In an audio system in a perfect world, all frequencies across the audible spectrum would be reproduced at exactly the same level with respect to all other frequencies. To say the least, this is not exactly a perfect world and the car audio environment is even worse. This means that even if you have a speaker system which will produce a perfectly flat response in an anechoic chamber, when it is installed in a vehicle the frequency response will be significantly less than perfect.

A spectrum analyzer (RTA) graphically indicates the level of the audio signal at various (usually predetermined) points. Many of the most popular RTAs are designed to indicate the level of audio at 30-33 different frequencies across the audio spectrum (also known as a 1/3 octave spectrum analyzer). Most of the portable units use LEDs as level indicators but as portable computers are getting more affordable, some people are moving towards PC based units.

The diagram below is what an LED based spectrum analyzer might look like. As you can see, all of the lit LEDs are centered at 0dB. This would indicate that there was no input or the input at each of the frequencies was exactly the same or at least within 1 dB of each other.

The diagram below is more likely what you would see from a system before it was fine tuned. There may be more dips and peaks and they would likely be at different points but this should give you an idea of what you might see.

Range Settings:
Spectrum analyzers have range settings to adjust the sensitivity to the test conditions. The range settings on the following analyzer determines how much signal level it will take to bring the LED indicators to the zero line. In this example the input signal would be the same for each of the range settings. On '95dB' it takes only 95dB of signal to bring the LEDs to zero on the display. On 100dB, it is slightly less sensitive and would take more signal to bring the display to zero dB. It moves down (relative to the 95dB setting) because the signal doesn't have enough energy to move the LEDs to the same level as before. The 105dB setting is even less sensitive. You can see that part of the display has fallen off of the 'screen'. This setting wouldn't be good for the signal we have. We would have to use one of the more sensitive settings to be able to see the entire display.