Horns for loudspeakers are fascinating devices. Horn designs can include a variety of horn sizes and shapes and, for many, it is a mystery what these horns are actually doing.
Horn design is an art form and a science, requiring a balancing act between the best audio directivity projection and loudness combination while maintaining good sound quality.
A good horn design for a loudspeaker has two key functions. First, the horn must direct sound in a required way and secondly, the horn must “load” the loudspeaker driver with the intent of making the sound louder.
This might seem very simple on the surface, however, horn design is complex. There is an infinite combination of horn profiles and lengths available and you can juggle horn design parameters all day long only to find that you still have to compromise.
In this article, I want to cover the basics of horn design and explain in very simple terms what the purpose of a horn is, covering:
- What is the purpose of a horn?
- How do horn loudspeakers work?
- How does a horn amplify sound?
- What speakers need a horn?
What Is The Purpose Of A Horn?
A horn for a loudspeaker has two main purposes:
1. To Provide An Acoustic Load To The Loudspeaker Driver.
In very simple terms, the volume of air in the horn provides resistance to the loudspeaker driver. This has the effect of improving the speaker efficiency (i.e. loudness in this case) and limiting the excursion of the speaker diaphragm at lower frequencies.
2. To Control Directivity Of Sound.
In the world of PA, this is really important as in a perfect world our entire audience will hear the sound well regardless of where they are standing in the audience or listening room relative to the speaker.
We want our horn to direct the sound equally and, more specifically, not “beam” where sound is directed in a very high intensity and narrow path.
In the world of horn design, and from a user’s point of view, the most important function of a horn is to control sound directivity.
Imagine standing in the audience at a concert where there are design problems with the speaker horns where it is “beaming”.
Horn beaming results in high frequencies concentrated on-axis in a set direction. Like all the frequencies in a certain range being focused on a point, just like a beam.
In some parts of the audience, you will not hear any or at least very poor high-frequency sound and in other parts of the audience, you will be overwhelmed with high-frequency sound.
How Do Horn Loudspeakers Work?
In very simple terms, here are the fundamental steps of how a horn works:
- The compression driver (loudspeaker) diaphragm vibrates to create sound.
- This moves the air in front of the diaphragm.
- The horn, mounted on the front of a speaker driver, creates an additional air mass or load. The air vibrating from the compression driver (loudspeaker) must now move through this extra air mass in the horn.
- The profile of the horn is carefully designed to “load” the compression driver (loudspeaker) in a way so that it can push air as effectively as possible. This will improve loudness and make the compression driver more efficient.
- The shape of the horn will help to direct the airflow, which controls the directivity of sound, ensuring everyone in your audience is hearing the sound at the same level.
To understand how horns work, we need to focus on what is happening at the diaphragm of a compression driver or loudspeaker.
When a diaphragm vibrates, it moves air in front of and behind the diaphragm. It is this movement of the air that generates the sound waves which travel through the air and we perceive as sound or music.
When discussing a vibrating diaphragm in a compression driver, this diaphragm has a radiation impedance.
In very simple terms, radiation impedance is defined as the ratio of the reaction force of the medium (i.e. air) to the velocity of vibration of the diaphragm or sound source.
So, in a nutshell, our loudspeaker diaphragm experiences a resistive force as it pushes air out (outward propagation) and a reactive force as it experiences the varying air in front of the diaphragm and in the horn.
This air in the horn appears to add a mass to the diaphragm.
Of course, when you add a mass, it will create phase and velocity discrepancies in the diaphragm and the diaphragm must now work harder to push air into the horn.
There are many different profiles and shapes of horns, but the most common and effective are:
- Exponential horns
- Conical horns
As with all horn profiles and shapes, there is not a perfect match when selecting a horn design for a compression driver and compromise is always needed.
In the world of horn design, exponential horns are known for providing excellent loading properties, while conical horns have poor loading but excellent directivity control.
How Does A Horn Amplify Sound?
A horn amplifies sound by matching the throat impedance of the loudspeaker driver. We call this “impedance matching” in the audio industry.
The horn does not require any power. It is purely amplifying the sound by coupling the air between the air in contact with the speaker’s diaphragm and the surrounding air.
When a compression driver loudspeaker diaphragm vibrates in free space, it will move the surrounding air. This air will radiate out, carrying sound waves to the ears of the listener.
A speaker diaphragm (also known as a dome) in a compression driver can have a very small radiation surface. We could make the compression driver louder by increasing the size of the diaphragm so it will move more air, however in reality, this is often not practical.
By placing a horn in front of the compression driver, it is creating a much larger connection (or surface area) with the surrounding air. This has the result of amplifying the sound, as the radiating surface of the air coming from the exit of the horn is much greater than the radiating surface of the air from the diaphragm on its own.
What Speakers Need A Horn?
Generally, speakers with small radiation surface areas, such as compression drivers, need a horn.
A horn is necessary for a speaker compression driver to work.
Unlike bookshelf speakers or tweeters, compression drivers are used for PA (public address) systems, such as those used at a gig or where loud music is required.
Horns are needed for such loud volumes and controlling the direction of the sound to ensure that you get the most loudness for your system.
At home, bookshelf speakers do not require such loud listening levels, therefore they do not need a horn, nor will they benefit from a horn.
For at-home speakers, the cone design is large enough to move the surrounding air and generate loud sound
Horn design may seem simple on the surface, but it is a complex and scientific process that needs considerable engineering skill to get right.
Horns are essential in certain applications where loud music is required, such as PA systems and other public address systems.
Horns allow for greater sound amplification by allowing air to be more efficiently moved from the speaker’s diaphragm to the ears of the listener, resulting in louder and clearer sound.
There are many horn profiles on the market and usually, when purchasing a horn, you will be given an indication of what area of sound it will cover and how well it will perform.