Rew acoustic test (IV): test principle of rew

More articles please pay attention to WeChat official account ：「 Smart speaker design 」

～～～～～～～～～～～～～～～～～～～～～～～～～～～～～～～～～

1. test signal

REW Logarithmic sine sweep test method is adopted . The logarithmic sweep signal is a signal that starts with a low frequency signal , A signal whose frequency increases steadily in logarithmic form to a high frequency . What makes the sweep logarithmic is the rate at which the frequency increases , It requires a fixed octave time （ for example , Sweep slave 20Hz To 40Hz Time and from 40Hz To 80Hz At the same time ）.

2. Test principle

In order to make measurements , We need a sound source （ Speakers or subwoofers ） And a microphone （ The microphone of some sound level meters can also be used as a test microphone ）. The sweep signal is sent from the sound source , The microphone receives the sweep signal directly from the sound source , At the same time, it will also receive sound waves reflected from the room .

2.1 Transfer function

When the microphone receives the signal , Start testing . By fast Fourier transform （FFT） To calculate each frequency of the original swept signal （ Amplitude and phase ）, At the same time, calculate each frequency of the signal received by the microphone . By comparing the amplitude and phase of the original signal and the signal received by the microphone , The influence of loudspeaker and measuring environment on each frequency can be calculated . This is called from the sound source to mic Transfer function of position .

Be careful , Different sound source positions or different microphone positions , Different transfer functions will be generated . Each measurement is only for the current sound source position and microphone position .

2.2 Impulse response

After calculating the transfer function , Can be reversed FFT Transformation , From the amplitude and phase information, a time signal describing any signal change form from the sound source to the microphone position is obtained . This time signal is called “ Impulse response ”. Same as transfer function , It is only valid for a specific sound source location and microphone location .

The impulse response is actually exactly the same as the signal we see , If we can make a short but loud click sound at the source , Then record the signal picked up by the microphone （“ Very short ” It means that the sampling rate we use for analysis lasts only 1 Time of samples , So for 48 kHz Sampling rate , It will only be 1/48000s）. The difficulty of using click tone is that the click time is very short , We need a very loud sound to get the change in the background noise of the room after the initial click . You need a starting gun or balloon to generate this click sound , The speaker cannot meet this requirement .

Technically speaking , Higher signal-to-noise ratio can be obtained by frequency sweeping . The SNR depends on the background noise level and the energy in the test signal , The energy in the test signal depends on the volume and duration of the signal . One pulse is very short , A few millionths of a second , So to get any important energy , It needs a very high volume . One sweep can last for many seconds , So even in a modest volume , Its total energy can also reach onemillion times that of a pulse .

Once the impulse response is obtained , You can analyze it , To calculate information about the room . The simplest analysis is the fast Fourier transform （FFT）, To display the frequency response between the sound source and the microphone position . The early part of the impulse response corresponds to the direct sound from the sound source to the microphone , The shortest path between them . The sound that bounces off the surface of the room must travel farther to reach the microphone , It takes longer , So the latter part of the impulse response contains the contribution of the room .“ window ” The medium impulse response only looks at the initial part , It shows us the frequency response of direct sound , Almost no contribution from the room . A window containing the latter part of the response allows us to see how the room's contribution changes the frequency response . Distinguish between direct sound and subsequent sound （ Reflection ） The contribution of sound is the ability of impulse responses to produce frequency responses with which we derive from RTA A key difference between the frequency responses obtained , for example ,RTA Only the total combined response of the source and the room can be displayed .

2.3 RT60

We can get other information from the impulse response , For example, waterfall chart , It generates a three-dimensional image of the response changing with time by gradually moving the window along the response and drawing various frequency responses , And the room “RT60” data , This is the sound attenuation of different frequency bands 60 The time required for decibels （ It means smaller than before 1000 times ）.

1. test signal

REW Logarithmic sine sweep test method is adopted . The logarithmic sweep signal is a signal that starts with a low frequency signal , A signal whose frequency increases steadily in logarithmic form to a high frequency .

What makes the sweep logarithmic is the rate at which the frequency increases , It requires a fixed octave time （ for example , Sweep slave 20Hz To 40Hz Time and from 40Hz To 80Hz At the same time ）.

2. Test principle

In order to make measurements , We need a sound source （ Speakers or subwoofers ） And a microphone （ The microphone of some sound level meters can also be used as a test microphone ）. The sweep signal is sent from the sound source , The microphone receives the sweep signal directly from the sound source , At the same time, it will also receive sound waves reflected from the room .

2.1 Transfer function

When the microphone receives the signal , Start testing . By fast Fourier transform （FFT） To calculate each frequency of the original swept signal （ Amplitude and phase ）, At the same time, calculate each frequency of the signal received by the microphone .

By comparing the amplitude and phase of the original signal and the signal received by the microphone , The influence of loudspeaker and measuring environment on each frequency can be calculated . This is called from the sound source to mic Transfer function of position .

Be careful , Different sound source positions or different microphone positions , Different transfer functions will be generated . Each measurement is only for the current sound source position and microphone position .

2.2 Impulse response

After calculating the transfer function , Can be reversed FFT Transformation , From the amplitude and phase information, a time signal describing any signal change form from the sound source to the microphone position is obtained . This time signal is called impulse response .

Same as transfer function , It is only valid for a specific sound source location and microphone location .

The impulse response is actually exactly the same as the signal we see , If we can make a short but loud click sound at the source , Then record the signal picked up by the microphone （“ Very short ” It means that the sampling rate we use for analysis lasts only 1 Time of samples , So for 48 kHz Sampling rate , It will only be 1/48000s）.

The difficulty of using click tone is that the click time is very short , We need a very loud sound to get the change in the background noise of the room after the initial click . You need a starting gun or balloon to generate this click sound , The speaker cannot meet this requirement .

Technically speaking , Higher signal-to-noise ratio can be obtained by frequency sweeping . The SNR depends on the background noise level and the energy in the test signal , The energy in the test signal depends on the volume and duration of the signal .

One pulse is very short , A few millionths of a second , So to get any important energy , It needs a very high volume . One sweep can last for many seconds , So even in a modest volume , Its total energy can also reach onemillion times that of a pulse .

Once the impulse response is obtained , You can analyze it , To calculate information about the room . The simplest analysis is the fast Fourier transform （FFT）, To display the frequency response between the sound source and the microphone position .

The early part of the impulse response corresponds to the direct sound from the sound source to the microphone , The shortest path between them . The sound that bounces off the surface of the room must travel farther to reach the microphone , It takes longer , So the latter part of the impulse response contains the contribution of the room .

“ window ” The medium impulse response only looks at the initial part , It shows us the frequency response of direct sound , Almost no contribution from the room . A window containing the latter part of the response allows us to see how the room's contribution changes the frequency response .

Distinguish between direct sound and subsequent sound （ Reflection ） The contribution of sound is the ability of impulse responses to produce frequency responses with which we derive from RTA A key difference between the frequency responses obtained , for example ,RTA Only the total combined response of the source and the room can be displayed .
​

2.3 RT60

We can get other information from the impulse response , For example, waterfall chart , It generates a three-dimensional image of the response changing with time by gradually moving the window along the response and drawing various frequency responses , And the room “RT60” data , This is the sound attenuation of different frequency bands 60 The time required for decibels （ It means smaller than before 1000 times ）.