# Influence of the Quality of Consumer Headphones in the Perception of Spatial Audio
## 6 Test 4. Frequency Response about Azimuth Localization  
### Test Description  
The results obtained in Test 2 are significant, but do not provide information about the accuracy in the localization of sources. For that reason, a test to evaluate the influence of frequency response on this accuracy was carried out.  
Attempts to describe different spatial attributes have been a constant pursuit in the field of spatial audio [18; 20; 21]. The diffuse term employed in Test 2 to ask about spatial characteristics (spatial impression) was intended to relate in a simple way the perception of quality with the feeling of spaciousness. A more specific study of spatial attributes is then necessary to better evaluate the performing of the different headphones. In this direction, the localization accuracy in azimuth is one of the most studied spatial attributes [22; 23; 24; 25] and therefore a good anchor point to contrast the previous Test 2 with a localization experiment. Therefore, this test tries to establish a relation of the influence of the frequency response on the azimuth localization in the horizontal plane.  
As commented on in Section 2.5, to simulate the position of the sound sources in the horizontal plane, recordings of BRIRs in a medium-sized room were done. Nine different azimuth angles, \(0^{\circ}\), \(30^{\circ}\), \(60^{\circ}\), \(90^{\circ}\), \(135^{\circ}\), \(225^{\circ}\), \(270^{\circ}\), \(300^{\circ}\) and \(330^{\circ}\), were used.  
Four types of sound were employed: door, voice (female), guitar and pink noise. A closing door is an impulsive sound with quite low frequency content, which can be useful for sound localization. The guitar sound was composed by various impulsive sounds in different main frequencies, one for each chord. Voice is an easily-recognizable common sound, and female was chosen to have some energy in high frequencies. The words _\"estimulo sonoro\"_ (_sound stimulus_ in Spanish) were employed. They present the repeated fricative phoneme /s/ with high frequency content and the phoneme /t/, a occlusive articulation that generates impulsive sound. Pink noise was employed to evaluate a wide spectrum signal.  
For this test, seven different headphones plus a hidden reference were simulated (Section 2.1). Besides these, an additional anchor auralization (low pass filtered (LPF) sounds at 7 kHz) for each angle was employed (ANC1).  
Figure 8: Difference between hidden reference and distorted signals _versus_ headphones.  
Therefore, the number of stimuli in this test was: 9 angles \(\times\) 4 types of sound \(\times\) (7 headphones simulation + 1 hidden reference + 1 anchor auralization) = 324 stimuli. These stimuli were presented in random order in two parts of 162 stimuli, with a rest in between.  
To perform the test, a GUI was developed in MATLAB, which allowed participants to freely listen to the stimuli from a random list as many times as they wanted. Participants should indicate the perceived angle of the sound source. The GUI consists of a circle of points, which represents the top view of the listener, with a 5\({}^{\circ}\) resolution. Additionally, it included a parallel control to freely listen to a reference sound (pink noise) in the angles of 0, 45, 90, 135, 180, 225, 270 and 315 degrees.  
The test was performed by 16 people, 10 men and 6 women (21 to 36 years, average age of 30). The average runtime was of 21 and 17 min for each part.