Creating ANC earphones and hearables with balanced armatures

Introduction:

As True Wireless Stereo (TWS) earphones become more mainstream, the consumer has increasingly high demands for performance and features, including Active Noise Cancellation (ANC), higher-quality sound, increased battery life, and quick charging.

Balanced Armature (BA) drivers bring improved fidelity and detail over traditional dynamic drivers. Balanced armature drivers use an electronic signal to vibrate a tiny reed that is balanced between two magnets inside a tiny enclosure. The motion of the reed is transferred to a very stiff aluminum diaphragm. This diaphragm is free of unwanted resonances in the audio band, allowing it to produce maximum clarity. The high frequency response from BAs contributes to the loud and clear audio we hear in our on-ear monitors and hearing aids for example.

Just as sound quality is important, premium features are also being added to TWS earphones. One of these features is ANC, which is gaining in popularity. Pre-pandemic, its growth stemmed from people using it for commuting or travel, and in the last year, was driven by people spending more time working from home and wanting a quiet, focused space.

Many headphone and hearable designers use dynamic drivers to design ANC devices. However, a balanced armature driver can also be combined with an existing dynamic speaker in what is commonly called a hybrid earphone. In this configuration, the balanced armature drivers focus on the critical high-end portion of the audio spectrum, bringing a new level of clarity and realism.

BAs have many technical benefits beyond just sound quality. They are smaller in size than other drivers, incredibly lightweight, and provide greater output with less power. Because they are incredibly small, BAs leave more room for other components or a larger battery inside hearable devices, and can allow multiple drivers to be combined to get the best sound and performance from TWS earphones.

Graph 1: ANC performance, Knowles BA earphone.

Knowles, the company best known for their microphones and balanced armature drivers for hearables, hearing aids, and earphones, has developed this design guide for creating ANC earphones and hearables with balanced armatures. Modern TWS devices on the market tend to incorporate complex hardware and there is always room for improvement. This design guide highlights how one can increase the quality of hearables without compromising the audio quality.

Within this design guide, acoustic engineers will find tips and techniques for designing high-performance ANC systems with BAs to create an optimum solution. Design rules for BA-based ANC systems follow the same precepts as for dynamic speakers, with proper accounting for the BA’s performance characteristics. With this design guide as reference, it is possible to create a competitive ANC system with BAs as shown by graph 1.

ANC headsets incorporate complex hardware and there is always room for improvement.  Within this design guide, acoustic engineers will find tips and techniques for designing high-performance ANC systems with BAs to create an optimum solution.

The first portion will discuss ANC headset designs using only balanced armature drivers.  With this design guide as reference, it is possible to create a competitive ANC system with BAs as shown by graph 1.  Design rules for BA-based ANC systems follow the same precepts as for dynamic speakers, with proper accounting for the BA’s performance characteristics.  The second portion will discuss designing ANC headsets using a hybrid design having a dynamic driver woofer and a balanced armature tweeter.  With proper crossover design, the ANC performance can be enhanced by the addition of the tweeter.

Graph 2: : Bass roll-off from various front vent sizes.

BA-Specific Design Considerations

An ANC headset using only a BA driver can have noise reduction performance comparable to one using a dynamic driver, after certain adjustments are made.  Concerns about BA phase response have proven to be unwarranted.  The BA driver can even provide better passive noise reduction than a dynamic driver.  Recommended adjustments include a reduction of the vent corner frequency, adding more low frequency boost and a stronger 3 kHz notch to the loop filter.  One may also choose to add a level compressor to make the BA output seem even louder.

Venting

A vent path from the ear canal to the outside world reduces the level of very low frequency occluded sounds (jaw motion / walking) and reduces the impact of leaks on ANC performance. However, just as a vent reduces occluded sounds, it also reduces the level of bass in music playback. The attenuated bass response can be compensated for by using corresponding electronic boost, but the maximum level of bass output from the driver is still reduced by the vent.

Referring to graph 2, vent size can be described by the corner frequency at which SPL in the measurement coupler is reduced by 3dB. Vents with a higher cutoff frequency can be used with drivers having higher maximum output capability. When a small BA driver covers low frequencies, a somewhat smaller vent allows a good balance of noise reduction and bass reproduction. A corner frequency in the range of 40Hz to 100Hz is recommended.

ANC Filter Design Requirements for BAs

A hybrid ANC system requires three filters; a feedback filter, a feedforward filter, and a music ANC compensation filter. The music filter requirements are the same for dynamic and BA drivers.

The transfer function of the driver, both sensitivity and phase, are critical to the performance of the ANC system. Designers used to working with dynamic drivers have been concerned that sensitivity and phase curves for BA look different than for dynamic drivers. Knowles has carefully studied these differences and found that this should not be cause for concern. The reason is that both dynamic and BA drivers are minimum phase devices in the frequency range needed for ANC. This means that both classes of drivers produce the minimum theoretically possible amount of phase shift for their sensitivity.

When a minimum phase type of equalizer is applied to flatten the response curves, the phase curves also become flat throughout the ANC frequency band. Due to their minimum phase properties, any gain and phase response achieved with a dynamic driver can always be matched when using BA driver after applying a suitable adjustment of the associated feedforward or feedback filter.  Once the gain of the BA driver is adjusted to meet the open loop filter target response, the open loop phase will look nearly the same as it would have for a dynamic driver.

Passive Attenuation

The backside of the headphone driver is typically vented to the outside world.  This allows the driver’s diaphragm to move more freely, increasing the bass output to the ear.  However, this also provides a path for outside sound to enter the ear.  A BA driver provides more passive attenuation of this sound due to its higher mechanical stiffness. This slightly reduces the amount  of active noise cancelling needed above the vent cutoff frequency.

Level compressor

While BA drivers offer very high efficiency for their size, small size BA drivers will have a lower maximum output rating than a 6mm or larger dynamic driver. This does not keep them from being effective in ANC in most situations but can result in sounds of distress when short term peaks occur. One very effective solution for this issue is to use a peak compressor. This will limit the level of signal sent to the speaker when loud sounds occur. This is a technique widely used with portable Bluetooth speakers to give the illusion that they are bigger than they really are. The audibility of the limiting can be minimized by using a fast attack time (<50 msec), a fairly quick release time (0.25 to 1 sec), and a fairly steep compression ratio (>5:1).

The peak limiter can be placed in different points in the signal chain. The placement may be limited by the flexibility of the device that is performing the ANC computation. The simplest method is to place the limiter after all signals have been combined, just ahead of the digital to analog conversion. This assures that the combined music, FB, and FF paths do not exceed a preset limit. No matter how the earphone is used and what sounds occur in the environment, the BA speaker will be protected and will not produce any sounds of distress. If an excessively loud sound occurs, the amount of noise reduction will be momentarily reduced. Full ANC will be restored as soon as the peak passes.

A potential limitation of this arrangement is that any one of the signals will change the performance of the others. For example, bumping the earphone will cause a large FB signal which will result in the music level being momentarily reduced. While peaks can occur in any of the FB, FF, and music paths, the FB path is the one most likely to have large peaks. Therefore, it may be useful to place the peak limiter solely in the FB path. This will assure that the FF performance and music performance will not be adversely affected by any infrasound occurring within the ear canal. The choice of limiter location depends on the expected signal levels for the various signal paths.

Multi-driver ANC

Hybrids employ a dynamic driver woofer and BA tweeter to leverage the advantages of both. Dynamics typically have ample low frequency output. BA tweeters provide high frequency extension, high dynamic range, and excellent detail. When a BA covers the treble, the dynamic driver can potentially be smaller and tuned for optimal low frequency performance.  It also provides the designer more freedom in placing or angling the woofer relative to the ear-tip exit of the housing.  One can also design a multi-way system using a large BA driver as the woofer, and a smaller BA as the tweeter.

When designed properly, adding a second driver does not interfere with the ANC system. The key factor is to minimize phase lag in the woofer channel. This is done by using a low order filter design for the woofer. Phase lag in the tweeter channel is much less important since it typically occurs above the frequency range where the ANC system is active.

In any multiway earphone a crossover network divides energy to appropriate drivers. Excellent performance can be obtained even without a low-pass filter on the woofer (or a simple acoustic filter) and a single capacitor acting as high-pass filter on the tweeter. An active crossover is also possible in TWS earphones by using both outputs from the DSP to drive woofer and tweeter separately. In either case it is possible to design the crossover to be compatible with ANC.

Knowles studied a wide variety of crossover designs using simulations and confirmed them with measurements. These experiments yielded some valuable observations:

• Zero or 1st order low pass woofer filtering works best for stability
• 4 kHz or higher crossover frequency works well for stability and audio performance
• Tweeter filter order is not critical

Graphs 11-14 show measurements for woofer and tweeter filters (xy in the legends) where x = woofer filter order and y = tweeter filter order. Graphs 3 and 4 show the amount of noise reduction. Graphs 5 and 6 show the music playback response with ANC turned off.

Graphs 3 and 4: Hybrid earphone noise reduction with various woofer and tweeter filter orders (xy=woofer-tweeter order)

Graphs 5 and 6: Hybrid response with ANC off and various woofer and tweeter filter orders (xy=woofer-tweeter order)

Higher order woofer filters cause excessive phase shift which degrades stability. Graph 7 shows the same earphone using a 3rd order filter. The ANC system shows net gain instead of attenuation from 300 Hz to 1.5 kHz.

Graph 7: Unwanted ANC gain as result of high-order woofer filter

Recommended BA Models for ANC

If using a BA for low frequencies, it is helpful to select a back vented design. This provides low frequency extension almost to DC and reduces low frequency distortion. Larger BA models will also provide greater output, enabling proper cancellation of louder noises, and/or lower resistance headset venting. Resistive back vents are popular for wired headphones, since they have reduced output near 200 Hz, which is useful for avoiding muddy bass. However, for ANC applications it is helpful to use a non-resistive vent to maximize the output near 200 Hz. Electrical equalization is then used to reduce music bass output near 200 Hz, improving timbre and system headroom.

When selecting a tweeter, one with a larger sound port can reduce the inertance of the device, boosting very high frequency output. If the system does not have room for an electrical crossover, there is a model with an acoustical crossover available.

The choice of microphones is also important for ANC performance.  Knowles engineers have developed a separate paper “Effects of Microphone Performance on ANC” that provides guidance in microphone selection for ANC earphones.

Table 1: Recommended BA models for ANC applications. The smallest full-range model, RAB, is listed here. Larger models with higher output are also available

Graph 8: Maximum output v. frequency for 10% THD, three BA families

Summary

Those experienced in designing ANC earphones with dynamic speakers will find that with the help of this guide, only small changes are needed to adopt a pure BA or hybrid configuration. Remember that the BA should be selected to meet the maximum output requirements. The front vent should be chosen for an appropriate tradeoff between occlusion reduction, user variations, and low frequency output requirements. The feedback loop filter should be adjusted to reduce phase shift at the low and high frequency extremes of the ANC range. At low frequencies, a 2nd order shelving filter boost may help to reduce the phase shift. At high frequencies, a notch filter matching the mechanical resonance of the driver is needed. If designing a system with a woofer and a tweeter, use a low order filter or none at all for the woofer to avoid excess phase shift.

Acoustic engineers working on TWS solutions should take advantage of balanced armature’s small size, low power, and premium sound for ANC earphones.

More information on Knowles balanced armature drivers for hearables & music earphones can be found at www.KnowlesPremiumSound.com

ABOUT THE AUTHOR: 

Thomas Miller is an Engineering Fellow for Knowles Corp’s Hearing Health Tech division. Thomas has worked in audio engineering for over 40 years, specializing in development and design for consumer audio, professional audio, and hearing health markets.