040 REPORT
expectations for sound in terms of its time, level, frequency and direction. In 2006 a Spherion Workplace snapshot found that one-third of all workers listen to music on headphones at work and since then, headphone sales have continued to increase so it is reasonable to assume that this percentage has also increased. The proximity of headphone drivers to the ear ensures that no refl ections from the room are heard and the lack of any variation in the time of sound arriving results in a very clear sound with no natural reverberation. As many people listen to sound on headphones at home it is unsurprising that one of the most commonly specifi ed criteria for audio systems is a measure of clarity - the speech transmission index (STI). This also considers the frequency content of the sound and is measured by playing a known set of modulated signals into the room, and comparing the sound recorded at a receiver to the test signal. STI values range from zero to one with 0.5 normally being considered as an acceptable benchmark and one being considered perfect. This score of one would be an excellent, if unlikely achievement for a system designed for voice alarms and occasional announcements. A perfect STI score indicates that the room has no effect on the output from the loudspeaker, but we know that we expect to hear reverberation from a space, and the absence of refl ections can be uncomfortable. This apparent anomaly in how we assess the quality of a sound is normally resolved by the addition of artifi cial reverberation, as it is much simpler to add apparent refl ections to a sound when required, than to remove it when it isn’t.
LEVEL
Level is often only simplifi ed to describe the maximum loudness of an audio system and peak sound pressure level (SPL) appears on many specifi cations. This only really gives a small portion of the picture as it is also important how quiet the system can go and still be audible, and what the average loudness of the source material will be. For an architectural audio system, the dynamic range will be defi ned by the difference between the peak SPL achievable and the noise fl oor of the building. The noise fl oor of the building will vary greatly depending on its use but roughly as- suming an average building has a background noise of around 50dB SPL at 1kHz, then a loudspeaker system capable of producing over 140dB would be required to fully replicate the variation between the loudest and quietest signal found on a CD. By careful design of ventilation systems, and using larger loudspeakers, this is just about achievable but at great effort and expense. While in building and audio system design there is in an apparent aspiration to increase the dynamic range, the opposite has occurred in music production and broadcast. Since the 1980s the average loud-
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ness in these areas has increased so that there is often very little dynamic range. The increased availability of compressors and revolution of the MP3 means that it is very unusual to listen to music with a large dynamic range. We then must consider, how much signal to noise is required from the audio system and is this likely to be utilised by the source that will be put through it? Interestingly this expands to beyond how loud the loudspeakers need to be, but also into the system design for playback and transport particularly in selecting the bit depth of the digital audio system.
DIRECTION
In the acoustic design of concert halls, much emphasis is put on refl ected sound that arrives to the side of the listener, known as lateral refl ections. With the increased availability of surround sound systems it is possible to recreate these refl ections in both a professional and domestic environment. The directionality of re- fl ections should not be confused with localisation of sound - its function is to create a sense of envelopment and enhance the listener experience. This is not to say that localisation is unimportant, particularly in systems in performing arts buildings where it is very important that the sound appears to come from the stage. As it is impracti- cal to have a loudspeaker in exactly the same position as a performer this localisa- tion is achieved by the psychoacoustic phenomena that allows us to use very small level and time of arrival differences between our two ears in order to localise sound. This is the same technique used in a two-channel stereo system, and interestingly means sound localisation is a function of time and level rather than the actual direc- tion the sound arrives from. In contrast, for cinema systems the centre loudspeaker is often used for speech, so the dialogue appears as a mono frontal source. This has been mirrored in acoustic design with large acoustic refl ectors providing a strong frontal component. In installed sound systems it is sometimes less important where the sound appears to be coming from, however, it is normally important to consider how sound will be localised, whether there is a preference to lateral or frontal sound and how enveloped the listener should feel.
INTEGRATION
Considering the time, level, frequency and direction of the produced sound is a very useful starting point for the design and analysis of architectural audio systems, but signifi cant integration with other design disciplines is also required. When selecting locations for loudspeakers the architect and client will normally have a view on the aesthetic required, and the preferred locations. It is easy for this to quickly become
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