Headphones

While the main text gives a brief description of how the measurements were made, some important practical details are missing.

With the isolator in circuit some of the sound pressure levels being measured are sufficiently low that the frequency responses obtained are susceptible to external interference. This was minimised in two ways.

First, the artificial head was compliantly isolated from the floor, using four coil springs, to reduce the influence of structure-borne low frequency interference from, for instance, passing cars and gusts of wind.

Second, while the reference capsule response (black trace) was averaged from ten measurements, an average of 100 measurements was used for the remaining responses in order to reduce variation at LF in particular.

Even so, some response variability remained between successive runs, mostly below 200Hz. To reduce this further the measurements would need to be performed in a structurally and acoustically isolated test chamber.