Stuck In The Middle

by Mithat Konar

Since the 1970s, the most common place for a loudspeaker system's high-frequency driver has been horizontally smack-dab in the middle of the cabinet and in-line with the low-frequency driver. This arrangement has the benefit of producing a completely symmetric cabinet, meaning that the manufacturer's production is slightly simplified and, perhaps more importantly, the need for the manufacturer--and retailer--to inventory systems as mirror-imaged stereo pairs is eliminated.

Unfortunately, what works to the advantage of the manufacturers and retailers in this case works to the disadvantage of the end user. To see what I mean, take a look at Figure 1. The figure shows computer predictions of the frequency response of an idealized high-frequency driver mounted on a 26cm (10-1/4") wide and 37cm (14-1/2") tall baffle equidistant from both vertical edges and 9cm (3-9/16") from the top. By "idealized high-frequency driver" I mean that the tweeter in the example has the low-end rolloff typical of a 19mm driver but that otherwise it has a flat frequency response, and that it has the radiation pattern of a perfect piston. The curves were produced by our proprietary diffraction modeling software--a state of the art modeling tool developed by engineers at Biro Technology specifically to study the effects of diffraction.

Figure 1. Frequency response of high-frequency driver in typical location.

In particular, notice the 6dB span between the dip around 2.2kHz and the peak just below 4kHz as well as the numerous smaller ripples above 4kHz. These response irregularities are the result of cabinet edge diffraction--a phenomenon which affects all drivers mounted on any sort of surface. As a sound wave propagates outward from the driver's diaphragm along the mounting surface (in this case the cabinet's front face), it eventually encounters the edge of the mounting surface whereupon it experiences an abrupt change in acoustic load. This change alters the way the wave propagates thereafter, thereby producing diffraction. As you can see, the resulting response aberrations can be significant--especially in the low-end of a high-frequency driver's range.

In an attempt to reduce these effects, some manufacturers employ rounded or chamfered edges along the cabinet's front face and sides. Unfortunately, these features are typically limited to a size of approximately 20mm (3/4") because of manufacturing and aesthetic constraints. Inasmuch as 20mm (3/4") corresponds to the wavelength of a 17kHz sound wave, it is not surprising that these techniques are completely ineffective at dealing with the most offensive diffraction-related problems, namely the ones which occur at significantly lower frequencies, that is, at longer wavelengths. To be truly effective in the problematic 4kHz range, a rounded edge would need a radius approaching 75mm (3"). While it is by no means impossible to construct a cabinet with such a feature (some in fact exist) it is not possible to do so in a cost-effective way.

The Biro L/1 addresses the issue of edge diffraction by largely side-stepping it. Mounting the high-frequency driver outside the cabinet in a compact housing essentially eliminates all cabinet-induced diffraction. The diffraction effects that remain (as a result of the small but still present driver housing) occur at frequencies so high that the driver is radiating almost no energy toward the boundary--and no energy at the boundary results in no diffraction. Problem solved.

For the L/2*, a different approach was required. To be manufacturable at a lower cost than the L/1, the L/2 needed to use a more conventional surface-mounted tweeter design. Cost constraints also precluded the use of large and expensive quarter-round features on the baffle. The solution arrived at involved using Biro's proprietary diffraction modeling software as the core of an optimization procedure. The primary reason that the diffraction effects experienced by a driver mounted on the center-line of a cabinet are so severe is that the effects from the left edge of the cabinet are identical to the effects of the right. In other words, the symmetry of the situation effectively doubles the intensity of the diffraction. However, if the driver is moved slightly off-center, the symmetry is broken up and the particular effects of the diffraction become distributed. Thus by incrementally moving the high-frequency driver off-center--and by adjusting its vertical placement to further distribute the diffraction effects--Biro engineers found the precise driver location which resulted in the "most distributed" diffraction and hence the smoothest frequency response. Figure 2 shows the result of this procedure using the baffle and driver from the example in Figure 1. As you can see, the 6dB anomaly around 3kHz has been replaced by a series of far less objectionable ±0.5dB ripples, and the response at higher frequencies is also slightly improved.

Figure 2: Frequency response of high frequency driver in optimized location.

Many listeners feel the high-frequency smoothness and resolution of the L/2 sets new standards for a speaker in its price class. Some of this is directly attributable to the "distributed diffraction" approach used in its design. Of course, the resulting asymmetry does place an added burden on our inventory management, but who cares? As a company dedicated to bringing you the best loudspeakers available anywhere, the last thing we want to be is stuck in the middle.

*Now discontinued [return]

Site copyright ©2005 Biro Technology–all rights reserved