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What is 'nominal impedance'?

post #1 of 27
Thread Starter 
Hello,

I was hoping you all would be able to help me out in relation to a question I have about 'nominal impedance'. Currently, I have a set of Sennheiser HD515 which has a 'nominal impedance' of 50 Ohm. I am looking to upgrade to a set of Sennheiser HD650 which has a 'nominal impedance' of 300 Ohm.

What exactly does the 'nominal impedance' affect? I have tried reading around about this, and it appears that the impedance is alternative resistance. I have no idea how this applies to headphones!

Any help will be greatly appreciated on this matter.

Thanks.



AudioRookie.
post #2 of 27
Yes, it's basically just the resistance when AC signal is applied.

It is called 'nominal' because all impedance curves vary with frequency, and nominal is really a way of saying 'average over most of the frequency range in music'.
post #3 of 27
Quote:
Originally Posted by chillysalsa
Yes, it's basically just the resistance when AC signal is applied.

It is called 'nominal' because all impedance curves vary with frequency, and nominal is really a way of saying 'average over most of the frequency range in music'.
i disagree.
it is the resistance in the dc range, ie when a resistance meter (or a dmm set to resistance) is applied to the drivers at rest, with no ac involved.

it is the mother of all nightmares, and the worlds most useless number when dealing with drivers that vary greatly in impedance over the desired frequency range.
post #4 of 27
I've read some specs. for speakers that state impedance as "Nominal, Min., Max.", and others that state "DC resistance, Nominal Impedance, Minimum Impedance", so it really depends on what they mean.

The definition of 'nominal':
http://dictionary.reference.com/search?q=nominal

2. Existing in name only.

So as for specifications - maybe it depends what the company is thinking when they write it.
post #5 of 27
I don't think you've gotten your answer yet. Essentially, the impedance of the headphone describes how hard it is to drive.

Let's say you had a cd player and listened to your music at a comfortable volume with a headphone with an impedance of 32 ohms. The volume was set to 1. If you listened to the same cd player wanting the same volume out of the headphones, with a headphone that had a higher impedance of like 300, you'd have to turn the volume up significantly. Some portable players will not provide enough power to drive really high impedance headphones, and thats why people get headphone amps.
post #6 of 27
Well, that's not the complete story either. How 'hard' it is do drive also depends on how efficient they are.

Efficiency is usually quoted in dB @ 1 mW of power. 115 dB is pretty much full symphony at peak volume with you sitting in the front row.

Ok, a low nominal impendance - say 32 ohm, will mean that a lot of current will be allowed to flow through the coils of the speakers in the headphones, ie: there is a low resistance / low impedance to the flow of current. So, for a given voltage output at the amplifier, it will be easy to pass 1mW through the headphones (and make a loud sound).

If the speakers happen to be efficient as well, in all likelihood you could get good sound quality and volume without needing an amplifier.

A high impedance like 300 ohms will allow a lot less current to pass through them - the resistance / impedance is high. So to get 1mW of power into the headphones, it requires more voltage to input that power.

Now, if all headphones had the same efficiency, all that would matter is impedance if you want to know how 'hard it is to drive'.

But, there are some headphones with low impedance AND low efficiency - these would also be hard to drive. For example, the speakers on your bookshelf are probably a low 8 ohm impedance, and not very efficient because they are so big.
post #7 of 27
Nominal impendance is pretty much just the equivalent impedance of all of the circuitry of a device.
post #8 of 27
Riddle me this - does anyone know what the efficiency of the drivers in their phones is? I sure as hell don't. It's hard to get specs on headphone drivers.
post #9 of 27
Isn't "Nominal Impedance" the impedance @ 1KHz?
Impedance varies widely over the audio spectrum.
Depending on the mechanical damping of the design, impedance will either show a peak @ resonant frequency (low mechanical damping - back EMF effectively raises the impedance) or twin peaks either side of resonance (high mechanical damping).

Edit: IIRC the sensitivity of HD650 is about 100dB/mW, Sony EX71=100dB/mW, Shure E3C= about 112dB/mW.
Funnily enough, that is just how the relative loudness sounds direct off HP out on my iPod photo: ie the E3Cs go very loud.
Also IIRC the Shure E5s come in at about 120dB/mW..
post #10 of 27
Quote:
Originally Posted by JWFokker
Riddle me this - does anyone know what the efficiency of the drivers in their phones is? I sure as hell don't. It's hard to get specs on headphone drivers.
Tricky tricky. Can turn into a game of hit or miss when for example buying headphones to run straight out of a portable =(.
post #11 of 27
Thread Starter 
Hello again,

Thanks very much for all of your replies. I can see that there is no definative answer in relation to 'nominal impedance'. Not even the official website has any information (it simply states to 'see impedance').

I am still unsure as to whether or not I should upgrade my headphones at this point in time. The Sennheiser HD5XX range all have a 'nominal impedance' of 50 Ohm (as stated above, I have the Sennheiser HD515), where as the Sennheiser HD6XX range has a 'nominal impedance' of 300 Ohm.

Does this mean that it is going to be harder to achieve the same volumes on the Sennheiser HD6XX that I am currently achieving on the Sennheiser HD515? If this is the case, why is the higher level of 'nominal impedance' used in the more expensive range of headphones?

Thanks thus far for your help, I greatly appreciate it.

post #12 of 27
http://www.answers.com/topic/nominal-impedance

http://www.google.com/search?q=%22no...en-US:official

Edit:
Quote:
In electrical engineering or audio, the nominal impedance of an input or output is the equivalent impedance of all of the output or input circuitry of a device lumped into one (imaginary) component. An impedance is a combination of resistance, capacitance, and inductance. It can be thought of as a resistor that changes values at different (sine wave) frequencies.
post #13 of 27
Thread Starter 
There is always one person who has to be a smart ass and post a link to google, originality is 'you' my friend. As I'm sure you haven't read the thread (because you did not touch on the question, simply the term), my purposes for posting here are because I do not understand how the theory behind 'nominal impedance' applies to a set of headphones.
post #14 of 27
Quote:
Originally Posted by AudioRookie
There is always one person who has to be a smart ass and post a link to google, originality is 'you' my friend. As I'm sure you haven't read the thread (because you did not touch on the question, simply the term), my purposes for posting here are because I do not understand how the theory behind 'nominal impedance' applies to a set of headphones.
Well he has actually answered your question best of all. The impedence of a set of headphones changes with the sound frequency it outputs from that explanation. Maybe if you weren't so quick to call someone a smart ass you'd get your answer?
post #15 of 27
Thread Starter 
Well if you haven't figured out by my forum name, I have no idea about the technical jargon in relation to audio devices. Whilst I'm sure his intentions may have been good, I am slightly insulted that he has implied that I have not bothered to search this out.

I have searched this out, but have not found a resource that was able to explain it in generalised terms for people (such as myself) with limited knowledge on the topic. I am uncertain of what the 'nominal impedance' affects within the headphones (clarity, distortion, volume, etc), and why the more expensive range of headphones have a higher 'nominal impedance'.

I don't know how or why headphones work the way they do, but what I do know is that a good set of headphones can really enhance the audio experience. If the above mentioned definition of 'nominal impedance' is as simplified as it gets, then I guess I am plain out of luck.

Thanks.
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