# Homework Help: Sound level

1. Jun 16, 2006

### teng125

the pain threshold of your ears is about 130 phon.The sound levelclose to a jet (r=1m) when it takes off is about 150 phon.How far do you have to go away in order to lower the intensity level below the pain threshold??

anyone pls tell me the formula to use so that i can calculate as i can't find and don't know which formula to use.

thanx very much

2. Jun 16, 2006

### Hootenanny

Staff Emeritus
Intensity is defined as power per unit area;

$$I = \frac{P}{A}$$

Now, if we assume the jet engine is a point source, which emits sound equally in all directions thus forming a sphere of radius r and a point r distant from the source, then the surface area of a sphere is given by $4\pi r^2}$, thus the intensity at a point r from the source is given by;

$$I = \frac{P_{0}}{4\pi r^2}$$

Where P is the power of the source. Can you go from here?

3. Jun 17, 2006

### teng125

nono,pls explain a bit further
thanx.....

4. Jun 17, 2006

### Hootenanny

Staff Emeritus
The phon is basically just a unit of intensity, it is actually a unit of loudness but in this case can be treated as a unit of sound intensity. You have been given the intensity at 1 meter from the source, thus you can calculate the power of the source (which is constant). Now you just plug in your numbers for power and intensity and solve for r.

Can you go from here?

5. Jun 17, 2006

### teng125

okok.i'll try
thanx