ELECTRONICS APPLICATION NOTE AN-6 FerrofluidTM Damped ED Receivers Introduction A new damping technique has been developed that provides adjustable damping without a screen. Ferrofluid injected into the magnetic gap of the receiver’s motor acts as a viscous damper to reduce the motion of the armature at resonance (Figure 1). The fluid is a suspension of microscopic magnetic particles (Fe3O4) in viscous oil that is retained by the magnetic field in the receiver’s motor. Fluid-damped Response appropriate fluid volume. Figure 2 on the next page shows that the range of damping options extends from as little as 2dB up to critically damped (i.e., delta peak ± 0.5dB relative to 1 kHz). The peaks in the response are reduced without affecting the nominal sensitivity at 500Hz. Figure 3 (next page) shows the same receivers under constant current drive conditions. “Damping” refers to the change in the mechanical resonance, which is the first peak in the frequency response with shorter ITE tubing (i.e., the peak between 2kHz and 3kHz voltage drive for the ED receiver). The amount of damping can be adjusted over a wide range by selecting the Figure 1: Cross-section of the Knowles FED receiver (i.e., ferrofluid-damped ED receiver) application note Ferrofluid-Damped ED Receiver 120 Constant voltage drive with ITE tubing (10mm x 1mm) into 2cc Coupler 115 Sensitivity (dB SPL) 110 105 100 95 90 delta peak = 8dB (undamped) delta peak = 6dB (2dB damping) delta peak = 4dB (4dB damping) 85 delta peak = 3dB (5dB damping) delta peak = -0.5dB (8.5dB damping) 80 100 10000 1000 Frequency (Hz) Figure 2: Frequency response curves under constant voltage drive conditions for fluid-damped FED receivers and a response curve for an undamped ED receiver. Labels in legend are the delta peak values. Ferrofluid-Damped ED Receiver 120 Constant current drive with ITE tubing (10mm x 1mm) into 2cc Coupler 115 Sensitivity (dB SPL) 110 105 100 95 90 delta peak = 13dB (undamped) delta peak = 10dB (3dB damping) delta peak = 8dB (5dB damping) 85 delta peak = 6dB (7dB damping) delta peak = 3dB (10dB damping) 80 1000 1000 10000 Frequency (Hz) Figure 3: Frequency response curves under constant current drive conditions for fluid-damped FED receivers and a response curve for an undamped ED receiver. Labels in legend are the delta peak values. Damping is calculated by subtracting the damped delta peak from the undamped delta peak and is specified as “dB damping relative to X”, where X = 8dB constant voltage drive for the ED receivers (see Equations 1 and 2). Delta Peak = Peak Sensitivity (dB SPL) - Sensitivity (dB SPL) @ 1kHz (1) dB Damping = Delta Peakundamped - Delta Peakdamped (2) Knowles specifies fluid damping under test conditions that include ITE or BTE tubing connected to a 2cm³ coupler and constant voltage drive. The amount of damping will 2 be similar under different acoustic loads (i.e., various tubing lengths and 2cm³ or Zwislocki-type couplers). Ferrofluid can dramatically reduce the mechanical resonance and the tubing resonances above the mechanical peak (i.e., > 2kHz). The amount of peak damping is shown in Figure 4 below for the FED receiver connected to ITE tubing. The second peak (tubing resonance) falls off much faster than the mechanical peak. However, when using longer tubing (i.e., BTE tubing), the fluid is not as effective in damping the tubing resonance below the mechanical peak (i.e., peak at ~1.3kHz). barometric relief), and Type III (modified barometric relief). For ITE and CIC applications, eliminating a screen in the port tube is desired because of potential problems due to wax build-up. Ferrofluid can be substituted for screen damping. Figure 5, on the next page, shows a magnified view of the peaks for several types of damping when ITE tubing is used. For BTE applications, the multiple resonant peaks created by the longer tubing are affected to different degrees by a screen (Type I), a diaphragm pierce (Type III), and ferrofluid. Ferrofluid alone is not as effective as Type II in reducing the first (acoustic) peak in the BTE response curve (i.e., peak near 1.3kHz). A combination of fluid damping and Type III damping is recommended to decrease all of the peaks when long tubing is used (Figure 6, next page). Damping Options The choice of damping method depends on the application. Until recently there were three damping options available: Type I (screen), Type II (screen + modified Peak Damping 0 Constant voltage drive with ITE tubing (10mm x 1mm) into 2cc Coupler 2 Damping (dB) 4 6 8 10 Mechanical resonance (2-3kHz) First tubing resonance (4-5kHz) 12 0dB undamped 1dB 2dB 3dB 4dB 5dB 6dB Damping (dB) Figure 4: Damping for the first and second peaks in the frequency response of the FED receiver connected to ITE tubing and a 2cm3 coupler under constant voltage drive conditions. 3 7dB 8dB 9dB critically damped ITE Frequency Response 120 Constant voltage drive with ITE tubing (10mm x 1mm) into 2cc Coupler Sensitivity (dB SPL) 115 110 105 100 95 undamped Type I ferrofluid Type II 90 1000 10000 Frequency (Hz) Figure 5: Frequency response of damped and undamped ED receivers connected to ITE tubing and a 2cm3 coupler cavity under constant voltage drive conditions (magnified view). BTE Frequency Response 125 Constant voltage drive with BTE tubing (8mm x 1mm + 28mm x 1..5mm + 25mm x 2mm + 18mm x 3mm) into 2cc coupler Sensitivity (dB SPL) 120 115 110 105 100 undamped ferrofluid + Type III ferrofluid Type II 95 1000 10000 Frequency (Hz) Figure 6: Frequency response of damped and undamped ED receivers connected to BTE tubing and a 2cm3 coupler cavity under constant voltage drive conditions (magnified view). 4 Hot / Cold Response Data Improved Shock Resistance The viscosity of the ferrofluid changes with temperature and will affect the frequency response. At higher temperatures there will be slightly less damping. At lower temperatures, the sensitivity of the receiver will be reduced. However, the receiver recovers as soon as it is warmed up again. For example, Figure 7 shows the data for an FED receiver at the limits of the operating range specifications (0°C and 63°C), room temperature (25°C), and body temperature (37°C). There is about a 1dB increase in the delta peak at body temperature. At lower temperatures, the stiffening of the fluid decreases the output of the receiver by 2dB. The ferrofluid-damped receivers have improved shock protection compared to standard receivers. Furthermore, there is no detectable fluid loss even under the most extreme conditions of shock testing. The data in Figure 8 at the top of page 6 illustrates the improved shock resistance of the fluid-damped ED receivers (FED receivers). Samples were oriented cover up and cover down and were dropped at progressively increasing heights starting at 12.7cm (5 inches) up to 196cm (77 inches). This is equivalent to decelerations of 5,000g to 20,000g. Temperature Variation 120 Constant current drive with ITE tubing (10mm x 1mm) into 2cc Coupler 115 Sensitivity (dB SPL) 110 105 100 95 90 85 25°C 37°C 63°C 0°C 80 100 1000 Frequency (Hz) Figure 7: Frequency response of an FED receiver at temperatures: 0°C, 25°C, 37°C, and 63°C. 5 10000 Shock Resistance 24 Undamped & 48 Ferrofluid-Damped Receivers 100 90 80 % Survival 70 60 50 40 30 20 ferrofluid-damped 10 undamped 0 0 5000 10000 15000 20000 Deceleration (g) Figure 8: Percent of surviving ferrofluid-damped and undamped ED receivers for increasing drop height. voltage drive conditions. Distortion is typically highest at 1/2 and 1/3 resonance where the effects of the second harmonic and third harmonic distortion are dominant. Therefore, these trends represent the “worst case”. The distortion at other frequencies will be less. Harmonic Distortion The total harmonic distortion will rise with increased damping (Figure 9). Figure 10 on page 7 shows the average percent total harmonic distortion (% THD) at 1/2 and 1/3 resonance versus damping under constant Percent Total Harmonic Distortion vs. Frequency 100 6dB damping relative to 8dB peak 5dB damping relative to 8dB peak % Total Harmonic Distortion undamped response 10 1 0.1 100 1000 10000 Frequency (Hz) Figure 9: Percent of the total harmonic distortion versus frequency for FED receivers under constant current drive conditions. 6 Percent Total Harmonic Distortion vs. Damping 100 % THD @ 1/2 resonance % Total Harmonic Distortion % THD @ 1/3 resonance 10 1 0.1 0dB 1dB 2dB 3dB 4dB 5dB 6dB 7dB 8dB 9dB Damping (dB) Figure 10: Percent of total harmonic distortion at 1/2 and 1/3 resonance versus damping under constant voltage drive conditions (FED receivers). Impedance 7000 ED receiver connected to ITE tubing (10mm x 1mm) into 2cc Coupler 8dB (undamped) 6dB (2dB damping) 6000 4dB (4dB damping) Impedance (Ohms) 3dB (5dB damping) -0.5dB (8.5dB damping) 5000 4000 3000 2000 1000 0 100 1000 10000 Frequency (Hz) Figure 11: Impedance curves for fluid-damped and undamped ED receivers connected to ITE tubing and a 2cm3 coupler. Labels in legend are the delta peak values under constant voltage drive conditions. APPLICATION NOTE 7 AN-6 Impedance Fluid damping also reduces the peaks in the impedance curve. Figure 11 on the previous page shows the impedance of FED receivers attached to ITE tubing and a 2cm³ coupler. Conclusion Ferrofluid provides a wide range of damping values and eliminates the need for a damping screen in the port tube. Fluid damping has the additional benefit of improved shock resistance. There is no fluid loss even under the extreme conditions of shock testing. The fluid viscosity will vary with temperature causing the delta peak to increase about 1dB at body temperature. There is a slight rise in the nominal harmonic distortion as damping is increased. Knowles Electronics LLC 1151 Maplewood Drive Itasca, Illinois 60143 Phone: (630) 250-5100 Fax: (630) 250-0575 www.knowles.com Knowles Europe York Road, Burgess Hill West Sussex, RH15 9TT, England Phone: (44) 1444 235432 Fax: (44) 1444 248724 Knowles Electronics Japan KK Kyodo Bloom Building 19-1 Miyasaka 2-Chome Setagaya-Ku, Tokyo 156-0051, Japan Phone: (81) 3 3439 1151 Fax: (81) 3 3439 8822 Micromax Pty. Ltd. P.O. Box 1238 Wollongong N.S.W. 2500, Australia Phone: (61) 2 4226 6777 Fax: (61) 2 4226 6602 Knowles Electronics Taiwan, Ltd. 53 Pao Hsing Road Hsin Tien City, Taipei, Taiwan Republic of China Phone: (886) 2 2911 4931 Fax: (886) 2 2918 6868 NOTE: Specifications are subject to change without notice. The information on this Application Note reflects typical applications. Specific test specifications defining each model are available by requesting Outline Drawing Sheets 1.1 and Performance Specifications Sheets 2.1 of that model number. Knowles’ responsibility is limited to compliance with the Outline Drawing and the Performance Specification application to the subject model at time of manufacture. Knowles Electronics LLC 8 Issue 01 - 0403