At the low frequency resonance of the driver, where to excursion is dominated by the compliance of the suspension instead of the mass of the cone, another rolloff appears, also at 12dB/octave. Since the cone’s excursion decreases by the square of the frequency, the resulting frequency response curve (excursion times acoustic resistance) rolls off at 12dB/octave. At some high frequency (where the acoustic wavelength is less than the circumference of the cone) the acoustic resistance levels off to a constant value. The two functions cancel each other to create a flat acoustic output between the roll off points. Treated as a mass controlled piston working into an acoustic load, the typical cone loudspeaker derives the flatness of its response from the cancellation of two opposing effects-its excursion (which decreases as the square of the frequency), multiplied by the resistance of the acoustic load (which increases as the square of the frequency).įigure 1 shows this relationship on normalized logarithmic scales. WHY USE CROSSOVERS?Ī necessary evil, the crossover network exists solely to ameliorate the inability of most loudspeaker drivers to cover the full audio range. In the belief that proper attention to phase does produce better sound, I will discuss the design of crossover networks which ever minimum phase distortion. Nonetheless we know that the ear is sensitive to phase and uses phase cues to help determine directionality. The question of the audibility of these distortions has become the object of heated discussions regarding the perceivability of absolute phase, frequency dependent phase shifts, and the rate of phase shift. Historically, the phase integrity of the audio signal has been considered much less important than amplitude and harmonic/intermodulation distortions, but as more of those problems are solved and the quality of reproduction improves, phase distortion stands out in greater relief. They can change the steady state waveforms of vocal sounds so that the singer seems to be ten feet wide. Phase shifts in the audio signal destroy the wave shape of the important attack characteristics of many instruments and hamper our ability to perceive the localization of the image,smearing the apparent source. All audio components distort the phase of the signal to some degree-even air alters the time alignment of a signal, but the biggest offenders are loudspeakers and their crossover networks. It is not particularly obvious that two different frequency components of a signal can go into a device at precisely the same time and emerge at different times, but it is extremely common. The importance of phase response in the audio chain has been brought to greater focus recently by equipment claims of phase coherency, (the output signal has the same phase relationships as the input signal).
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