Speaker Mms and Mmd are two important Thiele / Small parameters that are related to the moving mass of the speaker. These are important parameters to understand in speaker design.
Speaker Mmd refers to the total moving mass of the speaker. This is often called the effective moving mass and refers to the mass of the moving parts of the speaker, which include the voice coil, cone, dust cap, glues connecting parts, suspension and surround.
In this article, I will take a brief overview of these two parameters, covering:
- What is speaker Mmd?
- How is speaker Mmd measured?
- What is speaker Mms?
- How is speaker Mms calculated?
What Is Speaker Mmd?
Speaker Mmd is a measure of the total moving mass of a speaker system. This includes the mass of all parts that move when sound is produced, such as the voice coil, cone, dust cap, glue, suspension and surround.
It is a key parameter in speaker design and affects how well the speaker will produce sound, as it influences the resonant frequency of the speaker.
We typically measure Mmd in grams.
How Is Speaker Mmd Measured?
When measuring speaker Mmd of a physical loudspeaker, you need to take all the physical parts of the speaker that move and measure their weight.
These parts are the surround, suspension, cone, voice coil, dust cap, and any glue which fixes the parts together.
It is important to point out that it is just the moving mass of these components that actually contribute to the Mmd mass value.
For example, not all the surround or suspension actually move during speaker operation as the edges of these components are glued to the speaker frame.
In the following image, see how the suspension and surround are glued to the speaker frame.
In this example, to measure Mmd, you would measure the mass of the surround and suspension, excluding any material that is glued to the speaker frame i.e. the area outside of the red dotted line, as shown in the following image:
What Is Speaker Mms?
When a speaker moves, not only is it moving the mass of the speaker components, it must also move the air in front of the speaker.
The speaker Mms value includes the Mmd value, plus the mass of the air in front of the cone.
Mms = Mmd + Air in front of the cone
How Is Speaker Mms Calculated?
To calculate the Mms of a speaker, you can calculate the total moving mass of the speaker and subtract the Mmd value.
The total moving mass Mt of a speaker is given by the following formula: [source]
where:
- f0 = the driver free air resonance frequency
- Cm =is the driver mechanical compliance
Once Mt is calculated, Mms can be found from the following:
Mms = Mt – Mmd
Of course, you notice that we need other values such as the mechanical compliance and the free air resonant frequency to calculate the Mms value using this approach.
For a low frequency driver in free air (or baffle) you can calculate the air load as:
- 8/3 x AirDensity x Radius^3
Where:
- Radius = the radius of the cone
- Air Density = 1.204 Kg/m3 at 20°C [source]
Since there is an air load in the front and rear of the cone, for a low frequency driver that is not in a sealed box (i.e. in a baffle or free air), you will need to multiply by the air load by 2 since there is an air load on the front and rear of the driver.
To learn more about this calculation in greater depth, I highly recommend, “Fundamentals of Acoustics (Physics) By Lawrence E. Kinsler”
Why Is The Moving Mass Of A Speaker Important?
The moving mass of a speaker is an important factor to consider when designing loudspeakers and it contributes to the overall performance of the speaker.
The Mmd moving mass of a speaker affects the resonant frequency of the speaker and is particularly important when creating a low-frequency response. The lower the moving mass, the higher the resonant frequency and vice versa.
Therefore, knowing the Mmd value of the speaker can help you compare and select a speaker. For example, a speaker with a lighter Mmd value will have a lighter cone and moving parts.
As a result, this lighter mass will make it easier for the speaker to reproduce higher frequencies. Therefore, for mid to high end frequency reproduction, a lighter Mmd is generally better, as the speaker is more precise at high frequency reproduction.
Of course, with a lighter Mmd value comes a higher resonant frequency, so like all things, speaker-related a balance or compromise must be found between a resonant frequency that is acceptable to you and the audio reproduction capabilities of the cone.
By understanding the total moving mass of a speaker, engineers are better able to achieve maximum efficiency, output level, frequency response, and sensitivity.
Final Thoughts
Speaker Mmd and Mms are two very important parameters to consider when it comes to designing loudspeakers. Mmd refers to the moving mass of the physical parts that make up a speaker system, while Mms takes into account the mass of air in front of the cone.
By understanding Mmd, engineers are able to create speakers with optimised performance, ensuring they have achieved an optimal balance between output level, frequency response, and sensitivity.
Happy listening!
