Nominal impedance of the speaker is defined as its design intended impedance i.e. the designer intended it to be 8 Ohms or 6 Ohms or 4 Ohms.
i do see where you got the info that nominal impedance of a speaker is the lowest impedance after resonance....
but take a look at the following review of the B&W 804D speaker whose specs state that it is an 8 ohms nominal impedance. Look at the impedance-phase plots. the impedance curves are nowhere near 8 ohms after any of the 3 resonances!!
So, this flies in the face of the Wiki info you found!! ;(
http://www.stereophile.com/content/bowers-amp-wilkins-804-diamond-loudspeaker-measurements#4g61rRpjW4oTlfGm.97
what is resonance?
dial back a bit - a speaker driver can be modeled as a mass + spring. The cone being the mass & the spider network making the cone springy as you push (gently) on it & acts like a spring. (the voice coil suspended in the magnet also acts like a spring but just to keep it simple I mentioned just the spider network). For this driver there is an electrical frequency, which if feed into this driver, the cone will move back & forth at its maximum allowable excursion. If you measure this you will find a peak in this driver’s frequency response. This peak is called the resonance frequency. Every driver has a resonance frequency.
should you use 8 ohm post or 4 ohm post for your 6 ohm speaker? it really depends on what kind of speaker you have - one like B&W 804D that has some wild impedance/phase plots that require it to be driven by a high current amp OR if the impedance/phase plot is mostly flat thereby giving you the option of driving it with a lower output current amp.
In general, 6 ohms connected to 8 ohm tap - you will get higher output voltage & less current drive. 6 ohms connected to 4 ohm tap - you will get lower output voltage but higher current drive. The amp is more stressed now as (it has to deliver more current into a lower impedance). For most s.s. amps, the output power will increase (into 4 Ohms) but for tube amps the power could drop a little (10-20%) as tries to deliver more power (since tube amps run on the Power Paradigm & P = V*I, if you deliver more current, the output voltage drops & since the amp is stressed delivering more current, the output voltage could drop a little more than proportional/linear; hence the drop in output power by 10-20%).
What does your 6 ohm speaker need? More voltage or more current? If you have any knowledge of its impedance/phase plots you can better determine if you need the 8 ohm tap or the 4 ohm tap.
If you don’t know - try both & see which one sounds better.....
i do see where you got the info that nominal impedance of a speaker is the lowest impedance after resonance....
but take a look at the following review of the B&W 804D speaker whose specs state that it is an 8 ohms nominal impedance. Look at the impedance-phase plots. the impedance curves are nowhere near 8 ohms after any of the 3 resonances!!
So, this flies in the face of the Wiki info you found!! ;(
http://www.stereophile.com/content/bowers-amp-wilkins-804-diamond-loudspeaker-measurements#4g61rRpjW4oTlfGm.97
what is resonance?
dial back a bit - a speaker driver can be modeled as a mass + spring. The cone being the mass & the spider network making the cone springy as you push (gently) on it & acts like a spring. (the voice coil suspended in the magnet also acts like a spring but just to keep it simple I mentioned just the spider network). For this driver there is an electrical frequency, which if feed into this driver, the cone will move back & forth at its maximum allowable excursion. If you measure this you will find a peak in this driver’s frequency response. This peak is called the resonance frequency. Every driver has a resonance frequency.
should you use 8 ohm post or 4 ohm post for your 6 ohm speaker? it really depends on what kind of speaker you have - one like B&W 804D that has some wild impedance/phase plots that require it to be driven by a high current amp OR if the impedance/phase plot is mostly flat thereby giving you the option of driving it with a lower output current amp.
In general, 6 ohms connected to 8 ohm tap - you will get higher output voltage & less current drive. 6 ohms connected to 4 ohm tap - you will get lower output voltage but higher current drive. The amp is more stressed now as (it has to deliver more current into a lower impedance). For most s.s. amps, the output power will increase (into 4 Ohms) but for tube amps the power could drop a little (10-20%) as tries to deliver more power (since tube amps run on the Power Paradigm & P = V*I, if you deliver more current, the output voltage drops & since the amp is stressed delivering more current, the output voltage could drop a little more than proportional/linear; hence the drop in output power by 10-20%).
What does your 6 ohm speaker need? More voltage or more current? If you have any knowledge of its impedance/phase plots you can better determine if you need the 8 ohm tap or the 4 ohm tap.
If you don’t know - try both & see which one sounds better.....