What is the effect of impedance on headphone sound quality?

Question

Headphones come in various different impedance levels, such as 8, 16 and 32 Ohms. The power supplied by the audio source can be at different levels due to a variety of factors such as being limited due to being battery powered. At lower power levels, lower impedance in the headphones can allow higher volume levels (sensitivity being equal).

From my basic electrical engineering courses, my instinct is that higher impedance headphones would also create a more level frequency response, thus being more true to the original signal. Is this true?

In relation to achieving optimal sound quality, is there any advantage to pairing high-impedance headphones with an amplifier if the audio source has a lower power level rather than simply using low-impedance headphones?

Answer 1

High-impedance headphones are designed for studio-like applications where there may be multiple phones wired in parallel receiving an input signal from one source. Low-impedance headphones are designed to be plugged directly into a single source, and can more effeciently generate sound from the lower level input signal.

High-impedance headphones are a little more durable (electronically), but require higher signal levels to produce the same output level of low-impedance headphones.

Answer 2

The power required to operate headphones is a function of the driver size and mass, whether the phones are open, closed, or in-ear, etc. The impedance of headphones is usually designed to get an acceptable volume level (power = voltage^2 / impedance) at the voltage level expected on the driving equipment. It's not necessarily a straightforward relationship of low impedance = cheap, high-impedance = pro.

Many really fine headphones have large drivers (more power), are open-air (more power), AND expect to be run from higher-voltage professional/amplified equipment... which sort of cancels out the impedance question from a design perspective. Apple earbuds are 23 ohms... my fave studio headphones, Audio-Technica ATH-M50s, are 38 ohms... one of the most common studio headphones, Sony MDR7506, are 63 ohms. So, more "pro" headphones are higher impedance, but that just means that you will get less power (lower volume) on a low-voltage audio source.

There are other good reasons to use an external amplifier... RF/EMI noise isolation, etc... but I think the main reason you might want to with better headphones is that the volume may be low with higher-impedance headphones.

Answer 3

Snagged from Beyer-Dynamics website:

The impedance is determined by the voice coil (dynamic headphones), which is a winded copper wire (coated to avoid a short-circuit). This copper wire is available in nearly every length, but not in every gauge (thickness) and a thicker wire has less resistance than a thin wire ("less fits through"). The magnetic field of the voice coil depends on the number of windings of the coil, causing a low impedance system to use a thicker (also heavier) wire and since the membrane foil can't be infinitely light-weight, the moving mass (voice coil and diaphragm) is relatively high. It's pretty clear that a higher mass can't move as easily (following an audio signal) as a lower mass. This low mass can easily be accomplished with thinner (lower weight) wire, but the thinner wire has a higher impedance. This means that the DT 770 PRO with 250 ohms sound more natural, but plays (depending on the used headphone amplifier) not as loud as the 80 ohms version.

The transducers of the 80 ohms versions are stronger and more powerful, a bit more low-mid accentuated and therefore this version is ideal for powerful reproducing of low-frequency material f.e. coming from a bass guitar. The 250 ohms version sounds more smooth and voluminous and can be used for mixing situations within the studio to analyse the whole mix.

Answer 4

An amplifier works by multiplying the voltage of your audio source(CD, TV, etc...). When this voltage is applied to a load, such as a pair of headphones, then a current runs though the pair of headphones in relation to V=IZ where Z is the impedance. An ideal amp would maintain the exact same voltage regardless if the load is connected or not. However realistically the current causes a drop in voltage which leads to a different current then the one that was intended, this results in distortion. A set of high impedance headphones draws less current then low impedance headphones and therefore doesn't change the signal from the amp as much, which means lower distortion. This change in voltage is associated with a figure called the 'damping factor' which is the ratio of the headphone impedance with the output impedance of the amp.

Of course damping factor can also be improved by lowering the output impedance instead of increasing headphone impedance. However you can only push an amp as long as it can supply current. If you were to continue to lower the impedance Z you reach a point where you really can't get anymore current and wave cuts off. This cutoff is called clipping and it is a serious form of distortion. A high impedance headphone has an advantage in this case as well because of the higher impedance means a lower current draw and delays the onset of clipping.

The downside of high impedance headphones is the lower output for the same voltage. This is really only an issue if the sound level seems way too low even on max volume. In this case you need a more powerful amp to drive the headphones. Along with a limited amount of current, amps also have a maximum voltage they can reach. If you have a low powered amp with high gain then you could reach this maximum which would result in clipping again.

Just play close attention to the specs of your amp and headphones and you can avoid many of these issues.

Answer 5

In relation to achieving optimal sound quality, is there any advantage to pairing high-impedance headphones with an amplifier for listening on an MP3 player or laptop, as opposed to just using low-impedance headphones?

Yes, there is. In fact, there is an advantage to using an external amplifier for any headphones.

If you are resourceful, you could try building a CMOY amp in an Altoids box, like the picture below. The sound is awesome, and the cool factor is off the scale.

Why do amps like this improve the sound? I could talk about slew rate, total harmonic distortion and a whole host of other factors, but the simple fact is that having a booster amp like this means that your original amplifier doesn't have to work as hard. Most small audio output (including the headphone amps in the vast majority of computers and laptops) only produce about 100 milliwatts of power, and that's just not enough to insure high fidelity.

Here's how impedance affects the performance of headphones: a lower impedance allows headphones to capture more power from the amplifier. All other things being equal, they are going to be louder than headphones with a higher impedance, because higher impedance requires a higher voltage level from the amplifier to achieve the same relative loudness. This is why earbuds typically have a much lower impedance than other headphones; it is necessary to get adequate volume, given the low voltages provided by batteries.

Answer 6

It seems that the ratio of source impedance to headphone impedance, i.e. damping factor is primarily discussed in terms of effect on amplitude and frequency response. In addition, remember that a higher impedance circuit will likely exhibit a reduced slew rate and slower rise time. This will effect the clarity and openness of the sound and possibly dull transit response.

Answer 7

Consider the difference between consumer and line level:

You need to match the impedance in order to preserve the condition of the gear, but pro line level (+4 dbu) is a hotter signal than consumer (-10 dbv). The reason that professional monitors use the hotter signal, is because it is higher fidelity.

I assume that the same applies to headphones. you get a wider dynamic range and more clarity in softer dynamics when you have a wider spectrum of amplitude, which is the property of the signal propagating through your resistors and cable.