How to shop for speakers?

HiFiTime & HiFiHvn, most tube amps (that have output transformers) are designed to provide approximately the same maximum amount of power when a 4 ohm speaker is connected to the 4 ohm tap, as when an 8 ohm speaker is connected to the 8 ohm tap. There are several things to watch out for, though:

1)If an 8 ohm speaker is connected to the 4 ohm tap, which is sometimes done to achieve tighter bass or other effects on sound quality, then there would be a 3db volume drop (compared to the 8 ohm speaker being connected to the 8 ohm tap).

2)If the speaker sensitivity is rated with respect to 2.83 volts, rather than with respect to 1 watt (2.83 volts corresponds to 1 watt into 8 ohms), and if the speaker's nominal impedance is 4 ohms, then a 90db/2.83V/1m sensitivity would be only 87db/1W/1m.

3)Fluctuations in the speaker's impedance vs. frequency curve will assume different significance for a 4 ohm speaker connected to the 4 ohm tap than for an 8 ohm speaker connected to the 8 ohm tap, if the fluctuations are not in the same proportion. For example, if the impedance of a nominally 4 ohm speaker varies from 2 ohms to 8 ohms over the frequency range, a greater impact on frequency response would result (with a tube amp) than in the case of an 8 ohm speaker whose actual impedance had the same tolerance (-2/+4 ohms, meaning impedance varying between 6 ohms and 12 ohms).

Along the lines of Ralph's comment the matchup of the speaker's variations of impedance as a function of frequency, and the amplifier's output impedance, should be consistent with the designer's intention.

Also, I second Tim's comments about the importance of the phase angle of the impedance, and its variations with frequency. A low impedance magnitude coupled with a highly capacitive phase angle is a worst case load.

Regards,
-- Al

If an 8 ohm speaker is connected to the 4 ohm tap, which is sometimes done to achieve tighter bass or other effects on sound quality, then there would be a 3db volume drop (compared to the 8 ohm speaker being connected to the 8 ohm tap).

Slight correction to this statement in my earlier post. The statement should say:

If an 8 ohm speaker is connected to the 4 ohm tap, which is sometimes done to achieve tighter bass or other effects on sound quality, then there would be a volume drop of up to 3db, the exact value depending on the amplifier's output impedance (compared to the 8 ohm speaker being connected to the 8 ohm tap).

Best regards,
-- Al

Strictly speaking, neither interpretation of "efficiency" is correct. "Efficiency" would most properly be defined as acoustic power out vs. electrical power in.

However since both "efficiency" and "sensitivity" are being used in reference to acoustic PRESSURE, rather than power ("spl" = "sound pressure level"), it would seem that using "efficiency" to refer to the ratio of spl to electrical power input, rather than voltage input, would come closest to the correct usage.

And I believe that is the most widely accepted usage. Although the terms tend to be used so loosely and interchangeably that insisting on one definition or the other is probably pointless as a practical matter.

Best regards,
-- Al