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What It All Means: Frequency Response, Impedance, Sensitivity, and Drivers

As headphones of all types gradually become  fashion accessories, important technical measures of performance with unglamorous scientific measures seem to be preeminently targeted for exclusion by many manufacturers. Further still, even when manufacturers supply technical information, it can often be overlooked and placed beneath less pertinent factors to listening experience and sound reproduction such as model style, crafty novelty, and brand name. Just as one would not purchase a new set of reading glasses because they look “cool” without first considering the appropriate magnification, buying headphones requires that buyers are equally aware of the characteristics that best fit their particular needs. Whether it is a total suppression of relevant information or lack of knowledge surrounding the specifications that make a true difference in sound reproduction, purchasing headphones of any type based solely on emotive judgement can  result in an inadequate and unsatisfying listening experience. To help you make your next headphone purchase as a discerning and refined buyer, this blog will explain the importance and relevance of their most crucial technical specifications.

In the following sections we will define and explore the effect on listening experience of specifications including Frequency Response, Impedance, Sensitivity, and Drivers (which classify better as components rather than a characteristic, yet are imperative to sound quality). This list covers only the most common specifications and do not take signal quality or signal source into account.

Frequency Response

The Grado SR225i features a frequency response between 20 - 22,000 Hz, well within the average human range

Perhaps the most controversial specification among audiophiles, the frequency response uses Hertz (Hz) as a measure of sound waves per second where 1 Kilohertz (kHz) = 1,000 Hz. In reference to headphones, and speakers in general, the frequency response describes the frequencies a set of headphones can produce. Essentially, the lower end of the frequency response indicates the lowest frequency the speaker can produce while the larger number describes the highest frequency the speaker can produce. This, of course, does not necessarily translate into sound heard. The reason for this is because human hearing averages between 20 Hz to 20,000 Hz. What makes this measure particularly polarizing is the general assumption that a frequency response with a lower Hz measurement translates into a better bass reproduction. While a frequency response lower than 20 Hz can be taken as a strong indicator of the potential bass reproduction, measures beneath roughly 16 Hz are felt as sound pressures rather than audible tones. The same logic that applies to bass applies to the treble scale. Sound above roughly 22 kHz tend not to be audible. As a general principle when shopping for headphones, buyers should at the very least be sure to accommodate for the average audible range for human hearing with a safe amount above and below the human hearing range. For example, a set of headphones with a frequency range of about 16 Hz – 23 kHz is  adequate for most casual listeners. Nevertheless, frequency response should not be the lone factor in deciding a purchase, there is plenty more to consider.

Impedance

The Sennheiser HD 650 features an impedance of 300 Ohms making it practically impossible to play with only a portable device

Without complicating matters too much, impedance basically indicates the power demand of a headphone and is most useful to buyers as an indication of a headphone’s application. Impedance is measured in Ohms and when thinking of impedance it is useful to separate into low and high impedance categories.

Low impedance headphones ranging from around 32 ohms to around 100 ohms are particularly well suited for portable applications. Because low impedance headphones require less voltage to produce high volume, a battery powered MP3 player or a portable device like an iPod have enough voltage to allow the voice coil and magnet to push and pull the speaker in the appropriate direction.

On the other hand, high impedance headphones are intended primarily for high-powered applications. High impedance headphones require more voltage from the source in order to reach comparable volumes to a lower impedance headphone. Nonetheless,  at a higher impedance, the tightly wound, thin voice coil wire creates resistance and results in a greater magnetic field that allows a more responsive diaphragm allowing high impedance headphones to push and pull the speaker with greater ease. High impedance headphones can be used with portable devices if they are paired with an appropriate headphone amplifier that will provide the high impedance headphone enough voltage to convert the electrical signal into music.

Sensitivity

In direct relation to impedance is a headphones sensitivity. While impedance describes the voltage necessary to power the headphone at a specified volume level, sensitivity is describes the amount of electrical signal that is converted into sound.  The relationship between sensitivity and impedance is such that sensitivity is a direct result of voltage. In other words, sensitivity tells users about the volume level, measured in decibels (dB), at a specified voltage.  So, if you know the impedance of a set of headphones and the power supply, sensitivity can give users a good idea of the volume you can expect for a set of headphones.

Drivers

Finally, the driver. Easily the most important part of a set of headphones as this is the place where the music happens. Although there are many types of drivers available to consumers, perhaps the most common is the dynamic driver. All the specification discussed up to this point directly affect the performance of the driver. Of particular importance is the driver size, as this most often indicates the type of frequency the driver will best suited to reproduce.

The old adage that bigger is better is only partially true in regard to headphones. For instance, if the goal is to get the most bass out of a pair of headphones and have made sure to see that the frequency response of the headset is sufficiently low, then a large driver may be the best option. Because a large driver with a larger surface area will push more air through the chambers, the large driver is best suited to provide the best bass response. Moreover, because of the large surface area, the pressure of inaudible frequencies will be more noticeable.

However, if your musical tastes and demands tend toward the higher frequencies, a larger driver is antithetical to your purposes. Higher frequencies travel in faster and tighter waves than the broad bass frequencies. This means that your driver will need to respond quicker and fluctuate faster than it would need to with low frequencies. Smaller drivers, with a smaller surface area and reduced mass, are much easier to move making it the most appropriate option for mid to high frequencies.

Conclusion

Despite the continuing trend toward marketable and trendy headphones, these devices are complicated and technologically advanced pieces of equipment that have developed over many years. Understanding how to get the most of a set of headphones require a dedicated degree of commitment and personal investment. Ultimately, the diligence necessary to fully appreciate the characteristics of a pair of headphones can be the difference between just another mundane experience and a lifetime of enjoyment.

Staff writers at Sonic Electronix are experts in their field. In addition to a complete in-house training program, these experts typically have many years of hands-on experience in their specialty. Some come from car audio installer backgrounds, while others come with extensive retail experience.
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