Micro Speakers

What are MEMS?

MEMS is an acronym for micro electromechanical system. This term refers to structures including mechanically moving parts that are in the microscopic scale. That means, these structures are only visible with a microscope. Well known examples for MEMS are oscillators, gyroscopes, acceleration sensors, and microphones which all can be found in modern smartphones. Most MEMS are made from silicon, using semiconductor device fabrication technologies. 

What is the difference between a micro speaker, a headphone driver, a headphone transducer and a receiver?

The expression microspeaker, headphone driver, and headphone transducer are used in different ways by different people. As far as we know, the definitions are not very precise. 

Headphone drivers or transducers are usually small components in head- and earphones transforming (or “transducing”) an electric signal into sound. 

The term “receiver” has its roots in times when the telephone was invented. It was used for sound transducers in the telephone that would play the speech of the person you were talking to. The expression is still used for small speakers today, mainly in the field of hearing aids. 

microspeaker usually refers to a small sound transducer that has a diameter below 4 cm. Sometimes headphone drivers are considered as a subgroup of microspeakersHowever, due to the very different specifications depending on the applications, headphone drivers are considered to be a group of its own. In this case “microspeaker” refers to all other small speakers not used in headphones, e.g. small speakers in smartphones, tablets notebook computers. 

How relevant is the capacitance of micro speakers?

Modern hearables and smart in-ear devices need highly energy-efficient components. With a growing number of functions, more space and power are required for edge computing, artificial intelligence, and connectivity. 

MEMS speakers based on piezoelectric or electrostatic actuators are essentially electrical capacitors. Unlike resistors, capacitors only “consume” reactive power, meaning that they can be charged and discharged. However, in order to move these charges to and from the capacitor, real power is required in the drive electronics due to the inevitable losses. The effort to move these charges around depends on the size of the capacitor. A lower capacitance means that the capacitor can accommodate less charge, therefore the power required to charge the capacitor is lower.

What is an electrostatic loudspeaker?

For transforming electrical energy into mechanical movement and sound, different transducer mechanisms can be used. Using the electrostatic force – also known as Coulomb force – is one way of doing that. The coulomb force is the force acting between two charges (remember: balloon – hair).  

The beauty of electrostatic forces is that only electrical conducting and insulating material is required and no magnetic fields or piezoelectric materials are involved. This gives a huge advantage in terms of manufacturing, since only materials and processes are used that are CMOS compatible and widely adopted in the industry (see CMOS).

Macroscopic electrostatic loudspeakers use a very thin membrane that is moving between two fixed electrodes. Due to the different voltages between the electrodes, the force is acting on the charged membrane. Electrostatic loudspeakers are common in high end speaker systems and headphones because of their superb linearity and sound quality.


What are Hearables?

In our definition, hearables are smart, true wireless in ear devices with functions that go way beyond simply playing sound. Often, today’s existing earbuds are already described as hearables – but we think that is not true for all of them. However, many of today’s earbuds are comprising a growing number of AI functions and thus are turning more and more into true smart hearable devices. 

The hearable device of the future, in our vision, is a true wireless voice controlled in-ear device that can be worn all day. It will be directly connected to the Internet and thus be able to replace the smartphone and current wearables for many day-to-day functions. Future functionalities will include communication, navigation, searching the Internet, translations, payments, health monitoring etc.

What is TWS?

TWS is an acronym for TrueWireless System. It refers to earphones that consist of two separate earpieces that are connected by wireless technology (usually Bluetooth). There is no physical connection by wire between the earpieces and the signal source (e.g. smartphone).


How can you produce sound without a membrane?

Sound can be produced by a microspeaker without a membrane. Our solution, simply speaking: We cut the membrane into a multitude of thin stripes and put them into the silicon chip volume. The result is an array of very thin beams, enclosing air. The big advantage of that approach is that the chip has a large acoustic inner surface while the outside surface is kept at an absolute minimum. 

The beams are moved by electrostatic forces according to an audio signal. Electrostatic micro motors are used for that. The movement of the beams displaces the air between them. Each individual beam can only displace a small amount of air. The required sound pressure level is reached by the sheer number of beams in one chip. The air is released through openings in the chips’ bottom and lid. And that’s it: we have sound.

What sound pressure levels are possible with a silicon micro speaker?

Our MEMS µSpeakers can generate more than 120 dB sound pressure level with an active chip area as small as 10 mm².  

Don’t you need more volume to create great sound?

No. You can create great sound without a big volume when it comes to in-ear applications. Our µSpeakers are targeting in-ear devices only. The “standard” ear canal has a volume of 1.26 cm³. To generate a very loud sound of 120 dB sound pressure level, one therefore “only” needs to push approx. 0.5 cm³ of air. 

The specifications of the µSpeaker, like amplitude frequency response, resonance frequency, harmonic distortion, can all be influenced by the number and the design of the beams and the air chambers between them.


What does fabless mean?

Fabless means that a company designs and sells semiconductor chips but does not manufacture and process the silicon wafers. The actual production of the silicon chips takes place at a foundry and is purchased as a service.

What is piezo and why are you not using it, when other MEMS micro speaker producers are?

“Piezo” refers to elements that are based on piezoelectric materials. These materials show an interesting effect: Applying pressure leads to an electrical potential within the element. This effect is used e.g. in lighters (piezo ignition) to ignite a gas by a little spark that results from the electrical voltage that is produced by the mechanical force acting on a little piezo crystal. The effect also works the other way around: Applying an electrical voltage on a piezo material leads to a mechanical movement. This can be used to produce sound e.g. in piezoelectric beepers. 

Our philosophy is to use standard processes and standard materials that are widely adopted in the semiconductor and MEMS industry (see CMOS). Piezoelectric materials are not considered as standard materials and can only be processed in a very limited number of foundries. Furthermore, the use of lead, which is a component of lead zirconate titanate (PZT), one of the most commonly used piezoelectric material, is restricted in the European Union since it is considered to be a hazardous material.

What is CMOS?

The acronym CMOS (= complementary metal oxide-semiconductor) refers to both, a specific design of digital circuits and a group of processes that is used to manufacture these circuits. Usually, we refer to the processes that are involved in manufacturing.

What are the advantages of CMOS?

CMOS processes are semiconductor industry standard – despite some nichesMEMS designs that are using CMOS standard or are close to it benefit from the huge investments made in the past for machines and equipment in fabs and foundries all around the world. This is the reason why we are using standard CMOS processes and materialsto enable our designs to be manufactured everywhere.

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