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Power requirements
Power requirements
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Old 1st August 2012, 07:41 AM   #1
Vaughan is offline Vaughan
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Default Power requirements

I have a few questions on amp power that I'm hoping you guys can help me better understand. To my knowledge the amount of power you need for a speaker system is dependent on the following :

1)Room Size, 2)Distance seated from the speakers 3)Listening levels 4)System set up 6) Sensitivity and 5) Content

People often claim that you need to use lots of power to control the bass drivers in a big speaker. Let's take the B&W N803 speakers for example. Sensitivity is around 90 dB, which is rather high but it is nevertheless still considered by most to be a power hungry speaker for some reason.

Why is this? And does one absolutely require 200 or 300 watts to get a good "grip" on those bass drivers? Why wouldn't 100 watts suffice given a particular seated distance and/or listening level? Let's cut to the chase here.

Any engineers in the house who can shed light on this?
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Old 1st August 2012, 07:44 AM   #2
Mooly is offline Mooly  United Kingdom
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Power requirements
Give this a read and a try,
http://www.diyaudio.com/forums/multi...kers-need.html
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Old 1st August 2012, 09:30 AM   #3
Vaughan is offline Vaughan
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Reading through the thread it seems some don't agree with the testing procedure. Are there any intuitive explanations for the questions I raised?
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Old 1st August 2012, 10:21 AM   #4
sreten is offline sreten  United Kingdom
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Hi,

200 or 300 watts is only 3 or 5 dB louder than 100W and only necessary
if you want to push the speakers to their limits. For many 100W would
quite sufficient, albeit your really talking about near 200W into 4 ohms.

rgds, sreten.
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Old 1st August 2012, 11:34 AM   #5
Mooly is offline Mooly  United Kingdom
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Power requirements
Quote:
Originally Posted by Vaughan View Post
I have a few questions on amp power that I'm hoping you guys can help me better understand. To my knowledge the amount of power you need for a speaker system is dependent on the following :

1)Room Size,
Of course. To fill a large room is going to need more "power". Not just that though, the room contents and absorbtion play a part too.


Quote:
Originally Posted by Vaughan View Post
2)Distance seated from the speakers
Goes without saying. If your sat at the back of a huge room or hall it isn't going to sound as loud as up at the front.


Quote:
Originally Posted by Vaughan View Post
3)Listening levels
That is where the thread I linked to is so revealing. Even if you were 100% in error it still kind of puts into perspective the actual levels used.


Quote:
Originally Posted by Vaughan View Post
4)System set up
That can mean many things to different folk Such as lots of bass boost or playing heavily compressed music files etc etc

Quote:
Originally Posted by Vaughan View Post
5) Content
As reply #4 really. Certain types of music "need" or appear to need different overall amplifier power. Classical music with its high average to peak ratio for example.

Quote:
Originally Posted by Vaughan View Post
6) Sensitivity
Can play the biggest role of all. An 84db/m speaker and a 90db/m need approximately 4 watts and 1 watt respectively for the same level. The numbers grow big very quickly for the lower sensitivity. The higher efficiency speaker will manage around 99db for 8 watts input, the lower efficiency one needing around 32 watts.


Quote:
Originally Posted by Vaughan View Post
People often claim that you need to use lots of power to control the bass drivers in a big speaker. Let's take the B&W N803 speakers for example. Sensitivity is around 90 dB, which is rather high but it is nevertheless still considered by most to be a power hungry speaker for some reason.

Why is this? And does one absolutely require 200 or 300 watts to get a good "grip" on those bass drivers? Why wouldn't 100 watts suffice given a particular seated distance and/or listening level? Let's cut to the chase here.

Any engineers in the house who can shed light on this?
A power hungry load might mean the speaker presents a reactive load with difficult phase angles when provoked by a suitable drive waveform. It might mean the speaker has an uneven and low minimum impedance. That's not the same as a difficult reactive load though. Values as low as 3 ohm aren't unusual even though the speaker may be quoted as 8 ohm.

Amplifiers don't really "grip" or damp the speaker no matter what high damping factors suggest.
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Old 1st August 2012, 02:14 PM   #6
wg_ski is offline wg_ski  United States
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Quote:
Originally Posted by Mooly View Post
Amplifiers don't really "grip" or damp the speaker no matter what high damping factors suggest.
True, when the amp is operated linearly. What happens when you do drive into clipping is another matter.. and the "effective" damping factor will be a whole lot worse then the numbers claimed. If you drive an amp with a woosy power supply or inadequate reservior caps into clipping with heavy bass it will get VERY sloppy sounding. Look at the waveform on a scope, and it's a mess. Rail voltage jumping up and down, power supply ripple being conducted through saturated outputs (and of couse, producing mixing products with the signal). An amp with more behind it, even if it sarts clipping at a lower power level, will tend to stay cleaner sounding. The waveform will look better too - just flat-topping nice and clean, producing gragefully-degrading 3rd order products as it drives harder.
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Old 1st August 2012, 02:28 PM   #7
Vaughan is offline Vaughan
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Guys, thanks for the replies. I think I understand most of the theory, but it bothers me when people generalize and claim you need 250 watts or more for a particular speaker, that it will "provide more grip ... better control, etc etc etc".

I mean, if I run a 100 watt amp with a pair of N803's, jump to 200 watts, the levels should jump 3 dB's, but the mere fact that I added more power doesn't provide more control or "insert some audiophile term".

The other thing I'm not sure of (correct me if I'm wrong), is the amplifier damping. Wouldn't damping factor be insignificant compared to the damping in a speaker, generally?
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Old 1st August 2012, 04:52 PM   #8
Vaughan is offline Vaughan
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Quote:
Originally Posted by Mooly
That can mean many things to different folk Such as lots of bass boost or playing heavily compressed music files etc etc
Well bass management, for example. Setting speakers to "Large" vs "Small".
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Old 1st August 2012, 04:53 PM   #9
Vaughan is offline Vaughan
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Quote:
Originally Posted by Mooly
Amplifiers don't really "grip" or damp the speaker no matter what high damping factors suggest.
Please elaborate more on this.
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Old 1st August 2012, 04:59 PM   #10
Vaughan is offline Vaughan
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Just today, I had someone tell me "B&W crave power". But why? What makes it crave power and how can one make such a claim without knowing listening levels, room size and seated distance, system set up etc etc? It doesn't make any sense to me.

Click the image to open in full size.

90 db sensitivity. 8Ω (minimum 3.0Ω) Power handling between 50-500 watts. But again, how does one qualify that statement that they crave power? I don't get it.
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