Normally, speakers are measured in an anechoic chamber with the microphone calibrated exactly a meter away from the tweeter.

But in the case of headphones and earphones, it is often impractical to have the measuring equipment in this configuration. This is because headphones and earphones seal the air around the ears when worn, which changes their sound significantly.

Because of this, headphones and earphones are measured using a dummy, such as the Ear Simulator or the Head and Torso Simulator, which emulate the environment in which they are used. The microphones here are located so as to mimic the human body as much as possible, situated where eardrums would be in a person.

Bruel & Kjaer Type 4128C HATS (Head and Torso Simulator)

However, once the microphones are out of the anechoic chamber and inside the simulated ear canal, there is another variable: the microphone registers different frequencies based on where it is within the ear. This is due to the resonance caused by the structure of the canal, and tends to occur around the 3 KHz mark.

The distortion, of course, requires compensation, and a correct calibration is essential for an accurate measurement. The graph below is the calibration used with the Bruel & Kjaer Head and Torso Simulator Type 4128C, the model currently used at Golden Ears. The calibration is obtained from the characteristic response in a Diffuse Sound Field, as headphones and earphones have a sound similar to that in this environment.

The calibration chart for Bruel & Kjaer Type 4128C HATS (Head and Torso Simulator)

The strict usage of the word ‘flat’, as applied to speakers, implies that the frequency response curve is flat in a testing environment - namely, the anechoic chamber. Then the question is: would ‘flat’ speakers still sound flat when used in a listening room, a theatre, or a recording studio?

There are several data that answer this question - the first graph, courtesy of Etymotic Research, shows the characteristic response of a recording studio with high-end equipment.

Loudspeaker Room-Equalized Response
Five Recommendations

Notice that the ‘flat’ speakers actually represent progressively weaker responses in the studio as the frequency increases - this is similar to the X-Curve, the standard to which theatrical setups are tuned, and also to the Golden Ears office.

As master recordings are created in a studio with this characteristic, further calibration must be done in order to make the response more accurately resemble what we perceive.

At Golden Ears, all frequency response graphs are calibrated with this in mind.

Bruel & Kjaer Type 4128C HATS (Head and Torso Simulator)

GRAS IEC711 Coupler

Bruel & Kjaer Artificial Ear Type 4152 

Bruel & Kjaer NEXUS Conditioning Amplifier

Bruel & Kjaer Type 2669 Microphone pre-amplifier

Earthworks M30 Measurement Microphone

GRAS Artificial Ear Type 43AC