Bowers & Wilkins uses a technique known as “Scanning Doppler Velocimeter.” This technique measures velocity, not shape. However, the object’s shape can be simulated and superimposed over the image. Using Scanning geluidsmeting laten uitvoeren Doppler Velocimeter readings, the company can evaluate specific parts of a speaker or other object of interest. Sometimes the company looks at a single frequency and the outer ring of the diaphragm; other times it examines the entire audio band across the whole cone.
In addition to using a laser tool to get precise measurements, Bowers & Wilkins uses software to plot those measurements into two and three dimensional graphs. For example, Coherent offers a full range of laser measurement and control products including laser power and energy meters, laser beam diagnostics, laser tools, laser mode measuring, and spectral analysis instruments. By using an entire system, it becomes possible to better understand what all those measurements actually mean.
What about those invisible sound waves making their way across the room to your ears? How do you “see” those? Bowers & Wilkins used laser measurement tools and a series of lightweight cling film sheets suspended in front of a tweeter (2). These film sheets were coated with a reflective powder. Instead of using the laser tool to scan the surface of the tweeter, they used it to scan the sheets as sound played. As a result, you can actually see the sound waves as they move from the scanner and through the series of film sheets.