Is there an easy way to determine how many watts I'm playing my speakers at?

[RFP]

OK, I know the title is really lousy grammar, but I'd still like to know if there is an "easy way" to determine just how many watts I'm pumping into my Klipschorns. I have a pretty nice Fluke digital multimeter, but that's it.

Thanks a bunch!

Rob

[Guest " "]

if you have a ditigal multimeter, see attached chart for the formula for power.

[WMcD]

In short, no.

I assume you want to measure power while music is playing. With some sophisticated equipment, you could measure a short term voltage peak. That would require one of the VU meters which "hold" the peak. Or you might pick out a value on an oscilloscope.

That would get you a voltage peak.

The next issue is that the impedance of the K-Horn varies with frequency. So there is no simple R to use in Ohms law. And the peak is going to have a range of frequencies any way. So, again, there is no simple way.

= = =

There is a general rule that one assumes an 8 ohm load and then put in 2.82 volts. That would result in 1 watt. And the K-Horn should put out 104 dB. You could find a share ware program to turn you computer into a signal generator, feed the output to your amp at 1000 Hz as a start, and adjust your amp volume control until the meter shows 2.82 volts.

Don't do this, though. It is very painful.

Generally you can use

0.89 volts to get 94 dB. 0.1 watt

0.282 volts to get 84 dB (still too loud). 0.01 watt

Or 0.089 volt to get 74 dB. (tolerable). 0.001 watt. About as much as I can stand.

I would not suggest that anyone fool around with continuous test tones at more that that 74 dB level without experience. It is,though, fun to find the standing wave effects in rooms, and crank down to bass frequencies.

= = =

There is still a joker in this pack. These continuous tones are not music. The short term peaks of music can go up very much higher than average and it does not seem excessively loud. So you can't take a piece of music, say it is about as loud as the test tone, and then say the power is the same.

Gil

[RFP]

Thanks, guys...

My question is driven by the fact that I'm using TAD 2001 drivers (on Al K's Trachorns) and I want to be sure that I don't destroy a beryllium diaphragm ($$$'s, yikes) by playing them too loud.

Actually, with the lower crossover frequency of 500 Hz, they are supposed to tolerate up to 9-watts without thermal melt-down or slammin' the diaphragm into the phase plug.

Nine-watts of power into each of a pair of K-horns would probably cause deafness and blindness!

Maybe I need to find something else to worry about. []

All the best,

Rob

[DRBILL]

how many watts I'm playing my speakers at?

A chap from Texas A&M earned a fellowship for a year at Princeton. Wanting to find his way around he stopped a distinguished looking gentleman and asked, "where is the liberry at?". The man responded, "We speak English at Princeton and we know better than to end a sentence with a preposition!" The Aggie replied, "Oh, yeah." "Where is the liberry at a** h***?".

Sorry about that. But if something is funny, it's funny.

DRBILL

[SilverSport]

Bill

[RFP]

"We speak English at Princeton and we know better than to end a sentence with a preposition!"

Yeah... I know I used a preposition to end a sentence with. [:$]

I'll be more careful from now on! [:^)]

Rob

[DrWho]

If you are concerned about damaging your diaphragms, then you need to be worrying about the SPL and at what frequency, not the watts going into the driver...unless these drivers are wierd and there's something I don't know, frying a voice coil by too much power isn't going to be the end of the world. The difference is that power handling refers to the thermal limitations of the system, whereas you're concerned about the mechanical limitations of the system...

[jacksonbart]

wet your finger and turn up your amp to the usual spot, then stick your wet finger into the speaker out attachments of the amp. After a few attempts you will be able to judge it some what.

[RFP]

If you are concerned about damaging your diaphragms, then you need to be worrying about the SPL and at what frequency, not the watts going into the driver...unless these drivers are wierd and there's something I don't know, frying a voice coil by too much power isn't going to be the end of the world. The difference is that power handling refers to the thermal limitations of the system, whereas you're concerned about the mechanical limitations of the system...

I'm not really concerned about 'frying' a vioce coil, but rather destroying the beryllium diaphragm... Here's some stuff 'bout that (From Paul Butterfield's White Paper on the TAD drivers):

DISPLACEMENT LIMITATIONS. The maximum displacement of a diaphragm is determined by the gap between the diaphragm and the phasing plug. To assure highly reliable operation, definite measures must be taken to prevent drive signals that will cause the diaphragm to move far enough to contact the phase plug.

The magnitude of the diaphragm displacement depends on both the amplitude and frequency of the drive signal. the displacement (x) is directly related to the voltage (v) and inversely related to the

frequency () or x = v/. Since the power (p) dissipated by a

nominally resistive load is determined by the square of the voltage:

x = v p/.

As an example, if the crossover frequency is doubled, then the

instantaneous power can be quadrupled and the diaphragm displacement will remain unchanged.

The maximum instantaneous power vs. crossover frequency

characteristics are presented in the form of a table (see Table 1) for

compression driver model TD-2001. By choosing an appropriate power

amplifier/crossover frequency combination, the diaphragm will remain

in its Area of Safe Operation (ASO), assuming all items concerned are

operating properly and are not seriously overdriven. The maximum

instantaneous output of the power amplifier must be considered in the

above combination, rather then the rated output power, which usually

concerns steady-state, low distortion response.

TABLE 1 MAXIMUM POWER HANDLING VS. CROSSOVER FREQUENCY (-3dB, 12dB/OCTAVE) FOR COMPRESSION DRIVER MODEL TD-2001

FREQUENCY, POWER

(Hertz), (Watts)

400 3.00

425 6.00

450 7.00

475 8.00

500 9.00

525 10.00

550 12.50

575 13.00

600 14.00

625 16.00

DETERMINATION OF AN OVERALL SYSTEM LIMITATION. Both the continuous average power and the maximum instantaneous power must be maintained within the limitations described in order to assure highly reliable operation. When high instantaneous power and correspondingly high crossover frequencies are used, the possibility exists that the continuous average power will exceed the rating of the compression driver. The effects of acoustics and signal processing that tend to raise average signal levels or suppress peaks must be carefully monitored when high power is available to compression drivers.

Continuous drive signals, such as that from keyboard musical instruments, must also be considered.

PASSIVE PROTECTION FOR MULTI-AMPLIFIER SYSTEMS. A common cause for diaphragm failure in compression drivers is the application of low frequency power. This can result in the diaphragm hitting the phasing plug, even though the power may be within the rating of the driver. The selection of the proper crossover frequency and amplifier power rating are not sufficient in protecting the driver from low frequency power. Clipping, transients, hum, and amplifier failure can destroy a diaphragm even when the above precautions are taken.

For long term trouble-free operation, a passive protection network should be placed between the power amplifier and the driver. Basically, this would be a two or three pole high pass filter with a -3dB point one to one-half octave below the frequency band in which the driver will be used. The impedance of the driver/horn combination must be constant with frequency in order for a simple passive filter to produce the desired response. Realistically, many driver/horn combinations have an impedance rise in a narrow frequency band in the 500 to 2000 Hz range. This interacts with the high pass filter in such a way that the voltage across the driver terminals in the same narrow band is boosted, rather than attenuated. This seriously reduces the power handling capacity of the driver by increasing the chance of the diaphragm hitting the phasing plug.

[WMcD]

I'm not quite sure why he is putting those specific numbers in the table. They make sense generally.

One thing behind it may be that displacement of a diaphragm goes up as you slide down toward resonance, unless there is something to damp it. Unfortunately, horns are sometimes sliding down to Fc at about the same point and thus do not load the diaphragm. As you go down to Fc the acoustic load is disappearing.

You see this effect in the electrical impedance of a driver. At resonace there is a big peak. That is a way of picking out resonance by just looking at the impedance plot.

- - -

The -3 dB point a half octave below the working frequency does not quite make sense in that I'd expect that is a bit low in freq. He doesn't say how the normal x-over fits in, either.

Klipsch has similar issues with the K-77 tweeter. They went to a 3rd order crossover. In the AK-4 it looks like they're using seventh order, or so. The diagram I saw on the forum is not clear, to me.

What he is saying about the rising impedance messing up crossover operation is true too. In some of Klipsch crossovers they are using a Zobel to compensate for rising impedance.

I'll defer to Al. K, of course. His crossovers use a swamping resistor which solves some or all of this issue. And his extreme slope x overs do have very rapid roll over which should help too, particularly if their deep null is at the resonance freq. Al can tell us more.

- - - -

One favorable thing is that perhaps the TAD and certainly other drivers have to be knocked down 3 dB (half the input power) in any case. So that is some reduction from the start by the autotransformer or a pad.

= = = =

I was thinking about your concern of measuring. You might consider a cheapo, small analog meter. The lowest setting on AC might run up to 2 volt or so full scale (hard to say). You could hook this across the input to the driver. Again this is not reading peaks. None the less, I believe you will find even loud music will not show a reading above 1 volt or 2 volts. That should not create a problem.

Gil

[DrWho]

Well I suppose you could always calculate back from the sensitivity of the driver...It's what, 109dB or so? So with a 400Hz crossover you're limited to 3 watts which is about 111dB at one meter. Or a 475 Hz crossover would yield you 118dB? 625Hz would be 121dB. Of course this assumes no power compression...But still, even 111dB is rather loud.