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Sound wave theory 08
Speed of sound

Level of challenge Easy

 

Welcome to this tutorial on the speed of sound.

 

It is useful for us to understand how sound travels through different mediums such as the air and solid objects like walls. For example, we may wish to estimate the differing phase relationships between microphones placed at different distances from a source, or the effectiveness of sound proofing materials.

 

Caption - How the speed of sound varies

Sound waves travels at different speeds through gases, liquids and solids. This is because their molecules are spaced differently. For example, air molecules are much further apart than the tightly compacted molecules of steel. The closer the molecules are, the less distance they have to travel, or vibrate, to transmit energy to one another, and the faster the sound wave can travel.

 

Caption - Speed of sound in air

Sound travels relatively slowly through air at around 340 metres per second, which is equivalent to approximately 1 foot per millisecond.

 

Although not important for sound recording purposes, it should also be understood that the speed of sound is affected by altitude and temperature.

 

Caption - Speed of sound through solid objects

Sound travels much faster through a solid object. Through steel it travels at 5,100 metres per second.

 

It is important to understand that whilst sound travels faster through solid objects, its energy is absorbed faster than in a gas or liquid. It will therefore be either partially or completely absorbed. The amount of absorption depends on the nature and size of the object and the frequency of the soundwave.

 

Caption - Speed of electrical and digital soundwaves

Once converted into an analogue electrical, digital electrical , or optical signal, sound travels much faster. The speed of an electrical soundwave signal is approximately 200,000 kilometres per second.

 

It is therefore possible to accelerate the speed with which a soundwave is transmitted from one location to another.

 

Lets consider the example of a drummer recording in a studio. Assuming their ears are approximately 2 and a half feet from their snare drum, sound will take approximately 2 milliseconds to reach their ears.

 

Welcome to this tutorial on microphone polar pattern diagrams. The polar pattern diagram is used to show the sensitivity of a microphone to sound arriving at its capsule from different directions. They are primarily used by microphone manufacturers and displayed in a microphone's manual or technical specification. They can help us decide whether or not to purchase a microphone, and also to determine the best microphone to use for a recording. Caption - Diagram elements The polar pattern diagram takes the form of a two-dimensional contour map showing the microphone's output at different angles of incident of a sound wave. The centre of the diagram represents the microphone's capsule. This point is surrounded by a series of concentric circles which function as a scale with which to plot the output of the microphone. This scale is variable but typically shows 5 decibel increments. 0 degrees indicates the direction in which the capsule in facing, which is usually the angle at which it is most sensitive to sound and produces the greatest output. The sides of the microphone are shown at 90 and 270 degrees, and it's back at 180 degrees. A thick line plots the pattern of sensitivity. This plot is created by playing a test tone at a reference level through multiple loudspeakers which surround the microphone. The plot therefore shows how well the microphone picks up sound from all directions. The greater the plot from the centre the greater the output of the mic at that angle. Because a microphone's sensitivity can change according to frequency, the diagram may show different plots for different reference tone frequencies. Typically these will be at a range of frequencies such as 8kHz, 1kHz, and 200Hz. Caption - Common polar patterns The plot of sensitivity for a given microphone will depend on it's design, characteristics and quality. Most plots show variations on 6 common designs. Caption - 1. Omni-directional The omni directional microphone is designed to be equally sensitive to sound arriving from all directions. Caption - 2. Figure of eight The figure of eight microphone is designed to be equally sensitive to sound arriving from the front and rear. Caption - 3. Cardioid The cardioid microphone is designed to be sensitive to sound arriving from a wide range of angles at the front. Caption - 4. Hyper cardioid The hyper cardioid microphone is designed to be sensitive to sound arriving from a still narrower range of angles at the front. Caption - 5. Shotgun The shotgun microphone is designed to be sensitive to sound arriving from a distance at a very narrow range of angles.

 

But if the drum is close mic'd, and the drummer is using headphone to monitor the sound from the mic via a microphone pre-amplifier and headphone amplifier, the sound will be heard much sooner, with a delay of only 12 microseconds.

 

Welcome to this tutorial on microphone polar pattern diagrams. The polar pattern diagram is used to show the sensitivity of a microphone to sound arriving at its capsule from different directions. They are primarily used by microphone manufacturers and displayed in a microphone's manual or technical specification. They can help us decide whether or not to purchase a microphone, and also to determine the best microphone to use for a recording. Caption - Diagram elements The polar pattern diagram takes the form of a two-dimensional contour map showing the microphone's output at different angles of incident of a sound wave. The centre of the diagram represents the microphone's capsule. This point is surrounded by a series of concentric circles which function as a scale with which to plot the output of the microphone. This scale is variable but typically shows 5 decibel increments. 0 degrees indicates the direction in which the capsule in facing, which is usually the angle at which it is most sensitive to sound and produces the greatest output. The sides of the microphone are shown at 90 and 270 degrees, and it's back at 180 degrees. A thick line plots the pattern of sensitivity. This plot is created by playing a test tone at a reference level through multiple loudspeakers which surround the microphone. The plot therefore shows how well the microphone picks up sound from all directions. The greater the plot from the centre the greater the output of the mic at that angle. Because a microphone's sensitivity can change according to frequency, the diagram may show different plots for different reference tone frequencies. Typically these will be at a range of frequencies such as 8kHz, 1kHz, and 200Hz. Caption - Common polar patterns The plot of sensitivity for a given microphone will depend on it's design, characteristics and quality. Most plots show variations on 6 common designs. Caption - 1. Omni-directional The omni directional microphone is designed to be equally sensitive to sound arriving from all directions. Caption - 2. Figure of eight The figure of eight microphone is designed to be equally sensitive to sound arriving from the front and rear. Caption - 3. Cardioid The cardioid microphone is designed to be sensitive to sound arriving from a wide range of angles at the front. Caption - 4. Hyper cardioid The hyper cardioid microphone is designed to be sensitive to sound arriving from a still narrower range of angles at the front. Caption - 5. Shotgun The shotgun microphone is designed to be sensitive to sound arriving from a distance at a very narrow range of angles.

 

Caption - Relationship between frequency, wavelength and speed of sound

Some simple equations relate the speed of sound to frequency and wavelength ...


c = w x f (Speed of sound = wavelength x frequency)
f = c / l (Frequency = speed of sound / wavelength)
w = c / f (Wavelength = speed of sound / frequency)

 

Key: Speed (c), Frequency (f), Wavelength (w)

 

 

Caption - Thanks for watching

The script for this tutorial, with accompanying screenshots, can be found at projectstudiohandbook.com 

 

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Thanks for watching.

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