Comments on Simulation and Design Examples

A summary of the example data is shown below in Table 6.

Example Alignment fB, Hz VB, ft3 f3, Hz Ref. SPL, dB SPL
1 C4 with no HPF 17.7 11.7 17.4 100.2
2 C4 with B2 HPF 17.7 11.7 20.3 112.7
3 C4 with B3 HPF 17.7 11.7 18.6 112.0
4 C4 with B4 HPF 17.7 11.7 18.1 111.9
5 C4 with Inverse Chebyshev HPF 17.7 11.7 18.0 111.8
6 SC5 19.4 9.7 21.1 108.6
7 C6 Class 1 15.0 11.2 10.2 91.6
8 SC6 Class 2 19.8 12.5 20.6 113.4
9 SC6 Class 3 20.1 8.9 23.1 112.1
10 C7 Class 1 17.0 9.8 14.1 104.7
11 SC7 Class 2 21.6 10.2 23.7 115.5
12 SC7 Class 3 20.5 8.4 24.2 114.2
13 C8 Class {1, 2} 14.5 10.7 7.5 77.9
14 C8 Class {1, 3} 13.7 14.5 10.8 98.0
15 C8 Class {1, 4} 17.7 9.3 16.6 111.6
16 SC8 Class {2, 3} 22.7 11.2 24.6 116.2
17 SC8 Class {2, 4} 22.6 9.1 25.7 116.5
18 SC8 Class {3, 4} 20.7 8.2 24.9 115.5

Table 6. Performance Comparison of Example Alignments

The Thiele-Small parameters of the Dayton RSS390HF-4 driver used in these simulations are repeated below.

RE = 3.3 Ohms
fS = 18Hz
VAS = 9.95ft3
QMS = 3.10
QES = 0.49
QTS = 0.423
xmax = 14mm

In addition to testing both the design algorithm and the software that implements it, these examples also served as an exploration of potential designs to build. Given the large VAS value of the driver, it's clear the enclosure must be large to fit into one of the classic alignments. On the positive side, only a moderate amount of amplifier power is required to reach the system displacement limit. Since it's a given that the box will be large anyway, a design goal was to obtain the lowest possible -3dB frequency consistent with high SPL output. Thus a Chebyshev alignment is desired, since the Chebyshev alignments all have a -3dB frequency less than fS. I decided to aim for a system -3dB frequency less than fS even with the high-pass filter present. The standard C4 alignments with high-pass filters added, analyzed in examples 2 through 5, all yield a system -3dB frequency greater than fS. The fifth-order alignment of example 6 is of the sub-Chebyshev type, and its -3dB frequency does not meet the requirement. The only sixth-order Chebyshev alignment is the class 1 alignment of example 7, which has a filter with excessive boost below the box tuning frequency, making it impractical. Example 10 is a Chebyshev class 1 seventh-order alignment, but it too suffers from somewhat excessive low-frequency filter boost. Although it is somewhat better than the sixth-order class 1 alignment of example 7, its reference SPL is compromised by some 7dB compared to the C4 with add-on filter. Fortunately, the eighth-order Chebyshev class {1, 4} alignment of example 15 has a number of very desirable features. Its -3dB frequency is less than fS by a solid margin, its volume is less than the C4 alignment, and its reference SPL is not compromised by the slight low-frequency boost from the filtering, as this boost occurs near the box tuning frequency where the displacement function reaches a minimum.

Finding an assisted alignment with the desired properties can be like finding a needle in a haystack because of the discrete nature of the allowable parameters of the external filter. But in the case of the Dayton RSS390HF-4 driver, the search gave promising results in example 15.