Seeing a speaker driver by itself seems to be a rare occurrence as the vast majority of speakers are designed into boxes known as enclosures. To fully understand how speakers work, we must understand why they are designed into such enclosures and whether enclosure design is a major factor in loudspeaker performance.
Why do loudspeakers need enclosures? The truth is that speakers do not absolutely require enclosures to act as transducers but nearly all utilize enclosures to improve performance. Speaker enclosures have a great effect on the speaker’s sonic character and are largely required to mitigate phase cancellation at lower frequencies.
So, then, why do practically all loudspeakers utilize enclosures? This article will answer this question in detail, going over each of the reasons for the inclusion of an enclosure in speaker design. We’ll also touch on the various enclosure types.
What Is A Speaker Enclosure?
A speaker enclosure may resemble a box structure that houses the speaker drivers. Though that is one function of the enclosure, this seemingly simple box-like structure is much more important than that. In fact, it is essential for optimal speaker performance in nearly all speaker models.
A speaker enclosure is essentially a small hollow box that interacts with the rear of the speaker driver. Enclosures a meant to deal with the rearward sound waves produced by the speaker driver(s).
Speaker enclosures come in a wide variety of shapes and sizes. They range from simple sealed particleboard boxes for small drivers in home stereos to incredibly complex systems with internal baffles, horns, bass reflex ports and acoustic isolation that house multiple large-diaphragm drivers for concert-grade sound reinforcement.
Note that the enclosure is typically considered separate from the housing of the speaker that contains the speaker’s other components (crossover networks, amplifiers, inputs/outputs, EQ, etc.).
That being said, some regard the enclosure as being the entire housing unit of the speaker. In this article, we’ll be discussing enclosures as being separate.
Why Do Practically All Speakers Have Enclosures?
The fact that practically all speakers have enclosures should tell us how important they are to speaker design.
So why are practically all speakers designed with enclosures?
The answer has to do with phase cancellation and the enclosure’s ability to mitigate the loss of sound wave cancellation.
Let’s imagine a speaker driver for a moment.
The audio signal (AC electrical signal) applied across the conductive element (voice coil) of the driver causes coinciding movement in the driver’s cone/diaphragm.
As the cone/diaphragm pushes forward, it compresses the air in front of it and rarefies the air behind it. The opposite is true as the cone/diaphragm pulls backward.
This is effectively how the speaker acts as a transducer and converts electrical energy (audio signals) into mechanical wave energy (sound waves).
To learn more about speaker drivers and their role as transducers, check out my article How Do Speakers & Headphones Work As Transducers?
However, there’s an issue that needs to be addressed.
When the speaker pushes forward and creates a change in sound pressure, it simultaneously creates and equal but opposite sound pressure to its rear. The sound wave that would be produced, then, is largely cancelled out by the rear “anti sound wave.”
This is particularly true at lower frequencies, which are more omnidirectional by nature.
Of course, there wouldn’t be complete phase cancellation do to the physical design of the driver and acoustics of sound. However, the phase cancellation of speaker driver by itself would be significant enough to cause the driver to generate a rather thin sound.
By effectively closing off the rear of the speaker diaphragm, at least to some degree, an enclosure can mitigate much of, if not all, this phase cancellation and drastically improve the sound propagation of the speaker unit.
That’s the quick and dirty response to why loudspeakers need enclosures. Let’s now briefly touch on the various speaker enclosure designs.
The Different Types Of Speaker Enclosures
There are essentially two main types of speaker enclosures: sealed and ported. Let’s discuss each.
Sealed speaker enclosures, as their name suggests, are completely sealed off.
These enclosures are made airtight and are generally lined with sound-absorbing material. They are designed to trap the rear sound radiation and only allow the front sound radiation to emanate from the speaker unit.
With the airtight seal comes the main disadvantage of sealed enclosures. That is that, because the rear sound is completely trapped, the speaker will only produce half its potential in terms of output level.
This makes speakers with sealed enclosures inherently inefficient.
Of course, this has little to do with the sound quality of the speaker. Rather, it means that the speaker will require more signal level to produce loudness.
Greater signal levels means more signal gain. Amplifiers with excess clean gain can be rather pricey and so this low efficiency, though perhaps a minor detail in some instances, can end up costing more money when amplification is factored into the equation.
All of this talk of efficiency leads to a grey area though, since a speaker with no enclosure, as we’ve discussed, would suffer from significant sound wave cancellation. So even though a sealed enclosure, in theory, is less efficient, it is likely to actually louder and most certainly sound better than a driver on its own.
There are different types of sealed enclosures worth mentioning:
- Simple (acoustic suspension): a simple sealed box behind the driver(s).
- Infinite baffle: a driver with an, ideally (though impossible), infinite baffle. Imagine a driver in a wall that extends infinitely. In theory, this infinite wall would, by default, trap the rearward sound waves, making it a pseudo-sealed enclosure (though not necessarily so).
- Isobaric loading: A sealed enclosure with two identical drivers/transducers that act in unison. The air volume between the two drivers is ideally incompressible, making them perfectly coupled. Isobaric loading systems, then, can produce louder results with less gain (applied to two drivers instead of one).
Ported speaker enclosures, as their name suggests, are ported and, therefore, are open to the environment and allow sound to escape and enter the enclosure.
These enclosures are carefully designed to benefit their drivers.
A ported enclosure is designed with internal pathways and acoustic damping that effectively inverse the polarity of the rearward sound waves.
Therefore, upon leaving the enclosure, the rear sound waves now interact constructively, rather than destructively, with the front sound waves. In other words, the problematic phase cancellation has been solved.
This results in greater efficiency ratings and less reliance on amplifier gain.
Modern ported enclosures are designed with specific acoustic damping to eliminate nasty resonance frequencies that are naturally found in the acoustic labyrinths of the enclosures.
A main disadvantage of ported enclosures is a decrease in transient response and accuracy when compared to sealed enclosures. There is also less control below the tuning of the enclosure, which means that lower frequency signals could cause the cone to move too much and sound sloppy or even sustain damage.
Though sealed enclosures are relatively simple, ported enclosures can get quite complex. There are even various types of ported enclosures worth mentioning here:
- Simple (Bass-reflex): the basic ported enclosure that is carefully designed with front port(s) and tube(s) to allow the rearward sound waves to emanate from the speaker in-phase with the frontward sound waves.
- Passive radiator: a system with an active driver and a secondary driver with no voice coil. As the active driver produces sound, the resonance of the enclosure causes the passive radiator to oscillate and improves the low-end of the speaker.
- Compound (band-pass): a system where the driver’s frontward sound waves are sent into a ported tuned box. The rearward sound waves may be sent into a sealed enclosure (single reflex) or another ported enclosure (dual reflex).
- Aperiodic: a poorly sealed rear enclosure with damped vents that allow sound to escape. These enclosures are designed to be small but with low resonance.
- Dipole: dipole speaker enclosures are equally open to the front and back with enough of an enclosure to mitigate much of the phase cancellation that would happen with a naked driver. Dipole “enclosures” are common in ribbon and electrostatic loudspeakers.
- Open-back: this system is common in guitar cabinets, where the back is nearly as open as a dipole system but there are still some extra enclosure barriers to the rear of the driver.
- Transmission line: these ported enclosures utilize long acoustic transmission lines (acoustic labyrinths) within their enclosures. The sound from the rear of the driver must travel through the labyrinth before escaping via the port of the enclosure. This systems allows for excellent high fidelity, particularly at low frequencies.
Do door speakers need enclosures? Door speakers found in automobiles do not require their own enclosures and are rarely designed with dedicated enclosures. Rather, the door itself, though wildly non-ideal, acts as an enclosure for the door speakers.
Will a speaker work in a vacuum? Although there’s nothing to say that a speaker wouldn’t be able to vibrate in reaction to an audio signal in a vacuum, there certainly would not be any sound emitted from the speaker. Therefore, it is arguable that a speaker would not work as intended in a vacuum.