1. Frequency response:
First off, frequency response measurements are highly dependent on the measurement conditions. For example, how far away you measure, how long the measurement is, the size of the front baffle (the piece of wood you mount the Speaker to), diffraction, reflections, whether or not you flush mount the driver, etc.
The technique used here at DiyMobileAudio to measure frequency response is always the same. The Speaker is set on some heavy non-resonant surface and secured. There is usually no baffle, unless otherwise stated.
First a measurement is taken right up at the driver's cone. This is called the "nearfield measurement". The nearfield measurement gives an accurate measurement of the driver's low end response, free of room reflections and baffle diffraction. This line is typically represented in yellow. However, the nearfield measurement is only accurate up to
Frequency max = 4311 / driver diameter in inches. For a 7" driver that's typically 650hz.
Next, we take a series of "gated" measuremends. This just means that the measurement is cut short before any reflections have time to reach the microphone. We do this at 1/2 meter directly on axis with the driver, and both 30 and 60 degrees to the side of the driver. Now regardless of the distance, at DiyMobileAudio the gated measurement SPL will generally be represented at 1 watt, 1 meter.
So even though a driver is measured at 1/2m, the SPL will be adjusted so that it represents the true sensitivity at 1watt of power, at 1 meter distance.
Now some problems with the farfield measurement. First of all, having no baffle means the low end response will drop off rapidly. This is because low frequency wavelengths are long... and they will "wrap" around the speaker. So we have sound radiating both forward and behind the speaker, which causes a loss of spl. Another problem we have is diffraction. Sound waves will "bend" around objects in the room and around the speaker itself, causing dips and peaks in the frequency response.
So let's look at an example:
The yellow line is our nearfield measurement. Since this is a 7" woofer, we can assume it's accurate to about 650hz.
From there we notice the red line, which is our gated measurement at 1/2 meter. We can see that below 500hz the SPL drops off quickly, but we were expecting that due to lack of a measurement baffle.
Following the red line we notice a bump at 1.5khz, and another one at 5khz. This get's tricky. How do we know if this is diffraction caused by the lack of a baffle, or if it's part of the speaker's frequency response? Well, it helps to take a look at a number of other measurements for speakers of the same size. If they ALL have the same bumps at roughly the same places we can make a safe assumption that it's just diffraction. Also, bumps don't appear out of nowhere for no reason. Look at the distortion graphs. Does the bump coincide with increased distortion or energy storage? Is there a ripple in the impedance curve at those frequencies? If not, it's safe to say it's just diffraction.
Lastly, the 30 degrees and 60 degrees are off-axis measurements represented by the teal and blue line. They just show you the frequency response of the speaker off to it's side. Generally for a Car door, you listen at 45-80 degrees off axis.