Headphones' Power Draw Question

[Sales Dude McBoob]

Think of a series of rapids in a river, the water is the electron, the water's potential energy (how high it is from the GROUND) is its voltage, how fast the water's moving (kinetic energy) is current. The rocks in the water are like resistors.

And the guy with the inflatable arm rings who fell out of the dingy and is clinging to a rock, holding on for dear life, is me, in my attempt to explain impedance.

Thanks for this TD.

[Tone Deft]

As power consumption goes up, so does the power output (i.e. the perceived volume level). So for the same perceived volume level, headphones of different resistances will consume roughly the same amount of power (you'll just have the volume knob on different settings). Therefore, the impedance of your headphones has no effect on battery life. The efficiency of the headphones will make a difference, but this is not the same as the overall impedance.

However (since P=V^2/R) the larger the resistance of your headphones, the less power output (perceived volume level) you'll get for a specific voltage - for example, the maximum voltage across your headphone outputs. Therefore, higher impedance headphones won't go up so loud - so you might want lower impedance ones anyway.

You're saying that larger headphones are louder and take more power, so just turn them down and use less energy. Headphone impedence is completely independent of headphone size. Also, the problem is that if the impedence of the headphones is wrong then you can't get the power transfer you need, it's less efficient, so you have to drive it harder just to get a signal. It's not a linear relationship between SPLs produced, consumed power and headphone impedence. There's a sweet spot and then it falls off from there into unusable territory, for larger or smaller impedences. If you have headphones what are at the wrong impedence you will have to drive them harder than you're implying because they are less efficient, not to mention they won't sound as good. The same principles apply to using the right impedence on coax cables for your TV, use the wrong one and your TV picture gets ghosts and weird aritfacts.

It's the same thing as speaker selection, maybe someone with some roadie experience can chime in with some good practical advice on impedence matching and getting the most out of an amp.

[nebulae]

Headphone impedence is completely independent of headphone size.

From my experience, I've found penis impedance to be completely dependent on size.

[robbmasters]

You're saying that larger headphones are louder...

Er... no, I'm not. I said "the larger the resistance of your headphones" not "the larger your headphones"!

Headphone impedence is completely independent of headphone size.

But (loosely speaking) headphone impedence is the same thing as heaphone resistance (in fact, my post would have been more accurate if I'd said impedance instead of resistance.

[Tone Deft]

You're saying that larger headphones are louder...

Er... no, I'm not. I said "the larger the resistance of your headphones" not "the larger your headphones"!

Headphone impedence is completely independent of headphone size.

But (loosely speaking) headphone impedence is the same thing as heaphone resistance (in fact, my post would have been more accurate if I'd said impedance instead of resistance.

The resistance of headphones is something like 0.001 Ohms, the impedence varies from 2 to 16 Ohms (generally), they're completely different things. There's no 'loosely' about it.

The impedence of headphones have no effect on the loudness, there are different ways to make them (different permeabilies, different wire gauge and shape on the driver, different materials in the cone, etc.) and different levels of efficiency. You're making connections that aren't there.

Look here, for example
http://www.rane.com/note100.html
Headphones with the SAME impedence can have different SPL outputs.



Neb - It's not the size, it's the frequency.

[robbmasters]

The resistance of headphones is something like 0.001 Ohms, the impedence varies from 2 to 16 Ohms (generally), they're completely different things. There's no 'loosely' about it.

Really? I just measured the resistance of my 400 Ohm DT100s, and it was 400 Ohms. So it appears that the resistance and impedance values in my headphones (at least) are identical. Someone want to prove me wrong?

Impedance is a combination of capacitance and resistance. My multimeter can't measure capacitance (as my multimeter requires a current to flow) and I can't think why there'd be much capacitance in a pair of headphones anyway...

Headphones with the SAME impedence can have different SPL outputs.

Yes, headphones of different impedance can have the same maximum power output and/or the same maximum SPL. But that doesn't mean they will both produce that maximum for the same input voltage. The maximum volume you will achieve in the real world may be less than the maximum quoted for that device. Similarly, if you plug a 30W amp into a 100W cab, you only get 30W out not 100W. And if you've done anything like this, you'll know the impedance of the cabs makes a difference too...

An iPod is like the 30 Watt amp. Well, it's more like a 10mW amp (that's the rating on my Walkman, anyway). 450mW headphones aren't going to get any more than 10mW out of it.

The Walkman manual recommends 32 Ohm headphones. That suggests that the maximum voltage across the headphone outputs is a little under 0.6V (from P=V^2/R).

Put 0.6V across a load of 400 Ohms (like my DT100s) and the power falls to about 1mW. That will deliver about a tenth of the actual volume - but not a tenth of the perceived volume, as perceived volume is logarithmic. So it won't sound ten times quieter, just quieter.

[Edit: Added the following...]

Put 0.6V across an load of 16 Ohms and the power theoretically rises to over 20mW. This is more than the amp can deliver, so you risk damaging the amp. (Though I suspect there's some protection device to prevent that happening.)

Going back to our amp and cab analogy, this is why amps often give a minimum impedance rating for the cabs. Use a cab with too low an impedance, and you risk blowing the amp.

[End of edit.]

So impedance does make a difference to the volume level you will achieve in the real world. But it's not the only factor. Less efficient headphones will obviously be quieter than more efficient headphones of a similar impedance.

[robbmasters]

Here's a snippet from Sound On Sound:

Increasingly, people tend to use high-quality 'professional'-impedance headphones with portable equipment, and this is rarely a problem, except that the maximum volume will be reduced compared to a lower-impedance design — which is no bad thing in most cases and could potentially increase the battery life of the player.

Full article at:
http://www.soundonsound.com/sos/Jan03/a ... rkshop.asp

So impedance DOES affect maximum volume.

And note that they can only prolong battery life indirectly - i.e. if using them means that you end up listening quieter than you would otherwise. If you have your music at the same volume (by turning the volume knob up to compensate for the higher impedance) they will make no difference to battery life. And you could just as easily make your batteries last longer by turning the volume down on low impedance headphones.

[Tone Deft]

May I ask who I'm talking to? What's your background in electronics? Just curious, you never know who you're talking to on the interweird, for all I know you're Mr. Wizard. You definitely are coming from a direction with experience, I just feel that I'm chasing you around from fact to fact (you probably feel the same way ), we kinda agree. FWIW my background is a BS in electrical engineering and I've been working as an EE for 10 years making audio gear, FWIW. So, I'm coming at this from an academic background, one of many different ways to come to understand this stuff. What I'm not is a studio rat or a seasoned musician or producer. Just curious, you do have a good grip on this stuff.

Really? I just measured the resistance of my 400 Ohm DT100s, and it was 400 Ohms. So it appears that the resistance and impedance values in my headphones (at least) are identical. Someone want to prove me wrong?

Please understand that resistance and impedence are two different issues. Trust me, when my circuits classes went from resistors to caps and inductors, the homework assignments got a LOT LOT longer. Impedence is a much different science (until you hold the operating frequency constant and they become phasors.)

Impedance is a combination of capacitance and resistance. My multimeter can't measure capacitance (as my multimeter requires a current to flow) and I can't think why there'd be much capacitance in a pair of headphones anyway...

Impedance characterises inductance, capacitance and resistance.
You're right, I measure my ear buds and they measure 32Ohms, I'd never measured that before. What you're measuring is the resistance of the wire in the headphones, there's an inductive and capacitive component to that too. Capacitance is everywhere, we refer to 'parasitic capacitances' when modeling very exact circuits or high speed signals (>1 GHz). Anyway, headphones are nothing more than one long solid piece of wire wrapped around an iron core (or some core) that's attached to a diaphragm, move the diaphragm, you induce a current, blah blah, I'm sure you know. That's why I said that the resistance of headphones are really small, it's just a piece of wire, a bad call on my part. I learned something today!!

Yes, headphones of different impedance can have the same maximum power output and/or the same maximum SPL. But that doesn't mean they will both produce that maximum for the same input voltage.

But those numbers come from an agreed to and adhered upon standard. From that link I posted:

Code: Select all

Headphone manufacturers specify a sensitivity rating for their products that is very similar to loudspeaker sensitivity ratings. For loudspeakers, the standard is to apply 1 watt and then measure the sound pressure level (SPL) at a distance of 1 meter. For headphones, the standard is to apply 1 milliwatt (1 mW = 1/1000 of a watt) and then measure the sound pressure level at the earpiece (using a dummy head with built-in microphones). Sensitivity is then stated as the number of dB of actual sound level (SPL) produced by the headphones with 1 mW of input; headphone specifications commonly refer to this by the misleading term dB/mW. What they really mean is dB SPL for 1 mW input.

So.. the rating you see is a standard across the board, it's not about max SPL, we don't want Neb to go deaf.

The Walkman manual recommends 32 Ohm headphones. That suggests that the maximum voltage across the headphone outputs is a little under 0.6V (from P=V^2/R).

Standard line level is 700mV, but that can vary, there's no strict standard, FWIW.

So impedance does make a difference to the volume level you will achieve in the real world. But it's not the only factor. Less efficient headphones will obviously be quieter than more efficient headphones of a similar impedance.

So, we're agreeing now?? Dunno, it's been a long round-a-bout thread.

My only point is this - Impedence matching is KEY for good sound. Using headphones with a higher impedence WILL drain an amplifier faster. I think where I got off point was discussing the direct relation between impedence and SPLs produced, or something. Either way, interesting stuff.

I haven't read your new thread yet with the SOS article, will do that now. Please understand that if a system is not impedence matched you will not get the maximum power transfer and you WILL get distortion in the signal in terms of phasing.

Props for teaching me something today, thanks!

[Tone Deft]

Very good article, everyone should read it, it demystifies a lot of daunting topics musicians come across. Most of it I already know by trade and hobby, if there are any points in it you'd like to discuss, that's cool.

I dunno man, I think we agree, I respect your knowlege and for being cool. Should we just start insulting each other and get on with a good ol' fashioned flame war, this is all too civil and interesting for me.

[robbmasters]

May I ask who I'm talking to? What's your background in electronics?

I've got a B.Eng in Electronic Engineering (with Computer Science). But it was about 15 years ago, and I only use the Computer Science bits these days. So I'm undoubtedly a bit rusty...

Impedance characterises inductance, capacitance and resistance. You're right, I measure my ear buds and they measure 32Ohms, I'd never measured that before. What you're measuring is the resistance of the wire in the headphones, there's an inductive and capacitive component to that too.

If you're measuring the impedance, there's an inductive and capacitive component. But if you're measuring the resistance (which I think I am) there's not. Is there? (I didn't think I was that rusty!)

What they really mean is dB SPL for 1 mW input.

Yes, but if they have a higher impedance they'll get fewer milliwatts input (and therefore a lower overall SPL) for the same voltage (i.e volume knob setting) .

So impedance does make a difference to the volume level you will achieve in the real world. But it's not the only factor. Less efficient headphones will obviously be quieter than more efficient headphones of a similar impedance.

So, we're agreeing now?? Dunno, it's been a long round-a-bout thread.

Dunno. I thought you disagreed with that earlier...

Using headphones with a higher impedence WILL drain an amplifier faster.

Why? Both high and low impedance headphones need roughly the same amount of power to get the same SPL. High impedance headphones will simply achieve this with a higher voltage and lower current than lower impedance headphones. But since power consumption is the same why would battery life be affected? Even the snippet I posted from SOS suggests the opposite (but see my note as it's not as simple as it might first appear).

[Edit: Added the following] In fact, since batteries are rated in "ampere hours", and the higher impedance headphones draw fewer amperes, that suggests that batteries should last LONGER with high impedance headphones. But I feel like that's not the whole story. So I still believe headphone impedance makes no odds to battery life (unless they force you to listen more quietly thereby using less power.) And I think the maths backs this up.

[Tone Deft]

Cool. Yeah, I disagreed with some stuff and I think we've just come around and clarified some points.

iPods and whatnots are Class B amplifiers, so a higher load results in more current drain (I cheated and just asked a coworker about the overall question.)
http://en.wikipedia.org/wiki/Class_B_amplifier

Impedence is used when there's a real and a reactive element to resistance, IOW when components can create a phase difference between voltage a current. v=i*r no longer really applies. I need to look up class B amps. Always something to learn.