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    Elko - Wednesday 21 October 2021

    How to use measurement microphones, A basic guide to measure your (diy) audio system

    You have just purchased a measurement microphone, but you are not sure how to use it. Or you are wondering if you need one and you require some more information. This blog will explain what a measurement microphone is and also contains two mini-guides. One for setting up an acoustic measurement and another about a neat measurement example: the gated measurement.

     

    Part 1: What is a measurement microphone

    Let’s start with the basics. A measurement microphone is a microphone for capturing signals you wish to analyze. In general lines, it has a simple purpose and it does not differ that much from other microphones. A measurement microphone does not measure anything by itself. It just captures an acoustic signal and transduces the acoustic signal to an electronic signal.

    Yes, that’s right! A microphone is a transducer too, just as your speaker. It only transduces the other way around.

    If you would stop reading here, a measurement microphone appears not to be very special and you might consider your favorite bar’s karaoke microphone fit for acoustic measurements too. But hold on; A microphone becomes a measurement microphone when it has these special features:

     

    Flat frequency response over a wide frequency range
    Most measurement microphones are of the electret capacitor type. It uses a very thin film (diaphragm) that is placed close to a permanently charged plate, the electret. The film and charged plate form a capacitor. When the film is set in motion by soundwaves (or other changes in air pressure) it creates capacitance and a signal successively. A capacitor microphone can respond to static changes in air pressure and therefore is very suitable for low frequency capturing. The high-frequency end of the spectrum is easily captured thanks to the low moving mass of the thin film.

    Omnidirectional capturing pattern
    The small diameter of the capsule allows for equal sensitivity at different angles over a wide frequency range. This comes in handy because there are many situations where you capture from multiple angles at once. Even when measuring a single speaker at a single angle, the omnidirectional pattern can be used to create anechoic results in your living room.

    High sound pressure level handling
    Unless you are confident about the maximum output capabilities of your system and comfortable with high sound pressure levels, I would not recommend testing the maximum sound pressure level of your microphone. The high-level handling capabilities are useful because the distortion is very low at common measurement amplitudes. A higher maximum SPL comes at the cost of sensitivity. In the case of loudspeaker measurements, this is not a problem, as this makes the microphone less prone to pick up the background noise.

    Calibration
    Calibration ensures a flat frequency response. A calibrated microphone comes with a calibration file. This file contains information to correct the shortcomings of the capsule. The measurement software applies the correction on the measurement. If you wish to see how much correction your microphone needs, you can just open the file in a text editor. You will see a sensitivity correction and a two columned list. One column is the frequency that needs correction, the other is the amount (in decibels) of correction on that frequency.

     

    Connectivity

    As described above, the microphone itself just captures the signals. The actual measurement requires an analyzer. Nowadays this is a computer. How do does the captured signal get to the computer?
    Our most popular measurement microphones have a USB interface. The USB interface transfers the signal to your computer and supplies the 5 VDC voltage to power the microphone internal circuits. Besides being easy to use, a USB microphone has other benefits. The internal circuits allow for low noise performance, optimal hardware control, and minimize losses due to the stable digital signal transfer. You do not have any concerns about the compatibility and quality of a pre-amplifier or soundcard with a USB-microphone

    USB measurement microphones


    We also offer measurement microphones with analog outputs. These are still accurate, but they require a pre-amplifier and connection interface. It is common that the amplifier and interface are combined in one device, such as a USB audio interface or a soundcard.

    Analog measurement microphones:


    Scrolling through our Test & measurement products, you might have noticed the Audiomatica products. The CLIO pocket and CLIO 12 are the swiss army knives for loudspeaker measurements. They can measure both acoustic and electrical signals. The Audiomatica microphone is not a USB device, but instead, the included interface is. The CLIO software allows you to control the hardware and analyze both acoustic and electric measurements within the same software environment.


    What else do I need besides a measurement microphone?

    As the microphone only captures the signal and does not make any measurements by itself, you will need some additional tools:

    What

    Why

    Computer and software

    The computer does all the difficult work: generate the test signal, making calculations, and plot the graphs. Imagine how difficult it would be without a computer! The software is included with some microphones. If not, we recommend REW – Room Eq Wizard.

     

    Amplifier

    To amplify the test signal. Any modern audio amplifier that is compatible with your computer's output and that is powerful and stable enough to drive the connected speaker is just what you need.

    D.U.T.

    The Device Under Test. Usually a loudspeaker or sound system.

    Microphone stand

    Most microphones come with a tripod desktop stand. This is perfect for measuring room acoustics. However, if you wish to measure your loudspeakers a larger stand is a must-have.

    (Interface or soundcard)

    Only required with analog (non-USB) microphones

     

    Part 2: How to use a measurement microphone

    Do you have everything you need? Making a measurement is just a few steps away. Continue reading for a setup guide with REW – Room EQ Wizard

     
    1. Place your microphone in the stand and place the stand in a position suitable for your measurement.
    2. Connect your microphone to the computer. With a USB microphone, this is a direct connection straight from the microphone to your computer. Analog microphones require an interface or a connection to a soundcard in between.
    3. Make sure the computer's audio output is connected to the amplifier's input and the volume is set to a minimum. This can either be your amplifier's volume control or your OS volume control.
    4. Start the measurement application (we show REW in this example) and load the calibration file. REW should detect the microphone automatically and the software will ask you to load a calibration file. Most microphones come with a calibration file and it is recommended to load it.

    5. Set your levels.

      This is done by generating a test signal with the Generator

      -20 dB is a suitable level to start at. Click Play. Increase the volume until you can hear the signal properly.



      Keep the generator running and open the SPL meter

      Increase the volume until a level of approximately 75 dB is reached. You can also increase the level of REW’s signal generator until this approximate level is reached.



    6. Measure! Open the measurement tab. Set the level the same as the level of the signal generator on our previous step. Depending on your speaker, select an appropriate start and stop frequency. Make sure that Input is set to microphone and Output to the speaker you wish to measure.

      Click on check levels. REW will automatically verify if you’ve set your levels right. If no error shows up, you are ready to go ahead and click measure!

     

    The first graph will not be very pretty; it looks more like a seismographic event than a speaker system. This is because the microphone captures the direct output, but also reflections of the signal bouncing around the room polluting the direct measurement.

    A common method to make things look better is to apply smoothing. You can apply smoothing through the Graph menu in REW. A small amount of smoothing is not bad at all, but too much can distinguish the actual results. A gated measurement is a much better method for acquiring an accurate, but clean-looking graph.

     

    Gated measurement

    A gated measurement is a neat signal processing trick to make anechoic measurements in a normal environment. With this technique, you can create a time gate -or window- and force the software to only take the direct signal into account.  All you need are the tools described above and a reasonable amount of space. Most measurement software all have features for gated -or windowed- measurements. Because each software is unique, we would recommend checking the manual for a specific how-to on these features. The manuals are available on our product pages.

    With a gated measurement you create a time gate -or window- and take advantage of the propagating property of soundwaves, the omnidirectional property of the microphone, and powerful signal processing available in the measurement software. The fact that soundwaves travel (or propagate) allows you to make distinctions between the direct signal and its reflected counterparts. This is important as the reflections can add up or subtract on the direct signal resulting in comb filtering. The direct signal will always travel the shortest distance to the microphone. The length of your measurement gate is the time difference between the arrival of the direct signal and the arrival of the first reflection. Because sound propagates at a constant speed, you can easily calculate the extra time the reflected signals take to reach the microphone. Another easy and more accurate approach is to take a look at the impulse response graph to determine how much later the first reflection arrives. The image below shows the direct signal in orange and the reflected signal in blue: 


    There is a catch: A smaller measurement window results in a lower resolution and limits the lowest frequency you can measure. You can determine the lower frequency limit as follows: Fmin = 1/window length. To get a response on a wider frequency spectrum, you can combine -or merge- different measurement techniques. A near-field measurement is a perfect addition to extend the lower frequency limit of your measurement.


    The gated measurement is very useful, but just one example of many things you can do with your measurement microphone. I hope this blog gets your acoustic measurement adventures started!

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