Basic guide to speaker enclosures

The enclosure is instrumental in the functioning of any speaker. I started writing this blog to clarify the basic enclosures types and what they do, the good, the bad and the sometimes ugly, in a way that is accessible but also gives enough depth to base your decision on when starting your speaker project. Moreover, offer you a good understanding of the pros and cons of these designs. To keep things clear, a few variants and subjects like horn speakers have been omitted. These demand their own blogs in the future. This blog will mostly stay away from subjective observations of the different types of enclosures, although this might be difficult when talking HiFi. The following enclosures types and topics will be discussed.

• The need for an enclosure
• Finite "open" baffle
• Sealed box
• Vented/bass reflex enclosure
• Transmission line

The need for an enclosure

Why would you need one in the first place? When a loudspeaker driver converts an electrical signal into a mechanical pistonic action, it compresses air in front of the cone/diaphragm, creating a pressure ripple or wave known as condensation. At the same time, it causes the opposite effect on the other side of the cone/diaphragm resulting in a partial vacuum known as rarefaction. Furthermore, vice versa, when the cone travels the other way depending on the electrical signal, this action is called oscillation.

Newton's Third Law: "For every action, there is an equal and opposite reaction."

The easiest way to visualise this is to see the pressure wave as +1 and the back wave as -1. When you add these up, you end up with zero in sound terminology; they cancel each other out. (Note: This is a very basic explanation and does not take into account factors like directivity etc.). This is not a desirable effect. This effect will need to be mitigated like in an open baffle, controlled like in a bass reflex or acoustic labyrinth or absorbed like in a sealed enclosure to give a speaker a usable frequency range.

Finite "open" baffle speaker enclosure

The ideal situation for a speaker driver would be the Infinite baffle, where the front and back waves would never meet and would present an infinite beaming surface. This allows the speaker to behave without any negative effects an enclosure would have like diffraction, baffle step loss, cabinet resonances. Unfortunately, this is simply not a practical solution, so there was a compromise made: the finite or open baffle speaker,

This enclosure mitigates the cancellation effects and creates a figure 8 model response where the sound waves cancel each other on the baffle sides but still provide output at the front and the back. Depending on the baffle size, there is a frequency where the wavelength is so long that it does not "see" the baffle and still cancels out fully. This is called the cut-off frequency. To make this cut-off more gradual than -18 dB per octave, the driver can be mounted off-center.

The sealed box speaker enclosure

This is a variant of the infinite baffle concept where the back wave is separated from the front wave by fully absorbing it in a sealed enclosure. However, it does not present an infinite beaming surface for the driver.

The air trapped within the sealed enclosure acts as a spring for the woofer, which benefits from returning to its original position. Depending on the volume of air in the cabinet, this effect can be more or less. This is good for the woofers' transient response or accuracy. However, its performance is dependent on using the right woofer. Thankfully Dr. Richard Small made a guideline for the right kind of woofer. This is called the EBP formula or "efficiency bandwidth product." 

When the result is lower or equal to 50, the speaker is suitable for use in a sealed enclosure. This is by no means an iron rule; most modern drivers have a higher than 50 EBP because of the popularity of ported enclosure where a higher value is more suitable. An example of a suitable sealed enclosure driver would be the GRS 10PF-8 10" this speaker has an Fs of 43 Hz and a Qes of 1.66

"typical timed response of a sealed system"

The reflex enclosure speaker enclosure

These enclosures use a tuned port or slot to produce extended bass response around the drivers' Fs point, which is one of the most common enclosures. This reflex cabinet is also known as ported/vented enclosure.

The basic working of a port is very interesting. The common misconception is that it "pumps" air to create more bass. In fact, the air inside the port has a certain mass that will resonate at the frequency it is tuned at, usually the driver Fs point. This air mass in the port acts as a driver itself and will start to generate output at this point. Also known as a Helmholtz resonator. This output, however, is very specific and only happens near the frequency it is tuned for. This resonance effect also dampens the drivers' cone movement at the tuned frequency to a point where it becomes nearly motionless. This can be visible thru its impedance response, which shows a distinctive double hump shape. While extending the bass response in most cases, it can also create a "boombox" effect at a higher tuning frequency accentuating the lower bass frequencies.

1.1: Typical bass reflex enclosure response this example is from the SEAS Bitfrost kit based on the SEAS Excel W18NX001 woofer. The black line is the total system output, the blue dotted line is the woofer's output, and the red dotted line shows the port output.		1.2: This is a spectral decay plot showing the effects of a bass-reflex design. This also called "the time penalty" The resonances used to generate the bass extension does not dissipate immediately; this hinders the ability for a driver to start and stop instantly. This smears the drivers bass response in the time domain.

Transmission line enclosure

In most cases, this enclosure also referred to as an acoustic labyrinth, uses a damped quarter wavelength pipe to augment the bass response of a system. (Note that a common and practical definition of a transmission line is used in this blog.)

The theory behind it implies an infinite pipe to present the correct resistance to the driver at any frequency. Unfortunately, infinity is not a practical design criterium. There are also different implementations like a sealed quarter-length transmission line.

In this implementation, the back wave is guided thru the line. This delays it, and this delay shifts the phase to 90° degrees. The effect this has is that the back wave is back in phase with the front signal and adds to the output rather than subtracting from it. The length of the pipe is easily calculated with the following formula:

F =  Tuning frequency, the drivers' Fs point is commonly used 344 = is the speed of sound in meters per second measured in dry air at 20°C ƛ =  the length of the line in meters.

Whilst this is a basic calculation, the damping of the line also affects the speed of sound within the line. This could theoretically slow the speed of sound within the line to 50%, but more realistically, with a medium damped line, this effect is about 35%, effectively allowing the line to be shorter.
A transmission also has specific driver requirements because it is coupled to a mass of air that lowers its f3 and does not need a high compliance driver. It does, however, need a rigid cone to control this column of air without deforming at higher volumes; this would lead to distortion. An example of a driver which can be used with a transmission line is the SEAS Prestige L16RNX - H1488-04.

Hopefully, this blog has given you a bit more insight into the different types of enclosures. Condensing the information about these enclosures was difficult balancing depth and accessibility is a delicate thing. This may lead to every enclosure getting a dedicated blog on designing and tuning one, including MATH! However, in the meantime, I would recommend using software like WinISD to design your sealed and ported speakers.

Bibliography

Alden, R. (2017). Speaker Building 201 (1ste ed.). Audio Amateur, Incorporated.

Newell, P., & Holland, K. (2018). Loudspeakers. Taylor & Francis.

• Des

  Posted on wednesday 5 july 2023 08:41

  Interesting article, however I thought a golden rule was never to build a cabinet with the same dimensions as it may resonance at a specific frequency.

• Tech support

  Posted on tuesday 7 june 2022 09:40

  Hi David!
  
  We offer two subwoofer flatpacks under article numbers 300-7082 and 300-7078. Both cabinets dimensions are 48.8 x 48.8 x 50.8 cm (HxWxD).
  
  If you would like more information, please contact us at [email protected]

• David Crook

  Posted on monday 6 june 2022 15:52

  Do you do a flat pack speaker bass cabinet to take a 15inch speaker I need two ( don't need a big cabinet)
  Regards
  David Crook

• Tech support

  Posted on tuesday 18 january 2022 15:03

  Hi Luis,
  The parameters for the 10PF-8 allow for high Qtc (< 1.5) alignments in sealed enclosures. For two drivers 106 liters (F3 = 45 Hz & Qtc = 1,7) to 300 (F3 = 37 Hz & Qtc 1.5) would be our recommendation. We always recommend running simulations to see what respons fits your application. If need more information, please contact us at [email protected]

• Luis Leal

  Posted on saturday 15 january 2022 20:19

  Hi, Very good and simple article.
  Can someone tell me what is the best size for a closed box for 2 X GRS 10PF-8 10?
  For the same configuration, the best aligned(price and tech) tweeter with an horn?
  Best Regards
  Good projects for all! :)

• Igor Golenko

  Posted on tuesday 21 september 2021 21:07

  I like your guide!

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