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In Reply to: Stacked Advents - Efficiency? Suitability w/SET posted by FlaCharlie on January 17, 2004 at 08:36:54:
Charlie,Stacked Advents average around 90-91 dB sensitivity relative to 2.83 Volts (1 Watt into 8 ohms). The 6 dB number for paralleled speakers assumes mutual coupling which only occurs at some frequencies in the range of stacked Advents. Over most of the range it's 3 dB. Yes, the net load is nominally 4 ohms, but drops as low as 2.5 ohms in the 90-200 Hz and 1-3 Khz ranges.
Mutual coupling is a reinforcement in the sound level of an addition "free" 3 dB if the drivers are much less than a wavelength apart. In the case of stacked Advents, the woofers are 32" apart. So you would expect to see full effect mutual coupling below 100 Hz, and gradually reduced effect above that. The tweeters are 11" apart and some mutual coupling begins to occur in the lower end of the tweeter range. This accounts for some of the enhanced midrange in the stacked Advents. These numbers are for stacked original Advents. For your combo with the Advent and Advent/1, the spacings and crossover frequencies are different, so I'd expect mutual coupling to begin at a somewhat higher frequency in the bass, maybe 125 Hz, and there will be less mutual coupling the midrange because the Advent/1 tweeter is crossed over at 1500 Hz.
The effect of mutual coupling will be to boost the lower frequencies by 6 dB and in the midrange I'm guessing maybe 4-5 dB instead of the full 6. Over the rest of the range, the boost will be 3 dB as compared to a single. The extra output in those two regions is why I said in the beginning that sensitivty will be around 91 dB. Without mutual coupling, the overall would be the theoretical +3 dB.
Now, to make this a little longer: Mutual coupling occurs between the left and right speakers in a stereo pair in the bass region. Years ago, KEF published a Technotes paper that showed the respponse curves for a single speaker and a stereo pair of speakers with an 8' spacing. Over almost the whole range, the pair was precisely 3 dB higher SPL than a single speaker. But starting at about 200 Hz, the curve for the pair began to move upward, until by 50-100 Hz, the level was a full 6 dB higher. What this means is that a stereo pair has different frequency response than a single speaker. And the frequency at which the response changes is affected by the spacing. So the left right spacing in a stereo pair not only affects the imaging, but the midbass and bass balance as well. If you understand this, you can use it to fine tune the sound of your system.
Jerry
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Follow Ups
- Re: Stacked Advents - Efficiency? Suitability w/SET - Bold Eagle 01/17/0413:53:44 01/17/04 (0)
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