My system is 2.1 channel comprising 5.25” coaxial speakers at front doors and a subwoofer in the rear trunk. I love to hear treble, so I decided to install additional tweeters on the dashboard. The additional tweeters need high-pass filters to protect them. I simply add single 3.9uF capacitors to them, yes, it’s first-order filter. I tried listening to them with and without polarity reversal. The first-order gave clearly different result, so I could make decision easily. Yet, I have 0.3mH inductors unused, I decided to add them. Now, I have converted the filters to be second-order. Nevertheless, it’s not easy to decide like the first-order anymore. I found little difference between reversing tweeter’s polarity and without. One gave boosting treble, the other gave cutting it. Since this is car audio installation, I‘m not convenient to setup RTA equipment in the car. I’d like to ask whether there’s a way to determine tweeter’s polarity “in car audio” without using measurement equipment.
There may not be a definitive answer. The tweeters being at two different distances and the crossovers changing phase different amounts, depending on the frequency, listening and choosing what you like the most may be the best option. Post #2 is a good idea. Don't expect perfection with two tweeters playing the same signals. If you can disconnect the coaxial tweeters, that may be a good option.
General observation, sorry if this is completely understood already:
If those tweeters are 4Ohm types (important to note if they are not! you can simply measure their DC resistance with a standard multimeter if you're unsure, 4Ohm types will typically show around 3.2Ohm DC resistance, 8Ohm comes to around 6.4Ohm), then those 3.9µF caps without the inductors will result in a 1st order highpass with crossover frequency of ~10kHz - that surely is a bit too high to be useful anyway...? Combined, they result in a 2nd order 5kHz filter of ~Linkwitz characteristics which makes sense to me, so I guess those probably came from a finished crossover; you can't use them as separate items and expect them to work at the intended frequencies - i.e the 0.3mH inductors taken on their own would produce a ~2kHz lowpass for 4 Ohm speakers, leaving you with a huge gap in the mid if you decided to split this 2nd order HP-only crossover into a HP/LP 1st order crossover.
There are several crossover calculators online such as [this] to see which values of components you need to achieve specific frequencies and filter characteristics. I'd always recommend staying with Butterworth and Linkwitz-Riley if you're unsure what those characteristics do.
What's the problem with setting up a simple measurement system in the car? If your head unit has a bluetooth or aux input, then it's simple to plug in/connect a laptop with a measurement mic and determine the polarity and also other items such as effect of crossover frequency etc. Just hold the mic at head level, repeat a couple times, and you should be getting reasonable responses out of that thing. You could even get away with using a mobile phone app if for your phone there exists a known, halfway reliable mic calibration curve (iPhones, Samsungs, many other popular models...).
Since you're right now at such high crossover frequencies (5kHz for example means, a difference in the distance between woofer and tweeter of just 3.4 cm from their physical position to your ear results in a 180° phase shift on it's own), one cannot easily predict the resulting phase response anyway. It might also change every time you have a different person drive the car with a different seat position. However, I'd definitely try to measure and see what's up with your phase response, perhaps there are also a number of other problems such as holes in the response, or resonances in the car which occur at specific frequencies on specific trim/body parts, that you may like to fix.
If those tweeters are 4Ohm types (important to note if they are not! you can simply measure their DC resistance with a standard multimeter if you're unsure, 4Ohm types will typically show around 3.2Ohm DC resistance, 8Ohm comes to around 6.4Ohm), then those 3.9µF caps without the inductors will result in a 1st order highpass with crossover frequency of ~10kHz - that surely is a bit too high to be useful anyway...? Combined, they result in a 2nd order 5kHz filter of ~Linkwitz characteristics which makes sense to me, so I guess those probably came from a finished crossover; you can't use them as separate items and expect them to work at the intended frequencies - i.e the 0.3mH inductors taken on their own would produce a ~2kHz lowpass for 4 Ohm speakers, leaving you with a huge gap in the mid if you decided to split this 2nd order HP-only crossover into a HP/LP 1st order crossover.
There are several crossover calculators online such as [this] to see which values of components you need to achieve specific frequencies and filter characteristics. I'd always recommend staying with Butterworth and Linkwitz-Riley if you're unsure what those characteristics do.
What's the problem with setting up a simple measurement system in the car? If your head unit has a bluetooth or aux input, then it's simple to plug in/connect a laptop with a measurement mic and determine the polarity and also other items such as effect of crossover frequency etc. Just hold the mic at head level, repeat a couple times, and you should be getting reasonable responses out of that thing. You could even get away with using a mobile phone app if for your phone there exists a known, halfway reliable mic calibration curve (iPhones, Samsungs, many other popular models...).
Since you're right now at such high crossover frequencies (5kHz for example means, a difference in the distance between woofer and tweeter of just 3.4 cm from their physical position to your ear results in a 180° phase shift on it's own), one cannot easily predict the resulting phase response anyway. It might also change every time you have a different person drive the car with a different seat position. However, I'd definitely try to measure and see what's up with your phase response, perhaps there are also a number of other problems such as holes in the response, or resonances in the car which occur at specific frequencies on specific trim/body parts, that you may like to fix.
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