Once hearing loss becomes severe, though, and low- and mid-range frequencies are affected, Delta has drawbacks, he said.

The problem was that, while low-and mid-range frequencies were good, high-frequency sound caused ear-splitting reverberations.

Uptempo tracks like "Bye Bye Bye" and "Pop" are filled with blaring mid-range frequencies that resemble crowd noise.

A reed's resonant frequency is a mid-range audible frequency of perhaps 300 Hz.

A crossover was also built-in, with the Vibratone handling the mid-range frequencies, and sending the high/low frequencies to the driving amplifier.

A presence control boosts the upper mid-range frequencies.

As the level of "presence" was increased, so more and more of the higher frequencies in the negative-feedback loop were dumped to ground, leaving the low and mid-range frequencies.

Chimpanzees, for example, have been shown to hear best at about 1,000 hertz and 8,000 hertz, so humans' greater sensitivity to mid-range frequencies can be seen as an evolutionary adaptation to speech.

The M 50 featured a small, diffuse-field equalized pressure transducer embedded in the surface of a 40 mm hard plastic sphere, which gave it increasing directionality above the mid-range frequencies.

Jones built powerful amplifiers, with the technologically advanced capacity to distinguish and enhance treble, mid-range and bass frequencies.