There's been some discussion about a crossover point of 6,500 Hz for a 2-way speaker made with a 5" woven glass fiber mid woofer and a 1" dome tweeter set in a modest wave guide. Why did the maker choose such a high crossover frequency? Is it a good
idea or not? There has been some good discussion about this, and also some misinformation. There were several suggestions from various people, listed below. I followed them by my own opinions.
- Running 5" mid woofer up to 6.5 kHz would make for beaming. My opinion is yes, without any doubt, such a speaker would beam and generate, at best, a small stereo image.
- To allow using a cheap tweeter. There are plenty of cheap tweeters that can go as low as 2.5-3 kHz.
- Mistaken brochure. Always possible, especially if it was originally written in a foreign language.
- Good choice? wide band driver? While not impossible, I think this is unlikely. It requires a lot more info about that 5" mid woofer before I'd agree its a good choice.
- Crossover point is irrelevant ?!?! Whaaat ?
So, how do you choose a crossover frequency? Let's limit this to a 2-way speaker, but the same thinking applies for any multi-driver speaker.
First, and most important, find out what the unfiltered (no crossover) frequency response curve of the mid woofer looks like. Make sure this is done while the driver is mounted in the intended cabinet. All drivers have a so-called breakup frequency, called that because the cone material stops moving as a single piston, and its surface flexes instead.
View attachment 17388
This can sound really bad. Here's an example of a frequency response curve showing prominent break-up.
It peaks at just short of 5 kHz, but begins slightly above 3 kHz. The important thing to learn is how high can the mid woofer go? I would say, no higher than 3 kHz.
That graph only shows the on-axis response. If similar FR curves are made 30°, 45° and 60° off axis, what happens? The off-axis curves will show at what frequency the FR begins to fall. The more off-axis, the lower the FR begins to fall off.
For good imaging, with a wide stereo image, the 30° off axis sound should be within 3 dB of the on-axis sound. So, I'd guess that a good crossover point for that woofer might be about 2 kHz, perhaps as low as 1.5 kHz.
Once you know what crossover frequency is required by the woofer, then you can choose a tweeter that can handle frequencies that low.
In the case of that Fluance 2-way speaker, I really worry about a 5" glass fiber mid woofer playing up to 6.5 kHz. Glass fiber is known for having large break-up peaks. Unless this is a specially designed wide-range driver, modified so the break up peak has been moved well above 6.5 kHz, I think it would sound bad.