Tone Variator

TONE VARIATOR, an electric vacuum-tube generator of sound oscillations, used in surdotherapy for testing and exercising hearing. Properties of the T. required by the specified field of application: 1) sinusoidality of the produced oscillations (pure tones); 2) continuous change of frequency in the range from 50 to 10,000 oscillations per second (hertz); 3) change of sound intensity within wide limits (from the threshold of hearing to the threshold of unpleasant sensation); 4) stability; independence of frequency and sound intensity from internal and external conditions. The main parts of the T. are: the generator itself, the amplifier, and the output device. The main part of the device—the generator—has the task of producing an electric current with the specified frequency range and is built on the principle of beats. To form electric beats, two currents of different frequency are superimposed on each other, as a result of which a complex current curve is formed with an amplitude changing at a frequency equal to the difference of the frequencies of the fundamental currents. A similar phenomenon is observed in acoustics, when two sound sources of different pitch create beats, by which is understood the alternating amplification and weakening of sound. For the formation of beats in 573

TONE VARIATOR T. two high-frequency generators are used, of which one has a fixed frequency of 100,000 hertz, the other is variable for frequencies from 90,000 to 100,000 hertz (Figs. 1 and 2). The change in frequency of the latter is produced by changing the capacitance of an air capacitor in its oscillatory circuit. Thanks to the use of beats of specifically high-frequency generators, there is achieved, firstly, a continuous change

Figure 1. Schematic diagram of the tone variator of the Leningrad Scientific-Practical Institute for Diseases of the Ear, Throat, Nose, and Speech: G₁—fixed generator; G₂—variable generator; L₁—grid coil; L₂—anode coil; L₃—coupling coil; C₁—capacitor for fine-tuning; C₂—capacitor for frequency setting (frequency regulator); P—frequency scale switch; MC—coupling bridge; D—detector tube; F—filter for blocking high frequencies; A₁, A₂—amplifier tubes; V. C.—volume control; V. I.—voltage indicator; T—telephone receiver.

of frequency and, secondly, a change of frequency by one control element in the entire required frequency range. The obtained beat current is fed to a detector tube, after which the sound frequency, audible in the telephone, is obtained. According to requirements, the frequency range of the T. is divided into two regions: low tones, from 50 to 2,048 hertz, and high tones, from 2,048 to 10,000 hertz. In view of the insufficiency of the electric power developed by the generator part, a vacuum-tube amplifier with two stages of amplification is included after the detector. The amplifier has the task of equally amplifying all frequencies of the entire frequency range of the T. without distorting the shape of the current curve produced by the generator part. The amplifier, in turn, works on an output device,

Figure 2. External view of the tone variator for hearing testing.

which has the purpose of converting electric oscillations into sound ones with the possibility of adjusting the sound intensity. This task is performed by a telephone receiver and a volume control. To maintain stability in sound intensity, the output of the device is monitored by a voltage indicator. During manufacture, the T. is provided with a frequency calibration, giving the dependence of frequency on the divisions of the frequency regulator scale. Verification of the calibration during operation is performed by comparing the sound in the telephone at a certain division of the scale with a tuning fork at 512 oscillations (C²). The described construction of the T. is suitable both for examining hearing and for exercising it. A T. specifically for exercising hearing is produced in a simplified type with a motor drive for the frequency regulator capacitor. This eliminates the need for rotating the frequency regulator by hand when administering the procedure. Power supply for the T. used for hearing examination is carried out from storage batteries, while the T. for exercises can also be powered from the lighting mains, since in this case the requirement for the stability of the device is not essential. When installing the T., a dry room with a uniform temperature is chosen, removed from powerful electric power devices. The T. has found application in the fight against hearing impairment as a therapeutic and diagnostic apparatus. The relative simplicity of handling it and the speed of work give significant advantages over other methods of hearing testing, e.g., with the help of tuning forks. The T., manufactured by the means of the Leningrad Scientific-Practical Institute for Diseases of the Ear, Nose, Throat, and Speech, cost 2,500 rubles. Abroad, there are also devices similar to the T. The device closest in properties, known under the name oto-audion, is manufactured by the firm Audion-Krafft (Germany).

Medical application and clinic. The T., just like other types of devices built on the type of so-called electric sirens—oto-audions, audiometers (see)—constitutes for now only an accessory of acoustic laboratories and the acumetric equipment of ear clinics. Widespread distribution is hindered by relative expensiveness, complexity of the device, and insufficient portability; nevertheless, a great advantage of the T. is the possibility, when examining hearing, to obtain sounds of the desired pitch and intensity within a very short time over almost the entire volume of human hearing, and moreover, sounds that are relatively pure (devoid of overtones). A further benefit consists in the fact that when determining the threshold of auditory sensitivity, one can apply a gradually increasing sound, which eliminates to a certain degree the harmful influence of adaptation and fatigue, and the reading of data becomes more accurate. These properties of the device make it especially valuable for those cases of hearing examination when we are interested in obtaining a complete profile of it, or a curve of the sensitivity of a given hearing. To compile such a curve, or otherwise an auditory relief, the height of the sensation threshold is measured by means of an oto-audion for a series of tones, approximately at intervals of one octave or even better, one fifth. On specially graphed paper, a series of corresponding points are plotted and connected by a continuous line; usually, in healthy people, this results in a more or less symmetrical curve, indicating the greatest sensitivity of hearing to sounds in the treble zone of the tone scale (approx. 1,000–2,000 oscillations per 1 sec.) and a drop in sensitivity in the bass and ultramusical zone, reaching zero in the region of the lower and upper limits of hearing. In patients, the profile can also be symmetrical, although lowered throughout the entire extent of the curve, or asymmetrical. For example, in diseases of the inner ear, a sharper drop in hearing acuity may be revealed in the upper half of the tone scale. In some cases, an even more irregular curve is obtained with dips or even complete breaks. Such anomalies indicate more severe forms of damage to the auditory apparatus and are characteristic of deaf-mutism with remnants of hearing or, in general, afflictions of the sound-perceiving parts of the auditory apparatus. Furthermore, the device is convenient to use for a number of studies in acoustic physiology, when it is necessary to have a continuously operating and easily adjustable source of sounds, e.g., when studying the question of auditory fatigue, bone conduction (for which a special vibrating apparatus is attached to the oto-audion), binaural ability, etc. Thanks to additional devices—an amplifier, microphone, and telephone headphones—electric sirens make it possible to use them in exposing the simulation of deafness and for determining remnants of hearing in cases of a severe degree of its reduction. Therapeutic use of the T. is based on the possibility, with the help of this device, to obtain prolonged sound exposure of the desired intensity on the weakened organ of hearing, likened to the action of massage ('acoustic massage,' 're-education,' 'activation' of hearing). Indications are: underdevelopment of hearing, various types of dry catarrh of the ear, otosclerosis, and auditory neuritis of diverse etiology. The course of treatment usually lasts 2–3 months, after which a break of the same duration is made. The success is evaluated by authors differently. According to the data of Belogolovov, a good result (with improvement of hearing not only according to instrument readings but also for everyday speech) was obtained in approximately 1/3 of the cases followed up,

V. Voyachek.