Achieving loud, rich sound from micro speakers
While the video screens of mobile phones, tablets and notebooks have seen stunning improvements, audio performance has lagged far behind. Phone speakers still sound quiet and tinny, limited by their tiny size. Designers use various techniques to increase the volume and sound quality, but with limited success. They also bring risks: blown speakers are a common cause of failures in mobiles.
Simply limiting the output power makes for a poor user experience, and doesn’t protect against blocked speaker ports or high ambient temperatures. Temperature measurements can help but do little to improve sound quality. High-pass filters reduce the speaker excursion at the resonant frequency but cut out too much bass.
Feed-forward techniques can improve bass response but on their own aren’t enough and the can be a reliability risk. Additionally, clipping and low battery voltages can degrade sound quality even further.
This article will address these issues, as well as discuss NXP’s new TFA9887 – offered as the first IC to solve all these problems, using a combination of techniques including adaptive excursion control.
Speakers come full circle
Speakers and phones have developed hand-in-hand for over 150 years. The first speakers were used in telephone receivers, shortly afterwards they branched off into sound reinforcement and grew larger and more powerful.
In the 1980s and 90s things came full circle. Modern mobile phones have two speakers. One, still called a receiver, is in the earpiece. The second is for sound reinforcement, for things like ringtones, music playback and hands-free calling.
Micro speakers try to bridge the gap, aiming to produce room-filling sound from a tiny volume. What began with a move to play better polyphonic ringtones has now grown towards using a cell phone instead of a home stereo. These speakers are caught between two opposing trends, more output power and smaller size. As these trends accelerate, speaker designers are starting to look for new and innovative ways to get the best possible sound.
Modern micro speakers have a permanent magnet and a voice coil that is attached to a diaphragm that pushes the air to create sound. The entire speaker is enclosed in protective box that provides the "back volume" for the speaker to push against and project the sound from the speaker.
Output limited by temperature…
The first way to get more sound out of a speaker is simply to put more electrical power in. Small micro speakers rated at ½ Watt can generally handle many times that for very short periods. All the extra power going in has to come out somewhere, though.
Maximizing efficiency converts as much power as possible into sound. However, much is still wasted as heat in the voice coil. This ‘self heating’ is directly related to the current in the voice coil. If the temperature climbs too high, the glue holding the voice coil together can be torn apart (Figure 1).
Figure 1: Dissipating too much heat can tear the voice coil apart.
The speaker is cooled by conducting the heat out through the membrane, case and other components and by the cooling effect of moving air from the sound waves themselves. Lower frequencies generate more air movement causing more cooling and hence allowing higher powers.
This relation breaks down if the speaker port is blocked, the air movement is restricted or the ambient temperature rises. If the air cannot cool the coil, the internal temperature rises much faster than expected, and the speaker can be damaged in a few seconds. The relationship between coil temperature, power level, frequency, duration, ambient temperature, and airflow is complex, and is virtually impossible to reliably predict.
…and speaker excursion
Because micro speakers must be small, it is easy to move the diaphragm further than the maximum allowable excursion (typically around 0.4 mm). As speakers get thinner, the excursion becomes smaller, which is a major restriction on output sound level.
A speaker’s biggest excursion problem comes at and near its resonant frequency. At the resonant frequency the membrane moves easily, so small amounts of power can push the speaker beyond its limit. Micro speaker systems normally add a high-pass filter at around 1000 Hz to reduce the excursion. This can minimize the impact of the resonance peak, but losing the bass significantly degrades the sound quality.
The resonant frequency can change dramatically over the operating conditions, too. Temperature, ageing, a poorly designed phone case, and changes in the acoustic environment like blocking a speaker port will all cause shifts in the resonant frequency. Wear-and-tear on the phone case can also cause leaks in the speaker’s back-volume. Any of these changes can cause speaker failure in a fixed-filter system.
Clipping and power supply sag will affect sound quality
A 2 VRMS power level corresponds to a 2.8 V peak amplifier output. Nearing this level causes clipping and distortion. For higher frequencies, where excursion is not a problem, this is the main issue affecting sound quality.
It is worse with power supply ‘sag’ because of low battery voltage or high current drain. Since sag is often caused by the audio amplifier itself, this is hard to solve. Some systems lower the amplifier gain when the supply sags – however these cannot typically react fast enough for dynamic signals and still distort the peaks.
Boosting the supply level with integrated DC/DC converters can reduce amplifier clipping by adding headroom, but system designers must be careful not to damage the speaker by over-excursion. The voltage boost can also increase peak battery current and cause the voltage level of a partially discharged battery to drop low enough to cause a system reset, resulting in a dropped call or audio glitch.
Safe operating range for micro speakers without protection
All these parameters can delimit an area of safe operation (Figure 2). A temperature line limits the power amplifier to avoid the worst-case self-heating temperature, and a frequency line removes frequencies below resonance to prevent over-excursion.
Figure 2. Safe operating area for a speaker.
Allowing for changes in the acoustic environment and ambient temperature ensures safe operation, but with only a modest sound output.
For a typical system, limiting the power input to the speaker to 2 VRMS (2.8 Vpeak) and adding a 1 kHz high-pass filter will create a system that remains inside these limits. When used with an 8 ohm speaker it will result in less than 0.5 W (0.9 W peak) output.
Improving the volume output
Systems should always operate near peak output. Because audio signals are dynamic, they only rarely use the amplifier’s peak output voltage. Compressing the signal’s dynamic range (Figure 3) increases the apparent volume without changing the peak levels (which is why commercials sound louder than the rest of the TV or radio broadcast).
These dynamic compressors work by adding gain to the quiet parts of the music, and quickly reducing it at peaks (the ‘attack time’). The attack time is typically very fast (50 µs) and the corresponding decay time over which the gain is increased is typically much slower (5 seconds).
This approach again brings risks. Peak audio signals near the resonant frequency can see very large gain within the attack time. This increases the potential for over excursion and damaged speakers.
Figure 3. Sound sample before and after compression.
The output volume can also be increased by filtering out the resonant frequency. By removing the frequencies near resonance more power can be applied to the remaining signal. That drives more sound from the speaker, but the missing frequencies degrade sound quality.
The filter can be improved and narrowed by using models to predict the behavior of the resonant frequency and speaker temperature. However, any mismatch between the model and the real world can be catastrophic. A blocked speaker port, for example, changes the resonant frequency, with the filter then not protecting the speaker from damage.
Predictive models in these feed-forward systems can also calculate the speaker excursion based on the input signal. That can allow some frequencies below resonance back into the signal which improves sound quality, but it also compounds the risk, because high power signals can be delivered to the speaker where it is vulnerable to damage.
The feedback solution
Eliminating the differences between such complex models and the real world requires feedback. Feedback systems use real-world measurements to update the internal models that predict speaker behavior, and allow the system to produce more sound safely.
Key is to directly monitor the voltage and current to the speaker. This is not as easy as it sounds, since most portable audio systems use class-D amplifiers to reduce power consumption. The sample must therefore be taken after the signal is converted back to analog, which means using an external sense resistor after a power filter. This resistor lowers the system efficiency, because it consumes some of the output power.
Alternatively, more advanced current-sensing systems can be synchronized with the amplifier switching. This approach can provide more accurate results for small systems that don’t use power filters on the amplifier output. This solution can be fully integrated inside the amplifier, reducing output pins.
The first step in a feedback system is to measure the speaker voice coil temperature. Because coil impedance rises linearly with temperature, an accurate current measurement can provide a stable and accurate temperature measurement. This can accurately protect against thermal speaker overload as long as manufacturing variations are properly accounted for during production.
The next major step in protecting the speaker comes from controlling the excursion directly. Basic feed-forward systems can measure temperature to estimate the speaker resonance. More advanced systems use current sensing to accurately measure the impedance across all frequencies. The impedance spectra generate an adaptive model which can accurately predict the speaker excursion.
With direct information on excursion, a system can always drive low frequencies into the speaker without damage. If the speaker port becomes blocked, the resonant frequency changes and the system will adjust the signal to prevent damage.
The excursion information can also be used to optimize the output from the speaker, rather than optimizing a fixed electrical level. Here, the speaker can always use the maximum possible excursion for the desired signal. That also improves the sound quality by avoiding clearly audible distortion caused when the speaker moves beyond its limits.
Feedback can also use information from the DC/DC converter to optimize sound quality and system performance. Monitoring the current and voltage at the DC/DC converter can detect supply sag and adjust the peak output accordingly. This can ensure that the audio signal is never clipped, and sound quality (along with system performance more generally) will not degrade as the battery discharges.
Additional feedback points can further improve sound quality and system performance while also avoiding the risks of using higher supply voltages. This brings a huge performance improvement in SPL, sound quality and speaker reliability.
A feedback-based solution gives several key advantages by automatically adapting to changes in acoustic and thermal environments. A full solution, however, must use a combination of techniques.
Adaptive excursion control is needed to ensure that the speaker membrane excursion never exceeds its rated limit. Real-time temperature protection is needed to directly measure voice-coil temperature to prevent thermal damage.
A design must prevent clipping even with sagging supply voltage, and bandwidth extension must increase the low frequency response well below speaker resonance. And an intelligent DC-to-DC converter is needed to maximize audio headroom even at low battery voltages.
The benefits of an additional DSP
System designers would prefer to integrate any processing into the main system. Processing in the phone chipset will generally indeed give the smallest, most power efficient, and cheapest solution. However, accurate feedback is the key to successful speaker boost, and it needs low latency and high bandwidth.
Comprehensive speaker protection actually requires multiple input points – just knowing current and voltage isn’t enough. Furthermore, interrupts and system integration issues can become a major hassle. Multiple sensing points are needed to optimize the amplified signal. The processing must also optimize the performance of both amplifier and DC/DC converter.
To properly integrate this system into the central processor, all these signals would need to be converted and fed in to the chipset and all the controls properly taken out. A separate DSP can handle all these interactions automatically and can run continually even when the central processing shuts down.
The TFA9887 for speaker boost and protection
NXP’s TFA9887 (Figure 4) is offered as the first IC to dramatically boost output while fully protecting the speaker. It has an embedded CoolFlux DSP, Class-D amplifier with integrated current sensing, and intelligent DC/DC boost converter.
Figure 4. Block diagram of TFA9887.
The IC holds a software model of the speaker, and automatically adapts to any changes over the speaker’s lifetime including ageing, enclosure damage, blocked speaker ports, or whatever the world can throw at it. Better sound quality can also be traded against even smaller speakers and back volumes, giving smaller end products.
To confirm its performance, we compared the SPL of a speaker driven by the TFA9887 with a popular unmodified smart phone which uses a software compressor to enhance the volume. The test used identical test files and an identical phone (so identical speaker and enclosure).
Figure 5 shows more than 6 dB SPL increase in output volume. Optimized for bandwidth, bass output is increased by around 10 dB SPL – a huge improvement for a simple replacement in a state-of-the-art reference phone!
Figure 5. Volume and bass levels from the TFA9887 are noticeably improved.
Figure 6. Despite delivering around 2.5 W peak into a 0.5 W speaker (and over 5 W into a 4-ohm speaker), the excursion for music and speech clearly remains well within specifications.
This leap in performance illustrates an important design trend. The days of stand-alone amplifiers and converters designed in isolation have gone. The performance of phones and other portable devices have seen so many refinements that components must be treated as part of a bigger system. Each part of the system must sense and interact with the real world for best possible system performance.
So, audio systems must monitor the performance of the acoustics and adjust for the best user experience (Figure 6). Here as elsewhere, there is a clear trend to producing systems that measure and interact with the real world.
About the author
Shawn Scarlett is the marketing director for NXP’s mobile audio group, with a long history specializing in audio semiconductors including positions at Analog Devices and National Semiconductor, as well as start-ups such as Tripath and GTRonix. He has a B.S. in Electrical Engineering from the University of Arizona as well as an MBA from Santa Clara University. Before moving into semiconductors, he developed his audio skills as a professional sound engineer with the I.A.T.S.E working on sound reinforcement for major touring shows.
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