In the world of speakers, an infinite baffle is a term that often comes up, but what does it mean? And what consequences does an infinite baffle have for a speaker’s performance?
The baffle is the front or mounting face of the speaker.
An infinite baffle is a mounting face or front face that goes on to infinity; however, this is not practical in the real world. In practice, an infinite baffle implies a closed box that is large enough so it is not an air suspension system.
In this article, I will cover the basics of infinite baffles, and what they really mean, covering:
- What does infinite baffle mean?
- What do we mean by an air suspension system?
- Why do speakers need a box?
What Does Infinite Baffle Mean?
When we imagine a speaker working, we think of the cone moving up and down, creating a sound which comes from the front of the speaker.
In reality, this piston motion of the cone moves air in front of and behind the cone. This results in frequencies coming from both sides of the speaker, i.e. the front and back.
Without a baffle, the sound waves coming from the back side of the speaker can interfere with the sound waves at the front of a speaker. This can create phasing and cancellation issues which impact sound.
By putting in a baffle we are isolating these sound waves. To make the speaker work perfectly, we would have a baffle that would continue onto infinity so that the waves on the front and rear of the speaker never meet.
In practice, this is not practical, so we compromise by extending the baffle and turning it into a closed box.
Now we have a system where the waves on the front of the speaker never meet the waves on the rear of a speaker, equivalent to an infinite baffle.
What Do We Mean By An Air Suspension System?
With an infinite baffle, we have completely separated the sound emanating from the front and rear of the speaker, but it is now in a closed box.
For the system to be a true infinite baffle, the box must be large enough to ensure that the air in the box does not interfere with the speaker.
We need the speaker’s own suspension to be the dominant driving force of the speaker, with no interference from the rear air.
Air suspension systems (also known as acoustic suspensions) are those where the box is small enough that the high pressure of the trapped air in the box acts like a spring, and actually becomes the main restoring force of the speaker cone.
In other words, the air in the box is now the dominant driving force of the speaker, overriding the speaker’s own suspension system.
How Can You Tell The Difference Between An Infinite Baffle Or Air Suspension System?
So how do you know if you have an infinite baffle or air suspension system?
To understand the strict definition of when an infinite baffle becomes an air suspension, we need to talk about compliance.
In simple terms, compliance is how much the cone of your speaker moves in response to pressure.
When you gently press the cone of your speaker, if it moves a lot we say that it has “high compliance” and if it feels stiff with little movement, we say this speaker has “low compliance”.
If your speaker is sitting in free air, on your table, and you press on the cone, it is 100% the suspension system of the speaker you are feeling.
If your speaker is in a speaker enclosure and you press on the cone, then the air inside the box will have a spring effect and is contributing to compliance.
Compliance of a driver is specified as an equivalent volume of air, Vas [source]
The compliance ratio of a closed box will tell us if the box is an infinite baffle or an air suspension system.
If the compliance ratio of the closed box is greater than 3, then the air in the box is the greatest restoring factor (above the speaker suspension system), and this means that it is an air suspension system.
If the compliance ratio of the closed box is less than 3, then the speaker’s own suspension system is the dominant driving force and we have an infinite baffle.
The following table summarises this point.
| Compliance | Type Of System |
|---|---|
| Compliance > 3 | Air Suspension |
| Compliance < 3 | Infinite Baffle |
Why Do Speakers Need A Box?
In reality, speakers do not need a box to make sounds. However, the speaker’s performance can be greatly improved by being placed into an enclosure.
For anyone who has listened to a woofer while not in a box, you will fully appreciate the need for a box, especially for low-frequency drivers.
When a woofer is operated outside of a box, i.e. in free air, it will have very little bass output.
This lack of lower frequency for a woofer operating in free air is because of the sound radiating from the front and back of the driver interfering with each other and cancelling out frequencies.
We put speakers in boxes so that the front-side sound wave does not interfere with the back-side sound waves.
It is possible to place a speaker, just on a baffle board, with no rear enclosure. This type of system is called a dipole speaker. This simple introduction of a baffle to separate the front and rear of the speaker will cause more bass.
Final Thoughts
A speaker baffle is an essential part of any speaker’s design. By adding a baffle, we can improve the sound quality by isolating the sound waves coming from the front and back of the speaker.
An infinite baffle is a closed box that is large enough so the air enclosed in the box does not interfere with speaker suspension and performance.
A speaker enclosure, whether it is a box or baffle board, separates the front and rear sound waves to improve bass response. If you are looking for more bass from your speakers, try placing them in an enclosure rather than free air. A well-designed speaker enclosure will greatly improve the overall performance of your system.
Happy listening!
