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I recently added a pair of tweeters to my car's front pair and what "baffles" me is that after adding them, I hear more bass besides the added highs, so my question is whether adding these simple items in parallel can somehow affect the low frequencies too?

I guess the capacitor and tweeter offer another current path for high frequency signal, but low freq. should remain unchanged, or is it?.

The head unit and front speakers are OEM. The front speakers are marked 25W 4 Ohm, and are 5.5 inches diameter, dual cone (The type that has a single coil and a small cone attached to the center of the main one, where the dust cap usually is. This is not a 2-way coaxial speaker). And they remain installed in its default location, in the door panel.

The tweeters are "Audiopipe APHE-T300". I saw they have a 3.3 uF capacitor inside the case, but I could not see any other component in there, particularly, I did not notice a coil that some cheap piezoelectric tweeters come with.

The connection was done by "jamming" the tweeter's included cable female spade connectors around the factory speaker connector terminals, the cable coming from the radio was not affected in any way.

Could it be that adding the tweeter changes the impedance (or some other parameter) that the head unit sees and it changes equalization in response? This head unit has that feature that lowers the bass progressively as you turn up the volume. I understand this is a protection feature.

If this is the case, can this simple tweeter in parallel damage the radio or the speakers in the mid-long term?

I have fiddled several times with speaker drivers before, but never experienced a perceived bass response by adding tweeters. Sadly I do not have any measurement equipment, and the installation requires removing the door panel and some more screws, so a quick A-B comparison is not possible.

To illustrate what I hear: I usually had this radio tuned with the treble adjustment to the max (And it was never enough, so I added the tweeters) and the bass adjusted about 3 notches bellow max, somewhat 5 notches over it's neutral point. After adding the tweeters I use treble one or two notches up and bass about three notches up and still think I hearing louder bass than before.

If this is electrically nonsense, I'll accept this is some psychoacustic effect, but I'd like to know if it has a name in order to read further. I doubt it is simple "placebo" as I was expecting no effect on bass.

I have some experience empirically building bookshelf speakers (with mixed good and bad results, so I thrust my ears more than not). Also, I've had this car for more than 11 years. The H.U and front speakers are the original O.E.M ones, and the only prior change I made to the system was connecting aftermarket speakers to the previously unused rear channels, and it was done more than two years ago, that is, I'm pretty confident "I know how it sounds".

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so my question is whether adding these simple items in parallel can somehow affect the low frequencies too?

Unlikely.

I guess the capacitor and tweeter offer another current path for high frequency signal, but low freq. should remain unchanged, or is it?.

This is simply called "crossover". The simplest construction is to connect a tweeter with its series capacitor to woofer in parallel:

schematic

simulate this circuit – Schematic created using CircuitLab

NOTE: Ideally, the woofer should have a series inductor for HF filtering but in some applications it's not put. So I didn't illustrate it.

The series capacitor to the tweeter works as a blocker for LF signals, and the tweeter and the capacitor form a simple high-pass filter. Hence the name crossover. If the impedance of the tweeter is 8 Ohms and the series capacitor is 3.3 μF then the crossover frequency will be around 6 kHz. This means that almost anything above 6 kHz will go to the tweeter.

Could it be that adding the tweeter changes the impedance (or some other parameter) that the head unit sees and it changes equalization in response?

Unlikely. For low frequencies (i.e. below 500 Hz) the tweeter arrangement will have a relatively high impedance and therefore will not affect the net impedance of the load network (e.g. 4 Ohms in parallel with 500 Ohms is still around 4 Ohms).

To illustrate what I hear: I usually had this radio tuned with the treble adjustment to the max (And it was never enough, so I added the tweeters) and the bass adjusted about 3 notches bellow max, somewhat 5 notches over it's neutral point. After adding the tweeters I use treble one or two notches up and bass about three notches up and still think I hearing louder bass than before.

Normally, in sound engineering, above 3 kHz is considered as hi-mid and, in some cases, treble. So a crossover frequency of 6 kHz might be somewhat insufficient for your application (Note that if the speaker impedance is lower then the crossover will go further. e.g. 12 kHz for 4 Ohms. 8 Ohms of impedance is just an assumption here). For almost all genres, almost all music recordings have higher low frequency (below 500 Hz) energy compared to mids (500 Hz - 2-3 kHz) and trebs (2-3 kHz and above). So, with a crossover frequency of 6 kHz, you might be missing a very large amount of the HF content.

In conclusion,

  • a crossover frequency of 6 kHz for a tweeter is a bit high. Adding capacitors in parallel will reduce the crossover frequency but we don't have any info about the tweeter speaker's response on its own. The speaker itself might not have a good response at lower frequencies (e.g. 3 kHz), or even worse, the tweeter speaker (or the entire module) might be "dodgy".
  • The series inductor in woofer module might be missing. Remember that the series inductor should be placed to block high frequencies so that the woofer does not try to produce HF content (those speaker are generally terrible at those frequencies but who knows).
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Either Audiopipe APHE-T250 or APHE-T350 (how about getting those part numbers right?) are specified as having a built-in high-pass filter (a blocking capacitor that will not preclude more discriminate filtering), 93dB response, 3kHz lowest frequency and 4ohms of impedance.

They won't affect your bass response.

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