# Influence of the Quality of Consumer Headphones in the Perception of Spatial Audio
## 5 Test 3. Non-Linear Distortion  
### Test Description  
The objective of this test is to evaluate how the effect of harmonic distortion in headphones affects the spatial impression.  
Several stimuli with and without the simulation of their harmonic distortion were presented to the participants that had to score their perception.  
The effect of these distortions is very subtle. For that reason, the perceptual test was designed according to the recommendation ITU-R 1116-2 [19], which describes a method to assess small impairments in audio systems. This recommendation also establishes rigorous requirements of room, equipment and other arrangements. A continuous scale from one to five (1 - very annoying; 5 - imperceptible) is used to evaluate degradations with respect to a reference signal. The recommendation proposes an ABC test in which two stimuli, A and B, are presented to be compared against a known reference. One of these two stimuli, A or B, is always a hidden reference, and the other a degraded signal.  
One single question was presented to the participants: \"What degradation of quality and spatial impression do you hear with respect to the reference?\"  
The same five audio excerpts previously described in Test 2 were used here (see Table 1), as well as the same five virtual headphone simulations (b)-HQop, (c)-MQcl, (d)-BDso, (e)-LCmul and (f)-AirL (described in Section 2.1, with frequency responses in Figure 2). No anchors beyond the proposed scale were used this time.  
Two different versions of the headphones simulations were presented in this test. One without and the other with the distortion simulated with the method described in Section 2.4. These two versions of the same stimulus were presented each time to the participants. They have then to rate the distorted against the not distorted version of the same sound in a double-blind manner (A _vs._ B). In each trial, there was always a non-distorted version sound that acted as the known reference (C sound), which according to the recommendation [19] has to be compared to the A and B sounds.  
The number of stimuli of this test was then: 5 headphones simulations \(\times\) 2 versions (with and without distortion) \(\times\) 5 sound excerpts = 50 stimuli, presented in twenty five series of two stimuli plus the reference. All of these pairs were presented randomly to each participant.  
To perform the test, a GUI was developed according to the recommendation, which allowed participants to freely listen to each of the sounds to evaluate and the reference, as many times as they wanted.  
The five headphones under study were simulated (including distortion) to be reproduced with the reference headphones ((a)-REF, frequency response in Figure 2).  
This test was performed by the same 11 people of the previous Test 2; seven men and four women (21 to 37 years, with an average age of 30). The average runtime of the test was 16 min. Every participant did a training session before preforming this test, so all of them could listen to all of the stimuli and become familiar with the GUI and the assigned task.