Page 59 - Spring2022

These same arguments were undoubtably made about loudspeakers 40 years ago and until research proved lis- teners largely agreed on what is a good loudspeaker.
With the lessons learned from the loudspeaker industry, the author and his colleagues embarked on a seven-year research project to improve the consistency and sound quality of headphones. There were three fundamental questions we hoped to answer.
(1) What is the preferred headphone target curve? Should the reference be a loudspeaker in a FF, a DF, or a semireflective field (SRF) found in a typical listening room?
(2) Do listeners agree on what makes a headphone sound good? To what extent does listening expe- rience, age, gender, and geographical location influence sound quality preferences?
(3) Can listeners’ subjective ratings of headphones be predicted based on an objective measurement?
These research questions were addressed for the three main headphone types, but the scope of this article is largely restricted to AE and OE headphones. The pre- ferred target curve for IE headphones is almost identical to those for the AE and OE targets, except it has an addi- tional 4 dB of bass (Olive et al., 2016). Each question is addressed separately, followed by conclusions.
The Search for the Preferred Headphone Target Curve
Over the past 50 years, headphone researchers have
focused their attention on determining what the ideal ref- erence sound field should be for headphone reproduction and how to measure it. Three types of reference sound fields have been proposed: a FF, a DF and a SRF that lies somewhere between the two extremes. What these sound fields are, how they are measured or derived, and psychoacoustic investigations of headphone target curves based on them are described.
Free-Field Headphone Target Curve (1970s)
The reference FF was generated by placing a loud- speaker in front of the listener in a reflection-free room. A tedious subjective loudness-matching procedure was
used where a test subject would listen to narrow bands of noise at different frequencies alternately with the FF (with the headphone removed) and then with the head- phone. While listening to the headphones, the levels for each band would be adjusted to match the loudness of
the loudspeaker. This would be repeated for several test subjects to calculate the loudness transfer function that defined the headphone FF target curve.
Theile (1986) conducted formal listening tests and found the DF target to be preferred to the FF target, which produced an unnatural timbre and in-head localization effects. Although the FF target fell out of favor beginning in the 1980s, it remains part of the current headphone IEC (2010) standard today.
Diffuse-Field Headphone Equalizations (1980s to Present)
A DF occurs when a sound source is placed in a rever- beration room with little or no absorption, so the listener receives a random and equal distribution of sounds from all directions. The headphones are calibrated to the DF using a subjective loudness procedure or alternative methods. In one method, a probe microphone is placed in the ear canals of the listener to measure and then match the transfer function of the headphone to that of the sound field (Theile, 1986).
A second approach is to substitute the listener with a head and torso simulator (HATS); this produces faster, more reproducible, and safer measurements than putting probe microphones in the listeners’ ears. A third option is to use a headphone known to be DF calibrated as the reference and compare its performance with the headphone under test.
Møller et al. (1995) derived a headphone target curve based on different sound fields using a large set of head- related transfer functions (HRTFs) measured at the blocked ear canal. HRTFs define the transfer functions, both the frequency and phase responses at the entrance to the ear, for each direction and distance of a sound source. They capture both interaural time (ITD) and intensity (IID) differences and spectral cues that humans use to localize sound sources in space (Blauert, 1983). By selecting HRTFs from the appropriate directions and distances and integrating them, Møller et al. (1995) were able to derive transfer functions of reference sound fields ranging from the FF to the DF and anything in between.
This method eliminated the need for a physical reference sound field, making headphone calibration more practi- cal and reproducible. A headphone could be measured and equalized to the DF target curve using a calibrated dummy head or ear simulator.
Spring 2022 • Acoustics Today 59