How to Get Louder, Richer Sound from Micro Speakers
July 25, 2019
| By: Greg Mow
Business Manager, Maxim Integrated
| and Michael Tuason
Principal Member of the Technical Staff, Mobile Solutions Business Unit, Maxim Integrated
As you’re getting ready for work, you start streaming your favorite tunes from your smartphone. You brush your teeth, get dressed, and let the music transport you to the concert venue where you last saw this band play. The sound is so rich, so loud that you almost forget it’s coming from your phone.
As the trend continues toward smaller and sleeker form factors, the engineer in you wonders, “What will it take to ensure that my portable devices will continue to deliver the louder, richer sound that consumers demand? How can my device keep up with other sleek, high-performance consumer devices? When will my customers come to demand this same experience?”
The micro speaker has become ubiquitous in cellphones, tablets, PCs, wearables, gaming systems, and even IoT applications. Smartphones were the first to drive mass adoption of these tiny speakers, which are similar, but simpler and much smaller, than traditional speakers. The dynamic speaker drivers found in today’s traditional speakers have three basic components: a diaphragm, a voice coil, and a magnet.
- The diaphragm is a flat piece of paper, plastic, or metal. Excursion is the movement of the diaphragm that creates sound.
- The “voice coil” is a coil of very thin insulated wire that is used to make a variable electromagnet when a changing current is applied. The voice coil is attached to the diaphragm.
- The magnet provides a static magnetic field, so the voice coil can push and pull against it.
A micro speaker has the same three basic components as a traditional speaker, but is much simpler in construction due to its size. (Micro speakers are often smaller than a memory card.).
When the speaker diaphragm moves to produce sound, it makes sound waves in front. However, it also makes sound waves of opposite phase in the back. If the front waves are not separated from the back waves, they can cancel. This is why speaker drivers are mounted inside a box or enclosure; the enclosure ensures the back waves do not cancel the front waves. The actual size and shape of the box is typically dictated by the form factor of the end product. The smaller the box, the less efficient the speaker system will be, due to back pressure caused by the moving diaphragm pushing and pulling against the tiny air volume in the box.
Micro speakers inside smartphones and other compact devices can deliver louder, richer sound using a smart amplifier with Maxim’s Dynamic Speaker Management algorithm.
All speakers have a maximum power rating, the limit of which is driven by two key considerations: thermal (how hot the voice coil can get before parts of the micro speaker melt) and mechanical (how far the diaphragm can move before mechanical breakage occurs). As speakers get smaller, their loudness or sound pressure level (SPL) goes down while the resonant frequency goes up, which leads to less bass. Driving these speakers harder can increase loudness and bass response; however, this approach, if done without proper speaker protection, can easily damage the micro speakers, as it causes overheating and over excursion.
For proper speaker protection, the algorithm in the amplifier that boosts the loudness of audio signals must know the speaker’s characteristics (resonant frequency within its enclosure, excursion limit, and voice coil thermal limit, for example). Designers following a traditional approach would have to undergo a time-consuming, complex speaker characterization effort, or rely on suppliers to do so. When factoring in multiple projects with different speakers, one can picture how this effort can negatively impact time to market, with increased design complexity as well as an increase in the required design resources.
Maxim’s DSM Drives Speakers Beyond Maximum Power Rating
In this quickly evolving world, consumers are not willing to trade audio performance for form factor. Today, they demand both performance and sleek design. Luckily, there is a way to safely drive micro speakers beyond their specified maximum power rating using Maxim’s patented Dynamic Speaker Management (DSM™) technology. The DSM algorithm delivers louder sound and richer bass with industry-leading power consumption. Employing Maxim’s proven thermal protection, which models the speaker based on the temperature coefficient of the voice coil (Ohms/C) and DC resistance (RDC), DSM allows design engineers to safely push speakers well beyond their specified power rating to maximize the loudness. DSM also provides excursion protection that allows designers to drive their speakers to their specified excursion limits, which significantly improves low-frequency audio response, up to two octaves below the resonant frequency limit.
Maxim’s DSM smart amplifiers take our high-performance current and voltage (IV) sensing amplifiers and integrate our patented DSM algorithm into an easy-to-use, fixed-function DSP. Maxim’s newest smart amplifier with DSM is the MAX98390, a boosted, digital Class DG DSM smart amplifier. The MAX98390, available in a 6.3mm2 package, unleashes a system’s full audio potential by safely driving higher power levels (up to 5.1W) into tiny speakers typically rated for much lower power (up to about 3W). Compared to conventional 5V amplifiers, the MAX98390 delivers up to 2.5x loudness and two octaves more bass. Its ~24mW of quiescent power consumption is almost half that of its nearest competitor and the device also provides 86% peak efficiency, both of which help to extend battery life of end devices. In addition to these industry-leading power consumption specifications, MAX98390 features Perceptual Power Reduction (PPR), which saves up to 25% of the power by using the speaker’s SPL response acquired via DSM, coupled with the human hearing threshold, to dynamically remove the portions of the audio signals that are inaudible to the listener.
Implementing our algorithm in a fixed-function DSP eliminates the need for complex programming. Also helping to reduce design time and design resources is our easy-to-use DSM Sound Studio GUI. The DSM Sound Studio GUI enables fast and easy speaker characterization, acoustic tuning, and prototyping. The GUI also automatically generates a complete register map for the MAX98390, including speaker protection and tuning parameters that only need to be loaded into the amplifier upon startup. This eliminates complex programming: just turn on the amplifier, initialize the I2C registers, then start playing audio. The DSM Sound Studio has a Quick Demo feature which uses the included reference speaker from one of our partners, PUI Audio, and lets you hear the DSM difference in just minutes.