We recently contracted with a customer to build a set of small bookshelf boathull speakers utilizing a unique type of nonlinear wood construction that results in not only an acoustically superior enclosure but also a very aesthetically pleasing design. As you can see from the pictures above and below, the results are quite stunning.
Tortuga Audio intends to further pursue this type of speaker construction focusing on a larger floor standing model that utilizes a 10 inch Audio Nirvana full range driver. While all speakers designs involve a series of compromises and none are perfect per se, I am nevertheless convinced of the superior sound that can be achieved through the simplicity of a single full range driver without the complexity or artifacts of multiple drivers and crossovers.
These are labor and material intensive speaker designs that will appeal to those who are looking for distinctive and unique speakers that combine both high end performance with pleasing aesthetics. That’s a nice way of saying they’ll be expensive but worth it. These speakers will only be built to order.
What Is A Boathull Speaker?
A “boathull” speaker is a speaker whose cabinet takes on the longitudinal shape of a boat’s hull. And while boat hulls come in many possible shapes, we are talking here about a classic rounded shape. To be more specific, we are talking about an ellipse.
What’s an ellipse you ask? In mathematics, an ellipse is a curve on a plane that surrounds two focal points such that the sum of the distances to the two focal points is constant for every point on the curve. This is illustrated in the dynamic graphic to the right. A less elegant definition of an ellipse is “an evenly squashed circle”.
When we apply the definition of an ellipse to a speaker cabinet design, we get a plan view shape like the one to the right. Here we have two parallel semi-ellipses 1 inch apart, 10 inches deep, and 8.5 inches wide.
Why A Curved Speaker Cabinet?
A better question might be, why are square or rectangular speaker cabinets so common? The short answer is … cost! A rectangular box can be made relatively easily for the least cost. However, a rectangular box with parallel walls has arguably the worst acoustical properties in terms of undesirable resonance and internal reflected energy.
Minimize Reflected Energy – Consider that the primary purpose of a speaker enclosure is to project the sound energy from the front of a driver out into the room. At the same time it’s purpose is to also keep any sound generated within the enclosure from going out into the room. Internal sound is the sound generated from the back side of the driver cone. The sound from the back of a driver is 180 degrees out of phase with the sound from the front. For bass frequencies the reflected sound from the back can cancel the sound from the front, destroying the low frequency performance. At higher frequencies where the wavelength is smaller than the driver diameter, the situation is more complex. This reflected sound may add, cancel, or do something in between. The sound from the back is also delayed in time by a fraction of a millisecond, which can interfere with the stereo imaging. Curved walls help minimize reflected energy onto the backside of the speaker drivers.
Minimize Resonant Energy – The flat walls of rectilinear cabinets vibrate more easily and at lower frequencies than curved walls. This type of structural resonance adds its own voice to the sound – not good. Curved walls mitigate structural resonance. Moreover, curved walls made of literally hundreds of horizontal layers of vertically stacked and glued laminates are stiffer than any homogenous flat or curved wall of equal thickness. Last but not least, the 1+ inch wall thickness of these boathull enclosures exceeds the typical wall thickness and stiffness of most enclosures even before any additional internal cross bracing or dampening treatment.
Aesthetically Pleasing – Speakers are arguably the most visible component in any sound system. The eye is drawn to them. Simply put, curves are far more aesthetically appealing than….a box. Add in the unique warmth of curved hand-finished laminate hardwood edge grain and you get some really pleasing audio eye candy. Hard not to like. And we all know that aesthetic appeal can be a big factor when seeking that all important buy-in by your significant other.
A Bookshelf Boathull Speaker
Our first boathull speaker is a custom ordered 0.32 cubic foot bookshelf shown below. Although a two-way design, the customer required no internal crossovers be included since the intent was to use external electronic (active) crossovers and bi-amping. Because of this we were unfortunately not able to give these a listen before shipping them out.
The drivers used in this design are made in Denmark by Scanspeak and include their Revelator 15w/8530k00 5 inch low Qts woofer and their Revelator D2905/9900 1″ textile dome tweeter.
The enclosure is composed of multiple 1 inch wide milled open-ended semi-ellipses that are vertically stacked and glued to form the curved body of the speaker. One internal brace was also included. The material is 13 ply baltic birch hardwood plywood. The front baffle is a single sheet thick. The veneer used on the top, bottom and baffle is zebrawood.
All the elliptical parts plus the front panel are milled using a CNC router. The enclosure is lined with heavy felt prior to attaching the front baffle.
The finish is hand rubbed gloss poly oil – 10+ coats.