negative decibels?

[confusedcat]

I'm doing a science project where I am measuring the volume and pitch of cats around my neighborhood. I am trying to figure out wether male or female cats have louder purrs, and which have higher pitch purrs. I'm using audacity, but I am having trouble getting a straight number for pitch. When you go on analyse, you can get a graph that kind of shows you the pitch, but all the decibels on it are negative. I have had this problem with other computer programs as well, and I'm trying to figure out how it works. I don't get how negative decibels are even possible, since 0 dB is no noise. I tried to research it, but I still don't get it. Could you maybe explain this to me? Also, how do I make the graph into one flat number? Thank you.

[rmarz]

confusedcat - It's easy to understand your confusion, as the decibel can mean several things in several contexts. Primarily, an expression of Zero db does not necessarily mean 'zero' signal amplitude. In fact, various standards assign a measurable signal strength equal to 0 db. 0 db is just a reference point to compare to other measurements. Here are a few examples:

Zero-dB standards:

Audio industry: 0 dB = 1 mW in 600 Ohm resistance
(Measurements to this standards use the unit symbol dBm)

Television industory: 0 dB = 1 mV rms across 75 Ohm impedence

Radio engineering: 0 dB = 1 mW in 50 Ohm impedence
or 0 dB = 1 uV/m for electro-magnetic field strength

The use of the decibel scale is more to measure ratios of power or voltage over several magnitudes. Because it is a logarithmic scale, it is possible to express wide variations of power in small numbers. This makes it useful in sound measurement, as our hearing is very non-linear. The decibel scale makes it easier to manipulate these non-linear numbers. For example, when dividing large numbers by small numbers (often in several decimal point precision) its easier to subtract one db level from another, which is the same as adding and subtracting exponents to multiply and divide as in scientific notation. The difference in two db readings can be referenced easily to power ratios.

Here is site which will give you a table as well as an interactive way of calculating db numbers.

http://www.sengpielaudio.com/dB-chart.htm

Rick Marz

[deleted-71823]

An easier way to deal with the data is to take the absolute value of decibels but make sure you explain this in your conclusions. I do not quite understand what you mean by making the graph into one flat number? In this case there are many ways you could graph your results. Since your main experiment is to see whether male or female cats have louder purrs you would probably want to just graph the averages. You would want to include all the charts so that anyone could see all the date but sticking to just averages will make it easier to understand your results. Just a tip but you could do the three different breeds of cats and have five females and five males for each breed, meaning you would have to test 15 cats. This seems like a lot but would make your results more accurate.