Model-8121C generates " Pseudo Random Noise " by using Digital Technique,  this is the most  advanced method for generating the audio white noise.
The conventional noise generator practically generates only the white noise & pink noise, it needs to combine with weighting filter,power amplifier and true rms voltmeter ( a total of 4 circuits) before it can be used to test the loudspeaker. 8121C consists of all the aforementioned 4 circuits, it can generate pink noise, white noise & 3 other kinds of weighted noise that comply with the standards of the countries mentioned .
If special noise spectrum is needed, the user can insert a self-made weighting filter between the noise signal output and power amplifier input terminals.
Usage :
1.  Continuously  load  test  of  the  loudspeaker, headphone,speaker  system and crossover.
2.  Defines the rated input power of the under  testing sample based on the result of continuously  load  test.
3.  Injects noise signal to the loudspeaker , for measuring  the  sound pressure sensitivity ( dB value )  of  the  loudspeaker.
4.  Measures  the  frequency  response  curves of the loudspeaker by combining with the Audio  Spectrum  Analyzer.

Note :
Model-8121B is the old Model, which have already ceased to produce , it is now being replaced by Model-8121C. The only difference between the Model-8121C and Model-8121B is Model-8121B uses the old EIA 426A standard weighting filter (EIA-426A have been abolished) ; while 8121C uses the weighting filter that complies with the newly required EIA 426B standard. Sunlight also provides: Model-8121FT-1 Noise Filter, which complies with IEC-268-1 specification. (It must combine for use with either Model-8121B or Model-8121C).

 Model - 8121FT - 1