According to my probably antiquated knowledge I shouldn't be able to listen to anything if I plug my 300 Ω headphones (Sennheiser HD60) in my laptop or my phone, but it does play at a decent volume. Does it use a different technology from decades ago?
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6\$\begingroup\$ 300 ohms is not a high impedance. It is a normal coil and magnet little speaker used in fairly modern headphones. \$\endgroup\$– AudioguruCommented Oct 19, 2022 at 15:54
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3\$\begingroup\$ @Audioguru It is higher than normal/typical and portable equipment or normal/typical headphone outputs may drive these higher impdance headphones poorly because they expect lower impedance. \$\endgroup\$– JustmeCommented Oct 19, 2022 at 16:17
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1\$\begingroup\$ First, there is no "normal", there's just some intersection of what the market will bear and what the PC maker felt was a good idea. Second, AFAIK most audio outputs are fairly low impedance, or they're a voltage amplifier (i.e., low impedance) with maybe a series resistor for cheap short-circuit protection. \$\endgroup\$– TimWescottCommented Oct 19, 2022 at 20:10
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3\$\begingroup\$ @TimWescott There may not be normal but surely standards. For example Android devices limit headphone voltage to 150mV, have minimum load of 16 ohms and recommended range of 32 to 300 ohms. It means you get 1.4mW into 16 ohms and 0.7mW into 32 ohms etc. So the higher impedance phones you have, the less power there is available. So given 32 ohm and 300 ohm headphones with equal sensitivity in dB SPL at 1mW, the 32 ohm phones will be louder than 300 ohm phones. \$\endgroup\$– JustmeCommented Oct 19, 2022 at 23:50
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4 Answers
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For a given applied voltage on the headphone speaker (say 1 volt RMS) the power into an 8 Ω speaker will be 125 mW. Into a 300 Ω speaker it will be 3.333 mW. That's a decibel reduction in sound of nearly 16 dB.
A human is capable of perceiving a range of 90 dB before getting into things being too loud hence a 16 dB reduction between two relative sounds is not that troublesome.
Images from here.
And, it's quite possible that the higher impedance coils will be more efficient in a typical headphone socket that has dropper resistors to prevent acoustic shock on receiving speaker level voltages on your headphones.
answered Oct 19, 2022 at 16:09
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\$\begingroup\$ I believe I've alluded to that @Justme. Maybe you can detail that in your answer if you still think I haven't covered enough ground. \$\endgroup\$– Andy akaCommented Oct 19, 2022 at 16:39
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\$\begingroup\$ Oh you did in the end. Sorry I will remove my comment on that. But, the sensitivities are rather arbitrary. Sensitivity may not relate to impedance at all. Thus e.g. a mobile phone can drive some higher impedance phones loud enough and some not loud enough, while typically all standard impedance phones are loud enough. \$\endgroup\$– JustmeCommented Oct 19, 2022 at 16:46
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The impedance of the headphones isn't the only factor. You also need to think about efficiency, which tells you how much power you need to produce a certain loudness. Efficiency is measured in decibels/watt or decibels/milliwatt. The typical efficiency for over-the-ear high-impedance headphones is 100 dB/mW[1], So the few volts coming out of your laptop is more than enough.
answered Oct 19, 2022 at 16:19
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2\$\begingroup\$ Not neceassarily enough, that's the problem of higher impedance headphones, especially used with portable equipment such as mobile phones. The same voltage may be enough on standard headphones but too low on high impedance headphones, assuming they have same sensitivity in dB SPL per 1 milliwatt. \$\endgroup\$– JustmeCommented Oct 19, 2022 at 16:42
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Compare very old \$ 4000 \Omega\$ with modern \$32\Omega\$ headphones. When connected to the same audio power amplifier, they yield similarly-loud audio output.
Both use similar electrical construction - coil of wire, with magnet yielding a force driving the moving diaphragm to create sound.
The power amplifier is a voltage source. But it is current through the coil of wire that is the driving force to move the diaphragm. Those old 4k headphones only pull tiny current from the power amplifier, while the \$32\Omega\$ headphones pull far more current from the power amplifier. A simple matter of resistance.
The voltage-source amplifier is very good at ensuring voltage is delivered, at whatever current is demanded by its load (headphones in our case). So both loads get the same voltage, but far different currents. An \$8\Omega\$ or \$4\Omega\$ loudspeaker demands even more current, which some beefy amplifiers are able to deliver.
TLDR:
The key here is that 4k headphones are wound with many more turns of wire than those \$32\Omega\$ headphones. Lorentz magnetic force is proportional to number of turns:
\$N \times I \times B\$ where N is number of turns, I is current in the wire and B is due to the magnet.
More turns yields more force, but requires longer wire (which increases resistance). A stronger magnet can also give more force, so modern phones might give more volume. But manufacturers might also compromise volume for linearity, so that instead of being louder, modern phones might reproduce audio with higher fidelity.
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\$\begingroup\$ The question considers 300 ohms as high impedance. Which it is for modern era. Where did 4 kohm fit it? \$\endgroup\$– JustmeCommented Oct 19, 2022 at 19:11
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3\$\begingroup\$ @Justme I wished to emphasize that impedance scaling extends even further than 300 versus 32(standard). Vastly different impedance, yet similar sound output when driven from a voltage source \$\endgroup\$ Commented Oct 19, 2022 at 19:16
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\$\begingroup\$ OK, but still OP has no 4kohm headphones and talks about 300 ohm headphones. \$\endgroup\$– JustmeCommented Oct 19, 2022 at 19:19
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1\$\begingroup\$ I think this answer is best at illustrating the simple relationship that - all else being equal - power draw decreases in linear proportion to impedance, but sensitivity increases linearly because of the increasing number of turns. So - all else being equal - the impedance does not affect loudness. It is merely a trend (aided by physical circumstances other than coil impedance), that higher impedance headphones might opt to sacrifice loudness for linearity as you say. \$\endgroup\$– tobaltCommented Jan 11, 2023 at 7:32
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I don't know why you think you should not hear anything with "high impedance" headphones but that's wrong.
They just have higher impedance than normal headphones.
So any PC or mobile phone can drive voltage into standard (about 32 ohms) and what you call high impedance (about 300 ohms).
The problem might be that for example mobile phones may have limited maximum voltage drive ability, e.g. 150mV for Android standard.
It means that depending on headphone impedance, the same voltage causes different current. Larger impedance results into smaller current with the same voltage, and that results into less power driven to load with same voltage.
So what comes into play in addition to headphone impedance is the sensitivity. Sensitivity is usually given in dB SPL produced at 1 milliwatt of power, or sometimes given in dB SPL produced at 1 Vrms voltage.
Sometimes the selection which unit to use is done by marketing department and larger value of the two is used, but it makes comparing headphones more difficult.
Assuming the voltage output of headphone output has low impedance, it will give same voltage to all impedances and comparison can be done with dB SPL at 1V sensitivity. Equal value means headphones of different impedance are equally loud.
On the other hand, if dB SPL at 1mW sensitivity is used, conversion is needed because obviously higher impedance load gets less power so even if that value is identical the higher impedance phones are less loud.
So in either case, 300 ohm phones will output audio. But due to they being more difficult load to portable equipment, they may be less loud than you expect, but if the 300 ohm phones are relatively sensitive, the volume may be enough.
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2\$\begingroup\$ +10 :-). .............. \$\endgroup\$– Russell McMahon ♦Commented Oct 21, 2022 at 0:35