Articulation and Intelligibility Tests

As a result of articulation tests valuable facts have been determined. One of these is the relation between the intensity of speech sounds and the articulation, the results of this study being shown in Fig. 24. Zero on this scale is taken²¹ as the intensity level existing at the ear when a speaker talks in ordinary conversational tone with his lips ¹/₂ inch from the listener's ear.

Figure 24. Relation between the intensity of speech and articulation. (Courtesy Bell Telephone Laboratories and D. Van Nostrand Company.)

Articulation is independent of the intensity level over a wide range of variations. As the curve shows, the speech intensity may be increased 100 times (or 20 db) or decreased to about one one-millionth (60 db) of the initial intensity without greatly affecting the articulation.

A study was made to determine the relation between articulation and frequency by inserting electrical filters into the circuits being used. These filters were of two types - high-pass and low-pass filters. The high-pass filters were designed to pass only those frequencies of the speech currents above a certain cutoff frequency; and the low-pass filters were designed to pass only those frequencies below a certain cutoff frequency. By inserting into the circuits different filters having different cutoff points and by making articulation tests the curves of Fig. 25 were obtained. "Articulation L" and "Energy L" were obtained with low-pass filters; the "H" curves were obtained with high-pass filters.

Figure 25. The effects of eliminating certain frequency regions on the articulation and energy of speech.

This figure shows that the lower frequencies carry most of the speech energy but that they contribute very little to the intelligibility. Also, these curves show that the high frequencies contain very little speech energy but contribute greatly to the articulation. If only those frequencies above 1000 cycles are allowed to pass through a circuit, the articulation is about 86 per cent perfect but the energy content is decreased to about 17 per cent of the total. If only those frequencies below 1000 cycles per second are transmitted, then the curves show that about 83 per cent of the total speech energy is present but that the articulation is reduced to about 42 per cent. This can be summed up in the statement that the high frequencies contribute most to the intelligibility, and the low frequencies contain most of the energy.

Commercial telephone service is provided if a band from about 250 to 2750 cycles is transmitted.⁴⁵ The frequency band has been widened to about 200 to 3500 cycles for many modern telephone circuits, giving better intelligibility and naturalness.

The band-width requirements for broadcast program networks were discussed in the section on Music. Further information will be found in references 46 and 47. For frequency-modulation radio broadcast the band is arbitrarily set at 30 to 15000 cycles.