Understanding Earphone Specifications

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When choosing earphones for your device, understanding of the specifications can help you to the pick earphones that will perform their best with your audio devices.  Besides the enclosure there are a few basic specifications to look for Sensitivity, Frequency Response, Power-Handling and Impedance. Specifications can not tell us if we will love the earphones overall sound experience or not, but we do know we are hearing the earphones best performance.

Frequency Response
 The frequency response refers to the range in which the drivers can reproduce recorded sounds fairly uniform and undistorted, however the numbers alone (also known as Simple Frequency Response) are really not enough information and can only give us a small idea of the units true frequency response. A wide Simple Frequency Response does show value, however doesn't guarantee a great sounding driver. The value is seen by the wide response range. A stock earphone with a 20Hz-20kHz range does have full sound range, However, you are not receiving any actual bass responses or tweeter like responses.  Think of a 1-way speaker with no tweeter or sub-woofer compared to a 3-way speaker with tweeter, mid range and sub-woofer, the low and high end responses are now filled in, you get a bass response and a crisp tweeter like response in the air. That's really as far as the Simple Frequency Response shows us.
 The reason it can't guarantee a great sound is because it's missing some very important information. For us to understand a units actual frequency response we would need at least four parts: 1) Simple Frequency Response numbers to give us the range.  2).  +/-3dB single line graph showing us the driver response curves, not to sharp or to lose, ideally flat would be king. 3)  A MLLSA graph, which is basically a 3D sound graphic showing us the drivers drop-offs. If a sound is supposed to start instantly and come to a sharp end, this shows how well the drivers kept up. The 4th, Environment, the actual listening of the device in play. some drivers just don't response well to certain types of music and/or environments, also taking into account that we just might not like that certain earphones overall sound experience.

Sensitivity (dB)
 Sensitivity is how efficient the earphone drivers use the power they receive. A 95 to 102 dB (decibel) driver only efficiently uses about 4-10% of the power received, the remaining power is wasted by turning into heat. Although sensitivity is rated in decibels (dB), the sensitivity rating has nothing to do with the units peak volume or sound quality. Most of us think of a sound level chart when thinking of decibels, but for sensitivity rating it is solely a efficiency test. Manufacturers test for what is called "SPL" (Sound Pressure Level) of a speaker using 1 watt (or 1mW) @ 1 meter distance (shorter distance for earphones), this is where the dB rating comes in, basically how efficient did it take the 1 watt (or 1mW) of power and covert it to sound? Thus equals the specification "##dB SPL/1w/1m". If you take a 3dB higher speaker and give it the same power, the 3dB higher speaker would be 3 decibels louder at that given power. To make the 3dB lower driver produce the same volume (dB), it would require twice the power. So, If both drivers are designed the same and both receive their solid RMS power, both would peak the same loudness. The lower dB speaker would of course require a larger amp to get their but total loudness comes from a large mix of specifications, quality and driver purpose.
  A benefit to a higher sensitivity rating is your drivers will respond very well to a wide range of power output and do not require peak power for good performance. A draw back to high sensitivity is the drivers will not take a lot of abuse (listening at high levels of distortion, etc) . Lower sensitive drivers are more power hungry and force you to give them the full power to receive the full performance. The benefit to low sensitivity is that their simply harder to damage.
Earphones normally range between 92dB - 110dB, with a few exceptions.

Power-Handling & Impedance
There are two ways to look at Power-handling, Peak power and Continuous power (RMS/Rated). The peak power is also known as Maximum short term power, meaning the driver will only hold this power for a brief moment before damaging the drivers. Continuous power (also known as RMS or Rated power) is the power-handling that the drivers are designed to continuously run at. The continuous power is around 50-60% of what the peak power rating is. Knowing the audio device's power output is just as important here. Most MP3 players including the ipods(r) are around 40mW-55mW's headphone output, while hooked to a 32 ohm earphone. Now why did I put the earphone impedance in there? To understand the power handling, you also need a small understanding of impedance (ohms).
  Impedance is resistance, like water through a pipe, this is power through a wire. Lower impedance drivers simply draw more power from audio devices. all audio devices have limits to how low or high you can go with impedance, mp3 players earphones range between 64-16 ohms with a few exceptions.  The difference in ohms can make a large difference in power for example,  if the 64ohm earphone took 30mWs, the 32ohm earphone would draw around 50mW and a 16ohm earphone would draw around 80mWs.
 In the home and car audio amps work similar, for example by going from a 8 ohm driver to a 4 ohm driver would double the amps power output or at least pull up to the amps capacitor capacity. However, Mp3 players are actually a flat power like most headphone jacks and have no amplifier. This make our volume control more like a impedance control and we are simply extended the peak volume and start farther up the band. A lower impedance unit does not have more distortion or a lacking F/R, the only real drawback for mp3 players would be a shorter battery life.

Recommend power handling and impedance for mp3 player earphones
32 ohm earphone - 100mW peak / 50mW rated
16 ohm Earphone - 200mW peak / 100mW rated
These power handling recommendations are based on earphones with a sensitivity rating of 104dB and higher, going a little less if fine. If the earphones your going with have a lower sensitivity rating you would want the power handling to be less. Higher then these numbers and you might not be hitting the earphones continuous power handling.
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