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#386009 - 11/21/12 07:55 PM Power requirements for various channels
cpplain Offline
frequent flier

Registered: 11/20/12
Posts: 11
Loc: Bainbridge Island, WA
Calculating power for two channel setups seems to be fairly straightforward. But, what about multi-channel systems? If watching a movie with expected peaks of 100dB, should all channels be expected to reach 100dB? Or, will the center channel and rear surrounds peak at lower levels?

I've seen articles on how sitting distance from the speakers affects decibel levels, but I haven't seen anything on how the room size affects levels. With all things being equal (i.e., same equipment and speaker distance from listeners), will the decibel level change if the room is larger versus a smaller room? If so, what formulas are used to account for room size?

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#386012 - 11/21/12 08:26 PM Re: Power requirements for various channels [Re: cpplain]
dakkon Offline
connoisseur

Registered: 02/08/04
Posts: 1842
cpplain, i can tell you from my own experience, room size is a HUGE factor.. the current house i am in resulted in my upgrading from a single 5 channel amp. to 3 separate amps.. My current Great room is 2,400cu/ft.. But, since it's a Great room it opens to several other rooms within the house...


I went from 135W per channel, to 250W per channel... When i listen to music, the average sound level is about 95/96DB.. Movies are about the same volume. So, pretty close to your stated 100DB. For a given amount of power the room size will play a role in the over all DB level within the room.


Others will chime in as well. hope this gets the conversation rolling.

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#386019 - 11/21/12 11:34 PM Re: Power requirements for various channels [Re: cpplain]
JohnK Offline
shareholder in the making

Registered: 05/11/02
Posts: 10360
Chris, no, all channels don't peak simultaneously. This is essentially a non-event in actual movie and music program material, although of course it can take place during laboratory testing with test signals applied to all channels equally. This is why an "all channels driven" power spec is unrealistic for real world home listening and the FTC amplifier power regulations require a two-channels driven basis, explicitly rejecting a proposal to require the all channels standard.

Both listening distance and room size are factors in establishing power requirements, but in typically sized home rooms distance is by far the more significant factor. Studies by Dr. Toole(as reported in his book "Sound Reproduction")and others show that the combined effect of distance and room size lead to a reduction of sound level of about 3dB per doubling of distance, taking into account the contribution of room reflections added to the direct sound. This is sufficiently accurate for estimated power at various sound levels, and I use this for my calculations. For example, a speaker with a typical sensitivity of 89dB at 1 meter with 1 watt would have a comfortably loud 85dB level at a 9-10' listening distance(86dB at 2 meters and 83dB at 4 meters)with that 1 watt, and would use 100 watts at a brief 105dB peak.
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Enjoy the music, not the equipment.



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#386022 - 11/22/12 01:21 AM Re: Power requirements for various channels [Re: JohnK]
cpplain Offline
frequent flier

Registered: 11/20/12
Posts: 11
Loc: Bainbridge Island, WA
John,

I think I might not have been clear enough in my first question. I'm not necessarily asking if all channels will peak simultaneously, but rather, should it be expected that all channels can peak, at some point, to the same level. For example: if reference level is 105dB max, then should it be assumed that the center and rears could reach that level? Or, would it only be the front L/R to reach 105dB?

Alan's article on speaker power requirements (http://www.axiomaudio.com/power.html) lists 6dB loss per distance doubled. Is that an outdated or too conservative number?

When you're estimating power, do you use anechoic or in room speaker sensitivity ratings?

I have a rather large room of approximately 5500 cu/ft when taking into consideration the cathedral ceilings. How do you think the size will affect the power calculations if at all?

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#386024 - 11/22/12 02:22 AM Re: Power requirements for various channels [Re: cpplain]
JohnK Offline
shareholder in the making

Registered: 05/11/02
Posts: 10360
Sure, any channel can at a given instant be the loudest one. That's why amplifiers have equal power capabilities in each of their channels.

A 6dB loss per doubling of distance is correct for an omni-directional sound source in anechoic conditions. As mentioned above, the result in actual home listening rooms with reflections added to the direct sound is about 3dB per doubling.

Anechoic sensitivity ratings are used, since the 3dB figure takes into account the room contribution, and it shouldn't be counted twice.

Rooms aren't "filled" with sound at an equal level, despite the term being often used. The cathedral ceiling has a small effect on the level of room contribution from reflections. There's no easy way to determine how small the effect is, other than actually measuring with an SPL meter in your room at various distances to determine how close the loss in level is to the nominal 3dB per doubling figure.
_________________________
-----------------------------------

Enjoy the music, not the equipment.



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#386044 - 11/22/12 02:18 PM Re: Power requirements for various channels [Re: cpplain]
AdrianD Offline
old hand

Registered: 01/09/12
Posts: 85
Loc: Maple, Ontario
I have similar size listening room in terms of volume; my listening position is about 12 feet from the main speakers. I regularly listen at levels between 95 - 100 dB.
With M80 as main speakers, and an amp putting up 300W (all channels driven), I have a lot of power to spare. My preamp volume goes about half-way to reach that sound level.

I would say 200W on all channels should do, unless you have very, very inefficient speakers or you sit 20 feet away from your speakers.
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#386078 - 11/23/12 02:44 PM Re: Power requirements for various channels [Re: JohnK]
cpplain Offline
frequent flier

Registered: 11/20/12
Posts: 11
Loc: Bainbridge Island, WA
Depending on the amount of loss per distance doubled, I've calculated I would need 100W-200W to hit reference level. It looks like the best thing for me to do is to take some measurements to determine exactly how much my room affects decibel levels.

Would an iPhone SPL app be accurate enough for these types of calculations?

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#386079 - 11/23/12 02:51 PM Re: Power requirements for various channels [Re: cpplain]
J. B. Offline
connoisseur

Registered: 01/19/11
Posts: 1262
Loc: Quebec, Canada
right, i think that you should use the iPhone SPL;
it may not give exact actual reading, but in this case, it's the relative level you're looking after, and it should do an adequate job.
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or: Axiom Gallery

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#386101 - 11/24/12 08:48 PM Re: Power requirements for various channels [Re: J. B.]
cpplain Offline
frequent flier

Registered: 11/20/12
Posts: 11
Loc: Bainbridge Island, WA
I used two iPhone SPL apps. There were fluctuations in readings from both apps, but they were consistent enough to determine I've got about an 8dB loss from 1m to 4m (4dB loss per distance doubled).

Back to the gist of my first question. If I have a 5.1 system and each speaker requires 100W to reach reference level (105dB peaks), do I need an amp/receiver cable of 100W per channel, all channels driven?

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#386102 - 11/24/12 10:18 PM Re: Power requirements for various channels [Re: cpplain]
JohnK Offline
shareholder in the making

Registered: 05/11/02
Posts: 10360
No, Chris; as mentioned before, "all channels driven" isn't a realistic standard outside a test lab in the real world of home listening. When 1 or 2 channels happen to simultaneously hit a 105dB peak(which for most users may never happen, since reference is considered too loud for home use and a 10dB lower setting is common)the other channels aren't at that level.
_________________________
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Enjoy the music, not the equipment.



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