Compression Driver Loading (What is it and why it matters?)

When it comes to speaker compression driver design, there are a few key factors that you need to understand in order to create the best possible output, and one of these factors is compression driver loading, also known as compression driver ratio in some academic circles.

Compression driver loading, as it relates to compression driver speaker design, can be defined as the ratio of the diaphragm area to the gap area in the phase plug. The ratio of these areas defines our compression driver loading factor.

Although an important speaker design factor, this one calculation can often be overlooked by hobbyist compression driver speaker designers.

In this article, I will cover some of the most common questions around compression driver ratio, including:

• What is compression driver loading factor?
• How is compression driver loading factor calculated?
• Why does compression driver loading factor matter?
• What is a good compression driver loading factor to aim for?
• What effect has compression loading factor on compression driver output?
• Does compression driver loading factor impact speaker tonal quality?

What Is Compression Driver Loading?

Compression driver loading factor, also known as compression ratio, is the ratio of the diaphragm area to the gap area in the phase plug. This ratio is important in speaker compression driver design, as it can help contribute to the overall performance of the speaker.

When designing a speaker, it is important to consider the desired performance and then experiment with compression driver ratio that will help to achieve that goal.

How Is Compression Driver Loading Factor Calculated?

To calculate your compression driver loading factor, you will need to know:

1. The area of the radiating surface of your diaphragm. (Shown in red below)
2. The gap area of the phase plug channels. (Shown in yellow below)

In the above example, let’s call the diaphragm area Sd and the phase plug gap area St.

The compression loading factor (aka, compression ratio) in this instance is calculated from St/Sd [source]

For example, what is the compression ratio of a speaker with a diaphragm surface area of 14cm2 and a phase plug gap area of 1.4cm2?

First, you must convert the units to meters, as that is the SI Unit for length. [source]

Next, you simply divide the phase plug gap area by the diaphragm surface area as per the formula, St/Sd.

Therefore, 0.014/0.14 = 0.1

Our compression loading factor is 0.1.

Why Does Compression Driver Loading Factor Matter?

In a compression driver speaker design, the difference in area between the diaphragm radiating surface and the phase plug gap creates a differential that allows the impedance of the moving mass to be matched to the impeding of the air load of the throat of the horn.

We can work with this loading factor and experiment to optimise our compression driver design and ideally produce a high pressure and wide frequency bandwidth.

Mismatching the impedance loads can have a negative impact on the speaker output, for example, resonance.

What Is A Good Compression Driver Loading Factor To Aim For?

As a rule of thumb, a compression loading factor of around 0.1 is a good place to start.

From here, you can tweak the design to optimise and refine, but a compression driver loading factor of 0.1 will work well and greatly help you start an optimisation process.

What Effect Has Compression Loading Factor On Compression Driver Output?

There has been some work completed on compression driver ratios and the impact on output performance, but at the time of writing, the total amount of research is limited.

According to one study which looked at the maximum efficiency of compression drivers, higher compression ratios will raise high-frequency efficiency but may decrease mid-band efficiency. [source]

The quantity of data available is limited at present to make any sweeping conclusions.

Regarding tonal quality, it is impossible to say and a speaker designer must experiment with the compression driver ratio to optimise the design as best as they can.

Final Thoughts

Compression driver loading factor is an important aspect of compression driver design that can help contribute to the overall performance of the speaker.

When designing a speaker, it is important to consider the desired performance and then experiment with compression driver ratio that will help to achieve that goal.

A good place to start is with a compression loading factor of around 0.1. From there, you can tweak the design to optimise and refine it.

More research is needed in this area to make any sweeping conclusions, but speaker designers must be aware and experiment with the compression driver ratio to optimise the design as best as they can.

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