Thursday, May 22, 2014

Speaker Enclosures - Part 1

One of the challenges speaker designers routinely face is overcoming the laws of physics. How they choose to compromise their designs to address physical laws is an individual choice, some resorting to baffling, others to exotic shapes, and still others to exotic materials. All are good choices and none are free; there is always a downside to any choice in any design.

The biggest motivation a designer chooses something over something else is usually cost where if the best way to implement a design is this way but a good way is another less-expensive way, the latter will likely be chosen to permit higher profits and lower consumer costs. Is this bad? No, not at all. It's just a choice and you choose exactly the same exercise in everything you do when considering something to purchase.

For example, my wife loves purses and shoes and all of her life she bought those inexpensive accessories from mass-marketing stores - and she was happy. For her birthday, I sprung for an esoteric purse and that changed everything. When you step up in quality, you get more than a better-made product, you see how others choose compromises that such a purchase does not. Mercedes Benz automobiles, Brietling watches, and Gucci shoes are all examples of how everyday purchases can choose fewer or no compromises when implementing a design and owning such a product exposes the compromises made by inferior designs.

The single-most frustrating law of physics that plagues speaker cabinet makers is the internal resonances (aka dimensional resonances). Much like a tiny room, speaker cabinets are prone to vibrate at certain frequencies based mainly on parallel walls. The best thing one can do is to spread these dimensional resonances out so that they do not all occur at the same frequency, at the same frequency as a musical note, or with multiple resonances at the same frequency. You can measure the internal dimensions of your solid-rectangular speaker (subtract the thickess from the external dimensions) and calculate their existing resonances with the WINISD speaker design software available here. Such software automatically creates recommendations for cabet dimensions based on the volume required.

Such software (and designers) often use recommended ratios for height, width, and depth to build cabinets such as 1:1.14:1.39, 1:1.26:1.59, 1:1.28:1.54, 1:1.44:2, 1:1.60:2.33, 1:1.62:2.63, and 2:3:5. But such ratios are not really all that useful in predicting how many resonant modes will occur near each other (sum together creating a louder resonance), and what musical notes these resonances will exaggerate (over emphasize a note on the musical scale despite the anechoic response of the driver).

For example, plugging internal box dimensions of 20cm, 30cm, and 50cm into a simple resonance mode calculator yields harmonic resonances at 343Hz (near F4), 571Hz (near D5), 667Hz (near E5), and 685Hz (near F5). What this means is that such a box will artificially create louder notes at those points (especially D5, E5, and F5) independant of the driver's anechoic response.Changing the box dimensions to 19x30x48 better distributes the resonances in between the fundamental frequencies of notes on the musical scale. This means that all notes on a cello run will appear more uniform with the 19x30x48 cabinet as opposed to the D5, E5, F5 resonances with the 20x30x50 cabinet.

Other resonances can occur within a cabinet creating other issues above the operating range of the driver. For example, when harmonics of a resonance occur within a cabinet, the harmonic content of that fundamental note will also be exaggerated. So if a cabinet resonates at multiples of the fundamental frequency of D5 (667Hz), multiples of this frequency (2x, 3x, 4x, etc.) are its harmonics and resonances at these multiples change the way the character of that note is percieved (colors the sound and makes that saxaphone or piano note sound less real).

Aside from juggling the cabinet dimensions, what else can be done to minimize these internal resonances?
Logically, the simplest thing to do is to avoid using parallel surfaces (sides). Next to a solid-square box (one major resonance and the most resonant shape you can choose), the poorly-designed solid-rectangle vies for the second-worst shape since there are three parallel sides creating three characteristic resonances between them. But yet the vast majority of speakers built today use solid-rectangular enclosures. Why? In a word: cost. Straight cuts, square corners, and uncomplicated angles are mass-production friendly with low material waste. In other words, it is incredibly easy and highly efficient to build a solid-rectangular enclosure as opposed to a more esoteric shape.

Question: What can you do to improve your solid-rectangular speakers?

Answer: Build a new, properly-designed enclosure with the same volume and put the existing parts into it. For those of you with moderate wood-working skills, you can build another solid rectangle with minimal grouped dimensional resonances using the above calculator or hire someone to build you another cabinet. Tweak the dimensions to retain the required volume while adjusting the characteristic resonances to occur at fundamental frequencies that are not the same as those of musical notes.

For those of you with advanced woodworking skills, you can take this opportunity to build an alternative shape with fewer parallel surfaces. This approach will be covered in Part 2 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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