Combining multiple Speakers Impedance - what is the preffered way?

Question

I'm building a custom sound system using the HiFi Berry Amp2 which claims to [...] "work with all speakers or 4-8 Ohm impedance. It supports sample rates from 44.1-192kHz (16-32bit)." According to this graphic I conclude to have two channels (i.e. right and left) which each works well with an impedance of 4-8 Ohms.

So far I have four speakers, all with 4 Ohms impedance. Now I calculated for resistors in series a total resistance for each channel of my amplifier:

R_total = R_1 + R_2 = 4 Ohms + 4 Ohms = 8 Ohms

I connected the speakers and everything works well.

Now some people told me not to connect the speakers in series and a parallel connection would be preferably. Due to a more flexible design (being able to plug in and out additional speakers) I also would prefer a parallel connection using for example four speakers with 8 ohm. Since for resistors in parallel connection I calculate for each channel with two speakers:

1/R_total = 1/R_1 + 1/R_2 = 1/(8 Ohms) + 1/(8 Ohms) = 2/(8 Ohms)
=> R_total = 4 Ohms

Which also fits into the specification of the amplifier.

Now my questions:

1. Is the a preferred way of connecting multiple speakers to the amp? If so, how? Or: Is there any problem with connecting speakers in series like described in my current setup?
2. Would it be possible to have a mixture of 4 Ohms and 8 Ohms speakers, by maybe adding additional resistors into the circuit?** If yes, how?

** = Since I have one 4 Ohms speaker which kinda is in-build and cannot be swapped, but I also would like to give the parallel connection setup a try.

Answer 1

Don't work with resistors, and only put speakers of identical type in series. Impedance ratings are minimum ratings of AC impedance over the whole frequency range. Both magnitude and phase tend to vary widely over that range. This works because an amplifier acts close to an ideal voltage source within its specifications. If you put different speakers in series, they will distribute power unevenly over different frequencies and if you are unlucky, will create a resonant circuit at some frequencies that can blow up either.

That kind of blunder is somewhat frequent with amateur speaker builders who think they are doing piezo speakers a favor by "blocking" low frequencies with an inductivity (partly as part of a ready-made passive crossover not intended for piezos) that then forms a resonant circuit with the capacity of the piezo itself, doing exactly the opposite to "blocking" low frequencies.

For speakers of identical build, it doesn't matter whether you put them in series or in parallel, but in general you are better off connecting them in parallel as long as you meet the specs of the amp.