Sound _ Psychoacoustics

The decibel is commonly used to quantify sound levels relative to some 0 dB reference. 

The reference level is typically set at the threshold of human perception.

A reason for using the decibel is that the ear is capable of detecting a very large 

range of sound pressure.

The smallest sound that human can hear is 20x10-6 Pascal.

The ratio of the sound pressure that causes permanent damage from short exposure 

to the limit that (undamaged) ears can hear is above a million. Because the power in 

a sound wave is proportional to the square of the pressure, the ratio of the maximum 

power to the minimum power is above one trillion. 

To what degree do we hear?

- 20Hz ~ 20 kHz

- From the smallest sound we can hear (0dB) to the range where we feel

   ‘physical pain’ (130dB)

- The increase by 6dB means the double of sound pressure. 

   However, doubling the subjective loudness needs 10 dB increase. 

   Human perception can recognize from at least 3dB change.

- The subjective loudness or the loudness human can perceive is dependent 

   on several complicated factors.

- One of them is that human ears are not equally sensitive to the intensity of sound 

    in all the frequency. They are the most sensitive around 2~5kHz.

Equal Loudness Contours.

These curves illustrate the relationships in and our relative lack of sensitivity 

to low and high frequencies as compared with middle frequencies.

In order for the loudness to be perceived equal throughout overall frequencies 

in the range human can hear, each frequencies needs different energy.  

From the graph of the equal loudness contours, we can  realize that low frequency 

needs more energy in order to be perceived with the equal loudness to the 

loudness of other range.50 Hz needs to be 15 dB higher in order to be perceived with 

the equal loudness to the 1kHz of 70dB

Hass Effect

- When two identical sounds from different directions arrives at human ears 

  with a very short time interval, the sound that arrived later is masked by the 

  sound that arrives previously.

- The Direction of sound source is decided according to the direction of 

   the sound arrived ahead to ears.