Basic Speaker Dynamics Scenario/Question--Loudness

[mconn02]

First, thank you for everyone helping me as I learn some of these things.

I'm getting some practical understanding of power and impedance, and I'm wanting to understand the features that contribute to volume. (I will probably be looking to build an iso cab at some point this year).

So, an example: let's say I get a Marshall DSL15H (15 watt valve) amp head and add two MX112 1x12 cabinets. Both of these come with Celestion Seventy 80s, rated at 16 ohm, 80W rms, and 98dB sensitivity.

I'm trying to understand this, and currently this suggests to me that, if matched with a 16 ohm source, when running at 80W the speaker will create 98dB of sound. (Question 1) If this is read correctly, it seems to suggest that my 15 watt head will never come close to creating 98db of sound.

So, I'm not so concerned about this not being loud enough, rather the converse: Having heard them several times (and not caring much for their tone), I would probably replace the 70/80a with a nicer G12, such as Heritage G12Ms or a similar mix, each rated at 15 ohms, 20 watt RMS, and 96 dB sensitivity.

(Question 2) So, since the HG12Ms are rated to produce slightly lower volume (-2dB) at 1/3 the rms wattage, should I expect the speakers to be significantly louder at the same wattage when compared to the 70/80s?

(Question 3) Finally, given that the output of the head is 15W RMS, I think the quick rule is to allow double (30w) for peak handling. If so, a single cabinet with a 20w G12 might be risky at high volumes, where an 80W G12 would not. So the pair of G12s together for 40W seems safe, but getting back to total volume, this creates twice the cone surface pushing air as the higher rated 80W single speaker. Is this a separate factor, or is this included in the calculations for dBs for a given power level? (Perhaps it's just that, with the 15w rms is now split to 7.5w between two parallel cones, the two cones aren't pushing as hard as they would individually, hence less air moved=less sound.)

I appreciate any of the wisdom the community has to share with me. Many thanks!

Oh, and if on its surface your opinion is that this combination is problematic (especially would not drive the speakers well enough to get their true tone, etc.) I'd love you hear your reason why.

Thanks again!

[Scumback Speakers]

Just saw your post. First, speakers are usually measured with 1w of power sent to them, and the speaker decibel rating is then calculated with a measuring source such as a decibel meter with a mic attached, at a distance of 3 feet from the speaker.

From the Klipsch speaker site. http://www.klipsch.com/education/speaker-sensitivity

Speaker sensitivity—many times erroneously referred to as speaker efficiency—is used to determine the amount of power necessary to drive or operate a loudspeaker. It is a measurement of the amount of sound output derived from a speaker with one watt of power input from an amplifier. Sensitivity is usually measured with a microphone connected to a sound level meter placed one meter in front of the speaker. The resultant number is expressed in dB.

Second, there's a big difference in amp output watts. Some companies under rate their amps, some over rate them. I've heard wildly different output volumes from amps made by different companies, both claiming to be 15w. It's a crapshoot, seriously.

Third, all things being the same in your speaker scenario, the Heritage G12M's at 96 db are probably 10% or more less loud than the 98db 70/80 speaker.

Hope that helps!

Jim