Frequency Response
Frequency response is the measure of a headphones ability to reproduce all frequencies evenly. Theoretically, this graph should be a flat line at 90dB. The left hand side of the line is the bass, the right side is the treble. If the line is high on the left and low on the right, the headphones would be considered bass heavy. If the line is low on the left and high on the right, the headphones would likely be “bright” sounding with an emphasis on the highs and lean bass response.
To perform this test we drive the headphones with a series of 200 tones at the same voltage and of ever increasing in frequency. We then measure the output at each frequency through the ears of the highly-specialized (and pricey!) Head Acoustics microphone. After that we apply an audio correction curve that removes the head-related transfer function and accurately produces the data for display.
A "natural sounding" headphone should have a gentle hump in the bass (about 3 or 4 dB) between 40Hz and 500Hz. This compensates for the fact that headphones don't give you the physical punch or 'impact' that the sound waves from a room speaker have; so a slight compensation for increased bass response is needed for natural sound. Headphones also need to be rolled-off in the highs to compensate for the drivers being so close to the ear; a gently sloping flat line from 1kHz to about 8-10dB down at 20kHz is about right.
You'll notice all headphone measurements have a lot of jagged ups & downs (peaks & valleys) in the high frequencies; this is normal and mostly due to reflection cancellations in the folds and ridges in the outer part of the ear. Ideally however, the ups and downs of the frequency response should average out to a flat line. Large peaks or valleys over 3kHz in width usually indicate poor headphone response, and should be viewed as a coloring of the sound. Some small dips in the highs may actually be desirable and should exist in the 2kHz to 8kHz region.
Distortion Products
When you put a single tone, say 500 Hertz, into the headphones, you should only get a single tone out. But if the headphones are “non-linear” you will get some extra energy out at other frequencies. These are called “distortion products” and happen at multiples of the fundamental test tone. In this case the 2nd harmonic would be at 1000Hz; the third harmonic at 1500Hz; the forth at 2000Hz, etc.
Theoretically, the perfectly linear headphone would have no harmonic peaks whatsoever; in practice this is rarely the case. The full poindexter discussion is complex, but the general wisdom is that distortion is less audibly disturbing when the each peak gets smaller as the frequency goes up AND that the second harmonic is not nearly as disturbing as the third.
Generally, it is our experience that tight, clean, articulate-sounding headphones have few harmonic distortion products. Headphones that sound lush (thought to be the even harmonics) or hard and/or grainy (thought to be the odd harmonics) probably have a lot more harmonic distortion.
It is our experience, however, that some very good-sounding headphones have a lot of harmonic distortion, so it would be wrong to assume that just because there are a lot of distortion products that the headphones sound poorly. It's just not the case. The ears have to be the guide here!
Another thing to notice on this graph is the fairly flat baseline. Any significant bump on this baseline above 200Hz is likely to be mechanical rattling somewhere in the headphone assembly.
Impedance Measurement
The headphone impedance plot is a measure of the dynamic resistance in ohms of the headphones over the entire audible frequency range. This measurement is accomplished by measuring the voltage drop before and after the output impedance resistor of our Audio Precision tester and doing a simple voltage devider calculation at each frequency.
The large peaks in this plot generally represent the location of the diaphragm resonance. The overall height above zero is the impedance of the headphone. The general trend in headphones over the past decade is a gradual lowering of impedance. In general, the lower impedance the headphone, the easier it is to get higher volume. But, once a headphone goes below about 20 ohms it starts being a current hog and may become hard to drive again. The best measure for seeing how easy it is to drive a headphone is our “voltage required to get 90dB” measurement.
One important thing to notice in the headphone impedance measurement is the wiggles and glitches in the graph, other than that of the larger resonant peaks. Generally, these represent other acoustic or mechanical resonances of headphone enclosures and should be minor.
Isolation Measurement
This is a measure of a headphone's ability to isolate the listener from external ambient noise. If there is no attenuation the line will be flat. If the headphone attenuates outside sound, the graphed data will begin to get lower, which represents a reduction of sound level at those frequencies.
In this case we measure the specialized Head Acoustics microphone's response to pink noise generated by a speaker mounted about one meter from the head. Then we simply put the headphones on the head and measure the spectrum of sound the head "hears". We then calculate the difference between the two measurements and generate a numerical factor of how much noise attenuation the headphones are providing over the entire audible frequency range.
Even open headphones cut out a moderate amount of noise above 3kHz. Most sealed headphones will provide significant amounts of attenuation down to a few hundred Hertz. Noise-canceling headphones will sometimes extend this attenuation down just below 100Hz. In-ear-canal headphones (like the Shure and Ultimate Ears products) have the best isolation by far; this is important for the air traveler to know: UE and Shure headphones will provide far better isolation than noise canceling headphones with significantly superior audio performance!
