Audio Precision Portable One Plus Access User Manual

To make a THD+N measurement, we begin by having the device create or
reproduce a single sinewave called the ‘fundamental tone’, which represents the
desired sound. Some of the unwanted sounds will also be created, so the frequency
spectrum may look like this:
The signal is measured with a voltmeter at this point. The measurement will be
dominated by the amplitude of the fundamental tone, since it is at a high amplitude
relative to the unwanted signals.
The signal is then run through a narrow bandreject filter to eliminate the fundamental
tone. The spectrum then looks like this:
The fundamental tone has been reduced to a level where it will be insignificant. The
filtered signal is then measured with another voltmeter, and the reading is presented
as a ratio of the two voltmeter readings.
Notice that the final measurement is ‘wideband’. It measures the total of all the noise
sources within the frequency range of interest. It is not specific to the frequency of
the fundamental tone. While some devices may create more or less harmonic
distortion for different fundamental frequencies, the other sources of unwanted
sound will contribute to the reading no matter what fundamental frequency is used.
F
un
d
amenta
l
Tone
Hum
Interference
Distortion
Noise
d
B
V
-140
20 20k50 100 200 500 1k 2k 5k 10k
Hz
+0
-120
-100
-80
-60
-40
-20
-140
+0
-120
-100
-80
-60
-40
-20
d
B
V
20 20k50 100 200 500 1k 2k 5k 10k
Hz
F
un
d
amenta
l
Tone Greatly
Reduced
4 Operation
Operation Controlling the Analyzer : THD+N (Total Harmonic Distortion plus Noise)
4-30 Portable One Plus Access User's Manual
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Audio Precision Portable One Plus Access Specifications

General

Brand	Audio Precision
Model	Portable One Plus Access
Category	Test Equipment
Language	English