Effect Ohms Have on db and Speaker Sensitivity

First, be aware no spec gets fudged more than sensitivity! :) Most speakers are rated 3-5 dB higher than they actually are.

dB at 1W / 1m is "efficiency" and varies by load impedance, which as stated above, is 2.83V at 8 Ohms. Think of it as "power efficiency." With modern SS amps this is a meaningless measure really. You want sensitivity.

dB at 2.83V / 1m regardless of load (and therefore regardlesss of power) is "sensitivity." Again, think of this as "voltage sensitivity" to try to remember why they are not the same.  This measurement makes more sense, since most speakers impedance varies WIDELY at different Hz. Measuring between say 6 to 20 Ohms in the same speaker is quite typical. I don't really care about how much power is being dissipated at 2,450 Hz as much as how loud it will get with a 40 Watt amplifier.

Also, the exact Hz at which the dB are measured varies. I use 1 kHz, but some manufacturers may try to eyeball it.

As Kosst alluded to the question is somewhat unclear. But **if** you are asking what the difference in SPL would be on a per watt basis between an 8 ohm speaker rated at 97 db/2.83 volts/1 meter and a 6 ohm speaker rated at 97 db/2.83 volts/1 meter, and assuming those specs are accurate (which is often a bad assumption, as Erik indicated), the calculation would be as follows:

Power into a resistive load = Voltage squared/Resistance

Therefore:

2.83 volts into 8 ohms = (2.83 x 2.83)/8 = 1 watt

2.83 volts into 6 ohms = (2.83 x 2.83)/6 = 1.335 watts

The ratio of two amounts of power, expressed in db, is:

db = 10 x log(P1/P2), where "log" is the base 10 logarithm.

10 x log(1.335/1) = 1.25 db.

So if both speakers are provided with the same number of watts the 6 ohm speaker would produce an SPL that is 1.25 db lower than the SPL produced by the 8 ohm speaker, if both have sensitivities of 97 db/2.83 volts/1 meter.

Regards,
-- Al

I'm not sure if this is why you could be confused, but some driver manufacturers do offer 2 versions of the same woofer: an 8 and 4 Ohm.

The 4 Ohm allows twice the current to flow in the coil, which means 2x the power for the same V will be dissipated. The advantage in this particular case, everything else being equal, is the 4 Ohm version gains 3 dB of sensitivity.

It is only in this type of situation, where everything except the voice coil impedance is the same, that we can generalize and say that 4 Ohm versions are more sensitive. We cannot take this and compare drivers in general even within the same manufacturer. The magnetic strength, gap, weight, cone diameter, suspension, all come into play.

Thank you for all the feedback.

Thanks to Almarg for the math lesson.

Maybe I should have started by stating my issue:

I have speakers with 97db efficiency into nominal 6 Ohms. I'm running a pair of 300B mono amps......about 8 watts. 

I thought that 97db speakers at 6 Ohms would be efficient enough for the 300B's to really "sing,"  however, they lack the bass depth and slam that we all enjoy.

I may need a bit more power, or a speaker that is 97db into 8+ Ohms.