These pictures are generally too big to show in line, so view each one as you read each section.
White noise can be used with spectrograms to test frequency response. This is what the computer generated waveform looks like.
This is the spectral view of computer generated white noise. In a perfect recording, it will look like this.
A frequency sweep can be used with spectrograms and in waveform mode to test frequency response. This is what the computer generated waveform looks like. Note that the waveform is essentially a giant block on screen. This sweep starts with low frequency (0Hz) to the high phone frequency (4kHz) at a constant amplitude (which makes it look like a block). A perfect recording will also look like a very flat block (showing a constant recorded amplitude). If a newly recorded test doesn't look like a flat block (variable amplitude), it will have resonance and distortion problems at those frequencies where the amplitude deviates.
This is the spectral view of the frequency sweep in the lousy telephone audio format. This is as pure as it can get. Note the high noise floor from the low bit rate. An idealized recording will look just like this. The diagonal frequency line should be clean, straight, unbroken, and not deviating in intensity.
This is the same frequency sweep as above but in 16bit 44.1kHz CD quality format. Note the display can handle frequencies far above 4kHz. Also note the lack of a noise floor that is visible with the 8bit 8kHz version.