Here is a summary comparing nominal impedance and actual impedance when determining amplifier power output:
- Nominal impedance is the rated or advertised impedance of a speaker, usually 4, 6, or 8 ohms. It is an approximate average value[1].
- Actual impedance varies across the frequency range due to the speaker’s reactive components. It is not a constant value[2][3].
- Minimum impedance is the lowest point of the actual impedance curve. This determines the maximum current draw from the amp[4].
- Amplifiers should be matched to the speaker’s minimum impedance to ensure adequate power delivery across the range[5].
- If the minimum dips below the amp’s rating, it could overload the amp or cause distortion[2][3].
- Speaker power handling is usually based on the nominal impedance, while amp power output relies on driving the minimum impedance[4][5].
- Nominal impedance is useful for general system matching. Actual minimum impedance is critical for ensuring the amp does not clip or distort[4].
- Manufacturers sometimes misstate nominal impedance higher than actual to allow using lower-powered amps[6].
In summary, the nominal value provides a general guideline, but the actual minimum impedance determines the true amplifier load and maximum current draw. This should be matched properly to prevent issues.
Citations:
[1] https://en.wikipedia.org/wiki/Nominal_impedance
[2] https://audiosciencereview.com/forum/index.php
[3] https://geoffthegreygeek.com/understanding-speaker-impedance/
[4] https://www.talkbass.com/threads/impedance-vs-nominal-impedance.165368/
[5] https://us.kef.com/blogs/news/nominal-impedance
[6] https://dynaudio.com/magazine/2023/july/impedance-ask-the-expert