Radio Broadcast (May 1928-Apr 1929)

BROADCAST ENGINEERING BY CARL DREHER Loud Speaker Frequency Peak Correction SOMETIMES, in broadcast monitoring or other operations involving judgment of quality, loud speakers are encountered which would be suitable for the purpose except for a marked peak in response somewhere in the audio-frequency range. This undesired sensitiveness may readily be reduced to any desired degree by the application of an audiofrequency filter, which, in the form described below, is simply the familiar radio-frequency "rejector" designed to function at a lower frequency, i.e. , an audio frequency, the alternating-current principles remaining the same. Fig. 1 shows a radio-frequency rejector circuit intended to eliminate interference at a given frequency, f, to which the combination of inductance and capacity, L^Cr, is tuned. This path then presents a minimum impedance to incoming waves of frequency f, and the receiver tuning elements, LaCa, may be set for some other desired frequency without interference, the currents of frequency f passing to earth by way of the rejector path and hence causing no interference. Fig. 2 shows a loud-speaker characteristic, assumed to have been secured by such methods as those described by Wolff and Ringel: "Loud Speaker Testing Methods," Proc. I.R.E., Vol. 15, No. 5, May 1927; or Bostwick: "Acoustic Considerations Involved in Steady State Loud-Speaker Measurements," Bell System Technical Journal, Vol. 8, No. 1, January, 1929, and in numerous other articles. At 1000 cycles, it will be noted, there is a peak about 10 db above the general level of response at other frequencies. By an audio-frequency filter circuit such as that represented by the elements Lf, Cf, and Rf in Fig. 3,'connected in parallel with the audio coil of the loud speaker, such a peak in response may be smoothed out. The constants assumed are given only for purposes of illustration, although they are in the general range encountered in practice. + 30 " +20 « * a Clarence R. Clark, chief engineer for KJR, is pictured above at the controls of KJR's new 5000-watt transmitter The decrease in loud-speaker current required at 1000 cycles is first calculated from by some other standard method (see, for example, Ramsey: Experimental Radio), in which, however, care must be taken to feed the measurement circuit from a 1000-cycle source and to adjust the current through the loud speaker so that it will have substantially the same constants as under normal operating conditions. If we assume that the 1000cycle impedance is found to be 20 ohms, then the filter must have an impedance of 10 ohms at the same frequency. Part of this 10 ohms will be in the coil Lf, the resistance of which may be measured with d.c. on a Wheatstone Bridge; the remainder is made up in the separate resistance Rf. All that remains is to calculate Lf and Cr to resonate at 1000 cycles. The formula used is Response db = 20 log j-1 It is found that for a 10 db decrease the ratio between the two currents must be 3.16, or, approximately, the initial current should be f = 5033 Vlc (3) Rejector Circuit Receiver Fig. 1 reduced to one-third. This means that in the filter branch of Fig. 3 the current at 1000 cycles must be twice the current through the loud speaker voice coil. It is known that if the loud speaker impedance at 1000 cycles is Zs, and the filter impedance Zr, the current from the amplifier will divide in the ratio where f is in cycles per second, L in millihenries, and C in microfarads. The conditions will be satisfied approximately by a mica condenser with a capacity of 2.5 microfarad, and a 10-millihenry coil of the honeycomb type, or by a 1.0-microfarad condenser and a 25-millihenry coil. The resistance of the coil being measured, as outlined above, the proper series resistance may be added to smooth out the 10 bd peak in the loud-speaker response. Generally speaking, the sharpness of tuning of such audio-frequency rejectors matches the sharpness of peaks in the loud-speaker response curve, so that there is not much chance of doing damage to the frequency characteristic of the loud speaker by introducing troughs on either side of the peak which is eliminated. Above the resonant frequency the filter has an inductive reactance, which soon rises to several hundred ohms as the frequency is increased, while below the resonant frequency the same effect takes place with a capacitive reactance. Thus the filter serves to cut off the peak at the frequency for which it is designed, and has little effect on the response at other points in the band. Is If Zf Zs (2) Fig. 2 It follows that at 1000 cycles, for the purpose in question, the filter circuit in Fig. 3 should have only half the impedance of the loud speaker. The latter quantity is determined by measurement on an inductance bridge, or r 1 i f! is i L From Amplifier Output Loud Speake Audio Coil i Filter Circuit 1 'Zf Fig. 3 • april, 1929 page J02