 A Device for the Critical Midrange The purpose of this investigation is to describe a device that has been designed specifically for the frequency band that extends from about 200 Hz to 2000 Hz, which is commonly referred to as the "midrange." It shall be put forth that this decade is critical to sound reproduction since it contains the majority of audio information for speech and music. Support for this assumption will begin with an overview of the basic information from which it is drawn. The human hearing system is sensitive to modulations, or ripples, in the atmospheric pressure that surrounds us. This pressure can be "rippled" anywhere from less than once per second to many thousands of times per second. The typical human ear/brain system is sensitive to frequencies between 20 and 20,000 cycles per second, or Hertz (Hz). The purpose of a sound reinforcement system is to reproduce this spectrum with sufficient acoustic power to produce a desired level at some listener distance. The human ear, as marvelous a device as it is, does not cover the entire spectrum with equal sensitivity. In fact, the human ear/brain system is optimized for reception of the middle portion of the entire passband, most likely due to our dependence upon speech in our day-to-day lives. This middle portion, which we will call the midrange, contains the most critical information for the human receiver. With this in mind, it is interesting that this critical band has long been overlooked in the design and implementation of sound reinforcement components and systems. One purpose of this paper is to put forth the idea that the midrange is the most critical part of the spectrum, and that superior reinforcement systems can be constructed if developed around the midrange decade. The first step of this investigation is to determine the requirements for an optimum transducer for coverage of the middle decade of the audible spectrum. Such a transducer would serve as a complement to the already existing variety of excellent transducers for the other two decades of the audible spectrum. While there are many approaches to component design, perhaps the best course is to allow such a device to define itself based upon the physics of the sound that we wish to reproduce. This approach will discourage any temptation to take an existing transducer and stretch its parameters to include the middle decade, a common practice in the audio industry. As with the other two decades in the audible spectrum, the requirements are clearly defined by the physics of the sound that we wish to reproduce. The essential requirements for optimum midrange reproduction shall include, but not be limited to:
Let us first describe the desired passband of such a device based on the essential parameters of wavelength and distortion. The response of all transducers is frequency dependent. This simply means that they do not and cannot behave the same at all frequencies, due to the physical characteristics of sound waves. In everyday life, the physical size of something nearly always determines how we handle it. The same is true for sound waves. Since sound propagates at a velocity that is not frequency dependent, the physical size of a sound wave is directly related to its duration and velocity. This can be understood by considering that a single cycle of a 1000Hz tone lasts for about 1ms. Since this wave is traveling at approximately 1130 feet per second (344 meters/sec), distance (or length) is equal to the product of time and velocity, the length of the wave will be:
 A 100Hz tone has a period of about 10ms, and since it travels at the same speed as the 1000Hz wave (but lasts longer) its physical length becomes about 11 fee t(3.44 m). The common term used to describe the physical length of a wave in free space is "wavelength." Since wavelength is inversely proportional to frequency, as the frequency goes lower, wavelengths get longer. And, as wavelengths get longer, the physical attributes of the transducers that reproduce them must be altered accordingly. Since we are defining the characteristics of an optimal midrange device, the passband of such a device should be considered in terms of wavelength. While such a description will clearly define the physical dimensions of a pattern control horn, it will also serve to define the size and other physical properties of the piston that shall ultimately drive such a horn. Our optimum midrange device must be capable of generating large amounts of acoustic power from 250Hz to 2500Hz, and the waveguide must provide directional control for the corresponding wavelengths ranging from about 6 inches to 6 feet.
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