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ISEAT 2013: Future R&D methods and QC Testing for Micro Speakers Peter Larsen

ISEAT 2013: Future R&D methods and QC Testing for Micro Speakers Peter Larsen. Abstract.

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ISEAT 2013: Future R&D methods and QC Testing for Micro Speakers Peter Larsen

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  1. ISEAT 2013: Future R&D methods and QC Testing for Micro Speakers Peter Larsen

  2. Abstract Micro speakers are in general similar to traditional loudspeakers; however, their design is more complex due to the smaller size. Both the motor system, diaphragm and box design needs to be improved from the traditional design as well as the QC process. This article illustrates the design of micro speakers in detail with examples. Commercial programs, e.g. LOUDSOFT FINEMotor, FINECone and FINEBox, are introduced in the simulations. Also, the LOUDSOFT FINE QC is used for the production data.

  3. Typical Micro Speakers • Micro speakers are different from normal speakers by: • Having no spider (damper/secondary suspension) • Typically one-piece diaphragm (no separate surround or dust cap) • With self-supporting Voice Coil (No bobbin) • Are often rectangular shape (Race track, maybe also Oval Voice Coil) • Micro Speakers are typically < 2”/50mm (mini speakers > 2”/50mm)

  4. Example specifications for a given 15mm micro speaker

  5. Example 15mm Neodymium motor system modeled in FINEMotor Top plate is saturated. Voice Coil is offset 0.25mm up: Xmax = 0.141/0.613mm Bad / high distortion

  6. Problems found in the 15mm Neodymium motor: • The top plate is saturated, indicated by Bt =2.39T. • (Bt is the Flux density in the top plate and must be <2.1T) • The Voice Coil is not placed symmetrically in the air gap, but offset 0.25mm up • The Bl(x) curve is unsymmetrical, caused by the offset Voice Coil • Power rating is low

  7. Optimized 15mm neodymium motor system including top magnet VC moved for perfect symmetry (= Low distortion) Higher SPL with top neodymium magnet (Size optimized) Add Ferrofluid for higher power, damping and less compression

  8. Finally we may improve the 15mm micro receiver by using Ferrofluid (FF) in the air gap. 4 significant advantages with FF: • The power handling is increased due to the cooling effect from the Ferrofluid • The Ferrofluid can be selected to add damping at resonance (Fs), which prevents distortion • The air gap can be tighter, which will increase SPL and improve cooling. • Increased stability, due to less rocking • Remember venting! (To release air pressure)

  9. Typical free field responses of 30/40mm headphone speakers (capsules) The direct responses (no Artificial Ear) are far from flat due to break-up in the diaphragm, which is especially visible in the waterfall (Cumulative Decay).

  10. FEM Material input from Database The geometry of the 38 mm Headphone speaker was specified as a simple DXF file in the FINECone simulation. The materials for the diaphragm are inserted from the standard database

  11. 38mm Headphone speaker with break-up at 3165 Hz The first break-up takes place at 3165 Hz, where there is considerable break-up in the middle of the large surround. Since the surround area is a large percentage of the total cone area, the break-up here will influence the response considerably

  12. Response with break-up of the 38mm headphone speaker from FEM From 3165 Hz the response is dominated by break-up, which certainly is audible. The break-up is caused primarily by the shallow geometry. The diaphragm may be recalculated with an optimized geometry and other materials

  13. 38 mm headphone diaphragm with low Fs pattern, simulated in FEM After optimizing for the best geometry and material, the next challenge is lowering the resonance Fs. A pattern in the surround can help this by effectively making the diaphragm thinner, and the high profile may prevent some break-up

  14. Rectangular 18x13mm micro speaker 3D CAD file for 3D FEA simulation of this flat 18x13mm micro speaker

  15. FEA simulation of rectangular 18x13mm micro speaker 1 Here is shown mode no. 2, which is a rocking mode @ 209 Hz around the Z-axis. The surround must be thin to obtain a low Fs, which may cause this problem. Often the leads are also placed here, which may worsen the problem.

  16. FEA simulation of rectangular 18x13mm micro speaker 2 Here is shown mode no. 3, which is the second rocking mode @ 218 Hz around the X-axis.

  17. FEA simulation of rectangular 18x13mm micro speaker 3 Here is shown mode no. 37, which is one of many high frequency break-up modes in the surround. There are hundreds of these modes, but only a few will change the frequency response.

  18. FEA simulation of rectangular 18x13mm micro speaker 4 Special patterns can be made in the surround. In this example a deep pattern has been added across the long side (only). The purpose is to demonstrate the added stiffness to the surround in order to minimise the rocking. Here is shown rocking mode no. 3 around the Z-axis again. However now @ 566 Hz. This indicates much increased stiffness. The pattern may be angled etc to keep the Fs down while preventing break-up.

  19. 15mm driver data from FINEMotor Complete driver data, imported from FINEMotor. The thermal time constants of the Voice Coil (VC) and motor are automatically calculated.

  20. Example of 15 mm Micro Speaker Simulations in FINEBox Micro Closed___ Impedance peak at Fs= 400 Hz. Band pass___ Bandpass with hole (port) tuned to 4000 Hz. InterPort___ Bandpass with damping (cloth).

  21. Simulations of the 15mm unit in various enclosures • 100ccm closed box__ (Max SPL~81dB (0.1m) • 9ccm Bandpass__ (Max SPL~83dB (0.1m) • 9ccm Damped InterPort__ (Max SPL~84dB (0.1m) The VC and magnet temperatures are calculated. The VC is at 28.9C which is no problem

  22. FINEBox settings of port details for the 15mm unit in various enclosures InterPort Q is modified to simulate cloth/felt damping of port The ports may be changed by modifying the port diameters, and the length is automatically found according to the chosen tuning frequency.

  23. Excursion of 15 mm Micro Speaker Closed___ Excursion < Xmax from ~400 Hz. Band pass___ Excursion < Xmax from ~200 Hz. InterPort___ Excursion < Xmax from ~400 Hz. Use HP@400Hz

  24. Typical Earphone /Ear bud with front and rear chambers in FINEBox Micro Simplified model of typical (on-ear) ear bud with a 15mm driver, using lumped elements while assuming an infinite baffle (no Coupler or Artificial Ear). This method is convenient for verifying the acoustics by optimizing different cavities and holes.

  25. FINEBox Virtual simulation of Earphone/Ear bud Usually there is a very small cavity in front having holes to the outside. The rear cavity is still quite small, causing a driver resonance in the order of 2000 Hz, shown as the khaki green response___, with a sharp resonance around 2000 Hz when the small holes are closed.  The purple curve_ _ _ is the final simulated response including the small holes and the large tube (along the lead wire). The large resonance peak is considerably reduced and the tube acts as a bass reflex giving response down to 200 Hz. This is the free field response, but the near-field response may extend below 20Hz, when the earphone is attached without leak to the human ear.

  26. Micro speaker testing with Rub & Buzz in FINE QC Danish F. Leonhard derived in 1993 a new auditory model that corresponds to how the human ear perceives sound. The FINEBuzz detection method is partly using these principles, but having a completely new protected algorithm for finding the annoying sounds, which cannot be detected with THD, high harmonics or IM. The algorithm finds impulses in the Time Domain, normally masked by noise. Finally the result is filtered and transferred by FFT, whereby the Rub & Buzz phenomena can be correlated with the actual excitation frequencies.

  27. Learning mode for Rub & Buzz Limit in FINE QC Finding the Rub & Buzz limit is difficult. Therefore a new method has been developed: • First measure good speakers • Then measure some bad speakers • Automatically use that information to set the Rub &Buzz max level.

  28. Micro speaker production QC testing with Rub & Buzz in FINE QC Extensive testing with 1000’s of micro speakers conducted at ForGrand has verified that the electronic QC testing following these principles has successfully replaced the human testing by ear.

  29. Automatic Rub & Buzz Limit in FINE QC Measure good speakers and some bad ones, and use the new Auto Rub &Buzz feature to automatically set the limit to approve the good units, and reject the bad

  30. ISEAT 2013: The end Thank you for your attention. Peter Larsen

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