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Old 6th October 2007, 10:33 PM   #1
dscrobe is offline dscrobe  United States
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Default Speaker Watts

Sorry for getting technical here but I am definitely missing something here (feel free to try this at home). I checked a speaker for its resistance and got 5 ohms. So I am assuming that the nominal 8 ohm value comes from the reactance of the speaker coil to a particular frequency. I played a 1kHz sound at 50% on the volume control. I measured 1.2 VAC. Well, apparent power (which won't be that much higher than real power) would then be 1.2 squared * 8 ohms. That only comes out to .18 Watts. Doesn't seem like a whole lot of watts at 50% volume control. My system in an Onkyo TX-900 40 Watt Tuner/Receiver, pair of JBL 2500 8 ohm loudspeakers.
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Old 6th October 2007, 11:03 PM   #2
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Default Re: Speaker Watts

Quote:
Originally posted by dscrobe
Sorry for getting technical here but I am definitely missing something here (feel free to try this at home). I checked a speaker for its resistance and got 5 ohms. So I am assuming that the nominal 8 ohm value comes from the reactance of the speaker coil to a particular frequency.
Yes, but 'nominal impedance' is more of a convention that specific fact. Most 8R drivers are around 5-6R DC.
Quote:
Originally posted by dscrobe
I played a 1kHz sound at 50% on the volume control. I measured 1.2 VAC. Well, apparent power (which won't be that much higher than real power) would then be 1.2 squared * 8 ohms. That only comes out to .18 Watts. Doesn't seem like a whole lot of watts at 50% volume control. My system in an Onkyo TX-900 40 Watt Tuner/Receiver, pair of JBL 2500 8 ohm loudspeakers.
Position of the volume control knob has no relationship to the actual amount of power being supplied to the load, ie, full clockwise rotation doesn't mean that the amps full power is being supplied to the load. It depends upon the gain structure of the amp and the signal source level.
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Old 6th October 2007, 11:40 PM   #3
owdi is offline owdi  United States
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The first time I measured voltage across a speaker terminal, I too was surprised how loud 1 watt cat be.

Dan
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Old 7th October 2007, 01:11 AM   #4
TerryO is offline TerryO  United States
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Quote:
Originally posted by owdi
The first time I measured voltage across a speaker terminal, I too was surprised how loud 1 watt cat be.

Dan

Me too!

Seems that we get caught up in a "Horsepower race" without actually considering what the numbers represent. A lot of people, especially when they're starting out, feel that they need speakers that will go over 120 dB because they like to listen to their music at "realistic levels". The fact is that, depending on your room and how far you sit away from your speakers, 90dB is pretty darn loud.

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TerryO
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Old 7th October 2007, 04:27 AM   #5
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One of the reasons a lot of us can get away with <10W amps....

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Old 7th October 2007, 04:39 AM   #6
TerryO is offline TerryO  United States
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Originally posted by planet10
One of the reasons a lot of us can get away with <10W amps....

dave

And do it very well, I might add.
Anyone that's been to Dave's Vancouver Island DIY Meet or to one of the VSAC events really doesn't need much convincing.

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Old 7th October 2007, 08:20 AM   #7
dscrobe is offline dscrobe  United States
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I am glad to see the response I got to this. I've been looking at upgrading my system but I was curious to understand just what a Watt really is to my ears. My specs for one speaker show 88 Decibels at 1 Watt/1 Meter. Well, I take that to understand that if I am 1 Meter away from both of my speakers, then both speakers are handling 1/2 a Watt each and I am exposed to 88 Decibels. Now, of course, as you move farther away or if you are in a much bigger room, the speaker has to handle more Watts for you to hear the same dB level. My study room is 20' x 20' and I noticed that a volume control setting of 12 out of 40 was loud enough for me. From the voltage readings I took, the twelve setting represented only .6 Watt per speaker. 1.2 Watts time 88 dB gives 105.6 dB (true if you are 1 meter away; I was sitting 4.6 meters away). Studies have shown that a Subway Train is 100 dB and a Rock Concert is 110 dB. So I can't be that far off. So, voltage readings can give you some idea, but a power meter that I could wire in shunt would be nice (haven't found any though). Also, for a speaker that calls for 10-80 Watts of undistorted, continuous power per channel from the amplifier, a 40 Watt amplfier is incredibly plenty. 2 WATTS on a speaker is quite enough for me. That is what amazes the heck out of me!
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Old 7th October 2007, 08:20 AM   #8
dscrobe is offline dscrobe  United States
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Brett, I understand the convention on nominal impedance but I can safely assume that the impedance has to be something larger than the 5 ohm resistance I measured. The reactance of the coil to particular frequencies would cause an impedance of something higher than 5 ohms. But I can't imagine it ever getting higher than 10 ohms. I also understand that the volume control setting is not proportional to power handling of your speakers. I was just curious what my actual power was in relation to my volume setting. 30% of full setting on the knob and listening to a comfortable loud setting was interesting.
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Old 7th October 2007, 10:46 AM   #9
Nanook is offline Nanook  Canada
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Default there are no watts in speakers!

speakers have no watts, only the ability to use them, and to dissipate the heat generated in their voice coils.

comments made regarding efficiency ( dscrobe's included ), and how much power one needs are THE reason for amps such as the Nelson Pass "first watt" series (but Dave, Terry, et al don't need reminding ), Single Ended flea powered tube amps and small class "T" or class "D" to exist

the comments concerning the loudness of speakers at a distance is generally true, it is a relationship that follows an inverse square law.

comments regarding the apparent needs of most folks for an amplifier power output , of say 40 watts per channel RMS, are generally true too. Dave , please chime in here anytime...

as an audio salesman (years ago), I always thought that the majority of people would be well served by a good 50 watt per channel amplifier. It seems to me that's where most mass marketed brands started to get a little more serious. The build quality difference (usually) of a 50 watter vs. a 40 watter was substantial. Of course, typical of audiophile type brands, that was not always the case. Now with the advent of digital amps, that concept can considered dead (again usually, but not necessarily).

one of the reasons to go to a bigger amp than one might need is to allow for headroom, and also the ability to drive current. The more difficult the loudspeaker load is to drive, the more current is needed (at least in my experience), and hence more power.

as usual, I hope I didn't step on anyone's toes.

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Old 7th October 2007, 12:02 PM   #10
AndrewT is offline AndrewT  Scotland
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Hi Dsc,
your voltage readings could be right.
But keep in mind that many voltmeters cannot measure accurately outside their measuring range. Most DMM often quote 100Hz to 500Hz.
Your 0.18W into a 88db/W speaker would be an average SPL of about 80db @ 1m listening distance. If you're 4.6m away then at the listening position you're hearing roughly 67db. That's about conversation level.

Now on to headroom that a poster mentioned.
Most music has a peak level that is considerably in excess of the average level. Some types (heavy metal) can have a peak to average ratio as low as+10db, and some around +30db, many types of recordings have a +20db ratio of peak to average and for sake of argument we'll assume you are listening to music with this +20db P:A ratio.
This means your average level of 0.18W has occasional peaks of 18W. If you turn the volume up by just 3db (0.36W) then the highest peaks are only just short of clipping with your 40W amplifier.

It's to ensure that clipping is avoided that high voltage level capability is built into amplifier systems.
Some say that for realistic levels you should allow between 105db and 110db on peaks at the listening position. If you want that level at 4.6m and using 88db speakers, then average levels could be around 80db (starting to get loud) but the +30db peak would require 1650W. Even 165W would only manage peaks of 100db @ 4.6m.
All this assumes there is no reflective re-inforcement within your listening room.
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