Speaker Box Design (Which speaker enclosure type to use?)

When it comes to audio equipment, the speaker is one of the most important components. Without a good quality speaker, music and sound will not be as clear or enjoyable. But in order for speakers to perform at their best, they need an enclosure or “box” that helps them deliver better sound.

A speaker enclosure, or “speaker box”, is a structure that surrounds the speaker and helps to improve its sound quality.

Speaker enclosures work by trapping air and controlling the movement of sound waves so they are reflected and absorbed in certain directions. This helps to give deeper bass, reduce distortion and provide clearer overall sound.

Therefore, speakers need a properly designed enclosure in order for them to perform at optimum levels and deliver great audio.

Speaker boxes come in different shapes and sizes depending on what type of sound you are looking for. For instance, if you want a booming bass response, you would need a larger box with more air space inside it than if you were just looking for mid-range clarity, which can be achieved with a smaller box design.

This article will discuss the various types of speaker enclosures available and how they affect how your speakers produce sound, covering:

  • What is the purpose of a speaker enclosure? 
  • What are the different types of speaker enclosures?
  • What is an infinite baffle speaker enclosure?
  • What is a sealed speaker box? (Sealed enclosure)
  • What is a speaker bass reflex enclosure?
  • What is a speaker bandpass enclosure? 
  • What is a speaker transmission line enclosure?
  • How do I pick the right speaker enclosure type?

What Is The Purpose Of A Speaker Enclosure? 

Have you wondered why we put speakers in boxes? You may think that it is just a simple way to mount a speaker, however, the speaker box (aka speaker enclosure) is necessary to aid with the projection of sound from the speaker.

The purpose of a speaker enclosure is to contain the sound from a speaker, ensuring that it is projected in an efficient and balanced way. A speaker box or enclosure also helps dampen out any additional vibration, resulting in better sound quality overall.

Additionally, depending on the type of enclosure used, various acoustic effects can be achieved with different types of enclosures to further improve the sound quality.

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What Are The Different Types Of Speaker Enclosures?

The most common types of speaker enclosures are ported, sealed and infinite baffle.

Ported enclosures are designed to produce increased bass response from the speaker, by allowing sound waves to escape from the rear of the speaker box. This is achieved by adding a port or vent to the enclosure which allows sound waves to be directed outwards, resulting in an increased bass response.

Sealed enclosures are the most common type of enclosure and are designed to create a tight and uniform sound from the speaker. This is achieved by creating an airtight environment around the speaker cone, dampening any cabinet vibrations that could affect sound quality.

Here is a list of the main types of speaker enclosures:

  • Infinite Baffle
  • Sealed Enclosure
  • Bass Reflex Enclosure
  • Bandpass Enclosure
  • Transmission Line Enclosure

How you design your speaker enclosure and which type you use will impact the sound coming from the speaker.

What Is An Infinite Baffle Speaker Enclosure?

When a speaker operates, sound waves radiate from the front and rear of the speaker, since the speaker cone moves back and forth.

A speaker without a baffle (a baffle is a board onto which the speaker mounts) will result in the sound waves from the front of the speaker interfering with sound waves coming from the rear of the speaker as shown in the image below.

This is known as “acoustic interference”, and it can result in sound distortions.

An infinite baffle speaker enclosure eliminates this acoustic interference by providing an opening that allows the front of the speaker to operate independently from the rear. This way, sound waves can radiate freely from both sides of the speaker without any interference.

Here is an image of an infinite baffle. A theoretical infinite baffle will be large enough so that the soundwaves will never “reach around” to the back of the speaker.

For example, the lowest sound we can hear as humans is 20Hz. The wavelength of a 20Hz sound is 17 meters. If we have a baffle that is 17m square with our speaker in the centre, this is a theoretically infinite baffle as the sound wave is not large enough to interfere with the rear of the speaker.

However a 17-meter square baffle is not practical in reality in most applications, therefore placing the speaker in a sealed box where the internal volume of the box is greater than the Vas of the speaker is also equivalent to an infinite baffle.

To learn more about infinite baffles and speaker Vas I have covered this in greater detail in these articles:

What Is A Sealed Speaker Box? (Sealed enclosure)

A sealed speaker box, also known as a “closed box” or “sealed enclosure” is an enclosure designed to contain and isolate the sound of a speaker.

As the name suggests it is completely sealed.

It is typically made from high-density wood or MDF and lined with acoustic foam or other damping material.

Unlike an infinite baffle, the internal volume of the sealed box is less than the speaker Vas. This has the effect that the air inside the box acts like a spring which will dampen the speaker.

Overall a sealed box enclosure is the best when space is an issue. They have excellent transient response and overall are easy to build and difficult to get wrong.

The main disadvantage of a sealed speaker box is that this enclosure type is not the loudest speaker type. Due to the damping on the speaker as a result of the air sealed inside the box, it has low efficiency and will not be the loudest design you can choose.

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What Is A Speaker Bass Reflex Enclosure?

A bass reflex enclosure, also known as a “ported” or “vented” enclosure consists of a speaker mounted in an airtight box with one or more vents (or ports) which allow sound energy to escape from the back or front of the speaker.

The vent allows some of the sound energy that would normally be absorbed by the air within the box to escape. This energy is then redirected back into the room and adds acoustic output to bass frequencies, increasing low-end response.

You could say that the port acts as an additional speaker as it outputs or leaks sound.

A bass reflex speaker enclosure has much higher efficiency than a sealed speaker box. It’s louder and produces more mid-bass than a sealed speaker enclosure with higher power handling.

In addition, the bass reflex port design can allow a speaker to reach much lower frequencies, outside of what the speaker driver was designed for.

However, bass reflex enclosures can become noisy at higher volumes. They also require more accurate tuning to get the desired results and are usually larger than sealed boxes due to the ports.

What Is A Speaker Bandpass Enclosure? 

A speaker bandpass enclosure, also called a “fourth-order bandpass” enclosure, is designed to maximise the transfer of energy from a speaker driver to the air. This type of enclosure typically consists of two chambers that are tuned at specific frequencies.

The first chamber is sealed and contains the speaker driver. As sound waves travel through this sealed chamber, they are loaded onto the speaker cone and a low-frequency resonance is created. The second chamber consists of an opening in the box that is tuned with a specific port size to create a mid-range frequency response.

The idea behind the bandpass design is to combine the benefits of both sealed and ported enclosures. The sealed chamber provides a good low-frequency response while the tuned port creates a mid-range output that is much louder than a standard sealed box design.

The main advantage of using this enclosure type is its ability to produce loud, punchy bass with a tight response. However, it can be difficult to construct and tune as the box must be designed to exact specifications. In addition, the port can easily become noisy if not correctly tuned.

These enclosures can get impractically big and there is a risk that you can blow your speaker as you push it to maximum excursion but do not see it working inside the enclosure.

In addition to fourth-order bandpass speaker enclosures, there are also sixth-order and eighth-order speaker enclosures. In general, these speaker box types are difficult to design and a good level of knowledge and design criteria are required to ensure you get the sound you want from your speaker.

What Is A Speaker Transmission Line Enclosure?

A speaker transmission line enclosure is a type of speaker box that works similarly to a bass reflex enclosure. It consists of a sealed chamber, with the speaker mounted on one side, and a long duct or tube running from the back of the speaker to the other end of the enclosure. This duct allows sound energy to be “transferred” from the rear of the speaker and redirected back into the room, reinforcing bass frequencies.

The length of the duct should be ¼ of the wavelength of the Fs of the driver. 

For example, let’s assume you have a speaker with an Fs of 45Hz.

The wavelength of this frequency is 343/45 = 7.62m, where 343m/s is the speed of sound.

Therefore, ¼ of the wavelength of the Fs is calculated from 7.62m/4 = 1.905m

So the length of the port from the rear of the speaker to the exit of the box should be 1.905m.

Transmission line enclosures have several advantages over other types of enclosures. The long duct allows sound energy to travel further, resulting in more bass output than a standard sealed or ported box. This design will have an excellent low-frequency response.

However, transmission line enclosures are difficult to design and tune properly. In addition, they can become large and require a high level of acoustic knowledge to get the desired results.

How Do I Pick The Right Speaker Enclosure Type?

When it comes to picking the right type of speaker enclosure for your needs, there are a few key considerations you should keep in mind.

First, consider your space constraints. Different types of enclosures come in different sizes and shapes, so depending on where you plan to place your speakers, look for a size and shape that will fit your space.

Another key factor is the type of sound you want from your speakers. Different types of enclosures can affect the sound quality in different ways. For example, a bass reflex enclosure will likely give a more open and louder sound than an acoustic suspension enclosure, while sealed box enclosures provide a tighter bass response but less overall volume. Consider what your sound goals are and pick the type of enclosure that best suits them.

Finally, think about any additional features you might need in a speaker box. Do you require acoustic insulation to reduce external noise interference? Are there specific ports or damping materials you need added to the enclosure? Be sure to select an enclosure type that can accommodate these features.

When designing speaker boxes it is helpful to keep the Hoffman’s Iron Law in mind. [source]

Hoffman’s Iron Law states that for a given box volume and speaker, you can only optimise or choose two of the following three features:

  • efficiency
  • enclosure size
  • extended bass response (deep-bass)

Designing speaker boxes is a compromise, therefore if you want a small and compact enclosure box size with good bass extension, you will need to compromise on efficiency. In other words, you will need to feed more power to your speaker for it to perform.

When designing a speaker box, these three features are traded against one another. You can easily have two from the list, but not all three perfectly.

Final Thoughts

In conclusion, speaker box design is an important factor when considering what type of speaker to use. Depending on your application, the type of enclosure and size will play a crucial role in achieving optimal sound performance from your speaker.

When selecting the right type of speaker enclosure for your needs, it is important to consider several factors such as space constraints, sound goals and any additional features you may need.

Additionally, Hoffman’s Iron Law should be kept in mind when designing a speaker box – efficiency, size and extended bass response can all be optimized or chosen but not at the same time.

With careful consideration of these elements, you will surely find an optimal solution that meets both your practical requirements and listening desires.

If you are looking for a reference for speaker design, I highly recommend the Loudspeaker Design Cookbook as an excellent reference.

Happy listening!

Engineer Your Sound

We love all things audio, from speaker design, acoustics to digital signal processing. If it makes noise, we are passionate about it.

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