How many audio watts is 24 watts power draw from wall plug?

[dumpingg]

I have a watt meter similar to this one:
Watt Meter

My speakers happen to be Magnepan 1.6 which are not very efficient.

I am driving the Magnepans with a Hypex NC500x Class D amp. Which has a 93% efficiency rating or so at full power.

Playing loud music thru my speakers I am measuring about 24 watts electrical wall power. It would appear to me that at 93% efficiency this would be around 20 watts of audio power going thru my speakers. Are my assumptions correct?

 

[mhardy6647]

Efficiency needs to be referenced to acoustical power output (which is independent of distance/geometry). Acoustic watts out per electrical watt in (times 100%) will give the efficiency.

You're attempting to measure sensitivity (typically expressed SPL at a given distance per unit electrical power input into the load) which depends on distance, room geometry, number of driven channels, and, of course the signal variability itself.

The latter you can 'normalize' by using a sine wave -- but be careful if you do!

One nice thing about many (not all) Magneplanars -- they are a mostly resistive load.

Magnepan LRS loudspeaker Measurements | Stereophile.com

Sidebar 3: Measurements When a loudspeaker is measured, the underlying assumption is that the distance from the speaker to the microphone is significantly greater than the largest dimension of the speaker's drive-unit array. With conventional moving-coil loudspeakers, this assumption is almost...

www.stereophile.com

 

[LTig]

Only if you play at full power, what I don't think to be the case.

 

[mhardy6647]

The old rule of thumb for acoustic power: A symphony orchestra playing loud (is that ffff or fffff? I don't know?!) has an acoustic power output of ca. 1 watt.
If that level of acoustic power takes 100 electrical watts to achieve, the transducer (i.e., loudspeaker) is about 1% efficient.
Normal dynamic driver loudspeakers are typically one or a few percent efficient. Horn loading can improve efficiency by ca. an order of magnitude.

 

[dumpingg]

I am trying to get a sense of how hard I am driving (or not driving) my amplifier NCX500 which capable of significantly more than 24 audio watts. I always read that people would be surprised to find out how little audio power we are really using so I decided to do a rough measurement in my own listening setting.

 

[wwenze]

It would appear to me that at 93% efficiency this would be around 20 watts of audio power going thru my speakers.

More often that not, home users rarely use over a watt of power reaching their speakers electrically, unless blasting in a living room, in which case maybe 5W.

Most of the 24W you measure is quiescent power, or the power it draws when outputting nothing. And you can easily measure and confirm that by measuring while... you signal is nothing.

 

[mhardy6647]

If you measure the amplifier output voltage with a high impedance AC voltmeter (e.g., a DMM) the electrical efficiency of the amp, at least in principle, can be estimated.
Again, best numbers will come from sine waves, but the stress on the system will be relatively high.

 

[mhardy6647]

I am trying to get a sense of how hard I am driving (or not driving) my amplifier NCX500 which capable of significantly more than 24 audio watts. I always read that people would be surprised to find out how little audio power we are really using so I decided to do a rough measurement in my own listening setting.

Totally depends on the loudspeaker!
If you had Klipschorns, you'd need far less amplifier power on tap for quantitatively equivalent acoustic results.
Qualitative results will likely differ, though.

 

[wwenze]

I am trying to get a sense of how hard I am driving (or not driving) my amplifier NCX500 which capable of significantly more than 24 audio watts. I always read that people would be surprised to find out how little audio power we are really using so I decided to do a rough measurement in my own listening setting.

You want the actual power consumption? Use the wattmeter, measure power with zero volume, then measure power at whatever maximum volume you want.

You want the reactive power going into the speaker? Simple way is with multimeter hooked onto the speaker terminals so you get the volts and amps. Accurate way is... idk, a reactive power meter or something?
And I say reactive power, because a lot of the electrical power don't end up dissipated in the speaker, and instead flows back into the amp. And out of those dissipated, a lot is in voice coil resistance. The actual amount dissipated as kinetic energy transferred to air, is a low percentage.

 

[mhardy6647]

More often that not, home users rarely use over a watt of power reaching their speakers electrically, unless blasting in a living room, in which case maybe 5W.

Most of the 24W you measure is quiescent power, or the power it draws when outputting nothing. And you can easily measure and confirm that by measuring while... you signal is nothing.

Which is... a lot.

You want the actual power consumption? Use the wattmeter, measure power with zero volume, then measure power at whatever maximum volume you want.

You want the reactive power going into the speaker? Simple way is with multimeter hooked onto the speaker terminals so you get the volts and amps. Accurate way is... idk, a reactive power meter or something?
And I say reactive power, because a lot of the electrical power don't end up dissipated in the speaker, and instead flows back into the amp. And out of those dissipated, a lot is in voice coil resistance. The actual amount dissipated as kinetic energy transferred to air, is a low percentage.

True but do remember that many of the Magneplanars (especially the earlier, even less sensitive models) are basically big ol' resistors with relatively little reactive component to their impedance curves.

 

[Hayabusa]

I am trying to get a sense of how hard I am driving (or not driving) my amplifier NCX500 which capable of significantly more than 24 audio watts. I always read that people would be surprised to find out how little audio power we are really using so I decided to do a rough measurement in my own listening setting.

If you play music the average power will be much lower than the peak power, can easily be 10 to 100 times lower. So play a tone to see what happens

 

[NTK]

The old rule of thumb for acoustic power: A symphony orchestra playing loud (is that ffff or fffff? I don't know?!) has an acoustic power output of ca. 1 watt.

I saw this while I was wandering around the 'net. I was intrigued.

 

[mhardy6647]

I saw this while I was wandering around the 'net. I was intrigued.
View attachment 537273

That's probably -- as Rolls-Royce used to say of their engine (horse)power output -- adequate.


EDIT:
PS We can, if desired, speak of amplifier power output in horsepower. 1 hp = 746 watts (give or take).

 

[Flaesh]

saw this while I was wandering around the 'net

In the last millennium, EV published acoustic power in datasheets

 

[mhardy6647]

In the last millennium, EV published acoustic power in datasheets

They also quoted sensitivity specifications (as did their peers, back in the day) based on the EIA standard, which was SPL at 30 feet (ca. 9.1 m) with input power of 1 milliwatt (ca. 0.75 hp).

 

[Chris A]

It would appear to me that at 93% efficiency this would be around 20 watts of audio power going thru my speakers. Are my assumptions correct?

"Audio power" that you are referring to is input electrical power. In order to convert to acoustic power, here is a table (that is approximate) that will get you closer to the actual acoustic power output in-room:

Most people are not aware that their small loudspeakers are usually much less than 1% efficient in converting electrical power to in-room acoustic power.

For most Magnepan loudspeakers, their sensitivity ratings are in the 83-85 dB/(2.73V-m) regime (i.e., ~0.2% efficient). So you would have to multiply your 20w electrical power input figure by 0.002 to get approximate acoustic power output in watts, i.e., 0.04 acoustic watts per loudspeaker.

In my listening room, the left/right channel bass bins are ~105 dB/2.73V-m) (i.e., ~18% efficient), while the HF drivers on their horns are about 110 dB/2.73V-m (i.e., ~64% efficient). At 1 watt input to each bass bin, you're looking at 105 dB at 1m. At 20w input, you're looking at approximately 3.6 acoustic watts--which is really loud (above disco loud...per bass bin)--about 117 dB/1m.

JMTC.

Chris

 

[RandomEar]

OP does not have 20 W of electrical power into the speakers. The idle consumption of 2x NCx500 + 1x SMPS1200A according to their datasheets is 2x 5.3 W + 9 W = 19.6 W [1, 2]. Input buffers, some pretty LEDs and everything else may add another Watt to that. So with 24 W at the wall, the (average) power going into the speakers is below 4 W.

 

[JohnnyAudio]

Put an amp meter on the line to check it.

 

[dumpingg]

thanks all for the discussion and education. I love the technical depth of forum members here.