Digital Sound & Music: Concepts, Applications, & Science, Chapter 4, last updated 6/25/2013
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the sound’s intensity and helps turn harsh reflections into more pleasant reverberation. Usually a
combination of absorption and diffusion is employed to achieve the optimal result. There are
many unique types of diffusing surfaces and panels that are manufactured based on mathematical
algorithms to provide the most random, diffuse reflections possible
Putting these concepts together, we can say that the amount of time it takes for a
particular sound to decay depends on the size and shape of the room, its diffusive properties, and
the absorptive properties of the walls, ceilings, and objects in the room. In short, all the
aforementioned properties determine how sound reverberates in a space, giving the listener a
"sense of place."
Reverberation in an auditorium can enhance the listener's experience, particularly in the
case of a music hall where it gives the individual sounds a richer quality and helps them blend
together. Excessive reverberation, however, can reduce intelligibility and make it difficult to
understand speech. In Chapter 7, you'll see how artificial reverberation is applied in audio
processing.
A final important acoustical property to be considered is resonance. In Chapter 2, we
defined resonance as an object’s tendency to vibrate or oscillate at a certain frequency that is
basic to its nature. Like a musical instrument, a room has a set of resonant frequencies, called its
room modes. Room modes result in locations in a room where certain frequencies are boosted
or attenuated, making it difficult to give all listeners the same audio experience. We'll talk more
about how to deal with room modes in Section 4.2.2.5.
4.2 Applications
4.2.1 Working with Decibels
4.2.1.1 Real-World Considerations
We now turn to practical considerations related to the concepts introduced in Section 1. We first
return to the concept of decibels.
An important part of working with decibel values is learning to recognize and estimate
decibel differences. If a sound isn’t loud enough, how much louder does it need to be? Until you
can answer that question in a dB value, you will have a hard time figuring out what to do. It's
also important to understand the kind of dB differences that are audible. The average listener
cannot distinguish a difference in sound pressure level that is less than 3 dB. With training, you
can learn to recognize differences in sound pressure level of 1 dB, but differences that are less
than 1 dB are indistiguishable to even well-trained listeners.
Understanding the limitations to human hearing is very important when working with
sound. For example, when investigating changes you can make to your sound equipment to get
higher sound pressure levels, you should be aware that unless the change amounts to 3 dB or
more, most of your listeners will probably not notice. This concept also applies when processing
audio signals. When manipulating the frequency response of an audio signal using an equalizer,
unless you’re making a difference of 3 dB with one of your filters, the change will be
imperceptible to most listeners.
Having a reference to use when creating audio material or sound systems is also helpful.
For example, there are usually loudness requirements imposed by the television network for
television content. If these requirements are not met, there will be level inconsistencies between
the various programs on the television station that can be very annoying to the audience. These
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