Acoustics Assignment
Assignment Brief
To choose a room and analyse the construction materials and subsequent surface areas of that room, and using the given formula, show an understanding in the calculations involved in solving absorption coefficients, reverb times and standing waves of any given space.
Introducing Acoustics
Before any formula can be applied, or calculations analysed, a firm understanding must be grasped of the main components involved in this assignment, namely:
Standing waves
Nodes / Anti-nodes
Fundamental frequency
Reverberation time
Absorption
Absorption coefficients
Frequency
Wallace Sabine
Let us consider each heading
Standing Waves
The modes of vibration associated with resonance in extended objects like strings and air columns have characteristic patterns called standing waves. These standing wave modes arise from the combination of reflection and interference such that the reflected waves interfere constructively with the incident waves. An important part of the condition for this constructive interference is the fact that the waves change phase upon reflection from a fixed end. Because the observed wave pattern is characterised by points, which appear to be standing still, the pattern is often called a 'standing wave pattern.'
Nodes / Anti-nodes
One characteristic of every standing wave pattern is that there are points along the medium, which appear to be standing still. These points, sometimes described as points of no displacement, are referred to as nodes. There are other points along the medium, which undergo vibrations between a large positive and a large negative displacement. These are the points which undergo the maximum displacement during each vibrational cycle of the standing wave. In a sense, these points are the opposite of nodes, and so they are called antinodes. A standing wave pattern always consists of an alternating pattern of nodes and antinodes.
Fundamentals.
The first possible standing wave is called the fundamental, alternatively called the first harmonic, the second possible standing wave is called the 2nd harmonic, the third possible standing wave is called the 3rd harmonic, and so on. The fundamental is the lowest frequency produced by the instrument or source.
Reverberation time
Reverberation is the combined effect of multiple sound reflections within a room. After the source sound stops, reverberation in a room causes the perceived sound to decay at a smooth and gradual rate. Since vibrations of sound only get weaker and weaker without ever reaching zero, we cannot define reverberation time as the time it takes a source to stop putting out sound. Instead, we define reverberation time as the time it takes for a decaying sound to drop 60 dB below its initial level, or the rate of decay of sound. A drop of 60 dB in sound energy is the same as a drop of one-millionth of a sound's original intensity, which is the maximum audible energy ratio likely to occur in a concert hall, for example.
The diagram below illustrates various desirable reverberations.
Reverberation time is important because it can affect how well you understand speech, and it can change the way music sounds. The effect on speech intelligibility is noticeable in a gymnasium or arena, where you often can't understand someone who is only 10 or 15 feet away from you. Once you are an adequate distance away from a person speaking to you, the sound level of the room's reverberant field is as loud as their voice. The effect of this could be replicated by having a large group of people standing beside you repeating what you say at different levels and at different times. All this additional babble coming back from the walls, floor and ceiling mask and muddle what is being said. When the reverb time is long enough, it not only masks the next syllable, but it can mask the next word. The preferred reverberation time range for a space intended for speech is 1.0 second, or less. As the reverberation time (or RT60) becomes longer than that, it becomes increasingly difficult to understand unaided speech, and if it is much shorter the room sounds very dead. As the RT60 gets much longer, 3, 4, or even 5 seconds, speech becomes impossible to understand. This can become a life safety hazard in some environments like swimming pools, and industrial settings, where warnings cannot be understood.
Assignment Brief
To choose a room and analyse the construction materials and subsequent surface areas of that room, and using the given formula, show an understanding in the calculations involved in solving absorption coefficients, reverb times and standing waves of any given space.
Introducing Acoustics
Before any formula can be applied, or calculations analysed, a firm understanding must be grasped of the main components involved in this assignment, namely:
Standing waves
Nodes / Anti-nodes
Fundamental frequency
Reverberation time
Absorption
Absorption coefficients
Frequency
Wallace Sabine
Let us consider each heading
Standing Waves
The modes of vibration associated with resonance in extended objects like strings and air columns have characteristic patterns called standing waves. These standing wave modes arise from the combination of reflection and interference such that the reflected waves interfere constructively with the incident waves. An important part of the condition for this constructive interference is the fact that the waves change phase upon reflection from a fixed end. Because the observed wave pattern is characterised by points, which appear to be standing still, the pattern is often called a 'standing wave pattern.'
Nodes / Anti-nodes
One characteristic of every standing wave pattern is that there are points along the medium, which appear to be standing still. These points, sometimes described as points of no displacement, are referred to as nodes. There are other points along the medium, which undergo vibrations between a large positive and a large negative displacement. These are the points which undergo the maximum displacement during each vibrational cycle of the standing wave. In a sense, these points are the opposite of nodes, and so they are called antinodes. A standing wave pattern always consists of an alternating pattern of nodes and antinodes.
Fundamentals.
The first possible standing wave is called the fundamental, alternatively called the first harmonic, the second possible standing wave is called the 2nd harmonic, the third possible standing wave is called the 3rd harmonic, and so on. The fundamental is the lowest frequency produced by the instrument or source.
Reverberation time
Reverberation is the combined effect of multiple sound reflections within a room. After the source sound stops, reverberation in a room causes the perceived sound to decay at a smooth and gradual rate. Since vibrations of sound only get weaker and weaker without ever reaching zero, we cannot define reverberation time as the time it takes a source to stop putting out sound. Instead, we define reverberation time as the time it takes for a decaying sound to drop 60 dB below its initial level, or the rate of decay of sound. A drop of 60 dB in sound energy is the same as a drop of one-millionth of a sound's original intensity, which is the maximum audible energy ratio likely to occur in a concert hall, for example.
The diagram below illustrates various desirable reverberations.
Reverberation time is important because it can affect how well you understand speech, and it can change the way music sounds. The effect on speech intelligibility is noticeable in a gymnasium or arena, where you often can't understand someone who is only 10 or 15 feet away from you. Once you are an adequate distance away from a person speaking to you, the sound level of the room's reverberant field is as loud as their voice. The effect of this could be replicated by having a large group of people standing beside you repeating what you say at different levels and at different times. All this additional babble coming back from the walls, floor and ceiling mask and muddle what is being said. When the reverb time is long enough, it not only masks the next syllable, but it can mask the next word. The preferred reverberation time range for a space intended for speech is 1.0 second, or less. As the reverberation time (or RT60) becomes longer than that, it becomes increasingly difficult to understand unaided speech, and if it is much shorter the room sounds very dead. As the RT60 gets much longer, 3, 4, or even 5 seconds, speech becomes impossible to understand. This can become a life safety hazard in some environments like swimming pools, and industrial settings, where warnings cannot be understood.
