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Old 10th March 2013, 12:15 AM   #41
xnor is offline xnor
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Originally Posted by PlasticIsGood View Post
Not directly, because efficiency is not constant, so the proportion of power dissipation output as sound is not constant with respect to frequency. If that were not the case, then all headphones would have a flat frequency response with respect to the input, and we would have nothing to discuss.
Of course it's not constant with respect to frequency, but for all intents and purposes it's constants for a given frequency (what I called "ideal headphone" before).
So let me restate more precisely: for a given frequency, power is a function of voltage.

The headphone only "sees" the voltage. The power dissipation is a result of the input voltage, frequency, impedance ...

Just before that sentence in Wikipedia's entry on the dB is this one:

"Since the decibel is defined with respect to power, not amplitude, conversions of voltage ratios to decibels must square the amplitude"

Which is where the log comes from in the conversion from V to dBV. The dB is all about power. It is not at all about anything else but power.
If you quote Wikipedia do it properly:
The bel represents a ratio between two power quantities of 10:1, and a ratio between two field quantities of Click the image to open in full size.. A field quantity is a quantity such as voltage, current, sound pressure, electric field strength, velocity and charge density, the square of which in linear systems is proportional to power. A power quantity is a power or a quantity directly proportional to power, e.g., energy density, acoustic intensity and luminous intensity.
Since the decibel is defined with respect to power, not amplitude, conversions of voltage ratios to decibels must square the amplitude, or use the factor of 20 instead of 10, as discussed above.
I of course agree that decibel is defined in terms of power ratio, but dBV is not related to power like you said.

When I said it would be a similar shape I just had in mind that they go up and down together. The difference is that one would have a log vertical scale which, for low variance appears similar, just as sine squared looks similar to sine.
No they won't go up and down together.
Resistance at 100 Hz = 40 ohms
Resistance at 1 kHz = 30 ohms
1 V input (100 Hz) -> 25 mW
1 V input (1 kHz) -> 33 mW

Higher efficiency at 100 Hz (for example the resonant frequency) means that less power will be dissipated. The shape of the graph would be the opposite.

If headphones were ideal we would have nothing to discuss. I'm talking about real headphones. The discussion is about frequency response not amplitude response. Maintain the same voltage input over a range of frequencies and the output will vary. That's what I meant when I said that voltage input is not directly related to sound output.
I'm also talking about real headphones. I was saying ideal because you cannot double the voltage indefinitely. There's usually a limit around 120 to 130 dB SPL.

I do not understand the rest. Amplitude response is the absolute value of the frequency response.

Input voltage is related to sound output, just not in an absolute sense. That's what I meant, hence the "on top of the headphones' frequency response".

If you examine your maths you will also see how the dBV relates to power. Double the voltage doesn't result in double the dBV, notice? Why not? Where does the 6 come from, don't you wonder? Could it be because double the voltage results in four times the power, which is a rise in power of 6dB? How can you say the dBV is not about power when the relationship is so clear? dBV may be regardless of resistance, and therefore regardless of absolute power, but it assumes constant resistance, and is therefore directly proportional to relative power. The dB is essentially about power and nothing else but power.
There is no such thing as "your maths". Are you tinyman392 from ifans?

]That sneaks in a judgement that makes your argument circular. Headphones designed with 120 ohm source in mind may not be peaky in the bass output. If they were flat in the bass, then a lower source impedance would render them lacking at whatever frequencies the headphone impedance were higher, or peaky wherever it was lower.
If you look at the frequency response of dynamic headphones you will see that high output impedance will add a peak at the bass. One could also say the bass rolls off faster.

Rhetorical argument. Flat doesn't bother me. As I have said, my perception soon adjusts to quite a wide variation in response. Headphones isolate from the real world and create their own. We are adaptable animals. If I were to test lots of headphones in succession with the same source, I may be fooled into thinking differences are more significant than they actually are under normal circumstances. Anyone who chooses headphones by such a listening test is under an illusion. Much better to decide on clarity and reputation...but I digress.

For similar reasons I have no need for equalisation, and prefer simplicity.
So since our perception adapts to variations in response we don't need flat speakers or headphones.. ?

Also, I couldn't think of anything more simple than to EQ a headphone that I love everything about except for some frequency response imperfections.

Of course. As I said, added series resistance allows a normal opamp to drive any headphones without fear of overload. For headphones with 120 ohms source in mind, this gives cheap, bomb-proof high fidelity. The recent fad for low source impedance headphone amps rather fouls the plot.
Not op-amps but speaker taps. The resistors are just used as attenuation circuitry and since the manufacturers don't care about low output impedance (the headphone out is just an afterthought in such devices) the output impedance can be rather high.

120 ohm source impedance is not about high fidelity, it's about a stupid standard that nobody follows anyway, for example, to protect users to blow their ears with low impedance headphones.

This is an overly cynical view of how markets work I think. Headphones are not flat because flat doesn't sound flat. Since nobody really knows what flat is with respect to headphones, the market chooses whatever sounds best. One thing clear in my mind is that if you are hearing different from what most listeners hear, it ain't hi-fi. Elitism destroys fidelity like nothing else. Consequently Little Richard sounds daft on anything other than a Dansette. You may not agree. My handle isn't a joke, I mean it.
I don't think it's cynical at all, but realistic. We do have different equalization targets for headphones for centuries like I wrote.

Plastic is good? Plastic is a curse! Just look at the 1200 HD800 that is made of cheap painted plastic..

Certainly irrelevant for speakers, but not necessarily for headphones. I'll try to explain the commonly accepted view even though I don't necessarily agree with the common conception of fidelity:

The aim of speakers is to produce sound in the free air around you that is the same as the sound in the free air wherever it was recorded. It is reasonable to assume that ears in that free air around you would then hear the same as they would in the original free air, give or take all the stuff we know about the failings of speakers and rooms. It's generally and quite fairly accepted that that's the best that can be done, and therefore speakers should have a flat frequency response. No place for the "equal loudness" curves in that argument.

With headphones, you are no longer in free air, so much of what happens when your ears retrieve sound from free air in a room is bypassed. Now, a flat sound in free air is not flat in your ears, and it is that not-flat that headphones need to recreate if they are to approximate to that sound you would have heard in the original space where the sound was recorded. So we know flat is definitely not what we want for the generally-accepted view of hi-fi. What not-flatness do we need, exactly? Nobody knows, exactly, but we do know about the equal-loudness curves, so perhaps we should adopt them instead of flat. But external headphones do have a bit of outside-ear space, so maybe we should aim for something in between flat and equal loudness.
That's why we have ff and df equalization for headphones. It has nothing to do with equal loudness contours.

That appears to be the gist of the argument. A similar argument applies to the frequency-dependent cross-talk that happens in free air but not in headphones, but that's even more difficult to deal with so most headphones and amps ignore it, leading to a sound that, at best, is mostly pasted to the inside front of your head.
The answer to that is crossfeed. I hear you say: "oh no, more evil DSP like EQ!". To that I respond: "I can't stand listening without it. The unnatural stereo separation in headphones just sucks."

Considering the result is a confection, you may as well just forget hi-fi and go with whatever sounds best.

Better, in my view, is to follow the market...not the advertising hype, but rather what people actually buy and listen with. Considering music is essentially a social enterprise rather than a private phenomenon, I believe high fidelity must be socially defined. That's why snooty elitism can't work...you can't have higher fidelity than most people because most people define what high fidelity is.

There may be more chance of my argument finding favour in China, but we don't do politics here.
It's not a confection and high fidelity is not defined by what subjectivists prefer. They often prefer overpriced low-fi stuff targeted at them (audiophools).
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