How to Quickly Implement a High-Quality TWS Earbud Design with a Dedicated Tweeter and Woofer

In the early days of audio streaming, wireless data rates were limited, and users accepted a loss of fidelity as the price for the convenience of thousands of digital tunes in their pocket. But with the introduction of wireless technology that supports greater wireless throughput and enhanced compression algorithms, consumers have become more discerning. That means designers now need to offer true wireless stereo (TWS) audio earbuds to meet consumer expectations. TWS earbuds promised to reproduce sound more accurately across the audio spectrum, especially at the higher frequencies that were typically lost on older designs.

But sound quality is just one aspect of modern wireless audio reproduction. In a competitive market, headset developers must look closely at what consumers want, and use that insight to provide end-product differentiation as efficiently and cost-effectively as possible. For example, consumers also want effective active noise cancellation (ANC) and mitigation of occlusion effects so they can better enjoy their listening experience. For older listeners, automatic compensation (hearing personalization) for natural hearing loss at the higher frequencies is also increasingly in demand.

Meeting these demands requires a revised approach with designs that separate woofers for the bass and tweeters for treble. This is beyond the skillset of many development teams, resulting in extended time-to-market schedules and potentially lost opportunities as they hire or develop the expertise.

This article summarizes the developments driving commercial wireless audio and their impact on earbud hardware and software design. The article then introduces a reference design for TWS earbuds and shows how designers can use it to quickly bring headset solutions to market that allow for differentiated features, while precisely reproducing the strong bass and extended treble now captured by modern audio compression software.

Advances in digital sound

In the real world, sound is an analog signal, but our recording and playback equipment largely deals with digital signals. Sound is digitized using an analog-to-digital converter (ADC) powered by an encode/decode (“codec”) algorithm that governs the sampling rate in Hertz (Hz) and bit depth (bits). Sampling captures the sound’s analog waveform amplitude at specific intervals.

Sample rate is a tradeoff. Lower rates result in less data to handle but decreased resolution. The bit depth is the number of bits of information in each sample; again, there is a compromise required between the number of bits and the audio quality. Common bit depths are 16, 24, and 32 bits (Figure 1).

Figure 1: Analog sound is digitized by sampling at a given frequency and bit rate. Increasing the sampling rate and bit depth ensures the digitized information more closely mirrors the analog signal and enhances the quality of reproduction. (Image source: Knowles)

Sampling rate × bit depth × number of channels determines the bitrate in bits per second (bps). For acceptable music quality, the bitrate is typically greater than 192 kilobits per second (kbps). CD quality, for example, relies on a 44.1 kilohertz (kHz) sample rate and a bit depth of 16 bits. For stereo reproduction, the bitrate is therefore 1.411 megabits per second (Mbps).

Conventional codecs typically use compression techniques that discard information during encoding that has been determined won’t overly affect how the listener perceives the decoded audio stream. The aim is to lower the bitrate as much as possible without unduly compromising audio quality. Such codecs are called “lossy” because the decoder can never reproduce the original signal as it doesn’t have all of the original information. It is typically the higher (treble) frequencies that are eliminated by lossy codecs.

Thanks to advances in low power, short-range radio, wireless links can support greater throughput without compromising battery life. For example, Bluetooth LE Audio, a recently released form of Bluetooth LE-based wireless streaming, now offers both much higher audio quality than Classic Bluetooth Audio and lower power consumption.

Engineers have also enhanced the efficiency of their codecs. These newer “lossless” codecs, combined with higher throughput wireless connectivity, have enabled much higher wireless audio (Table 1). Audio services from companies such as Apple, Amazon, and Spotify now offer high-quality audio lossless streaming. However, the designer should note that the encoded bitrate for the lossless codecs is often higher than the wireless link can reliably support. For example, Sony’s LDAC codec encodes at a bitrate of 6.1 Mbps (32 x 96 x 2), but the bitrate of the wireless link is limited to 990 kbps.

Table 1: Comparison of “lossless” codecs (Sony, Savitech, and Qualcomm) with CD quality and lossy codecs (Qualcomm and Bluetooth SIG (SBC)). Note that the maximum bitrate for the lossless codecs is limited by the capability of the Bluetooth wireless link. (Image source: Knowles)

ANC and personalized sound

Consumer expectation for TWS earbuds extends beyond superior sound. High-end products must also offer ANC and other features. ANC is popular because it offers users a quality listening experience when there’s a high level of background noise, such as in an aircraft cabin. ANC operates using microphones built into the earbuds that pick up low-frequency noise and cancel it out before the user is aware of its existence. Cancellation occurs when the headset generates a secondary sound that's inverted by 180˚, relative to the original noise.

Another key enhancement now offered with wireless earbuds is personalized sound. Users with hearing disabilities that they are born with, or which develop with age, might have particular difficulty hearing higher frequencies (Figure 2). There are smartphone apps and other tools that allow a user to boost specific frequencies to compensate for hearing loss, but they tend to be rudimentary and offer poor results. But now, high-quality products take this further with algorithms that set hearing levels across the full range of frequencies by subjecting the user to a detailed listening test. The result is earbuds with outputs perfectly adjusted to compensate for hearing deficiencies.

Figure 2: As users ages, they gradually lose the ability to hear higher frequencies. Personalized sound boosts selected frequencies to compensate for loss of hearing sensitivity. (Image source: Knowles)

A final technical development in modern earbuds is occlusion reduction. Occlusion effects occur when an earbud seals the outer portion of the ear canal. This is a common issue with products that are designed to fit relatively tightly into the ear. The earbud effectively increases the acoustic “impedance” of the ear canal, which in turn boosts the amplitude of acoustic pressure, particularly when the ear is subject to low-frequency sound generated by the user (examples include talking, walking, and swallowing). The result is an echo-like “boom” in the ear that’s annoying and distracting.

Earbud makers have worked to reduce occlusion effects by mechanical design, such as adding a small opening between the earbud and ear canal to reduce acoustic impedance, as well as through software design, such as including occlusion reduction in the ANC routines.

The advantages of separate woofers and tweeters

Until recently, designing wireless headphones was less challenging compared to designing full-size speakers connected to high-end audiophile sound systems. Users accepted lower quality in their headphones as the price for convenience, and that made it easier for designers to develop products in a small form factor at a reasonable cost. For example, it was commonplace to use a full-range driver in place of a separate woofer and tweeter, saving space. Higher-frequency reproduction was potentially sacrificed, but this was hardly a problem when those frequencies were absent from the wireless audio stream.

However, with the advent of lossless codecs and high-throughput technologies such as Bluetooth LE Audio, wireless audio now offers a full range of bass and treble frequencies (Figure 3). Reproducing this audio demands much more from the earbuds. Moreover, consumers expect ANC, personalized sound, reduced occlusion effects, and suitability for a wide range of use cases, including music, TV, video conferencing and voice calls—all in a highly compact form factor, and again at reasonable cost.

Figure 3: Lossless codecs provide more high-frequency information, enabling better reproduction of treble tones during music playback in suitably designed earbuds. (Image source: Knowles)

Many of these requirements demand design tradeoffs. For example, to deliver effective ANC in noisy environments such as an aircraft cabin, the speaker drivers need to produce high bass output with low distortion. Semi-open designs that address occlusion place further demands on the bass output. At the same time, lossless audio playback requires the speaker driver to handle treble outputs of up to 20 kilohertz (kHz) and above. Meeting both requirements with a single dynamic speaker driver in a tiny form factor is virtually impossible.

The solution is to split the bass and treble frequencies across a dynamic woofer and a separate balanced armature (BA) tweeter. The BA tweeter is a specialized component that was originally developed for hearing-aid applications, which is now increasingly being used to boost treble response in high-quality earbuds. In a BA tweeter, an electronic signal vibrates a tiny reed that’s balanced between two magnets inside a compact enclosure. The motion of the reed is transferred to a very stiff aluminum diaphragm which produces the sound.

With a dedicated woofer and BA tweeter configuration, the woofer can be designed to focus on providing strong bass to support lossless reproduction, ANC, and occlusion effects reduction, while the BA tweeter output is optimized for clear and distinct treble. This reduces the need for equalization, which in turn saves power and increases dynamic headroom (Figure 4).

Figure 4: Separating the speaker system into a dynamic woofer (green) and BA tweeter (blue) produces a flat frequency “hybrid” response (red). (Image source: Knowles)

A further advantage derives from separating the speaker drivers: the designer has a greater degree of freedom in the driver arrangement. For example, the woofer can be less directly aligned with the ear tip, thereby allowing the BA tweeter to be positioned near the ear opening to minimize the volume of air trapped between the tweeter and ear tip, limiting occlusion effects (Figure 5).

Figure 5: Separating the woofer and tweeter in the earbuds allows for the tweeter to be positioned toward the front of the device, which helps to limit occlusion effects. (Image source: Knowles)

In addition, the separation of woofers and tweeters allows designers to refine the frequency response. For example, they can shape the acoustic features near the tweeter opening to refine the high-frequency response. Then designers can adjust the crossover for smooth blending of the woofer and tweeter signals. Designers can also adjust the sensitivity of the tweeter to get a better match to the woofer by selecting a higher or lower coil impedance. Final shaping of the earbud’s overall frequency response can be accomplished using digital signal processing (DSP)-enabled tuning.

Moreover, since many Bluetooth ICs have dual outputs, the woofer and tweeter can be driven by individual amplifiers for even more flexibility in shaping the frequency response.

High-quality wireless audio reference design

Engineers accustomed to a single-speaker driver in their wireless designs will be challenged by the additional complexity brought about by the separate woofer and tweeters needed to reproduce high-quality audio. Yet, the trend is clearly toward higher quality audio capabilities, so a path toward a dual-driver design for quality reproduction of lossless audio streaming must be considered.

To assist designers moving in this direction, Knowles, a BA tweeter manufacturer, has introduced the TC-35030-000 True Wireless Stereo earbuds reference design. The reference design shortens the time-to-market for TWS earbuds by including many of the key advanced features users demand, thereby removing many of the common design challenges.

The reference design includes Knowles’ own design of a BA tweeter for good high-frequency sound, along with a 10-millimeter (mm) dynamic woofer for solid bass. The unit also includes microelectromechanical systems (MEMS) microphones for ANC and voice calls. The reference design offers 13 hours (hrs.) of playback time or 8 hrs. of talk time from its built-in battery, and it is Bluetooth 5.2 compatible. Additional features built into the kit include touch controls and integrated voice assistant technology (Figure 6).

Figure 6: The TC-35030-000 TWS earbud reference design features a BA tweeter for good high-frequency sound and a 10 mm dynamic woofer for solid bass. (Image source: Knowles)

The BA tweeter provides a response extending well above 20 kHz. When comparing the treble output of the Knowles product to a typical 8 mm dynamic speaker, the BA tweeter provides the increased treble output and extension necessary for high-quality audio, including the capability to support hearing personalization or enhancement (Figure 7).

Figure 7: Shown is the high-frequency response of Knowles’ BA tweeter compared with that of a dynamic speaker. (Figure source: Knowles)

Conclusion

Advances in wireless semiconductors and codecs have changed the landscape for earbuds. Consumers now expect deep bass, refined treble, and wide dynamic range from their in-ear TWS devices. Moreover, users expect advanced features such as ANC and personalized sound and are less accepting of effects such as occlusion.

To better meet the frequency-response requirements of TWS headsets, designers need to move to dual-driver designs with a dedicated tweeter and woofer. While doing so is technically challenging, Knowles’ TC-35030-000 TWS earbuds reference design can help. Combining a BA tweeter, a woofer, and MEMS microphones, it offers a good foundation for the design of high-quality audio earbuds with features that enable clear product differentiation.