Phase and Interference...

What is wave interference?
when two or more waves from different sources are present at the same time in the same space to create a new wave. The more useful part of this question is what happens and what types there are...
Constructive Interference
when the compressions and the rarefactions match they create a wave of higher intensity.
Destructive Interference
When the waves are out of phase, as in the opposite to constructive interference but the sound is louder in some places and softer in other places. Often leading to pulses and beats of sound.
What is a Phase?
Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.
What is Phasing?
The relationship between the waves to form the new wave, often used to describe the resulting sound, commonly constructive or destructive.
Dead Spots?
This is where the compression of one wave matches or is in phase with the rarefaction of another and so the sound is cancelled out and nothing is heard.
What else to consider?
Sound waves also change speed due to rarefaction from entering different mediums such as air to water, thus changing angle and bending the wave. this leads onto:
The critical angle..
When a waves entering angle reaches a certain point it's called the critical angle. The rarefaction is parallel to the dividing line between the mediums. The greater the difference in speed from the sound in the two mediums, the greater the critical angle.
What does this mean?
If the sound hits the new medium with any angle smaller then the critical angle it will not be able to enter and so be reflected from the dividing line. Even if it enters the medium some will be reflected because of waves rarely propagate in straight lines only.
Example...
A wave travelling through the air hits a building at 20 degrees, which for this example is less than the critical angle, then it is all reflected at 160 degrees as the total angle is 180 degrees of the wall.
If the wave is above the critical angle then most will enter the brick and speed up because the molecules are closer and so is refracted. The rest bouncing off because it won't all be entering with in the critical angle. 

Hope this all flowed nicely,
Thanks,
Matt Price

Reverberation Time

A reverberant sound in any closed space like a room or concert hall diminishes as the sound energy is absorbed by multiple interactions with the surfaces of the room. Though the speed it takes to diminish depends on the variable of the rooms surfaces, for instance a reflective  or "live" room will take longer than a very absorbent or "dead" room. The time for the sound to completely die away will depend how loud the sound is at the start and how well the observer hears. 


The standard reverberation time has be established as the time it takes for a sound source to diminish by 60 dB below its original level. This is established at 60dB from testing in auditoriums where the loudest crescendo in an orchestra is 100dB and typical background noise for a music-making area being around 40dB. 60dB is also roughly the dynamic range for orchestral music.


In terms of time it depends on the rooms use. In a classroom you want the room to have low reverberation time to keep articulation clear and reduce build up from people talking. In an auditorium you probably want a bit more so you can really feel the music. As a general rule you wont get long reverberation time in a small room because of how fast the energy is bouncing off the surfaces and being absorbed a bit each time.


Reverberation time is just one part in creating pleasing environments for sound, you also need to consider the absorption in terms of frequencies.


Thanks,


Matt Price

Parabolic Refectors

A Parabolic microphone is a microphone with a parabolic reflector fitted to it. This collects and focuses sound on the microphones head like a TV dish picks up satellite signals. This has many advantages for picking up and amplifying distant sounds that you want to isolate from the rest of the environment. A classic example is bird song because parabolic reflectors are very good at picking up high frequencies. this is also used in sports broadcasting, eavesdropping and espionage. 

The high frequencies are picked up more due to the direct physical laws of sound waves. This is because they only focus waves with a wave length much smaller than the diameter of the parabola. To obtain hi-fidelity sounds including the lower end frequencies (down to 20Hz) you need a parabola around 17 metres. This is because if we say the speed of sound is 342 m/s through the air (speed of sound) 342 ms / 20 Hz = 17 metres. Parabolic reflectors will sacrifice the lower end frequencies to be more manageable sizes.

Representation of how a reflector works with microphone facing the central point  for maximum pickup.

Thanks,

- Matt Price

Q+A: Basic Sound Questions...

This is going back to basics with what is sound anyway?...


What is sound and how is it created?
Sound is a vibration and/or a wave of air molecules caused by motion of an object. This wave is a compression wave where the molecules are bunched up like a wave in the sea, A sound wave is created when a series of these pressure changes/waves move through the air. This wave or waves speed depends on the temperature, e.g. at 15 degrees Celsius the speed is 340.276 m/s but at 25C the speed is 346.13 m/s. (Calculator here).


Through solids the wave will bounce back or echo/reverberate and the energy can change from acoustical to electrical energy which is how microphones and telephones are able to work.


What is frequency?
When drawing a sound wave, like with any wave there is a peak and a valley which vary in distance from each other. Sound sources vibrate at different rates or frequencies as they move through the air. We measure frequency in cycles per second or Hertz. This is named after Heinrich Rudolf Hertz (February 22, 1857 – January 1, 1894) who was a German physicist who was the first to satisfactorily demonstrate the existence of electromagnetic waves by building an apparatus to produce and detect VHF or UHF radio waves.
The faster an object vibrates the higher the frequency and thus higher the pitch of the sound. a common example is a tuning fork for A above middle C will vibrate at 440 times per second and so has a frequency of 440 Hertz.




What is amplitude? 
We already understand by now that sound is made by pushing air molecules together in varying degrees of strength, this is amplitude. for example if something is struck really hard the wave will carry some of that energy through the air and it will sound louder than if it is struck gently. Sound waves with the same frequency can have different amplitudes.


Feel free to add any comments if i haven't covered anything in enough detail.


Thanks,


- Matt Price