26 Aug Hi-Res Audio: Why Solid-State MEMS Speakers are Ideal for High-Definition Audio Experiences
By: Mike Housholder, xMEMS Labs, Inc.
In recent years, the emergence of hi-resolution and lossless audio features in popular music streaming services from Apple, Tidal and Amazon (among several others) has brought renewed focus to Hi-Res Audio certifications in consumer earphones and headphones. In fact, according to Qualcomm’s 2021 State of Sound report, 52% of consumer respondents to the survey are now looking for high resolution or lossless sound quality in their next product purchase.
The Hi-Res Audio certification process for audio devices defines requirements for the digital signal chain (codec, DSP, DAC, etc.), but what is often overlooked is that there are equally important requirements for the analog signal chain (speaker and microphone) in order to produce Hi-Res audio for the listener.
When it comes to Hi-Res audio, much of the attention in the media and in audio product specifications have been around the codec to ensure it can achieve the bit-depth and sampling rates necessary to deliver Hi-Res audio (Figure 1). This gets even more scrutiny in the fast-growth TWS (True Wireless Stereo) earbud market due to the challenges with achieving Hi-Res using low-bandwidth, low-power wireless standards, such as Bluetooth.
Figure 1: Dynamic Range and Bandwidth for Hi-Res Audio
Source: Wikipedia (Wikimedia Commons, Aquegg)
It’s the Speaker, @#$%!
However, while the codec and the rest of the digital signal chain is responsible for delivering the highest quality data stream, the speaker is ultimately responsible for using that data to produce what we hear. If the audio stream is of high quality and studio production level, can the speaker accurately render and reproduce it for the human ear, as the artist intended? Can the consumer actually perceive and appreciate next-level detail, harmonics and clarity promised by the Hi-Res standard? This question is critical to the fate of Hi-Res Audio. Is it just an empty marketing logo, or does it deliver a next-level audio experience? Will consumers hear better quality and be willing to pay more for the difference?
This is why the analog side of the signal chain, in particular the speaker, deserves to be brought into focus when we talk about Hi-Res. A new generation of solid-state, high-fidelity MEMS speaker technology may be critical in delivering to consumers tangible value and elevated audio experiences.
Hi-Res Requires a Robust Ultrasonic Frequency Response
Achieving the Hi-Res standard requirements with in-ear micro speakers is difficult for traditional coil-based speaker architectures. The standard specifies that the speaker’s 20kHz-40kHz response should degrade no more than -20dB from the speaker’s response at 1kHz (Figure 2). Note that others may interpret the standard differently and reference an alternate frequency (e.g. 10kHz instead of 1kHz) or may choose to target a more aggressive degradation limit (e.g. -10dB). In the future, achieving this -20dB target may become even more challenging as codecs and streaming services adopt higher sampling frequencies, like 192kHz. In this case, a speaker’s response out to 96kHz may also come under scrutiny.
Figure 2: Hi-Res speaker requirement illustration (20kHz-40kHz: -20dB or less from 1kHz reference)
In general, the typical behavior of coil-based speaker architectures is such that there is a natural roll-off (loss of SPL/loudness) at higher frequencies as shown in Fig. 3. The resonant peak shown is the acoustic peak which accounts for the air stiffness within the sealed internal ear coupler. The roll off at higher frequencies is a characteristic of the mechanical resonant frequency of the coil-based speaker which typically is below 2 kHz. The high frequency degradation makes it extremely challenging for a coil speaker to achieve the -20dB target to 40kHz, and even more so to 96kHz.
Figure 3: Reduced drive magnitude of dynamic drive tweeter at higher frequencies, beyond the resonant peak.
Hi-Res and Solid-State MEMS Speakers: A Perfect Match
For the advancement of Hi-Res Audio, a technology evolution in speakers may have arrived at the right time. The traditional coil speaker architecture has been around for about 100 years. Since the invention of the balanced armature speaker 50 years ago, no significant advancements in speaker technology have come along until the launch of the world’s first solid-state micro speaker by xMEMS in July 2020.
Solid-state MEMS speakers offer many compelling attributes and performance advantages vs. coil-based technologies. The key characteristic relevant to this article is the near-flat ultrasonic and mechanical response above 20kHz due primarily to the piezoMEMS drive and the material stiffness of the silicon speaker diaphragm. Coil-based speaker operation is designed for a single mechanical resonance and results in limited bandwidth due to sound pressure level decay at frequencies higher than the resonance, as shown above in Figure 3. By comparison, xMEMS speakers with piezoMEMS actuators, are designed with multiple resonances in order to achieve wide acoustic bandwidth. This provides earbud system designers with a straight-forward and low-risk path to achieving Hi-Res product certifications and enables consumers to more easily discern and appreciate Hi-Res audio experiences vs. traditional CD or MP3-quality formats.
Figure 4 below demonstrates that xMEMS speakers can drive near constant magnitude across the frequency spectrum.
Figure 4: Consistent drive magnitude of xMEMS speaker across the entire frequency range, even beyond the resonant peak.
Figure 5 below demonstrates that the consistent drive capability of xMEMS speakers correlates to the final frequency response of the speaker, achieving a near-flat response out to 80kHz! xMEMS speakers can actually deliver gain, not degradation versus the 1kHz response.
Figure 5: 20kHz to 80kHz xMEMS Montara free-air baffle frequency response measurement. Net SPL gain vs. the 1kHz response, no degradation.
Consumers today have greater awareness about the possibilities for enhanced audio experiences via the Hi-Res standard, thanks, in part, to the support of Hi-Res in popular streaming music services.
However, for the Hi-Res standard to deliver real, not just perceived value, it will be important to bring increased focus to the ultrasonic performance characteristics of micro speakers for consumer earbuds. The highest quality and most true sound reproduction is critical if the earbud system is to take advantage of the higher resolution audio streams coming from the codec. Anything less could lead to Hi-Res Audio becoming an empty and forgotten initiative. The emergence of piezo-drive, solid-state MEMS speakers has the opportunity to deliver on the promise of enhanced, Hi-Res audio experiences.
xMEMS is reinventing sound with the world’s first solid-state MEMS speakers for TWS and other personal audio devices. The monolithic architecture implements both actuation and diaphragm in silicon resulting in unmatched part-to-part frequency response consistency and reduced speaker matching or calibration time at manufacturing. This innovative transduction mechanism has also produced the worlds’ fastest and most precise µspeakers, eliminating spring and suspension recovery of coil speakers which improves audio quality and sound field reproduction. xMEMS speakers are mass-production ready. Please contact us to learn more. http://www.xmems.com; firstname.lastname@example.org
 As defined by the Japan Audio Society, JEITA RC-8140B-1, and IEC 60268-7:2010+A1:2020 (2020-09-15)