So maybe you are already familiar with the various types of headphones that are available in the market. But, do you actually know how they function in order to reproduce sound? Having a deeper understanding of how the technology works, would be a valuable asset to any budding audio engineer. Hence, ask yourself this – How do headphones work?
Welcome to another awesome article on audio equipment (It will be awesome!). Today, we will take a closer look at the various technological principles behind headphones. Understand how the inner workings of a headphone works, in order to reproduce high quality audio. So, without further ado, let the learning begin!
Introduction – Design Principle
At its core, a headphone is very much like a sound system’s loudspeaker. It essentially functions as an electroacoustic transducer, which converts electrical audio signal to sound waves, that are then heard by the listener. However, in contrast to a loudspeaker (which emits sound into the open air), headphones allow the user to listen to audio sources (music, speech etc.) privately.
When you take a look at the different headphone types out there, you might think that they all work exactly the same. Well, today you will learn that this is not true at all. Several factors such as size of the headphone, sound quality and function, will affect the final design of the transducer. Let us now take a look at the various transducer types used in most headphones:
- Moving coil
- Balanced armature
The most common transducer type used in headphones is the moving coil driver (also known as dynamic driver). This design is made up of a stationary magnetic element that is attached to the frame of the headphone, which functions as a static magnetic field. The magnetic element is usually created using ferrite or neodymium.
Materials such as cellulose or polymer, is typically used to create the diaphragm (materials usually have a high stiffness to mass ratio). It is then affixed to a coil of wire (voice coil) which is immersed in the static magnetic field of the stationary magnet. When an audio electrical signal passes through the voice coil, it will start vibrating, thus causing the diaphragm to move back and forth.
What actually happens is that the current through the coil produces an alternating magnetic field, which reacts against the static magnetic field. This causes the voice coil and the diaphragm (attached to the coil) to vibrate and cause a disturbance in the air, thus producing sound.
The moving coil technology works very much like a dynamic loudspeaker. Hence, a typical loudspeaker diagram can be used in order to further illustrate this concept.
Electrostatic transducers are composed of a thin diaphragm, which is suspended between two perforated metal plates (referred to as electrodes). Both of the diaphragm and electrodes are electrically charged with a constant voltage, where the diaphragm has a positive charge, and the electrodes has a negative.
When an electrical audio signal is sent to the electrodes, it causes one of them to momentarily have a greater charge over the other plate. This causes the diaphragm to be attracted to the plate with the greater charge. The electrodes will keep alternating in terms of the level of charge, causing the diaphragm to move back and forth between both plates, thus producing sound waves.
As compared to moving-coil headphones, electrostatic ones are usually more costly and are not as widely available in the market. Take note that a special amplifier is also needed, in order to amplify the electrical audio signal to deflect the membrane. This often requires the amplifier to be capable of producing 100 to 1000 volts.
Generally, electrostatic headphones are able to produce very clear, crisp mid to high end frequencies with minimal distortion. However, due to the small displacement of the diaphragm, they usually lack in low-end frequencies. A well-designed, high end electrostatic headphone is capable of a significantly higher sound reproduction quality as compared to other types.
Essentially, electret transducers share the same functioning principles as an electrostatic driver. The main difference is that the electret transducer has a permanent charge incorporated into it, as compared to electrostatics, where the charge is constantly supplied by an external amplifier.
Both electret and electrostatic headphones are very rare and not readily available in the market. Original electret headphone designs were also typically cheaper and inferior in terms of technical capability and fidelity as compared to electrostatic headphones.
Here’s a great video explaining the technology behind headphones!
Designed with the intention of increasing the transducer’s electrical efficiency, the balanced armature works by nullifying the stress on the diaphragm characteristic, inherent in most other magnetic transducer types. This transducer contains a magnetic armature that is pivoted, allowing it to move within the field of the permanent magnet.
No net force is applied on the armature if it is precisely positioned in the centre of the magnetic field, hence the term “balanced”. When an electrical audio signal passes through the coil, it causes the armature to be magnetized one way or the other, and it will then rotate slightly about the pivot, one way or another. Ultimately, this also causes the attached diaphragm to vibrate and produce sound.
Due to their very small size, balanced armatures are typically incorporated into in-ear headphones (monitors) and hearing aids. Take note that in general, they are limited at the extremes of the hearing range (could produce frequencies below 20Hz and above 16kHz) and needs to be better sealed for isolation as compared to other types of drivers, in order to maximise its potential.
You can look for higher-end models that are designed with multiple armature drivers. A passive crossover network is used to divide the different frequency bands between them. This design is often implemented in order to provide a higher fidelity sound. Some models may also include an armature driver with a small moving-coil driver for an enhanced bass response.
We have reached the end of this article. Did you guys learn a lot today? Well, I sure hope this has been very beneficial for you guys in some ways. Now, get out there and get your ideal headphone!
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