Basic Understanding and Preparation for Measurement

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This account describes the “basic knowledge of measurement” that should be understand before using the measurement instrument. Please read to become a measurement expert quickly.

1.Measurement Attitude

Before you start measuring, the points you wish to keep in mind are summarized so that you can gain accurate and more effective data.

Measure AccuratelyIt is important to aim at measuring accurately.
Don’t RushIf you rush, your attention may be distracted.
Follow each step and check your work steadily.
ObservationMake full use of your observation ability and five senses for the surroundings and measured results.
Measurement EnvironmentPay particular attention to noise, temperature and acoustic space conditions.
Well OrganizedBeing well-organized reflects the readiness of the operator.
Sort out DataSort out to make good use of measured data.

2. Points to Keep in Mind for Measurement

What is a good measurement for gaining good sound? Here are important points to achieve good measurements. Please keep them in mind and try to make improvements.

GoalWhat is measured for what purpose?
ItemsArrange appropriate measurement systems, connections and signals, etc.
ObjectDefine the under-test device and point
ConditionsSet up the conditions and level, etc.
AccuracyEnsure reliability and reproducible of measured data, measuring operation and environment.
ResultsHow are the measured results?
Can you estimate the meaning and reason of data?
ExperienceWhat to do next? How can you improve?

3.Basic Knowledge of Measurement

3-1 Types and Features of Measurement Sound Source

Let us understand the characteristics of each signal and select one that suits the purpose of your measurement.

Sine WaveClean sound without distortion. Fit in easily to the hearing. Easy to recognize rub & buzz by the ear.
Sine SweepA sine wave with a constant change in the frequencies .
Since measurements can be performed while smoothly changing the frequencies, it is suitable for detailed analysis of frequency responses of the under-test device.
Pink Noise*Noise in which the total band components are flat when viewed by the spectrum analyzer. Since all band components are included, it is used when you want to observe the frequency characteristics in realtime. Since the peak value of signal is about twice as much as the effective value, attention should be paid to saturation.
White Noise*This has the same power no matter what frequency band you take across the entire bandwidth. As a result, since the energy of 20Hz ~ 5kHz band, and the bandwidth of 5kHz ~10kHz, are almost the same, the input load to the speaker becomes higher at high frequencies. So be careful not to damage the tweeter during measurements. Also, since it is noise, similarly with pink noise, the peak value of signal is about 4 times as much as the effective value. So further care is needed.
Time Stretched Pulse
This is a sine wave whose frequency changes with a constant frequency width per unit time. A high S/N ratio can be gained using TSP in an impulse response measurement. As the signal is a sine wave, the amplitude of peak value is 1.4 times the effective value and the power is twice as much. This reduces concerns about the saturation of peak level. But note that power per unit time, is the same as white noise because the frequency change is linear.

3-2. Setting up Appropriate Measuring Level

Let us understand the effect of measuring level and achieve better results.

Meaning of Level: S/N Ratio

Let us consider what level to measure when testing. If the signal (S) is lower than the noise (N), you will measure a higher noise level than the actual sound. On the contrary, if the signal is too high, sound will saturate, and you will measure crushed characteristics rather than actual characteristics. The high/low level varies depending on the content of measuring signal, so you need to measure within an appropriate range that matches the signal being measured.

What Determines Dynamic Range

The maximum to minimum ratio of measurable values is called the dynamic range. Ensure a dynamic range as wide as possible while paying attention to the number of bits in the A/D and D/A converter, and the number of significant digits calculated internally.

Ambient Noise and Electric Noise

In the actual measurement, the dynamic range is determined by the ambient noise, and the “noise” that passes through the connected signal lines. Clearly, it becomes harder to measure with a signal below noise. In the actual measurement environment, you need to preliminary measure the noise level. If the measuring level fluctuates, you should suspect the presence of noise.

Highest Frequency of Effective Band

The converted sampling frequency determines the maximum frequency of test signals. When the sampling frequency is 44.1kHz or 48kHz, the highest frequency is approximately 20kHz. Note that frequency signals and noise that exceed this, do not appear as measured values. When analyzing harmonic distortion in distortion analysis, the maximum measurement frequency is defined according to the harmonic order and sampling frequency. For example, if the sampling frequency is 44.1kHz or 48kHz, the test limit is approximately 10kHz for the fundamental wave that is used to measure the 2nd harmonic, and approximately 7kHz for the fundamental wave that is used to measure the 3rd harmonic. Similarly, when the sampling frequency is 96kHz, the maximum frequency of test signal is 44kHz, the fundamental wave of 2nd harmonic is up to 22kHz, and the fundamental wave of 3rd harmonic is up to about 12 kHz.