# Influence of the Quality of Consumer Headphones in the Perception of Spatial Audio
## 3 Test 1. Sensitivity Disparity between Left-Right Transducers  
### Test Description  
The idea of this test is to evaluate how sensitivity disparity between the left and right transducers affects the perception of the source azimuth. To do that, a subjective perceptual test was carried out applying some volume level variations to different binaural sounds and checking how this affects the accuracy of horizontal localization.  
In this test, participants had to listen, wearing headphones, to some binaural recordings obtained with a HATS on specific angles in the horizontal plane. Different variations of the original level between left and right transducers were applied to these sounds and then presented to the listeners. Participants should then indicate the direction of arrival, marking the angle in a Graphical User Interface (GUI).  
The volume level variations applied were 0 (no modification), 1, 2 or 4 dB more on the left channel than the right one. Four different angles of direction of arrival were chosen, \(-30^{\circ}\), \(0^{\circ}\), \(65^{\circ}\) and \(90^{\circ}\) of azimuth in the horizontal plane. Besides, the influence of different types of sounds was also studied.  
These sounds were specifically recorded for this test using a binaural mannequin (B & K Model 4100) at the specific angles under study. A 44,100-Hz sampling frequency was employed, obtaining full audio band recordings. The mannequin was in a semianecoic room, and sources were placed around it at 1 m apart. Four different sounds were recorded: a timbal drum hit, voice, a whistle and pink noise. The impulsivity of the timbal hit is an interesting characteristic regarding sound localization, also interesting for its low frequency content. Both voice and whistle are easily recognizable common sounds, which make them useful for the test. Moreover, the reduced spectral content of the whistle can be an interesting feature that can affect the test. The voice signal was the syllables \"ba-be-bi-bo-bu\", pronounced by a male voice. This sound has diverse vocalic contents and bilabial consonantal phoneme /b/, which produces impulsive sound. Pink noise was employed to evaluate a wide spectrum signal. All of these sounds were reproduced by the Sennheiser HD800 reference headphones.  
According to the different types of sounds described above, the total number of stimuli presented to each participant in this test was: 4 angles \(\times\) 4 types of sounds \(\times\) 4 level variations = 64 stimuli. These stimuli were randomly presented, and the participant could listen to each of them as many times as he or she wanted.  
During the test, participants also had the possibility of hearing a reference stimulus at any time, choosing between \(-90^{\circ}\), \(-45^{\circ}\), \(0^{\circ}\), \(45^{\circ}\) and \(90^{\circ}\) of azimuth.  
To perform the test, a simple Graphical User Interface (GUI) was developed in MATLAB that brings the user full control of the test. The participant could select the perceived sound source direction angle in an arc of \(-90^{\circ}\) to \(90^{\circ}\) of azimuth (with a \(5^{\circ}\) resolution). It was also possible for the subject to freely control and listen to the reference stimulus.  
The test was performed by 20 people, 10 men and 10 women (21 to 45 years, with an average age of 32). The average runtime of the test was 9 min. Every participant did a training session before taking the test, so all could listen to all of the stimuli and become familiar with the GUI and the assigned task. Some preliminary results of this test were previously published by the authors in [16].