.jpg)
Image Credits: ioat/shutterstock.com
Sound; the phenomenon of traveling pressure waves by which we communicate. However, this governing aspect of our life is not a tangible thing we can observe, an invisible concept of nature. Yet the power of science has made possible the seemingly fictional concept of visualizing the invisible. This article presents the method of Schlieren optics to observe sound waves.
How does Schlieren Optics Work
The underlying principle behind Schlieren optics is the refraction of light waves when traveling into a region of differential density. This concept is often demonstrated by placing a straw in a glass of water and noting the apparent shifting of the submerged half. However, the refraction of light does not only occur when traveling through different mediums, but changes in density can also arise from changes in temperature of the fluid for example. Refraction can be described mathematically by the equation where K is taken to be a constant and n refers to the refractive index. Due to this relationship, the effects of changing density and refractive index are referred to interchangeably in literature.
The above concept was exploited in 1864 to achieve what is known as the Schlieren effect. The experimental procedure is as follows: A point light source is directed at a parabolic mirror which reflects this beam towards a thin wire or knife edge where this object acts as the light block. A camera is placed directly behind this light block with its lens focused directly on the test area in front of the mirror. Any change in refractive index in the test area will result in the refraction of the beam. If this refraction is great enough, the additional light that passes over the light block will appear in the camera image as a streak of light originating from the exact point in the test area at which the change in density occurred. What’s more, this procedure not only identifies the presence of a variation in density, but there is also a direct correlation between the brightness of this light streak and the magnitude of the change in density.
Visualizing Sound
Since its initial discovery, the Schlieren effect has been extensively used in the aerospace industry to model the formation of shock waves in wind tunnel tests. Since shock waves are defined as thin regions of dramatic change in pressure, it is therefore only a logical progression that sound waves should be able to be modeled given that they are fundamentally waves of pressure.
The experimental set up as described above is used with the addition of a sound source placed in the test area and a camera boasting high-speed frame rates used to capture a time lapse of Schlieren images. Once these images have been enhanced to increase the contrast and sharpness, the resultant sequence of photos provides a visual representation of the propagation of sound waves from the source. Inevitably, the ability to visualize this sound field comes with its limitations. The contrast of the produced images decreases with the frequency of the emitted frequency. Research undertaken by the Massachusetts Institute of Technology has demonstrated the capability to model frequencies down to 10KHz at 110dB. This is within the range of frequency detection for humans, however pressures above 85dB can cause permanent damage to the ear hence these test conditions cannot be classed as audible.
More recent research by the University of Oxford has shifted the focus from the replication of the sound field itself to identify a further acoustic application in relation to the recovery of the audio signal. Each pixel of a Schlieren image can be described as a spatial derivative of the sound field itself. The fluctuations in refractive index associated with audible sound are very low which results in a diminishing signal to noise ratio of each pixel’s time series and a consequential barrier to the representation of audible sound. However, the proposed approach by Oxford is to computationally combine all the series into one Schlieren output effectively eliminating the effect. This can subsequently be integrated to recover the original signal and thus has been deemed the “Schlieren microphone”.
Alternative Acoustic Visualization Techniques
Schlieron optics is certainly not the sole method of sound visualization nor the most advanced in terms of its development. Most notably, laser Doppler anemometry and laser Doppler velocimetry have demonstrated their capability to calibrate pressure microphones. However, these techniques are reliant on expensive, specialist equipment such as lasers. In contrast, Schlieren photography can be achieved with 4 simple pieces of equipment. Hence, with greater research, there is the potential that Schlieren optics could be the key competitor as a more practical mode of acoustic visualization.
Disclaimer: The views expressed here are those of the author expressed in their private capacity and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and conditions of use of this website.