# Influence of the Quality of Consumer Headphones in the Perception of Spatial Audio
## 4 Test 2. Frequency Response about Quality and Spatial Impressions  
### Test Description  
In this test, participants listened to some excerpts of sound with headphones and rated their quality and their sound spatial image. These different headphones were simulated as described in Section 2.3 by means of the convolution of their frequency responses with the stimuli sounds, and all of them were reproduced with the reference headphones. Due to the fact that different frequency responses produce noticeable effects, the perceptual test was designed according to the recommendation International Telecommunication Union, recommendation by Radiocommunication sector (ITU-R) 1534-2 [17], which describes the MUltiple Stimuli with Hidden Reference and Anchor (MUSHRA) perceptual test. This kind of test describes a method to assess intermediate quality audio systems and also all of the requirements needed to accomplish the test with rigor. Besides, this test sets a zero to 100 continuous scale (zero-bad; 100-excellent) to evaluate quality and other parameters of sounds and systems, always using a reference sound. All systems are compared to a reference of maximum quality, and the different systems are also compared between them. Two different tasks were evaluated during the test by the participants. The first task was to indicate the quality of the sound with respect to the reference. The second task was to evaluate the spatial impression (locations, sensations of depth, immersion, reality of the audio event) [18] with respect to the reference. Five different excerpts of audio (12 to 14 s) were employed as source material (see Table 1), and all of them were reproduced simulating the different headphones under study. All of these sound fragments were chosen by their spatial, stereophonic and timbral attributes. In this test, five headphones simulations were done, corresponding to headphones (b)-HQop, (c)-MQcl, (d)-BDso, (e)-LCmul and (f)-AirL (described in Section 2.1, with frequency responses in Figure 2). Each of the five sound excerpts previously mentioned were reproduced by the virtual headphone simulation described in Section 2.3. A virtual headphone simulation for each sound was presented randomly in series to the listeners, as well as a hidden reference ((a)-REF) and also two anchor signals. The first Anchor signal (ANC1) was a 7-kHz low pass filtered version of the sound (according to the mid-quality anchor of the ITU recommendation 1534-2 [17]), and the second Anchor signal (ANC2) was a monaural version of the sound. This second anchor was determined to set a reference for the spatial impression question. To perform the test, a GUI was developed in MATLAB according to the recommendation [17], which allowed participants to freely listen to each of the sounds and to the reference, as many times as they wanted. The different sound fragments were presented randomly as a series with all of the different headphone simulations, to compare to the reference sound. Once the participant had scored all of the simulations of a series, a new sound excerpt was presented to be evaluated. This process was repeated twice, once for each question of the test (the first about quality and the second about spatial impression), with a pause in between. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Artist** & **Track** & **CD** & **Description** \\ \hline [PERSON] & _Haugebonden_ & Women en Mi & female voice and guitar \\ [PERSON] & _Zambra Gitana_ & Canción Andaluza & male voice and guitar \\ [PERSON] & _Night Boarders_ & OST The Mummy & high dynamic orchestral \\ The Chad Fisher Group & _Basin Street Blues_ & live & jazz (binaural) \\ Smashing Pumpkins & audience sound & live & audience and drums (binaural) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Music program used for listening Tests 2 and 3. The number of stimuli of this test was: (5 headphones simulations + 1 hidden reference + 2 anchor signals) \(\times\) 5 sound excerpts = 40 stimuli, presented in five series of eight stimuli plus the reference.