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9. Investigation of standard Electret Mics in the Ultrasonic Range V1.4 UFO Doctor, Nov. 27th, 2011 1. Introduction

Miru and UFO Doctor developed an innovative distance and direction detection method by ultrasonic FSK (Frequency Shift Keying) and PLL (Phase Lock Loop). This method is less sensitive to echo-reflections and does not depend on the received sound amplitude as long it is above the 2 mV threshold of the PLL input. A first application of this method is the autonomous control of a model baby duck by acoustic signals transmitted from a model mama duck. The sound frequency range is 21 to 29 kHz, well above the human hearing capability. Both speaker and microphone should show omnidirectional characteristics. This report shows the preliminary result with low cost speakers and microphones. 2. Material and method

- HS: High woofer speaker Visaton TW 6 NG, 3.5 k to 30 kHz (See data sheet on internet: specified for 1 to 20 kHz only) - Definitions: Mode VERTICAL = HS membrane vertical, see Fig. 2 Mode HORIZONTAL = HS membrane horizontal, see Fig. 6 - Mic: Courtesy by JLI Electronics Inc., Harleysville, PA, USA, four types:

(see data sheet on internet: specified for 1 to 20 kHz only) A1/B1: JLI-60AY107 A2/B2: B6022AP-C A3/B3: JLI-64PNT A4/B4: F6027AP423-31 - Sound absorbing test setup environment, by household methods, see Fig. 2 - Signal generator, sinus 2Vpp (about 125 mW), Test 7 HS inside box with 6Vpp. - Supply for Mic: 2V, 2.2kOhm according to data sheet JLI - Storage oscilloscope, amplitude measurement by visual reading of the received sinus amplitude (do not laugh: it’s the best method in a noisy environment!) 3. Test Setup

3.1. Microphone Tester

A1 A2 A3 A42 1: 3

Fig. 1. Microphone Tester 1: Mics A and opposite Mics B A1/B1: JLI-60AY107 A2/B2: B6022AP-C A3/B3: JLI-64PNT A4/B4: F6027AP423-31 off: Capacitor 33 pF only 2: Shielded cable 3: Rotary Switch (Design details in Appendix 9)

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3.2. Sound absorbing test setup Precise acoustic experiment should be performed in a large, silent room with sound-absorbing panels. But for qualitative measurements a room with soft woolen blankets and tilted side walls (against total reflections) are permitted.

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Fig. 2a. Test Set-up Front view 1: High Woofer Speaker Mode VERTICAL 2: Mic Tester 3: Soft woolen blanket 4: Storage Oscilloscope 5: Signal Generator 6: Supply Mic 2V

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Fig. 2b. Test Set-up Side view 1: High Woofer Speaker Mode VERTICAL 2: Mic Tester 3: Soft woolen blanket (angle about 30 Deg) 4: Storage Oscilloscope 5: Signal Generator 6: Supply Mic 2V

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4. First Experiments on Acoustic Signal Transmission (Original EXCEL measuring date available on request!) 4.1. High woofer speaker (HS) directed to Mic tester (Mode VERTICAL)

Fig. 4a. Frequency Response 0 to 40 kHz Face to Face A1 to A4: Different JLI Electret microphones

Fig. 4b. Frequency Response 0 to 40 kHz Face to Mic Rear 180 Deg A1 to A4: Different JLI Electret microphones

Fig. 4c. Frequency Response 0 to 40 kHz Face to Mic side 90 Deg A1 to A4: Different JLI Electret microphones

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Mic

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pp

Frequency kHz

HS Face to Mic Face, 0 degree

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A2

A3

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HS Face to Mic Rear, 180 degree

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5. Experiments on Microphone Directivity 5.1. Directivity test with High woofer speaker (HS) Mode VERTICAL

Fig. 5.1a. HS VERTICAL Mic A1 Amplitude Frequency Response 20 to 30 kHz 0: Mic at front 180: Mic at rear side 90: Mic at left/right side

Fig. 5.1b. HS VERTICAL Mic A2 Amplitude Frequency Response 20 to 30 kHz 0: Mic at front 180: Mic at rear side 90: Mic at left/right side

Fig. 5.1c. HS VERTICAL Mic A3 Amplitude Frequency Response 20 to 30 kHz 0: Mic at front 180: Mic at rear side 90: Mic at left/right side

Fig. 5.1d. HS VERTICAL Mic A4 Amplitude Frequency Response 20 to 30 kHz 0: Mic at front 180: Mic at rear side 90: Mic at left/right side

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5.2. Directivity test with High woofer speaker (HS) Mode HORIZONTAL

Fig. 5.2a. HS HORIZONTAL Mic A1 Amplitude Frequency Response 20 to 30 kHz 0: Mic at front 180: Mic at rear side 90: Mic at left/right side

Fig. 5.2b. HS HORIZONTAL Mic A2 Amplitude Frequency Response 20 to 30 kHz 0: Mic at front 180: Mic at rear side 90: Mic at left/right side

Fig. 5.2c. HS HORIZONTAL Mic A3 Amplitude Frequency Response 20 to 30 kHz 0: Mic at front 180: Mic at rear side 90: Mic at left/right side

Fig. 5.2d. HS HORIZONAL Mic A4 Amplitude Frequency Response 20 to 30 kHz 0: Mic at front 180: Mic at rear side 90: Mic at left/right side

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6. Some ideas to improve the horizontal sound radiation An improved omnidirectional sound radiation in the horizontal plane is possible by reflectors. Such a reflector improves the line of sight reception (0 degree), BUT decreases the reception at the rear side (180 degree). The received acoustic signals at front, left/right side and back of a microphone should not differ very much for further signal processing. Thus, it is better to radiate an ultrasonic sound vertically in the air! Anyhow, here are the experiments with 3 different reflectors. The reference amplitude (without reflector) is 0.2 mV. The mode is HORIZONTAL, Mic at 0 degree:

Fig. 6a. Best horizontal radiator: Paper Cup, spaced about 20 mm to speaker Amplitude at 0 degree: 0.6 mV@25 kHz Comment: Very difficult to implement this resonator in practice!

Fig. 6b. Aluminum cone 90 degree, spaced 10 mm to speaker Amplitude at 0 degree: 0.5mV@25 kHz

Fig. 6c. Ping-Pong Ball placed directly on HS Amplitude at 0 degree: 0.4 mV@25 kHz

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7. Experiments with HS speaker inside of a cardboard box, with Mics looking upwards. 7.1. New test situation The two ears of the baby duck are now aligned vertically upwards! (Not horizontally left and right as before!). A rotation of the baby duck head will not cause amplitude variation of the received sound.

Fig. 7. Frequency response Mic A1 to A4, looking upwards, HS speaker free, mode HORIZONTAL, speaker supply: Sinus 6Vpp, transmission distance 0.5m. All Mics are ok, but A4 (F6027AP423-31 JLI Electronics) is the best, However, the received signals are small and further experiments with a band pass filter are need! 7.2. Experiments with speaker mounted inside of a box The (HS) high woofer speaker of 132 grams should be mounted at the bottom of mama duck in order to keep the center of gravity as low as possible. C1 Speaker HS free, HORIZONTAL C2 HS inbox, totally closed C3 HS inbox, HS at bottom, hole D55 on the top of the box C4 HS inbox, HS at bottom, paper tube between HS and top hole C5 As C4, but top hole protected by an inox plate with D4 holes C6 As C4, but top hole protected by a bread board with D1 holes Table 1. New test situations (see Figs 7a-f below), Dimension of the cardboard box: 95x145x195 mm

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Fig. 8a. Situation C1 Fig. 8b. Situation C2 Fig. 8c. Situation C3

Fig. 8d. Situation C4 Fig. 8e. Situation C5 Fig. 8f. Situation C6 Experimental results:

Fig. 9. Experimental results with situation C1 to C6; Mic A4 showing vertically upwards. Situation C5 (with protection mesh with 4mm holes) looks good! 8. Discussion: - The reception of ultrasonic signals by Electret microphone is possible - A band pass filter is needed since the sensitivity (especially in case Fig.9.) is 30 dB below the audio signals arriving in the axis of the Mic. - Outdoor experiments in a noisy environment are urgently needed!!

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9. Appendix Drawing Mic Tester

PAN00.00.00PAN25.11.11

C1

1:1

Mic Tester

(Gewicht)

Blatt

Ersatz durch:Blätter

Maßstab

(Rohteil-Nr)

(Zul. Abw.)

Bearb.Gepr.

Datum Name

(Oberfl.)

Ersatz für:

Norm

Ursprung

(Verwendungsbereich)

DatumZust. Änderung Name

(Werkstoff, Halbzeug)

(Modell- oder Gesenk-Nr)

red 3V

white rightblackGndgreen left

shield Gnd

Rotary SwitchBottom view

A1A2

A3

A4

B1 B2 B3 B4

A1A2A3A4

Rotary Switch

A1A2A3A4

B1B2B3B4

Shielded Cable

Mics under TestA1,B1: JLI-60AY107A2,B2: B6022AP-CA3,B3: JLI-64PNTA4,B4: F6027AP423-31C: Capacitor 33pF only for noise testR: 2.2 kOhm

Plastic Box 50x50x30 mm Mics under Test

Shielded Cable

R

R

Shielded Cable

left side

right side

Mics courtesy by JLI Electronics Inc, Harleysville, PA, USATester: UFO Doctor, Switzerland (pan.neukomm@bluewin.ch)

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0 13 21 29 37

25

005

D4.3

D6

0 25 35

D10D3

C

C

sticker

12 3 4

off

JLI Microphone Testerwww.UFO-Doctor.ch

Fig. 10. Sketch of the Mic Tester