With the rise of mobile audio production, headphones — with their extreme portability — have become the preferred listening method in the field. With this sharp rise in headphone use, a question comes up frequently. Is a dedicated headphone amplifier necessary for high quality sound monitoring?
Neumann, a maker of professional studio headphones, says the answer is “yes,” especially in two cases: getting more volume in the headphones and hearing better audio quality.
Two key parameters determine how much volume headphones can generate:
• One is sensitivity ratings. This means the volume level for the headphones at a given input level. Modern headphone designs tend to have more volume than older models. Sensitivity is specified as either sound pressure level per volt (dB SPL/V) or sound pressure level per milliwatt (dB SPL/mW). Both of these are complex formulas for measurement. Buyers should know that modern headphones are often around 105 dB-SPL/mW, which is ample level.
• The second parameter is impedance. Headphones typically have an impedance ratings ranging from about 16 ohms to 600 ohms. Lower impedance models produce higher volume than high impedance headphones.
Until the 1990s, studio headphones tended to be high impedance, ranging from 250 to 600 ohms. Much has changed since then. Today, high impedance is rare. This is largely because of the sharp rise of battery powered audio devices. These devices tend to require low impedance headphones to achieve adequate volume and the market answered. Most modern headphones now range between 16 and 100 ohms and generate good volume from virtually any headphone output.
This is where the headphone amplifier comes in. The interplay between the headset and the amplifier has an effect on the sound quality delivered. The lower the headphone impedance, the higher the load it presents to the headphone's output stage. The higher the load, the harder the output circuitry must work. This can generate more distortion. The bottom line is low impedance headphones are good in terms of volume but are bad in terms of distortion.
The following graph shows the frequency responses of a low impedance headphone (red) and a medium impedance headphone (blue), measured at various driving impedances. The low impedance headphone is affected more than the medium impedance headphone, especially at low frequencies.
Impedance can vary over frequency. The impedance rating found for headphones is usually measured at 1 kHz. At lower frequencies, the actual impedance may be much higher, sometimes by a factor of two or three. This can affect the frequency response. How much depends on the output impedance of the headphone amplifier. If it is above 50 ohms, it can cause audible changes in the headphone’s frequency response. This effect is typically stronger for low impedance headphones.
Another influence on headphone sound fidelity is impulse response. A good audio system should reproduce impulses as accurately as possible. This requires proper damping to keep the membrane from oscillating after the impulse has ended. Apart from acoustical damping, which is part of any headphone's design, there is also electrical damping. Proper electrical damping is achieved if the output stage’s impedance is no more than one-eighth of the headphone's impedance.
This is not the case with very low impedance headphones. These require a super low driving impedance to achieve proper damping. For optimal sound with all types of headphones, the headphone amplifier’s output impedance should be as low as possible. A dedicated high quality headphone amplifier should achieve this. But the outputs on headphone audio interfaces are often around 20 ohms. This is too high for adequate electrical damping with low impedance headphones, which may result in a muddy midrange and poor bass response.
In general, all headphones benefit from a powerful headphone amplifier with low distortion and ultra low output impedance. The question is can the serious field recordist do without a dedicated amplifier, especially when using the headphone jack on a personal computer?
The answer is it depends. High impedance headphones will often require a dedicated headphone amplifier to achieve adequate listening levels. Low impedance headphones, on the other hand, have no problems achieving high volume. But they do profit from a high quality headphone amplifier in terms of overall sound quality. The amp brings lower distortion, full bass and a fast transient response.
Good advice is to use medium impedance headphones (100 – 250 ohms). Such headphones will deliver enough volume on the vast majority of audio interfaces and other devices, and most headphone outputs will have a low enough driving impedance for optimal sound.
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