Carbon microphone. Roman Doronin Vitaliy Matiunin Aleksandr Severinov Vladislav Tumanov Maksim Tumakov. Russia IYPT

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1 Carbon microphone Roman Doronin Vitaliy Matiunin Aleksandr Severinov Vladislav Tumanov Maksim Tumakov Russia IYPT

2 The problem 2 For many years, a design of microphone has involved the use of carbon granules. Varying pressure on the granules produced by incident sound waves produces an electrical output signal. Investigate the components of such a device and determine its characteristics.

3 3 The principle of a carbon microphone

4 Design of a carbon microphone 4 Front plate = diaphragm Carbon granules Back plate Battery Output voltage Transformer

5 Edison s microphones 5

6 Concept of a loose contact 6 The electrical resistance of a carbon powder changes under strain due to: a change in the number of microscopic hills which are in contact with each other; a change in the area of their junctions.

7 7 Pressure experiment

8 Carbon microphone MK-16 8

9 Scheme of experimental setup 9 Buttery and ammeter Pressure recorder Microphone Pressure sensor

10 Pressure & current vs. time 10 Pressure (kpa) Current (A) Time (s) Time (s)

11 11 Frequency response

12 Frequency response graph 12 Frequency Ratio of output/input intensities

13 Measuring the frequency response 13 There are some ways to measure the frequency response of a microphone. The first way is to sweep a constant-amplitude pure tone through the operating range. The second way is to apply a signal with a constant power spectrum density and to observe the spectral response.

14 White noise 14 White noise is a random signal with a flat power spectrum density.

15 Experimental procedure 15 Spectrum analyzer White noise generator Microphone

16 MK-16 frequency response db +5 db 0 db 5 db 10 db 100 Hz 1000 Hz Hz

17 Frequency response of audio path 17 Sound card Loudspeaker Microphone Sound card

18 Test procedure 18 Sound card Loudspeakers Microphone 1 Microphone 2 Sound card

19 Electret microphone MIC db +5 db 0 db 5 db 10 db 100 Hz 1000 Hz Hz

20 20 Hand-made construction

21 Design of our microphone 21

22 Frequency response db +5 db 0 db 5 db 10 db 100 Hz 1000 Hz Hz

23 23 High-frequency response

24 Forced oscillations 24 Equation of motion Steady oscillations mx = F e i ω t x() t = 0 0 i t x e ω x F ( ω ) = m ω Amplitude decreases as the square of frequency

25 Resistance current power 25 The variable component of the resistance is proportional to the amplitude of oscillations: a R= R0 + e ω ω 2 i t The variable component of the current is proportional to the amplitude of oscillations: U a I = I0 1 e 2 R R0ω iωt The power of oscillated signal is proportional to the square of the amplitude: 2 N a 4 ω

26 High-frequency response 26 0 db 10 db 10 4 times 20 db 30 db 10 times 40 db 100 Hz 1000 Hz Hz

27 27 Resonant frequencies

28 Frequency response db +5 db 0 db 5 db 10 db 100 Hz 1000 Hz Hz

29 Oscillations of the diaphragm 29 Video 1000 fps oscillation frequency = 95 Hz

Natural tones of the box 30 10 cm Sound wave length 4

30 Natural tones of the box cm Sound wave length 4 0,1 m = 0,4 m Oscillation frequency 340 m/s : 0,4 m 850 Hz

31 Frequency response 31? 800 Гц = air oscillations in the enclosure +10 db +5 db 0 db 5 db 10 db 0 Hz 1000 Hz 2000 Hz

32 Filling the box with foam rubber 32? Decreasing of the resonance +10 db +5 db 0 db 5 db 10 db 0 Hz 1000 Hz 2000 Hz

33 33 Quality of carbon

34 Coking anthracite db +5 db 0 db 5 db 10 db 100 Hz 1000 Hz Hz

35 Pounded charcoal db +5 db 0 db 5 db 10 db 100 Hz 1000 Hz Hz

36 36 Microphone with charcoal tablets

37 Design of the microphone 37 Charcoal tablets, R Om

38 The surface of a charcoal tablet μm

39 Frequency response db +5 db 0 db 5 db 10 db 100 Hz 1000 Hz Hz

40 40 Response linearity

41 Scheme of experimental setup 41 Voltage Loudspeaker Sound level meter Microphone Current

42 Resistance vs. time 42 Nonlinearity 145 R (Om) 85 Frequency = 1000 Hz, volume of sound = 113 db

43 Excitation of high harmonics Hz 1000 Hz 2000 Hz 3000 Hz 4000 Hz

Response to 1000 Hz 44 24 db 26 db 33 db

44 Response to 1000 Hz db 26 db 33 db 31 db 1000 Hz 2000 Hz 3000 Hz 4000 Hz 5000 Hz

45 45 Summary

46 Conclusions 46 The carbon microphone operates on the principle of varying the resistance of loosely packed carbon granules as they change their contact area under the varying pressure of sound waves. To study the frequency response of an acoustic path a white noise can be used. Response of a carbon microphone decreases sharply at frequencies > 3000 Hz. This decreasing is explained by the inertia of the mechanical parts of the system.

47 References 47 Goucher F. S. (1934) The carbon microphone: An account of some researches bearing on its action. J. Franklin Inst. 217, Calvert J. B. (2003) Microphones.

48 48 Thank you for your attention!

3D Intermodulation Distortion Measurement AN 8

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