The frequency response of your headphones probably isn't neutral, but at least it's always the same. Or is it?
Two decades ago, headphones were a secondary listening peripheral. Today, they're the means by which most music is consumed, and they're an ever-more important part of the production process too.
Compared to monitor speakers, headphones provide numerous advantages, such as freedom from room effects, portability and isolation from external sound. Yet they also present unique challenges. One is the lack of meaningful standards in tonal response targets and measurements, which has led to a very Forrest-Gumpian chocolate–box situation. Another is that of properly providing them with an electrical signal, which is the main theme of this article.
The sound quality or fidelity of a headphone generally is determined by its physical design, and as with every loudspeaker, there are thousands of factors that determine its performance. Every headphone designer worth their salt has put a lot of effort into designing drivers and earcups that are free of unwanted resonances and reflections. Yet there remains a factor outside the designer's control that can alter the frequency response of most headphones: their load impedance interaction with the circuit of the amplifier that's driving them.
Passive loudspeakers generally come in three flavours, load impedance-wise: 4Ω, 8Ω and, rarely, 16Ω. Headphones, however, are way more varied, starting at 4Ω and spanning a range up to 800Ω. The key specification for determining how loud a pair of headphones will go is sensitivity. Expressed in dB/V, this measures the output level for a given voltage swing. This is related to impedance, which determines the power generated for a given voltage swing, and also to efficiency (dB/mW), which measures how much of this power is delivered as sound.
Usually, high-impedance headphones are meant for use in studios or with high-end hi-fi systems, where good-quality, mains-powered audio interfaces and dedicated headphone amps should have no trouble providing the higher voltages needed to drive them. As battery and USB-powered devices take over the market, however, lower headphone impedances have become the norm. Impedance is a useful shorthand for determining whether a headphone will play nicely with a smartphone but, as for most rules, there are exceptions.
Technical specs usually list headphone impedance as a single 'nominal' number, like 300Ω for the Sennheiser HD650, or 32Ω for the low-impedance version of the Beyerdynamic DT770. In reality, though, impedance changes with frequency. For most headphones that use a standard moving-coil driver, the impedance can rise to more than 1.5 times its nominal value at the self-resonance frequency, which is typically around 100Hz. Usually the impedance also increases slightly at higher frequencies, due to the inductance of the voice coil.
A special case in this regard is in-ear monitors, which often employ a passive crossover filter feeding multiple balanced-armature drivers. IEM impedance curves can have many dips and peaks as all of the drivers have their own resonances, and passive filters always alter the electrical impedance of a system as well. There are two types of headphones which generally have a flat impedance curve: orthodynamic or...