In the world of loudspeakers, bigger isn’t always better. There’s growing demand for mini speakers that can be put in small spaces and still deliver exceptional audio. To get this right, you need a speaker that is designed well, built well, and tested to ensure it will work in real-world conditions.
The process begins with identifying the dimensional constraints you’re dealing with and communicating those to a loudspeaker original equipment manufacturer (OEM). The space available is typically the single greatest limiting factor and variable in getting the right speaker for an application.
The next question: What sounds will the speaker handle? Will it be voice, music, a simple sine wave, square wave tones, or a sweep of tones? This helps determine the resonant frequency needed.
The questions after that relate to the volume and how much amplifier power, or voltage, the customer has available to use. This helps determine how the speaker will handle power and its sensitivity requirements.
Once we know the size, sound, and volume specifics for a speaker application, we can figure out the right materials and design.
Lots of the micro/mini speakers have either an aluminum cone with a polyether surround, a polyester/PET (or mylar) cone and surround, or an acrylic-treated cloth cone and surround. Each of these offers some level of water resistance (or is waterproof), which is often a prerequisite for customers who need mini speakers.
Effects of an Enclosure on a Speaker’s SPL
The physics of audio dictate that a larger enclosure is typically better suited for low frequencies (woofers are bigger than tweeters). The air inside a speaker enclosure acts as a pneumatic spring. The smaller the box, the less air inside and the stiffer the spring. If an enclosure is too small, it will increase the resonant frequency, meaning it prevents it from playing the full bandwidth it was designed for.
A good audio engineer can take advantage of this, though, and tune a speaker for maximum output at a higher resonant frequency by using too small of an enclosure. This is especially useful in alarms or sirens where maximum sound pressure level (SPL) is desired over maximum frequency response.
In many cases, customers want to use the internal space of their product’s housing as the speaker enclosure, which can be a great solution. You just have to make sure to secure or cover any loose parts inside that could buzz or rattle.
Improving the low frequencies of a small speaker takes design expertise. We often use a bass reflex enclosure design. You may know this as a ported or vented enclosure. These designs take advantage of the volume of space in the enclosure, tuning it with a port or a vent and turning it into a Helmholtz chamber that resonates. The air in the vent or port acts as a secondary air piston that augments the speaker’s frequency response. A small speaker may require a long vent, or port, depending on the tuning frequency.
Workarounds for Small Speakers
Increasing the SPL of micro/mini speakers is not as easy as dumping more power into them. Their voice coils are smaller, and their mechanical parts are more fragile. Besides, you only gain 3 dB of SPL for each doubling of amplifier power. An easier approach is adding another speaker. Two speakers, wired in parallel (most amplifiers today are 4-ohm, if not 2-ohm, stable), immediately increases the system SPL by 6 dB, which is a noticeable difference.
The advent of DSP (digital signal processing) and “smart” amplifiers has literally changed the game for micro/mini speakers. Most modern smartphones use a 15x11mm or 18x13mm speaker in an ultra-small 1 to 2cc enclosure. To maximize the output and low-frequency response of these speakers, DSP and/or “smart” amplifier technology boosts the signal of lower frequencies (or higher frequencies if needed) going to the amplifier to offset the natural roll-off of these smaller speakers. This is just like the tone controls or EQ in your car audio infotainment system.
Today’s smart amplifiers don’t just boost signals. They can also monitor how far a speaker moves (Xmax), the voice coil temperature (to prevent overheating), and provide Pro Audio-quality limiting, compression, filtering, and equalization. Isn’t technology great?!
As demand grows for mini speakers, it’s important to work with people who really get the science behind the technology.