haha.. got it myself here
Frequency Response
Frequency response of headphones, as we measure it, is the apparent loudness of sound produced by a set of headphones for all frequencies at a fixed drive voltage. Headphones that produce a lot of energy in the lower end of the frequency scale have more bass and are said to be “warmer†or (if the excessive energy is concentrated in the very low end) “boomy.†Headphones with excessive energy in the high frequencies have more treble, and are said to be “bright.†A frequency response curve (also called a sweep tone plot) is made by putting a pair of headphones on a high quality dummy-head microphone, playing back an audible sweep of all frequency tones from 20Hz to 20kHz (driven at a fixed voltage), and recording the response with an Audio Precision System Two Cascade ($20,000 of the coolest piece of audio test gear you’ve ever seen). The measurements gathered are then compared against an ideal headphone measurement made from the averaged frequency response of ten very good headphones. The resulting difference between the measured response curve and the “ideal†is then plotted. This basically means that when you look at a headphone graph of a particular pair of headphones, you’re seeing how they vary from the ideal.
Square Wave Response
Because a square wave contains equal amounts of all odd harmonics in a very specific phase relationship, deviations from ideal performance are quite easy to see. Using a 1 kHz square wave, hi-frequency losses are shown by loss of steepness on the leading edge of the square wave (called “slew rate limitedâ€), and phase incoherence is shown as ripples in the waveform (usually caused by acoustic and/or mechanical resonances). Using a 50 Hz square wave, you can see poor bass response as a falling off at the top of the square wave. You can also see a bass boost as a hump on the top of the square wave.
Impedance Response
The impedance response of a set of headphones is their load resistance to the drive voltage of the amplifier as frequency changes. We normally think of this measurement as a single number (for example, 18 Ohm, needing a low drive voltage, for portable player headphones, or anything from 32 Ohm to 600 Ohm, needing a significantly higher driver voltage, for high performance headphones). However, like any dynamic speaker, headphone drivers have electrical resistance that varies significantly with frequency. In other words, a headphone’s impedance varies depending on what sounds they are producing at any particular moment. Upward peaks in impedance response graphs represent driver resonances. The shape of the curve actually changes significantly depending on whether the headphones are on a head or in open air.
Voltage at 100dB
We developed this measurement because more and more headphone listening is being done on portable players, which tend to have headphone jacks with limited drive voltages. This measurement indicates the drive voltage needed for the headphones to sound very loud. We believe that in order to be musically satisfying with a wide variety of music types, headphones have to be able to be driven to at least 100 dB during musical peaks. Thus, for any given pair of headphones, the “voltage at 100dB†is the minimum required power supply voltage of the headphone amplifier driving the headphones. If the headphones need 15V at 100 dB, then a 5V portable player power supply won’t drive them. [Note that this is the power supply voltage of the portable unit, not the power output of the portable unit’s headphone jack.]
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