Impedance in headphones and speakers

Jimbo Mahoney · 16 Jan 2006 at 11:44

OK, I think I understand impedance in speakers, but I'm confused because it seems the situation in headphones and speakers is different. I'm probably just looking at things a bit backward though....

In headphones, a lower impedance requires less power to drive them, so low impedance phones are normally used with portable audio to maximise battery life, right?

In speakers, a lower impedance requires MORE power to drive them.

:confused:

tom_nieto · 16 Jan 2006 at 14:17

It's to do with ohm's law etc. I can't explain it fully, but when the impedance drops very low for example when a deep bass note is played loud you'll need more current from the amp. Try asking in the hi-fi forum or google for a more detailed answer.

Logsi · 16 Jan 2006 at 14:48

Ohms Law

V= I x R

Volts= Current x Resistance.

So if we had a 4 ohm amp which is 100 watt shall we say and then powering 1, 4 ohm speaker, the 4 ohm speaker will be louder because the amp can power it, so full 100 watts being used there will be 100 watts going to speaker.

So if we plugged a 8 ohm speaker to the 4 ohm amp there will only be 50 watts being played because of the extra resistance. It will sound quieter because the speaker needs more current for it to be powered.

High Impedance/Low Current Flow
Low Impedance/High Current Flow

Logsi · 16 Jan 2006 at 14:53

^ EDIT

Jimbo Mahoney · 16 Jan 2006 at 15:14

I'm still confused.

When I plug my 85 ohm headphones into my MP3 player, I need to turn the volume up higher to achieve the same loudness as I do with my 24 ohm headphones. Thus, my 24 ohm headphones are easier to drive, right?

When I plugged my hifi amp into a set of 4 ohm speakers, I needed to turn the volume up more to achieve the same loudness as I did with my 8 ohm speakers.

So, with headphones, I get louder music with a lower impedance, but the reverse seems true with speakers an an amp.

:confused:

EDIT - Ah, unless my confusion lies with the fact my amp (a NAD C320) has impedance detection of some sort...

So lower impedance = easier to drive and louder, right?

daz · 16 Jan 2006 at 15:48

A lower impedance speaker is easier to drive for an amplifer. In some cases, it's *too* easy for the amp to drive, thus creating much more heat than would normally be expected, which can damage the internal components and/or power supply if the amp wasn't designed with that impedance load in mind.

Jimbo Mahoney · 16 Jan 2006 at 17:26

OK, I guess I'm just confused because I need to turn the volume knob on the NAD higher when it's connected to 4 ohm speakers, which I guess makes sense if it needs to supply more current.

Just confused me how my mp3 player needs to be on, say 20 (out of 25) to drive my 85 ohm Sennheisers vs. 16 to drive my 24 ohm Sonys.

Logsi · 16 Jan 2006 at 22:08

What ohmage is your amp.

Jimbo Mahoney · 17 Jan 2006 at 11:00

5bjoshua said:
What ohmage is your amp.

It says

• 2 x 40 Watts continuous into 8ohms
• 90 Watts dynamic power into 8ohms
• 125 Watts dynamic power into 4 ohms
• 160 Watts dynamic power into 2ohms
• Impedance Sensing Circuitry (ISC)

I've used it to drive 4 ohm speakers, which I assume is OK, especially if I don't play my music very loudly.

daz · 17 Jan 2006 at 17:12

Jimbo Mahoney said:
OK, I guess I'm just confused because I need to turn the volume knob on the NAD higher when it's connected to 4 ohm speakers, which I guess makes sense if it needs to supply more current.

It also depends on the sensitivity of your speakers. I have a pair of Wharfedales that are 91dB/w at 1 metre, which are 8 ohms. If you had a pair of speakers that were 94 dB/W (always at 1 metre...), it would take the same power for them both to achieve the same volume (3dB increase, double the power

).

Jimbo Mahoney · 17 Jan 2006 at 19:14

Mine are Celestion F20's - 89 dB, 8 ohms

The pair I tried (lucky bugger who owns them!) were Dynaudio Audience 62's - 86 dB, 4 ohms

daz · 17 Jan 2006 at 19:59

Jimbo Mahoney said:
Mine are Celestion F20's - 89 dB, 8 ohms

The pair I tried (lucky bugger who owns them!) were Dynaudio Audience 62's - 86 dB, 4 ohms

Weird, your amp should put more power into them, and they should be easier to drive at the same time.

Logsi · 17 Jan 2006 at 22:14

Celestion's are nice ive got two B18 1000's very mean, 20 years old in fact, older than me! Sound ace though, cant beat a classic.

megatron · 17 Jan 2006 at 23:11

5bjoshua said:
Ohms Law

V= I x R

Velocity= Current x Resistance.

Its v for volts, and having matching impedances on source and load gives maximum power transfer according to the maximum power transfer theory.

Logsi · 18 Jan 2006 at 12:10

megatron said:
Its v for volts, and having matching impedances on source and load gives maximum power transfer according to the maximum power transfer theory.

Your right, i am getting mixed up with wavelength formula.

C = F x Lambda(half life sign) This is finding out how long a wavelength is.

Velocity = Frequency x Lambda

Lets say a 20hz sine wave, will be 17.2 metres long.

Velocity 344m/s (speed of sound at 20*c with no wind)

344m/s / 20hz = 17.2 metres long etc.

I do sound engineering at college and get mixed up with loads of formulas, there also absorbtion coefficient of a room.

RT60 = 0.161 x V/A

But thats for another time very long comlicated equation.