Lab 1: Sound Questions

Welcome to my second blog on the topic of "Sound" for my university module "Audio, Image and Video Processing". My first blog introduced the major principles of sound and was largely theoretical. In this blog, I will attempt to showcase some practical examples of what was covered in the first lecture in question and answer form. For usability purposes, please find the questions noted in black, whereas the answers will be noted in red.

In a recording room an acoustic wave was measured to have a frequency of 1KHz. What would its wavelength in cm be?

As explained in my first blog, there is a formula for working out wavelength. The above diagram shows the three way calculation method between Velocity, Frequency and Wavelength. Velocity is worked out by multiplying frequency and wavelength and hence is stationed above those in the diagram. This means in turn, working out wavelength and frequency would be done by dividing velocity by the other two elements.

In this particular calculation, the wavelength is the required product. The information in the question highlights that the frequency is 1KHz or 1000Hz depending on scaling. For the purpose of calculating, I am going to refer to in Hz form. The question also specifies that the sound is travelling through a room, more specifically through air. The speed of sound in air is roughly 333 metres per second. Therefore, to acquire the wavelength, you would divide 333 by 1000, which tells you the wavelength is roughly one third of a metre. I understand this may seem quite contrived for a simple calculation, but I was aiming to cover to cover all the particulars of working out wavelength. Noted below, is a more concise version of the calculation.

Velocity = 333 m/s Frequency = 1000Hz (1KHz) Wavelength = ?

Wavelength = Velocity / Frequency

Wavelength = 333 / 1000

Wavelength = 0.33 m 

If a violinist is tuning to concert pitch in the usual manner to a tuning fork what is the likely wavelength of the sound from the violinist if she is playing an A note along with sound from the pitch fork?

Again, working out the wavelength involves the equation where velocity is divided by the frequency, like the above calculation the velocity of sound in air is roughly 333 metres per second, but this question would require more research, so I visited a website http://liutaiomottola.com/formulae/freqtab.htm to find out more.

This website was able to tell me both the frequency and the wavelength of the A note, based on the velocity of sound at 340.29 metres per second, which although still roughly 333 metres per second as above, would give an answer which is slightly out.

Going by these conditions, the formula for calculating the wavelength of the A note would be as follows:

Wavelength = Velocity / Frequency
Wavelength = 340.29/27.5
Wavelength = 12.37 metres

How long does it take a short 1KHz pulse of sound to travel 20m verses a 10Hz pulse?

They will both take the same amount of time to travel, as Hz or KHz is the unit used to measure frequency and has no effect on the speed of sound.

Why are decibels used in the measurement of relative loudness of acoustics waves?

Sound is measured in decibels mainly so that a large as possible amount of values can be represented, discussed and graphed. Decibels measure the sound intensity level and are calculated using a logarithm, as the human ear uses a logarithmic scale to calculate sound.

 

If an acoustic wave is travelling along a work bench has a wavelength of 3.33m what will its frequency be? Why do you suppose that is it easier for this type if wave to be travel through solid materials?

Upon the presumption that the bench is made of steel the calculation would be as follows:

Frequency = Velocity / Wavelength
Frequency = 5000/3.33
Frequency = 1501.50

Sound travels quicker through solid materials as the molecules are closer together than they are in other mediums.

Sketch a sine wave accurately of amplitude 10, frequency 20Hz. Your sketch should show two complete cycles of wave. What is the duration of one cycle? What is the relationship between the frequency and the duration of one cycle?

Research the topic “Standing Waves”. Write a detailed note explaining the term and give an example of this that occurs in real life.

A standing wave, also known as a stationary wave, is a wave which remains in a constant position. This type of wave can occur in one of two situations, firstly if the wave is moving in the opposite direction from the medium and secondly if two waves travelling in the same direction interfere with each other.

If you were looking for a suitable comparison of the two, the first occurence (moving medium) would be seen often in river rapids, whereas the second occurence (opposite directions intersecting) would be more likely to occur in open ocean waves.

What is meant by terms constructive and destructive interference?

Constructive Interference is when two waves interfere with each other and the shape of the medium is determined by the amplitude of each separate wave. The shape of the wave is the sum of the amplitude of both of the waves which intersect. At areas where the waves do not intersect, they take the shape of the original wave.

Destructive interference is the opposite, when two waves do not intersect, cancelling each other out. The shape is determined by a subtraction calculations of both amplitudes.

What aspect of an acoustic wave determines its loudness?

Amplitude determines how loud a sound from an acoustic wave is. The higher amplitude, the louder sound.

Does sound travel under water? If so what effect does the water have? 

It travels quicker than in area due to the molecules being more compact under water. In water, sound travels at the velocity of (insert speed here) whereas in air it is only 333 m/s.