Sound Waves, Acoustic Treatment and Bass Placement

Sound Waves

1. What is a sound wave

Sound is produced when something vibrates. Vibrations in air are called traveling longitudinal waves, which we can hear. Sound waves consist of areas of high and low pressure called compressions and rarefactions, respectively.

The shaded bar above it represents the varying pressure of the wave. Lighter areas are low pressure (rarefactions) and darker areas are high pressure (compressions). One wavelength of the wave is highlighted in red. This pattern repeats indefinitely. The wavelength of voice is about one meter long. The wavelength and the speed of the wave determine the pitch, or frequency of the sound. Wavelength, frequency, and speed are related by the equation speed = frequency * wavelength. Since sound travels at 343 meters per second at standard temperature and pressure (STP), speed is a constant. Thus, frequency is determined by speed / wavelength. The longer the wavelength, the lower the pitch. The 'height' of the wave is its amplitude. The amplitude determines how loud a sound will be. Greater amplitude means the sound will be louder.

Vibration inside a tube forms a standing wave. A standing wave is the result of the wave reflecting off the end of the tube (whether closed or open) and interfering with itself. When sound is produced in an instrument by blowing it, only the waves that will fit in the tube resonate, while other frequencies are lost. The longest wave that can fit in the tube is the fundamental, while other waves that fit are overtones. Overtones are multiples of the fundamental. The areas of highest vibration are called antinodes (labeled 'A' on the diagram), while the areas of least vibration are called nodes (labeled 'N' in the diagram). In an open pipe, the ends are antinodes. However, in a pipe closed at one end, the closed end is a node, while the blown end is an antinode. Thus, closed pipes yield only half the harmonics.

If a string that is fixed on both ends is bowed or plucked, such as in a violin, vibrations are formed that are in a standing wave pattern, having nodes at the fixed ends, and an antinode in the center. Several harmonics are also produced, in a similar way to the standing wave.

2. What are the properties of a sound wave

• They are longitudinal in nature - that means the disturbances are parallel to the direction of motion of the wave.
• The cannot travel through a vacuum. They need a medium to travel through.
• They travel fastest through solids and slowest through gases.
• They obey the wave equation 
  $v=f\lambda$
   and their speed through air at sea level is approximately 
  $340m/s$
• They can be reflected and obey the law of reflection.
• They can be refracted, diffracted, undergo superposition (interference), etc.
• Their frequencies may appear different to observers in relative motion due to the Doppler Effect.

3. What makes us perceive pitch 

Periodic - pitch

Randomised - not pitch

*Drums do not perceive them as a pitch because their overtones are irrational 

4. What are the common acoustic issues in a studio

- Overtones  (Overtones go up depending on each instrument)

- Flutter Echoes 

- Ambience 

- Reflections 

5. Where to place a bass guitar and amp

A: 440

E: 330

41 for a four string bass

* If you want more attack on a bass guitar, turn up the 550Hz and above