Let's continue looking at the science of musical acoustics. In Chapter 2.1, we looked at the physics of sound, progressing from simpler sine waves to more complex sound waves. In this blog, we'll finish up the rest of the chapter, which looks at how the properties of sound change as a function of differences in musical instruments and venues.
The acoustics of musical instruments
We now move on to applying acoustic principles to musical instruments. The key principle that Tan and colleagues put across is that of coupled acoustics. The idea here is that, within most instruments, there exists at least two sources of vibration. While the first source is what generates the initial sound wave, the second vibrating device is responsible for amplifying that sound wave. The reason for this coupling is that the initial sound wave (activated when the performer vibrates the first device) often does not have enough energy to be propagated through a large space and be heard by others. To solve this, instruments have a secondary device, called a resonator, that is coupled to the first and that vibrates alongside it, thereby amplifying the initial sound wave.
The choice of resonator is also very important. The frequencies that are preferentially amplified will depend on the make-up of the resonator, including its size, shape, and material. In other words, from what we've learnt about complex sounds, the resonator not only amplifies the sound wave, but also contributes to its timbre and sound quality!
The authors then move on to describe the characteristics of different classes of instruments. Briefly, these include strings (including violins, guitars, and yes, pianos!), woodwinds and brass (which can be classified into those that produce edge tones and reed tones), as well as percussions (which typically receive short bursts of energy for sound to be produced). Across these instrument classes, the theme of coupled acoustics is consistently reiterated. Here, some examples of resonators of different instruments are provided, as well as changes in timbre as a function of changing the shape, size, and material of the resonator.
The acoustics of musical venues
The chapter ends with a discussion of architectural acoustics, which explores how designing buildings with acoustic principles in mind can not only affect sound clarity, but also the subjective evaluation, enjoyment, and satisfaction of music by audience members and performers. Specifically, the authors explore 3 areas, namely: direct and reflected sound, sound absorption, and reverberation time.
Direct and reflected sounds
Direct sound refers to the sound that, once produced by an instrument, travels straight to the listener without any disruptions. Meanwhile, reflected sound is any sound that reaches the listener after bouncing off one or more surfaces (of the musical venue). The time period at which the first reflected sound takes to reach a listener after the direct sound (known as the initial-time-delay-gap, ITDG) can affect both the quality and clarity of the music. When the ITDG is long, the music becomes muddy and unclear. Too short, and the music, while being clear, might sound too empty. Research suggests that an ITDG of approximately 15 milliseconds is optimal for most concert halls.
The shape of the concert hall also affects the musical experience for the listener. Here, audiences and performers typically prefer shoebox-shaped (Beranek, 2004) over fan-shaped venues. This is likely because the narrow walls of the former create stronger lateral sound reflections that, when paired with the direct sounds coming from the front, create a rich and full 'surround sound' experience for the audience members.
Sound absorption and reverberation
Other than being directly transmitted and reflected, sound can also be absorbed. Based on the material of the absorbing surface, high- or low-frequency sounds can be preferentially suppressed. Additionally, audience members (specifically the size of the audience and their seating arrangement) can affect the level of sound absorption.
Finally, sound can also reverberate through space. Here, reverberation refers to how long a sound persists after its initial production. More formally, reverberation time refers to the time it takes for a sound to decay by 60 decibels. The reverberance of a musical venue can have a nontrivial influence on musicians and performers. For example, excessively long reverberation times can make it difficult for musicians to hear what everyone is playing. Here, we can describe this either as a loss of horizontal clarity (i.e., not being able to distinguish successive tones, such as a melody) or a loss of vertical clarity (i.e., not being able to distinguish simultaneously played tones, such as when playing chords or singing harmony).
On the other hand, short reverberation times (where sounds disappear very quickly) can make it difficult for musicians to produce rich and full sounds that fill the space of the musical venue. In addition, short reverberation times also require higher degrees of accuracy from performers, since they can no longer hide their mistakes behind reverberation effects.
Concluding remarks
And that's a wrap for Chapter 2! One interesting remark by the authors is that because the musical experience is so subjective and psychological in nature, physical acoustics alone is unable to help us design a perfect acoustic environment that guarantees maximum satisfaction and enjoyment from listeners. Shoebox-shaped halls are a good example of this. A physical analysis of the shape actually predicts non-optimal conditions for audience members and performers. And yet, this shape is frequently the most highly rated by music critics, performers, and seasoned listeners alike. Hence, in the next chapters, we'll start to look at how sound gets into our auditory system, and how we come to perceive more psychological and subjective qualities of musical sound.
References
Beranek, L. L. (2004). Concert halls and opera houses: music, acoustics, and architecture. Choice Reviews Online, 41(08), 41–4446. https://doi.org/10.5860/choice.41-4446
Tan, S., Pfordresher, P., & Harré, R. (2010). Psychology of Music: From Sound to Significance. http://ci.nii.ac.jp/ncid/BB01824497
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