Speaker Xmax is often included as part of the specifications of a speaker driver, but what does this Xmax value mean and how does it affect our speaker performance?
Speaker Xmax is a measure of how far the speaker voice coil can move in the speaker’s magnetic gap. Once the voice coil moves beyond the Xmax distance, then distortion and undesirable outputs will affect speaker performance.
Xmax can be simple to understand in theory; however, the implications are less widely understood.
In this article I will explain what speaker Xmax is in simple words, showing how you can calculate it and what it means for your speaker, covering:
- What is speaker Xmax?
- Is Xmax the same as speaker excursion?
- What is Xlim or Xmech?
- How is Xmax measured?
What Is Speaker Xmax?
To understand speaker Xmax, we must know what the inside of a speaker looks like and how it operates.
In simple terms, a speaker comprises a cone and voice coil assembly and a magnet assembly.
A voice coil is simply a copper wire that is wound around a piece of material, which is called the former.
The voice coil sits within the magnetic gap of the magnet assembly, as shown in the image below.
When a signal is input into the speaker, it flows through the voice coil. As now we have a current flowing through a wire in a magnetic field, the voice coil will oscillate.
Because the voice coil is attached to the cone, the cone will also oscillate, moving air and, as a result, producing sound.
Xmax refers to how much the voice coil can move within the magnet gap.
We typically have two types of voice coil designs, which are an overhung voice coil and an underhung voice coil.
Overhung Voice Coil
With the overhung voice coil design, some of the voice coil extends beyond the magnetic gap. In this instance, Xmax is calculated as the amount of voice coil that extends beyond the gap.
The following image shows a close-up view of an overhung voice coil sitting in the magnet assembly of a loudspeaker, with the Xmax dimension highlighted.
With an underhung voice coil design, there is a distance between the top of the voice coil windings and the top of the magnetic gap; this distance is the Xmax, as shown in the following image.
Is Xmax The Same As Speaker Excursion?
Xmax and excursion are not the same things.
Although Xmax and excursion refer to movement (or displacement), where Xmax refers to how far the voice coil can move out of the speaker gap, speaker excursion refers to how far the speaker cone can move in operation.
Xmax is called “maximum linear excursion” within the speaker industry, but it refers specifically to how far the voice coil can move from rest until it is out of the magnetic gap.
The total excursion is how much the cone assembly can move in total, which could mean moving far beyond Xmax, until mechanical stops are hit.
The speaker’s voice coil, and as a result the cone, can move beyond the distance of Xmax in the real world.
You really do not want to exceed movement that is equal to Xmax, as otherwise you will be beyond the designed operating parameters of the speaker and will get unpredictable results and distortion.
What is Xlim or Xmech?
Xmech and Xlim mean the same thing and refer to the maximum amount a cone can move until it hits a mechanical stop.
Xmech stands for Maximum Mechanical Excursion and Xlim stands for Maximum Excursion Limit, but they both mean the same thing.
It is the amount that the cone can move until it can go no further.
You really do not want to reach the Xmech of a speaker, as you are pushing the cone assembly far beyond its margin of safety until the voice coil actually hits the metalwork, which can cause damage.
As a general rule, Xmech is usually greater than Xmax, with enough margin to ensure that Xmech is never reached in a perfect design.
We never want to hit the Xmech, but during operation we can exceed Xmax at times.
Speaker designers have to allow some margin, so the speaker can comfortably reach Xmax by allowing the speaker to move slightly beyond it.
We then rely on the suspension design to restore the speaker movement back into balance, so it never hits Xmech.
How Is Xmax Measured?
If the voice coil is an overhung type, then Xmax is measured by measuring the distance from the top of the coil to the top of the front plate (also known as a top plate) as shown in the following image.
If the voice coil is an underhung type, then Xmax is measured by measuring the distance from the top of the front plate to the top of the voice coil, as shown in the following image.
Another way to state this is to subtract the total height of the voice coil from the total height of the front plate and divide your answer by 2.
Of course, this can be very difficult to do with a fully assembled working speaker with the underhung voice coil type, as the voice coil will be hidden in the magnetic gap and you cannot access it in order to measure the Xmax.
Here is a really nice video showing how you can use a pair of Verniers to measure the Xmax of a speaker.
Final Thoughts
Speaker Xmax is the maximum linear excursion (or displacement) of a speaker’s voice coil within the magnetic gap of the speaker.
It is important to never exceed this distance, as doing so can create undesirable frequency output, such as distortion.
Xmax is measured in different ways depending on the design of the voice coil; however, the fundamental principle is to subtract the voice coil height from the front plate (i.e. top plate) height and divide by 2.
Although Xmax and excursion refer to movement, they are not the same thing – Xmax refers specifically to how far the voice coil can move in the speaker gap, while excursion refers to how far the cone can move in operation.
