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1) Audio Quality

1.6 Sound quality

In this white paper, the term ‘sound quality’ refers to the perceptional characteristics of the hearing sensation invoked by an audio signal. Using Crosby’s definition of quality, stating sound quality means stating in what degree the hearing sensation conforms to what we specified as requirements. And here things start to become tricky: every individual has different requirements.

In this white paper we propose the following requirement for sound:

Requirement for sound
An audio signal should satisfy either the expected or the preferred hearing sensation of an individual listener.

For the definition of a system’s sound quality, the sound quality of the original signal has to be considered as well. We will name the sound quality of the original signal ‘source quality’. The requirements for the sound source then read as follows:

Requirement for a sound source
An audio signal generated by a sound source should satisfy either the expected or the preferred hearing sensation of an individual listener without limitation or change by an audio system

The satisfaction of listening to a sound source - without a system in between ears and source - depends on individual hearing abilities and preferences, and also on the sound characteristics - or the ‘sound’ - of the source. For example, when listening to a solo violin performance, the hearing sensation is influenced by the composition played, the proficiency and virtuosity of the player, the characteristics of the violin. All these parameters together constitute the sound characteristics of the source. Although a statistical average appreciation can be found, for example by assessing the popularity of the solo violin performance by counting the number of persons who bought a concert ticket, every individual will assess source quality in a different way.

Knowing the requirements for the sound source, the sound requirements for the audio system can be defined:

Requirement for an audio system’s sound
The intended change of an audio signal by an audio system should satisfy either the expected or the preferred change in the hearing sensation of an individual listener with a given source sound.

In the case of the solo violin performance, the acoustics of the concert hall constitutes an audio system. If the performance needs amplification, then the PA system constitutes an audio system. In both cases, the audio system intentionally changes the audio signal produced by the sound source, contributing positively to the hearing experiences of the audience: the concert hall adds reverberation, the PA system adds loudness.

Sound requirements for sources and systems use perceptional characteristics ‘warmth’, ‘transparency’, ‘definition’. Note that in real life, multiple sound sources as well as multiple audio systems are involved. In this white paper we propose the following definition for sound quality:

Sound quality
The degree of satisfaction of the expected or the preferred hearing sensation of an individual listener as a result of hearing an audio signal.
Source sound quality
The degree of satisfaction of the expected or the preferred hearing sensation of an individual listener as a result of hearing an audio signal from a sound source disregarding the limitation or change by an audio system.
System sound quality
The degree of satisfaction of the expected or preferred hearing sensation of an individual listener as a result of the intended change of an audio signal by an audio system with a given source sound.

In words: if sound quality and audio quality are summarized as a percentage ‘Q’ - with 0% as ‘minimum quality’ and 100% as ‘maximum quality’, the sound quality of an audio signal experienced by a listener is the product of the source’s sound quality, the system’s audio quality and the system’s sound quality:

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