A comparison of four full range drivers on an open baffle

Aim

To compare the frequency response of four highly regarded full range drivers in an open baffle application. Hopefully the results will help me choose a final driver to use in my OB-1 system, at least for the next few years!

Method

From left to right: Mark Audio Alpair 6 metal cone driver, Tang Band W3-1364 bamboo cone driver, Tang Band W4-1320 bamboo cone driver and CSS FR125S paper cone driver.

With my OB-1 speakers very nearly finished it is difficult to change the size of the mounting for the full range driver without making a real mess. I decided to compromise and mount each driver on a baffle the same width as the OB-1 enclosure, and the same distance from the top edge. I would then mount each baffle on top of the OB-1 speaker in turn and take measurements. Although the driver would not be in the exact location where it will be mounted in the final speaker, it would be reasonably close.

I measured the frequency response of each driver at an approximate distance of 2.3m, pretty much at the listening position for the OB-1 open baffle speakers. Sound levels and mic sensitivity were kept constant between runs in order that differences in sensitivity would be apparent.

Note that while the absolute measurements, due to lack of a reference volume level as well as the substantial influence of the enclosure may not be much use on their own, the variation between the different drivers is significant and interesting.

Results

Frequency response from the front of the cone. Notice the striking similarity between the Tang Band W4-1320 and the CSS FR125? The frequency response curves are almost identical with the Tang Band being about 5dB more sensitive. When I first purchased these drivers, the physical similarity was obvious. The CSS uses a ferrite magnet while the Tang Band uses a neodymium. Otherwise they appear pretty much the same; basket, cut-out and mounting hole locations for the two drivers are identical!

Frequency response from the rear of the cone. For this test I simply turned the boards around so the back of the driver faced forwards. I did not reverse the polarity of the drive signal so the bass in the crossover region with the bass driver cannot be relied upon. Best to look at 500Hz and above only.

Output from all drivers drops significantly above about 2kHz. The Tang Band W3 manages a little more extension.

Measurements were made from the rear of the driver simply because I was curious. There has been some discussion recently about the significance (or lack thereof) of the presence of high frequency content in the rear wave coming off an open baffle speaker. These measurements demonstrate that if you want a full range signal being projected rearwards you had better introduce a rear facing driver to do it; the full range drivers alone cannot be relied upon to carry out the task.

Conclusion

The Alpair has a limited top end compared with the other drivers, making for a very laid back sound. The CSS and Tang Band W4-1320 seem to be identical except for the difference in sensitivity. The CSS has the much larger X-max of the two. The upshot being the CSS is possibly more suitable for producing bass or large amounts of it, compared with the Tang Band. The Tang Band W3 has much better high end extension than the other drivers.

It remains to be seen which driver sounds the best, but I'll be putting both of the Tang Band units at the top of my short list.