Newbie amp math

[sjtm]

Yesterday I visited my friend who has inherited my first round of purchases - a Marantz PM7005 integrated amp rated at 60WPC and a pair of Focal 716V speakers rated at 91.5db@8ohms. He has set these up in a fairly large family room open to his kitchen. The approximate dimensions are 18' deep x 24 wide 'x 12'high. His main listeneing position is about 10' from the speakers, which are 2' from the back wall. There is about 6' between his couch and the rear wall which is mostly glass onto his patio. ( BTW this is a beautiful New England farmhouse fom the 1700's that he personally restored over 4 years, including building over 400' of handlaid stone walls around the perimeter of the property. Building stone walls is his hobby - he is certainly not afraid of a little hard work)

While discussing the power needed to drive the speakers at a level suitable for both personal listening and while entertaining, I suggested the following rough math:

To deliver 91.5db @1meter takes 1w of power. To increase by 3db requires doubling of power and to maintain same listening level 1 meter farther away also requires doubling of power. Therefore to listen at 97.5 db (likely as loud as you would ever want for a party) and to maintain that at 3 meters from speakers ( ie midpoint of room and his typical listening position)power must double 4x or to 16w. To maintain sound level at far side of room would require another 3x doubling or 128w. If he wants to maintain listening at 94.5db he needs 64w. Less power than that and he will experience clipping. Given the Marantz is reported to be conservatively rated, it may be sufficient for most listening, but undersized for " filling the room".

So is my rough math completely misguided for estimating amp size? And if this is generally ok, how do you account for dips in speaker efficiency ( I believe mmany Focals have dips down to 3ohms at certain hz which place additional demands on power)?

[blackraven]

That Marantz is rated down to 4 ohms and it should be have no problems with dips to 3 ohms. Those Focals have a high efficiency rating of 91.5dB which should be no problem for the amp. A more powerful amp will deliver better dynamics and clarity at high volumes. Is he experiencing any clipping, distortion or loss of dynamics at high volumes?
If so a more powerful amp with high current may help.

Alternatively, he may need larger speakers to fill the room. A subwoofer may help fill in the bass if he is having issues with it. Also, he should play around with speaker placement.

[sjtm]

That Marantz is rated down to 4 ohms and it should be have no problems with dips to 3 ohms. Those Focals have a high efficiency rating of 91.5dB which should be no problem for the amp. A more powerful amp will deliver better dynamics and clarity at high volumes. Is he experiencing any clipping, distortion or loss of dynamics at high volumes?
If so a more powerful amp with high current may help.

Alternatively, he may need larger speakers to fill the room. A subwoofer may help fill in the bass if he is having issues with it. Also, he should play around with speaker placement.

Appreciate the advice, but actually trying to figure out if there is in fact some rough math approach to determining how to size an amps power to fit the need other than just trial and error. For instance, if my example has any validity, how mathematically does the amps ability to handle 4 ohm loads calculate in? In the case of the PM7005, it is rated to 80w @ 40hm which seems to meet my calsulation of 94.5db at 6 meter (requires 64w) but not 97.5 db (128w). Is this the correct way to assess this or am I completely off track?

[blackraven]

These 2 links may help=

How Much Amplifier Power Do I Need? | Crown Audio - Professional Power Amplifiers (http://www.crownaudio.com/en-US/how-much-amplifier-power)

The one below is a calculator from the link above=

Crown Audio - Professional Power Amplifiers (http://www.crownaudio.com/en-US/tools/calculators#amp_power_required)


Here is another calculator-

Amplifier, Speaker & SPL Calculator - Geoff the Grey Geek (http://geoffthegreygeek.com/calculator-amp-speaker-spl/)

[sjtm]

These 2 links may help=

How Much Amplifier Power Do I Need? | Crown Audio - Professional Power Amplifiers (http://www.crownaudio.com/en-US/how-much-amplifier-power)

The one below is a calculator from the link above=

Crown Audio - Professional Power Amplifiers (http://www.crownaudio.com/en-US/tools/calculators#amp_power_required)


Here is another calculator-

Amplifier, Speaker & SPL Calculator - Geoff the Grey Geek (http://geoffthegreygeek.com/calculator-amp-speaker-spl/)

Thanks - these are very helpful. The two calculators result in similar results to my rough math. The first link gives a narrative that recommends power as a multiple of the speakers "continuous power rating". Focal publishes a "RMS" figure but no continuous power rating. If these are the same thing, the multiples recommended result in dramatically higher power recommendations from 280w to 450w per channel depending on music type, whereas the calculators are in the 75w - 175w depending on db levels and distance from speaker to listener. In fact the multiple recommendation exceeds the max power recommended by Focal. I think I qwill stick with the clculators.

[frenchmon]

Raven...those are some nice links...Thanks!

[blackraven]

Thanks - these are very helpful. The two calculators result in similar results to my rough math. The first link gives a narrative that recommends power as a multiple of the speakers "continuous power rating". Focal publishes a "RMS" figure but no continuous power rating. If these are the same thing, the multiples recommended result in dramatically higher power recommendations from 280w to 450w per channel depending on music type, whereas the calculators are in the 75w - 175w depending on db levels and distance from speaker to listener. In fact the multiple recommendation exceeds the max power recommended by Focal. I think I qwill stick with the clculators.

RMS is the same as continuous power and it is the average power and is a more relevant rating of what a speaker can handle rather than peak power which is usually about double the RMS.