Odd power ratings on PA speakers?

BahHumbug

I’ve come across a couple of general purpose powered monitors recently, for which the power ratings have got me scratching my head.  I wonder if anyone here can tell me what’s going on.

The first is a Behringer monitor that I bought for use in the band.  It’s marketed as a 200W unit. However, the rating next to the kettle lead socket says 35W.  The only rating info in the user manual says ‘Power consumption at 1/8 full power = 35W

The second is a Yamaha monitor/PA speaker that a friend just bought.  It’s similar in that it is marketed as 400W, but the rating next to the kettle lead socket says 60W.

Both units are plenty loud, certainly way louder than I would have expected from the 35W or 60W ratings on the back of each, so neither unit is particularly deficient.  I just like to see a rating that means something.

i get that this type of equipment is often quoted with peak and RMS ratings, but the consumption at 1/8 full power seems like witchcraft to me.

can anyone enlighten?

Thanks

olafgarten

Basically it is technically possible for them to have a really high peak output but they will only sustain it for a very small amount of time. They do this by using capacitors to store energy.

The Kettle Lead ratings are most likely 'average' ratings.

But it's all a bit unclear how they test these things. 

Musicwolf

As olafgarten says, consumption is a sustained level and will typically be much lower than RMS figure.  You always need to take the 'power rating' on PA speakers with a large pinch of salt.  There doesn't seem to be a standard and many of the budget systems quote peak power or that great marketing invention - music power.

As well as the amplifier's power you also need to consider speaker efficiency.  The most reliable measure is spl (measured in dB).  Again, not all manufacturers will measure this the same way but it's a better indicator.  It's also quite telling when no figure is quoted.

dB is a logarithmic scale.  It is generally accepted that a system with between 6 to 10dB additional level will be perceived to be twice as loud.

robinbowes

These are two different things that share the same SI unit (the watt, W)

https://en.wikipedia.org/wiki/Sound_power

https://en.wikipedia.org/wiki/Electric_power

R.

ICBM

I used to think that the power output was the more likely to be ‘creative’, because I assumed that the power draw would have to be correct for regulations compliance - but that appears not to be the case. Not only can the quoted power output be a lot higher than the average power consumption, it seems that the quoted power consumption can actually be lower than the continuous average power too... so both figures could be ‘correct’.

I tested my ‘200W’ Ampeg Micro VR - which is about as loud in use as my old 100W Peavey Century - and has a quoted power draw of 150W. I was expecting the true power output to be somewhere around 100-120W, but to my great surprise it *does* actually put out 200W... and draws 288W, ie almost double the quoted power.

So the problem is that you can no longer rely on adding up the quoted power consumption on all your equipment if you need to assess whether you can run it from a single socket or a particular extension cable... because the numbers are now basically meaningless.

Nerine

I don’t think audio wattage and electrical wattage are the same thing, are they? 

pintspiller

The rating beside your power lead is to do with the current draw and hence what you will pay your electricity provider.

The wattage of your monitors is confusing. For marketing purposes, the rating that is generally the product name is usually the max output of a powered and unpowered monitor. Something to do with ohms. A single powered monitor is usually half that figure.

robinbowes

Nerine said:

I don’t think audio wattage and electrical wattage are the same thing, are they? 

Nope, see my earlier post.

R.

Danny1969

A Watt is a Watt, one 1 volt potential difference with a current of one amp will produce one Watt in DC terms 

The trouble with audio is your dealing with AC currents across reactive loads and feeding the system a sine wave of 50 \ 60 Hz so you need some maths to do the correct calulations

For AC 230V \ 240 it's power factor X amps X volts roughly  ... inductive loads have a PF of 0.7  \ 0.8  ... to get watts

But you see stupidly low wattage ratings next to IEC sockets all the time ... the Mackie SRM1801 are marked 150W or so despite the fact the unit's will draw 700 odd watts at full power

ICBM

Danny1969 said:

But you see stupidly low wattage ratings next to IEC sockets all the time ... the Mackie SRM1801 are marked 150W or so despite the fact the unit's will draw 700 odd watts at full power

My guess is that the specifications must be for average power over quite a long time - eg the length of a gig, rather than the length of a song! That way the average power can be quite low since there will be periods of very low levels.

This actually makes sense if the risk factor is overheating the supply cabling, since that is cumulative over quite a long time (since PVC is a very good thermal insulator), but it's going to be a real problem if you hook up a total of "2KW" or so - from adding up the power consumption figures - to a 13A socket, and the short-period power draw is more like 4KW... resulting in a blown fuse.

BahHumbug

Thanks for answers folks.  One thing dawned on me while reading through them.

One or two of you have mentioned acoustic power/energy.  It occurs to me that I’ve always assumed that amplifier output ratings are stated in terms of electrical power delivered to the speaker(s).  I appreciate that not all speakers are equal in terms of efficiency,but I must admit I had never thought of the output rating as taking into account the speaker efficiency.

@ICBM, I’d be interested in how you measured/calculated the input and output power of the two amps that you mention.

jpfamps

From memory I think audio products are tested at 1/3rd maximum output power.

The logic is that for music reproduction, the average power when the peaks are clipping is about 10% of maximum power, hence the surprisingly low apparent power requirements for many products.

robinbowes

Some interesting information here: https://www.astralsound.com/power.htm

ICBM

BahHumbug said:

@ICBM, I’d be interested in how you measured/calculated the input and output power of the two amps that you mention.

Very simply - and fairly crudely I know since it doesn't take account of phase angle, but that's not that important really - by measuring voltage into a dummy load at the same time as mains input current, with a 1KHz sine wave input and a scope to check for the point of visible clipping - again not a precise THD figure, but not too bad.

Not totally accurate but close enough to tell which of the two quoted power ratings was true.

BahHumbug

Thanks @ICBM , I thought it would be something like that.  You are measuring electrical power, nothing acoustic.  Fair enough.

The website that @robinbowes linked to has answered my original question quite nicely.  Apparently, in the audio/PA world the 1/8 full power measure indicates the power consumed when a device is occasionally clipping a musical signal, meaning that the output from the device will be as loud as it will go while sounding acceptable from the distortion point of view.  It makes sense to assume that this refers to a typical dynamic music signal and therefore represents power consumption in typical usage.

ICBM

BahHumbug said:

Thanks @ICBM , I thought it would be something like that.  You are measuring electrical power, nothing acoustic.

Speaker efficiency has nothing to do with the power of the amp or the power rating of the speakers. It's certainly a factor in the overall volume a system will produce, but not what we're talking about here.

BahHumbug said:

The website that @robinbowes linked to has answered my original question quite nicely.  Apparently, in the audio/PA world the 1/8 full power measure indicates the power consumed when a device is occasionally clipping a musical signal, meaning that the output from the device will be as loud as it will go while sounding acceptable from the distortion point of view.  It makes sense to assume that this refers to a typical dynamic music signal and therefore represents power consumption in typical usage.

The problem is that 'typical' is not a very good reflection of real-world use.

Suppose you're going to use the same PA for a folk band, and a metal band. The ratio of average to peak power will be completely different in both cases.

Musicwolf

robinbowes said:

Nerine said:

I don’t think audio wattage and electrical wattage are the same thing, are they? 

Nope, see my earlier post.

R.

A watt (W) is a unit of power (1 joule per second).   1 Watt is the same amount of power whether it is electrical or mechanical (as in moving air).  The method of measuring or calculating will be different although the dimensions of the watt remain the same (M*L*L/T*T*T).  What you can't say is that 1 watt of electrical power will convert to 1 watt of 'acoustic' power since heat will be generated during the conversion and the laws of thermodynamics rein supreme.

robinbowes

Sound power or acoustic power is defined differently.

See https://en.wikipedia.org/wiki/Sound_power

R.

olafgarten

robinbowes said:

Sound power or acoustic power is defined differently.

See https://en.wikipedia.org/wiki/Sound_power

R.

It's the same measure, putting 1 watt of electrical power through a perfectly efficient speaker will produce 1 watt of sound power.