Black Box Explains...Speaker sound quality..
A human with keen hearing can hear sounds within a range of about 20 Hz to 20 KHz. But most human speech is centered in the 1000 Hz range, so most old-fashioned analog telephone networks provided audio bandwidth only in this range. This range transmits most voice information but can fail to register voice subtleties and inflections.
Because these older analog phone systems had such a narrow bandwidth, headset manufacturers built their products to operate only in those particular frequencies.
When digital networks and fiber optic connections came into use, however, they provided a much wider bandwidth for voice transmission. This led to a corresponding increase in headset sound quality.
Today, quality headsets take advantage of increased network bandwidth and typically can reproduce sounds in the 300 Hz to 3500 Hz range. This makes voices far easier to understand and enables you to pick up all the nuances and inflections of your caller’s voice.