What is a balanced armature? How is that different than a speaker?

In my travels I’ve learned a lot about IEMs and sound devices of all kinds. But I have to admit I was always a little confused about the pros and cons of Balanced Armature vs Dynamic Speakers, and I know I’m not alone.


In the world of IEMs there is a healthy debate and in the overall world of headphones the debate really rages. Lucky for us at UE University, we got a chance to ask some questions from the folks over at Knowles corporation. Knowles was started in 1946 by Hugh Knowles and by 1954 they developed first miniature microphone and receiver for hearing aids. Knowles Balanced Armatures also power many of the UE PRO monitors.


We sat down with Tim Wickstrom ( Global Applications Manager and with the company since 1988) and Andrew Bellavia (Director of Market Development) and tried to take some of the mystery out of BAs and point out some key differences between them and Dynamic Speakers.


Can you explain a little bit about the difference between Balanced Armature (BA) and Diaphragm sound reproduction?


In a balanced armature earphone, the metal armature moves driving a paddle. The coil is fixed. In a dynamic speaker the coil moves along with the diaphragm.


So BA is a bit like a reed where a diaphragm is a more like a drum head or a speaker?


The BA diaphragm is hinged at one end while the dynamic diaphragm moves axially.


Balanced Armatures were developed for hearing aids first and then applied to earphones, is that correct?


Correct, and they still are used essentially 100% in that application due to their small size, efficient drive, and tuning versatility to match different hearing impairments.


What are some of the advantages to Balanced Armatures versus small dynamic speakers?


In an ordinary earphone the primary advantage is that they are so small that multiple drivers can be fit in a single earphone. Just as with home tower speakers, more faithful sound is reproduced with separate speakers for lows – mids – highs. There is also wide tuning latitude which is good even with single driver earphones to achieve the sound the designer wishes. The diaphragm mass is light, giving more precise response.


So am I correct in thinking that BA has a higher or more efficient output than small dynamic speakers?


Correct. You see that in the output graph for same input power.

Can you explain the concept of “back volume” a little for our readers?


The stiffness of the paddle (or diaphragm) is partly due to the stiffness of the armature/paddle structure and partly due to the stiffness of the air volume trapped behind the diaphragm. In a non-vented BA, these two stiffness components are typically similar in magnitude and total stiffness is the sum. For a back-vented BA, air stiffness is removed, allowing greater excursion of the paddle for more low-end sensitivity.


So if I am understanding this correctly BA can be used in a smaller space than small dynamic speakers, how is this an advantage in In-Ear Monitors?


As told before, the ability to use multiple drivers is the big advantage. For example UE900 has four and UE custom IEMs can have more. But even with single drivers the small size can be used to create light, comfortable earphones with excellent sound quality.


Can you explain a little about what we're are seeing in the attached screenshot?

Impedance vs. frequency into the MHz range. The high impedance of BA at ultrasonic frequencies is a big advantage when using Class-D drive like some hearing aids are doing today.


The lower impedance of dynamic compared to BA at ultrasonic frequencies acts as a load on the amplifier, drawing more power for no benefit in performance.


Can you tell us a little more about this idea: “As health and fitness enthusiasts take notice of the benefits of “wearable computing” devices, Balanced Armature driver technology stands ready to optimize their in-ear listening capabilities.” What are some of the new technologies that might be developed in this way?


That’s kind of a “teaser” for the details in the white paper WP03. In an in-ear wearable where there is not only a speaker but also some DSP, bio sensors, onboard music player, etc. (think Bragi Dash), the need to keep the size down becomes more important so the device is comfortable with long wear. Reducing power consumption can also lead to longer battery life or the ability to use a smaller battery.


Think hearing aids again. They have to run continuously from dawn to dusk on a single button battery and their speakers have to be small enough to be inserted in one’s ear all day without discomfort. The same considerations are coming to the fore in in-ear wearables, in addition to the desire for a quality listening experience.


Wow, that is really incredible and very informative. Thanks for taking a lot of the mystery and jargon out for us and making easier for everyday audio folks like myself. I really appreciate it and the time you’ve given over to us for this UE University.

If you want to find out more, check out the Knowles website and definitely download those white papers. They are super informative.

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