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If two sound waves arrive 1/2 wavelength apart, or 180 degrees out of phase, they subtract and cancel, resulting in no sound level. If two sound waves arrive at the same exact time, or in phase, they add together and become louder. Since the entire dish surface is collecting sound energy, many separate wave fronts are being combined at the focus. Within each wavelength, the sound pressure will be increasing during half the wavelength, and decreasing during the other half, see Figure 2, forming a sine wave. Their wavelength determines their behavior and gain in a parabolic microphone. The sound waves develop an actual length called their wavelength, Wavelength in inches = 13560/Frequency, see Table 1. This motion causes a very important principle in understanding how the sound waves will behave when encountering the reflector. Now, let's look at what sound waves actually are: Sound waves are pressure fluctuations in the air, and they travel at the speed of sound, which is about 1130 Feet per Second. With a telescope, the image would become blurred, and with sound the gain will suffer losses as frequency increases. A spherical section or any other shape will not yield a proper focus. This principle is exactly analogous to an astronomical reflector telescope, which is also a perfect parabola. These two intrinsic properties allow sound collected by the parabolic dish to concentrate at the focal point, where you then locate your microphone, see Figure 1. 2) The angle at each point on the dish allows sound waves to reflect exactly to the focal point. This focal point has: 1) Equal path lengths for incoming sound waves from the distance, from anywhere on the dish. Paraboloids Used with Microphones and Accuracy RequiredĪ Paraboloid shape has TWO very important properties that make it suitable for use with a microphone that no other shapes have: Referring to Figure 1 again, you will notice there is a virtual focal point (F), which is easily calculated, F = 1/4a.
RECORDING AUDIO IN REFLECTOR 3 HOW TO
Later we will show you how to check any dish for compliance with a Parabola, using Equation 1), simple rulers, and a hand calculator. A different shape factor will give you a different depth and focal point. Notice that the example dish is 6.0 inches deep, and the focal point is 4.17 inches. Figure 1 is a graph of the resulting shape. Now, Y is the vertical point on the curve that can be calculated for each X radius up to 10, for say our example 20-inch diameter reflector. For a simple parabolic reflector example, b always = 0, a= the shape constant, say. When the Parabola is spun around on it's axis, creating a bowl-like shape, it is then called a Paraboloid. Equation 1) Y = aX^2+b, is the Parabolic function, it is used to calculate the Y value, for any X value which is the radius. Don't get worried, it is a very simple function, see Figure 1. First you may want to read our other articles for background information about the basic concepts of dBs and sound which will not be explained fully here.Ī Parabola is a type of mathematical function know as a Quadratic Function. We hope this information will improve your understanding of the subject, and help dispel the misconceptions, deliberate misleading information, and resulting confusion about parabolic microphone dishes and their use. You will also learn a simple method to check the accuracy of any dish. Then gain, the main reason a parabolic dish is used, will be discussed, and the accuracy requirements to realize that gain. The concept of a Parabola will be explained first, and how it functions with a microphone.
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The information will be confined to only address the dish accuracy numerous other issues can also affect parabolic microphone performance even at lower frequencies – see our other Articles for more. This article will explain the requirements of a parabolic reflector for use with a microphone, and answer the question, “How accurate does a parabolic dish need to be, and how good should it work?”.