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.

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).

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.