Saturday, June 7, 2014

Speaker Enclosures - Part 3

In Part 1, we saw how all rectangular solids resonate at frequencies determined by their physical dimensions (dimensional resonances). And we saw how online calculators help determine where these resonances occur in your speakers and  if these dimensional resonances occur (group together) around notes on the musical scale. We also saw how to correct for these dimensional resonances by building new rectangular boxes of different dimensions but with the same internal volume and then stuffing it with the existing hardware.

In Part 2, we found that solid rectangular cabinets resonate in three planes: against two parallel walls (axial), against four walls (tangential), and against all six walls (oblique). We explored an alternative physical shape (the wedge) and learned why such a shape was even better (had fewer clustered resonances) than that of even a well-designed solid rectangle (fewer parallel walls meaning fewer axial mode resonances).

Here in Part 3 we will take this concept of removing parallel walls another step by exploring yet another cabinet shape to eliminate axial mode resonances. If you have anticipated this, you get a gold star and are beginning to understand audio, specifically acoustic resonances. Making a cabinet with zero parallel walls eliminates all axial resonant modes. But does this assure you that the remaining tangential and oblique resonant modes are random enough not to cluster? Maybe...we'll see.

First, a completely non-parallel surfaced speaker looks odd and few people are attracted to the radical departure from the solid rectangular reference although most artists and right-brained people readily embrace this departure (myself included). Such cabinets can be very expensive to build and beyond the means of the average aspiring garage-housed woodworking shop.

A Speaker Cabinet with Non-parallel Walls

But variations of the simple wedge shape can create an enclosure that has no parallel walls and is much easier to build. Sloping either the left, right, or both otherwise parallel sides of the basic wedge shape results in a cabinet resembling a truncated pyramid and a shape that is still within the abilities of the average DIYer to create in even the most humble garage.

Another Speaker Cabinet with Non-parallel Walls

Although you could also slope the remaining 90-degree faces on the basic wedge, this complicates construction and may not be required. Such alternative designs completely eliminate axial mode resonances and this is why they are used. However, this does not eliminate all major resonances.

No matter what shape a cabinet is, another major resonance creeps in: that of the fundamental resonance of the entire enclosure. If you tap anything - a wine glass, a fender on a car, a rubber ball - everything resonates at a frequency inherent to that shape. Even the earth has a natural resonant frequency and Nikola Tesla leveraged this resonance to transmit electricity from Colorado to Australia without wires. Such is the power of understanding resonances!

If this natural resonance occurs within the operating band of the driver, it will be excited by the driver at some time causing an emphasis at that resonant frequency. Several products are available to help reduce these resonances typically applied to the large surfaces of automobiles but are also useful in cabinet design.

A Typical Self-adhesive Sound Damping Material

So after careful construction of your truncated pyramid enclosure you abruptly rap its side and it rings like a bell, there is hope to recovering from an otherwise surmised disaster. Covering at least 1/3 of the internal surfaces with self-adhesive sound absorbing material will help lower the frequency of or totally eliminate cabinet natural resonances and salvage your back-breaking brow-sweating project. Making the shape of the damping material random rather than uniform may also help in distributing these peak of a resonance. Here trial and error will tell you if you have succeeded and your cabinet is as "dead" as possible.

Understanding the extent of a cabinet's natural resonance is as simple as rapping it with your knuckles while holding up your smart-phone's RTA application. If a natural resonance occurs within the operating range of the driver, this resonance will be excited at some time during a listening session (when the "note" matches the resonant frequency of the cabinet). Shifting the natural cabinet resonance off of the same frequency of a note on the musical scale will help minimize the audible effects of such resonances.


Speaker cabinets do more than create a known volume in which a driver will optimally operate; it also introduces resonances to the speaker that can easily influence its sound. Changing the shape of a cabinet can help control the internal resonances especially by eliminating parallel walls in its design. Adding internal sound damping can reduce effects of remaining natural resonances by moving them off of a musical note or totally eliminating them entirely.

There is one more issue regarding cabinet design we will explore in Part 4 of this series.

Related ArticlesSee all entries about speaker enclosures in Part 1Part 2Part 3, and Part 4. Also, a related article on the effects of crossover network components on driver performance.

Yours for higher fidelity,
Philip Rastocny

I do not use ads in this blog to help support my efforts. If you like what you are reading, please remember to reciprocate, My newest title is called Where, oh Where did the Star of Bethlehem Go? It’s an astronomer’s look at what this celestial object may have been, who the "Wise Men" were, and where they came from. Written in an investigative journalism style, it targets one star that has never been considered before and builds a solid case for its candidacy.

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