What Does A Phase Plug Do? (Explained)

Phase plug design is a wonderful topic and a core component of compression driver and general speaker design. 

Phase plugs are nearly always found in modern compression drivers, as they are fundamental to operation. Some cone drivers also use phase plugs to help manage sound. 

Phase plugs are designed to improve the dispersion of sound as it leaves the diaphragm (or cone) and reaches the listener. From helping to extend the high-frequency response to reducing internal reflections, it is a fundamental component of compression driver design.

In this article I will describe what a phase plug is and how it works in simple terms, covering:

  • What is a phase plug?
  • How do phase plugs work?
  • Does a speaker need a phase plug?
  • What are the two types of phase plugs?

What Is A Phase Plug?

Phase plugs are generally found in the middle of a compression driver, or in the centre of a speaker driver cone with the aim of improving the dispersion of sound and reducing internal sound wave reflections.

Sound waves exiting the speaker driver’s diaphragm can cause interference if they’re not properly managed. This can lead to sound cancellation, which reduces the overall output of the speaker.

Phase plugs help to prevent this by managing the sound waves as they leave the diaphragm or cone.

By doing so, they can help extend the high-frequency response and reduce internal sound reflections, which, if left uncontrolled, can cancel each other out, resulting in poor audio output.

In the audio industry, you will typically find phase plugs in two locations:

  1. In the centre of a speaker driver. This usually looks like a bullet or alternative metal shape.
  2. Inside a compression driver. Generally, you cannot see the phase plug in a compression driver unless you dismantle the speaker.

How Do Phase Plugs Work?

My experience in phase plug design relates to compression drivers, so I will describe the principles of operation in terms of compression speaker drivers.

Just like all speakers, compression drivers create sound by a voice coil oscillating in a magnetic gap. This voice coil is attached to the speaker diaphragm (or cone).

Because of the oscillation of the voice coil, the diaphragm will also resonate. This causes the air in front of the diaphragm to move and travel in sound waves until it reaches our ears.

Phase plugs sit between the diaphragm and the horn, or audience. As sound waves move as a result of the diaphragm oscillation, the phase plug will “channel” and direct those sound waves so that they do not interfere with each other, which would cause cancellation.

In addition, we can manipulate the position and channel shape of the phase plug to promote a better frequency response, including an extended HF (high-frequency response) in some cases.

Phase plugs are greatly researched, but even today, speaker designers are finding new alternative shapes and configurations for this old speaker component.

Phase Plug Design

Phase plug design in modern compression drivers is an artful skill.

When designing a diaphragm for a compression driver, speaker designers will typically aim to meet the following criteria:

  1. Create a diaphragm that is very stiff. This prevents component breakup at higher frequencies.
  2. Create a diaphragm that is very light. A lightweight diaphragm will have less mass to move, therefore can produce a better result in the high-frequency range,

In order to design a phase plug for a compression driver speaker, a speaker designer will consider how the diaphragm behaves at certain frequencies and try to create a phase plug design that optimises sound wave travel within the speaker.

Using finite element analysis simulation software, a speaker engineer can inspect the modes of the diaphragm and decide on the following features:

  1. The position of any air channels in the phase plug.
  2. The size of the air channels.
  3. The length of the air channels.
  4. The geometry of the air channels.

With a careful and skilled design, a loudspeaker engineer can create a compression driver with extended HF (high-frequency) and smooth frequency response by manipulating the phase plug design.

Does A Speaker Need A Phase Plug?

The requirement for a phase plug depends on the specific speaker.

For example, all compression drivers need a phase plug in order to manipulate the sound from the diaphragm. Without a phase plug, much of the high-frequency extension will be lost and the audio quality will be severely affected.

For woofers and hi-fi cone speakers, there is some debate about the need for a phase plug.

On a very practical note, I can see how a phase plug on a cone speaker could be useful before we even consider the ability to improve directivity or clean up the top-end frequencies.

By using a phase plug, we can reduce the moving mass of the speaker since it generally replaces the need for a dust cap.

In addition, by removing the dust cap, we get a free moving cone without interference, however, in some designs, speaker engineers may use the dust cap to control modes or other speaker performance parameters, so again, it depends on the speaker design.

You get a better high-frequency response and directivity with a cone driver that has a phase plug, but this really depends on how good the speaker designer is. Most high-end manufacturers will use simulation software and measure the impact of the phase plug on speaker performance.

On the other hand, I am sure there are plenty of cone speakers on the market with phase plugs, just because they look cool.

As a speaker engineer and one who works with compression driver design, phase plugs really matter on compression drivers from my point of view. They absolutely need a well-designed phase plug to perform.

What Are The Two Types Of Phase Plugs?

There are two main types of phase plugs that are found in compression drivers: radial and circumferential.

Radial phase plugs have ribs that extend outward from the central axis in a radial direction.

Circumferential phase plugs have slots that extend in a circular direction around the circumference of the central axis.

Image Via Wikipedia

Both types of phase plugs serve the same purpose, but radial phase plugs can be more common as they are more cost-effective for manufacturing. In comparison, we make the circumferential phase plug in three parts which slot together to create the air channels, which is more expensive.

The type of phase plug you choose will depend on your specific needs. The best speaker designers will experiment with both and use the optimal design for their specific speaker specifications.

Final Thoughts

Phase plugs play an important role in the design of a compression driver speaker. In the simplest terms, they are used to manipulate sound waves and improve audio quality.

There is some debate over whether they are necessary on woofers and hi-fi cone speakers, but most high-end manufacturers will use simulation software to measure their impact on speaker performance.

Radial phase plugs are very common and less expensive to manufacture, while circumferential phase plugs are more expensive but provide better high-frequency response and directivity.

We need phase plugs to extend the HF (high frequency) response of a speaker, reduce any standing waves in the air cavities, and flatten the frequency response to ensure a smooth and best possible acoustic output.

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