Sound Localisation

Human ears are a unique pair of organs to look into. Without opening our eyes, we are able to detect sound from various positions around us. Being able to localize sound enables us to detect a vehicle approaching without the need for us to turn our head. Imagine a sound source is on the right hand side of you, sound will reach your right ear before the left ear. Also, the sound from the right ear has a higher level than the left ear because the head shadows the left ear.

There are two main ways for our ears to localize sound.

(1)Interaural Time Differences (ITD)

(2)Interaural Level Differences (ILD)

Low frequency sound which is approximately below 1500Hz gives rise to ITD.  In contrast, high frequency sound which is approximately above 1700Hz gives rise to ILD. This is because our head acts like a low pass filter and much of the high frequency sound gets reflected off thus reducing the sound levels being reached to the other ear. These mentioned frequencies vary as the size of the head differs from one person to another. For frequencies in between 1500Hz and 1700Hz, both roles ITD and ILD play a part in being able to localize sound.

Knowing the fundamental of sound localization, it is possible to use this critical information in audio production. In stereo recording, both panning and level are used to simulate a real live band performing in front of us.

Youtube is a good website to find some audio and acoustics related stuffs. Some of the users provide an easy way to understand the materials so that you would have some idea about it. The video below explains and demonstrate how sound localization is being tested.

Do you agree with my blogpost?  Feel free to comment it. 🙂

 

References

(1)  Sound Localization

http://www.open.edu/openlearn/science-maths-technology/science/biology/hearing/content-section-12.1

(2) Perception

http://www.lifesci.sussex.ac.uk/home/Chris_Darwin/Perception/Lecture_Notes/Hearing5/hearing5.html

Hypersonic sound

I’ve found an interesting video link in TED website which is related to audio. It is an approximately 15minutes video which explains about hypersonic sound. Loudspeakers tend to be poor in its directivity. However hypersonic sound is able to control and focus the sound to where the user wants it to be. A good analogy would be light bulb. Light bulb tends to emit light in all directions. We call this an unfocused light as it is perfectly diffused in all directions. However, when a reflector is placed behind the light bulb, it is able to focus the light towards the front of the lightbulb. Woody Norris has invented a loudspeaker which is able to focus sound directly in front of the loudspeaker. In his video, he shares more about how his invention works and the theory behind it.

I will highly recommend everyone to watch this video. 🙂

Sonic the Hedgehog (Supersonic)

For those who don’t know Sonic the hedgehog, he is an animated character who has the ability to run at supersonic speed. Have you ever heard of supersonic speed? It is the speed that is faster than the speed of sound in air which is approximately 343m/s. Some of the fastest airplanes are U.S Navy F/A 18, F-111 Aarkvark, F-15 Eagle, etc.

In relation to my Doppler effect blog post, a person on the ground would hear a high frequency noise as a commercial plane is approaching and a low frequency noise when the plane flies past him/her. However when an airplane breaks the sound barrier and flies faster than the speed of sound, he/she would hear a sonic boom which is a loud noise like thunder. Pressure wave fronts are formed at the front and back of an airplane. At supersonic speed, both the pressure wave fronts will be forced to merge together to form a large single wave. Therefore all the sound waves that would have normally propagated ahead of the plane are then combined together so at first one would hear nothing, and then one would hear the boom it creates. You can learn more about sonic booms by imagining the ripples that a boat leaves in the water.

At times, it is possible to see a cone-shaped clouds occurring behind the airplane. It is reacting to the temperature and pressure change induced by the airplane’s body by sliding past.

Do you agree about what I have written about supersonic speed? Feel free to make a comment about my blog post. 🙂

Fig 1. A bullet traveling faster than the speed of sound in air.

Fig 2. Cone-shaped clouds formed behind the aircraft.

 

References

The 10 fastest Military Airplanes.

[1] http://www.livescience.com/39829-fastest-military-airplanes.html

What is supersonic flight?

[2] http://www.nasa.gov/audience/forstudents/k-4/stories/what-is-supersonic-flight-k4.html#.U1ocbRb-uZw

Video of supersonic.

[3] https://www.youtube.com/watch?v=gWGLAAYdbbc

What causes a sonic boom?

[4] http://science.howstuffworks.com/question73.htm

Picture of a bullet traveling faster than the speed of sound in air.

[5] http://www.nasa.gov

Why do airplanes leave tracks in the sky?

[6] http://wonderopolis.org/wonder/why-do-airplanes-leave-tracks-in-the-sky/

Back to Basic (Waves)

What is acoustics? In a nutshell, it is the physics of sound. In acoustics, we deal a lot with waves. Waves?? Isn’t that a boring topic to learn? It’s just particles oscillating in space. Let me share with you how waves impact our life. Lets start with the basic question “What is a wave?”. Well a wave is an oscillation that travels through a medium. A wave can be longitudinal or transverse depending on the direction of its propagation. Now I am not going to bore you with all the physics and theory about waves. You can just Google them to know more about it.

Waves are used widely in all aspect of life. Firstly, human audible hearing range is taken approximately 20Hz to 20kHz.  For instance, we are able to hear the sound waves that is emitting from a loudspeaker. Frequencies lower than the audible hearing range are called infrasound whilst above it is called ultrasound.  Infrasound waves can be used to monitor earthquake or petroleum formations below the earth. On the other hand, ultrasound waves can be used for animal, chemistry, medical, destructive and others.  One of the many wonderful things ultrasound wave is able to do is to detect pregnancy. Lastly not to forget about sonar waves. They are used widely for underwater acoustics. It is a sound propagation used to navigate, communicate with or detect objects on or under the surface of the water.

http://en.wikipedia.org/wiki/File:Ultrasound_range_diagram.svg

So the next time you think about how dull waves are, do think about the many wonders it can do in our life.  🙂

 

Reference:

[1] http://www.physicsclassroom.com/class/waves/Lesson-1/What-is-a-Wave

The Doppler Effect

Have you ever come across a loud screeching siren from an ambulance or police car heading towards you and then it doesn’t get that screechy after it passes you? Do you ever wondered why is that so?

Well that’s because of this term Doppler effect. It is the apparent change in the frequency of a wave caused by relative motion between the source of the wave and the observer.

Lets go back to the basics of sine waves. It consists of compressions and rarefactions and can be illustrated with a simple sine wave. The amplitude of a sine wave will tell you how loud (sound pressure level) the wave is and the wavelength of a sine wave (the distance of a full cycle sine wave) is another way to represent the frequency of the wave. The longer the wavelength, the lower frequency it produces whilst the smaller the wavelength, the higher frequency it will be. Frequency is related to the pitch we hear. The higher the frequency, the higher pitch we perceive and vice versa.

Now back to the siren example. When the vehicle is stationary, the siren emits a constant sound wave to the observer. However when the siren is moving towards the observer, the sound waves are clustered together, resulting in a shorter wavelength and hence the frequency increases. However when the siren is moving away from the observer, the sound waves are spread out, resulting in a longer wavelength and hence the frequency is reduced. The picture below would show you a better understanding of what I meant.

http://www.physicsclassroom.com/class/waves/Lesson-3/The-Doppler-Effect

Now if you ever hear the siren coming when you are walking along the street, you will remember the Doppler effect and understand why this occurs. 🙂

Active Noise Control

Noise occurs in everyday parts of life. It can be heard everywhere from school, workplace, traffic, malls and even at home. For some of them it may be of low levels or low frequency that you don’t pay much attention to it, where for the others it may be of high levels and at mid-frequency which irritates your ear while hearing them. Nevertheless noise poses a negative impact in our life and harms our body. A high level of noise for a long period of time can induce hearing loss. There are ways to reduce noise levels happening around us. One way would be the use of active noise control where it is a method to reduce noise by adding another sound source, specifically to cancel the noise.

The noise is recorded by a microphone and the signal is then processed and inverted before transmitting to the loudspeaker. Once the noise from the source and the speaker is added together, it forms a destructive interference which results the wave effectively canceling each other out. With that, noise is reduced drastically in a noisy environment. However, in order to do that, the signal is processed in real-time so that the source and speaker noise are in anti-phase at every point in time.

It does seems that active noise control is the future technology to reducing noise occurring in our everyday life. In fact it has already been implemented in some parts of our life. Can you think of any? Well, the common ones are in airplanes, cars, factories and even headphones.

http://esmog-responders.co.za/tag/destructive-interference/

 

Below is a Youtube link that explains more about active noise control in a headphone.

Measuring modes on a string and room modes.

 

I did an experiment lab for Principle of Acoustics 2 weeks ago. We were conducting an experiment for both modes on a string and room modes.

Setup of Melde’s experiment

For the modes on a string (Melde’s experiment), I connected a string on Melde’s apparatus where one end was tied to the apparatus and the other was tied to a pan where the weights were placed in it to increase the tension of the string. The heavier the weight that is added to the pan, the higher the tension will be, and that will lead to a higher frequency being produced. You can think of the analogy of a guitar, except that the guitar string is tightened to increase the tension of the string. The aim of this experiment is to compare and explain the error between the measured and calculated values of mass per unit length, M. One of the main errors would be human error as we determine the resonant frequencies with our eyes and that the dial on the oscillator is very sensitive.

Setup of room mode experiment

For room modes, the speaker was placed at the corner of the reverberation chamber to excite all modes in the room. A pure tone will be oscillated from the oscillator to the speaker and we will have to find the first order axial mode for all three dimensions of the room (length, breadth and height). With the aid of a sound level meter, the resonant frequency would provide a maximum pressure at the edges of the room and a minimum pressure in the middle of the room. With the resonant frequency, it is possible to calculate the dimensions of the room and compare it with the measured value. Similar to the previous experiment, you should get a close but not perfect comparison due to some errors in the experiment. The error is due to the walls not being parallel to each other and the ceiling is slanted to one side, thus it is not rectangular shaped.

From both experiments conducted, there are nodes and anti-nodes occurring at resonant frequencies. However for Melde’s experiment, nodes always occur at the edges of the string, whilst for room mode’s experiment, anti-nodes always occur at the edges of the room. This is because of the different measurement that was actually measured. Melde’s experiment is measuring the displacement whilst room mode’s experiment is measuring acoustic pressure. The experiments conducted in this report are just a different way of looking at standing waves. It is possible to look at standing waves in either by its pressure or displacement. At boundary conditions, the pressure anti-nodes are found where the air particles are strongly compressed in the room. However at the same point, since the air never moves, it is also a displacement node. On the other hand, the pressure nodes are found where the resultant pressure never changes which occurs when the air particles are evenly spaced around the room. However at the same point, the displacement and velocity amplitudes are greater than anywhere else, which is also called the displacement anti-node.

Reference

• H.Kuttruff, Room Acoustics, 4^th edn., Spon press(2000).
• Lawrence E. Kinsler, Austin R. Frey, Alan B. Coppens and James V. Sanders, Fundamentals of Acoustics, 3^rd edn., Canada(1982)
• Donald E. Hall, Musical Acoustics, 3^rd edn., (2002).