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Old 14th January 2013, 07:08 PM   #51
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Quote:
Originally Posted by JMFahey View Post
To begin with the worse offender:
Written that way, without qualifications, it can be read as a statement that each and every harmonic will be 30V.
Fact is, if the 100Hz signal is 30V , harmonics will always be a lot lower than that, depending on "distance" from 100 Hz.
There's also another misstatement mentioning:
In fact we will have only odd harmonics, meaning 2100, 2300, etc , but not 2000, 2200, etc.
Don't remember exactly but I think the transistor equation was like

output= Gain* input + b/input +c/input*input + d/input*input*input ......goes on....

B,C,D are harmonics and they reduce in amplitude as you go higher in harmonics
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Old 14th January 2013, 08:58 PM   #52
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Quote:
Originally Posted by JMFahey View Post
... Now let's see what the poor tweeter will have to stand: ...
Worst case, it will have to withstand a bit under 3.75 watts. Even a poor tweeter should be able to manage that.

(100 Hz square wave, 40 volts, 8 ohms (200 watts), crossover 2 KHz)

Last edited by Don Hills; 14th January 2013 at 09:01 PM.
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Old 14th January 2013, 09:56 PM   #53
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I think the idea of sounding twice as loud implies a subjective impression of loudness that will probably prove impossible to generalise and quantify.

Though not logarithmic, it reminds me the idea of a day's weather feeling twice as hot; impossible to know what this means subjectively, just that it bears no relationship to physics.
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Old 15th January 2013, 02:05 AM   #54
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Originally Posted by Robert Kesh View Post
I think the idea of sounding twice as loud implies a subjective impression of loudness that will probably prove impossible to generalise and quantify.

Though not logarithmic, it reminds me the idea of a day's weather feeling twice as hot; impossible to know what this means subjectively, just that it bears no relationship to physics.
I agree, Its too tough to tell what is half when it comes to sound.
used a tone generator
I thought -4.8db is half as loud at 200hz
my friend thought -5.0db at 200hz and then after hearing 1khz for 30 secs he said -6.6 db is half as loud at 200hz

10db is a biggg difference. Just as hot weather example loudness is too subjective.
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Old 15th January 2013, 03:41 AM   #55
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I would say that your informal assessments with only 2dB variation overall was good correlation.
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Old 15th January 2013, 09:37 AM   #56
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That is better than I would have thought, but would be interested to know how that would change at different initial volumes.

Harmonic content is also very important in assessing sounds

And the ear contains its own dynamic range compression, making things even more awkward.
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Old 15th January 2013, 12:36 PM   #57
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Ok JMFahey but lets just look at the square waves rise time for a moment:

If the rise time is 50 microsec to go from zero V to 40V we must have a 20kHz component that reaches 40V. If it would be lower than 40V we would get a longer rise time because only a 20kHz sine is fast enough and all waveforms in the end are made up of sines.

I may have miscalculated the actual numbers but the idea is the same: Look at the actual rise time and find a sine of frequency x that satisfies that. The faster the rise time the higher the frequency needed to achieve it.
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Old 15th January 2013, 01:26 PM   #58
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Just in case it isn't clear you need to be careful (or specific) when using "dB" and "perceived volume".

Electrical dB values (eg watts) do not correlate directly to acoustic dB values (eg SPL).

It's a generalization, but useful none the less:
To gain a perceived doubling of loudness (+3dB acoustically) you need 10x the power electrically.

Now, you can take into consideration a bunch of factors and come up with somewhat different results. If you were designing a system using electronic crossovers, for example, that might be useful and even recommended. But in the grand scheme of things, with a single amplifier and individual full-range speakers, you're better off using the quick guesstimate and using your time dealing with other things that might be part of your design considerations. It won't be far off results-wise and you won't be wasting money on unused power or risk not spending enough under-powering.
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Last edited by Johnny2Bad; 15th January 2013 at 01:37 PM.
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Old 15th January 2013, 02:48 PM   #59
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"all waveforms in the end are made up of sines."

A square is all the odd harmonics out to infinity. In practice it looks square after the 29th harmonic, and each higher higher harmonic has a lower value.

Assume a 100W square wave at 100hz. The 3rd harmonic is -9.54dB, the 5th is -13.98, the 7th is -16.9dB, the 9th is -19.1dB.

Assume a 1Khz crossover. Now the 11th will be -20.8dB (0.83W), the 13th is -22.3dB (0.589W), etc. these go to the tweeter. As you can see, the total amount of power left to make it through an ideal 1kHz crossover (and on to the tweeter) is less than two watts (0.83 + 0.589 = 1.419W). Hardly a problem.

Yet the tweeters still blow up.

Driving the amplifier harder cannot increase the power, so why did the tweeter burn out?

Could it possibly have been to the increased power during the un-clipped portion of the program material?

The brief duration of clipping peaks on program material does not cause the failure of the tweeter, it is the increase in long-term average-power during the un-clipped portion of the program material the causes the failure.

In reality, most of the failures in the field were of a mechanical nature, the single-strand lead-out wires broke.

PS

Calculations beyond the 13th harmonic are left for the reader to add up. Hint: the sum is still trivial.
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Last edited by djk; 15th January 2013 at 02:56 PM.
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Old 15th January 2013, 03:27 PM   #60
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Those high freq corners of the square wave might be really high freqs at full output voltage, but their duty cycle is very low. You might look at that 20kHz spike, but it occurs only at the worners of thge 100Hz signal in question. SO it isn;t the same thing as feeding a full power 20kHz signal into the tweeter.

There used to be a Rane Note on clipping and speaker failure, I'll post it if I can find it. It made the case for compression blowing the speakers. The typical speaker faces music with huge low freq spikes and the treble content considerably lower in amplitude. As you turn up the material into clipping, those lows clip, but the smaller treble range continues to grow - relative to the lows. Yes, the clipped lows add some high freq stuff, but the whole high freq portion of the music is free to get louder and louder. If you had a 200 watt woofer and a 20 watt tweeter in an enclosure, and a 200 watt amp0lifier, at clipping you might have those power levels into each driver. Now raise the power to the cab to 400 watts. Or what would have been 400 watts. The woofer drive is clipped at the 200 watts the amp can create. The tweeter would now see 40 watts, well within the amp's ability, but a stress for the tweeter. Now push the signal to the 800 watt level. The woofer drive still clips at the amp's 200 watts, but now we have 80 watts into the tweeter, and the amp not even sweating. The idea is that it is the 80 watts of compressed material that blew that 20 watt tweeter, not the high freq components of the clipped low end.
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