Piezoelectric MEMS Technology FAQs

How does Piezoelectric MEMS microphone work?

A piezoelectric material is one that develops an electric charge in response to a mechanical stress and vice versa. In our piezoelectric microphone, acoustic pressure enters the sound port of the microphone and deflects the cantilevered piezoelectric plates. As the plates bend, stress develops in the film. This creates an electric filed or an electric displacement depending on the electrical boundary conditions of the circuitry. The magnitude of this electric field is determined by the amount of stress generated by the acoustic pressure and the coupling coefficient. This electric field therefore inherently generates an electric voltage at the output of the MEMS transducer directly via piezoelectric effect which is then amplified by the ASIC circuitry within the package.

How are piezoelectric microphones different from Capacitive MEMS microphones?

The primary difference between piezoelectric and capacitive MEMS microphone structures is that the Piezoelectric MEMS do not require a backplate. In piezoelectric MEMS microphones, the signal is generated in the moving diaphragm while, in capacitive MEMS microphones, the signal is proportional to the change in the gap between the moving diaphragm and stationary backplate. This absence of a backplate and air gap, inherently provides reliability and robustness to piezoelectric MEMS microphones. In a capacitive MEMS design, any environmental contaminants such as dust, water, moisture etc. can get into the air gap and cause the diaphragm to stick to the backplate. Even if the diaphragm does not stick to the backplate, particles can remain between the diaphragm and the backplate, causing changes in sensitivity and frequency response. Whereas, the single layer design of a piezoelectric MEMS microphone retains its performance even if dust particles settle on the top of the crystal.

Why is Vesper’s piezoelectric Microphones better than those built by others in the industry?

Piezoelectric MEMS microphones offer several advantages compared to Electret Condenser or Capacitive MEMS microphones used in the industry

  • Due to a single layered design, Piezo microphones are inherently robust to environmental particles such as dust, water, moisture, kitchen oil etc.
  • Our microphones do not require a charge pump to produce bias voltage. This provides an ultra-low startup current and a fast wakeup. This feature is used in Vesper’s ZeroPower Listening Technology.
  • Piezoelectric MEMS sensors are highly linear until high sound pressure levels. These sensors can withstand sound pressures as high as 170 dBSPL. Therefore, the Acoustic Overload Point (AOP) on these microphones is only limited by the ASIC voltage rail. This linearity performance provides high stiction immunity to piezoelectric MEMS microphones

Why is robustness important for MEMS microphone components in a device

Consumers are adapting voice user interface devices rapidly and want to use their devices wherever they go. Robustness is therefore critical for using these products in the outdoors. Robustness gives three-fold advantages in building Voice UI devices –

  1. a) Adds immunity to dust or other particles entering the acoustic port during assembly. Capacitive MEMS microphones require protective films during assembly to avoid failures adding up to manufacturing costs.
  2. b) provides long term stability and durability for microphone arrays used in far-field devices, increasing the life time of microphones used in these products.
  3. c) Enable the use of microphones without a mesh/membrane resulting in overall BOM savings and a simplistic design. A mesh/membrane adds to the cost of a capacitive microphone-based solutions and requires additional modelling to compensate for the changes in sensitivity and frequency response characteristics due to adding a mesh/membrane

How high of an Acoustic Overload Point (AOP) can a piezoelectric MEMS microphone achieve?

The piezoelectric MEMS sensor is inherently linear until sound pressure levels as high as 170 dB. The linearity of piezoelectric material, therefore, is only limited by the ASIC voltage rail and the required sensitivity. If the required sensitivity is -38 dB and the supply is 1.8V, Vesper microphone’s AOP will be comparable to the industry standard. However, higher AOP can be achieved with piezo material by a trade-off with the sensitivity of the microphone. In a capacitive MEMS microphone, any changes to the stiffness of the diaphragm will significantly tradeoff Signal to Noise Ratio of the microphone, making it impossible to achieve higher AOP.

Contact Vesper team to get information on roadmap products that can provide AOP as high as 152 dBSPL.

How about the performance of Piezoelectric microphones for ultrasonic demodulation noise?

Vesper piezoelectric microphones reject ultrasonic frequencies better than capacitive MEMS microphones. Our team has performed several experiments to characterize the performance of piezo microphones for ultrasonic demodulation, which can only be shared under an NDA.

What does IP57 rating mean?

Ingress Protection (IP) rating classifies the degree of protection provided against intrusion of environmental particles such as dust, liquid etc. by mechanical and electrical enclosures. The first letter in IP57 rating indicates protection against solid particles such as dust. Second letter indicates protection against liquid particles. Vesper microphones are IP5x rated for dust and IPx7 rated for liquid. Note that due to the need to have an Acoustic port for the microphone, dust can still settle on the MEMS structure, but does not impact the performance of the microphone. Additional details on IP rating can be found on Wikipedia link here – https://en.wikipedia.org/wiki/IP_Code

What is the IP rating of Vesper’s Piezo MEMS microphones?

Our microphones are currently IP5x rated for dust and IPx7 rated for water immersion upto 1 meter for 30 minutes. Our IP57 rating makes the microphones suitable for applications where the device is directly exposed to the environmental contaminants including dust, moisture, kitchen oil, water etc.

What is the ASIC process node used for manufacturing Piezoelectric microphone?

We use the standard 18 nm process node for ASIC manufacturing.

Where can we get data on performance and reliability characteristics of piezoelectric microphone?

Basic performance characteristics of our MEMS microphones are already provided in the corresponding product datasheet. For additional information on these characteristics or to request data for specific tests, please reach out to info@vespermems.com.

Are there any specific manufacturing and assembly guidelines for devices using piezoelectric MEMS microphones?

Our manufacturing follows a similar process as capacitive MEMS microphones. Assembly guidelines for our microphones are provided in an application note under the resources section of our website.

Does Vesper have evaluation boards to test the microphones?

Yes. Vesper offers its microphone products on a PCB, which we call Coupon board, and includes all the soldering connections to the pinout. These coupon PCBs can be interfaced with your hardware boards using standard CW Industries CWR-170-10-0000 edge connector. Description of these boards can be obtained from the Resources section on the website. The boards can be directly purchased through our distributor network.

Are Vesper mics available with distributors?

Yes. Vesper microphones in production are now directly sold via our Global distributors including Digi-Key and Mouse Electronics. Please go to Buy section of our website to purchase our products from distributors.

What is the typical lead time to order Vesper microphones?

Our microphones in production can be purchased with a 12-week Lead time. Please check with our distribution partners on lead times for purchasing on their websites. Any inquiries regarding Engineering versions of our microphone products can be obtained by directly contacting Vesper sales team or an authorized sales representative for your region. Lead times for engineering version might vary based on the product.

ZeroPower ListeningTM FAQs

How does ZeroPower Listening (ZPL) work?

ZPL uses the piezoelectric effect to make the acoustic transducer operate as an acoustic switch. When a soundwave hits a piezoelectric cantilever, it moves the cantilever. This motion creates a voltage via the piezoelectric effect. This voltage is sensed by a very low-power comparator circuit, which sends a wake signal to the rest of the system.

Does ZPL wake for any kind of sound?

Any kind of sound will create a voltage on the piezoelectric MEMS cantilever, but not all sounds will activate the companion circuit. The circuit is designed to only respond to sounds between 250Hz and 6kHz, which includes the human vocal range. Sounds outside of this range such as wind, HVAC, etc. will not activate the microphone. The sound must also exceed a specified sound pressure level (SPL) between 65 – 89 dB. Choosing an external resistor value configures this SPL setting. Refer to Application note “Introducing ZeroPower Listening Technology using VM1010” for additional details.

Does ZPL do keyword check?

Keyword check is still done in the digital domain by a companion DSP. In one example of our reference design, the DSP from DSP Group running the Sensory Truly Handsfree keyword algorithm will accomplish this.

Does ZPL miss any of the keyword?

No. Piezoelectric MEMS microphones start up very quickly at around 200 microseconds and do not miss the keyword. Capacitive MEMS microphones take about 250X more time to start up because they require time to charge their MEMS to a high bias voltage. Piezoelectric MEMS microphones, in contrast, do not require any bias voltage and are low-voltage devices.

Why don’t capacitive microphones have this feature?

Capacitive MEMS microphones require a high bias voltage generated by a charge pump. This charge pump must be controlled by a larger circuit and cannot be activated by voice energy. It is impossible to have ZeroPower Listening without a piezoelectric MEMS microphone.

How does ZPL work with low power voice activity detect?

ZPL is compatible with existing low-power voice-detect algorithms. The ZPL mode is a power mode that runs below the lowest power voice activity detect (VAD) modes. In a very noisy environment, the system will move into VAD mode.

Does ZPL really extend battery life?

Yes. ZPL greatly extends the life of a battery in systems that are often in sleep mode. For example, a voice operated TV remote in a living room will be in standby mode for the most amount of time in a 24-hour period and will go to full power mode only a few times in a day when a ZPL microphone activates the rest of the system to full power mode. Whereas an alternate system without ZPL is always running the voice activity detector to look for the wakeword. If a system is used more frequently, then the battery consumption from active use mode will dominate the battery life. Our analysis shows that ZPL improves standby battery life from 3x-7x based on the design parameters when compared to alternate listening solutions. We can provide models to analyze the battery life performance of any device with ZPL mode.

Are there any design parameters to control ZPL mode?

Yes, the Wake On Sound(WoS) mode in ZPL microphone can be controlled using Acoustic Threshold and hold time metrics. Acoustic threshold is based on the value of external resistor used with VM1010 whereas hold time is a metric controlled by the DSP on a system level to decide when to go back to WoS mode from Normal mode once it wakes up. A detailed description of the two metrics and some example usecases are provided in the application note “AN4 – Design parameters for ZeroPower Listening using VM1010”

What if I want to dynamically change the acoustic threshold on the fly?

Currently, the acoustic threshold is adjusted using a set value of an external resistor. However, we can provide recommendations to dynamically change the threshold using additional system components. Contact the Applications team to get more details on this recommendation

Is ZPL technology currently offered on a digital microphone?

Currently, ZPL is only offered on Analog microphone – VM1010. We are working on a digital version of ZPL microphone. Contact sales team for product schedules.

What are some good applications for ZPL?

ZPL is great for applications that:

  • Use voice as an interface or do acoustic event detection
  • Are battery-powered or benefit from reduced system power consumption

Some great examples include?

  • Smart TV remotes
  • Security cameras/standard cameras
  • Noise monitors
  • Hearables with voice interface
  • Augmented reality systems with voice interface
  • Battery-powered smart speakers

Can ZPL be used in systems that are plugged into an external power source?

Yes. Although ZPL is primarily intended for systems with tight power budgets, but systems do not need to be battery-powered. A huge percentage of household power consumption goes to plugged-in devices with high-power hibernate modes. For example, TV sets consume a lot of power even when they are turned off. Using ZPL would save these systems a tremendous amount of power by powering them off completely when nobody is home.

Are there circumstances where ZPL can increase power consumption?

No. As long as a system has an always-listening voice interface, ZPL will decrease power consumption and extend battery life. If, however, a system is constantly and continually used, the power savings will be negligible.

For additional queries related to our technology, please contact  info@vespermems.com

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