100 Things #98: MEMS Speaker Measurements

MEMS speakers are one of the biggest innovations in speaker technology in recent years. Offering full range performance with compact size and low power, they are rapidly being adopted for use in devices such as earbuds, hearing aids, smart glasses and more. With SoundCheck you can make exactly the same MEMs speaker measurements as you can with conventional mechanical speakers. Watch this short video where we demonstrate frequency response, impedance, and distortion measurements on the xMEMS Montara MEMS speaker.

MEMS Speaker Measurements

 

We’d like to thank Michael Ricci, Sr. Director of Electroacoustic Engineering at xMEMS for the technical guidance on Piezo-MEMS transduction.

Learn More About SoundCheck’s Advanced Features

Read more about more measurement features in SoundCheck.

Learn more about the Normalized Distortion Measurement technique mentioned in the video – we have a short video explaining this, or a longer (but rather old) technical paper.

More information is also available in the  SoundCheck Manual.

 

Video Script: MEMS Speaker Measurements

SoundCheck is one of the most widely used loudspeaker and microspeaker measurement systems in the world, but did you know that it can also measure MEMS micro-speakers? MEMS micro-speakers are rapidly becoming popular for devices such as hearing aids, earbuds, smart glasses and more as they offer full range performance with compact size and low power, and they are also SMT reflowable. They’re constructed in an entirely different way to conventional miniature speakers – rather than using inductive coils and magnets, they use a voltage driven capacitive actuator to provide full range performance.

I’m going to demonstrate a MEMS micro-speaker test using the xMEMS ‘Montara Plus’ full-range Piezo-MEMS microspeaker, that uses a monolithic solid state fabrication. These devices are entirely manufactured with MEMS processes in a semiconductor wafer foundry. When you’re testing these devices, the xMEMS provided driver circuit delivers Voltage bias and boost converter to step up the voltage as piezo-MEMS devices have a very high input impedance and draw very low current.

Here, I’m going to use xMEMS’ own charge amplifier. You’re also going to need to build the speaker into an earbud or make your own test jig in order to test it. I’m going to demonstrate using this test jig, which is actually the one that xMEMS uses for their own measurements, and we’re going to put an ear simulator coupler on it to simulate an in-ear measurement. Aside from that, the test setup’s very similar to what we would use for any other speaker. We have an AudioConnect 2 interface which will power the coupler, and that’s connected to SoundCheck for analysis.

So we have a test sequence that will play the stimulus and analyze the response. You won’t hear it as it’s all in the coupler. And here we can see the results.

Let’s start with the frequency response. You can see it has a very flat response at low frequencies, and then in the higher frequencies you have a resonance due to the piezoelectric material and the resonance of the coupler.

We can also look at the impedance. You can see here that it’s a very different shape from a conventional loudspeaker impedance. The values are much higher but it’s very linear, which makes it easy to compensate for.

We can also look at distortion. The total harmonic distortion is also very linear right up to where we get into the ear canal response.

And while we’re on the subject of distortion, I just want to use the measurements on this device to highlight the importance of using frequency normalized distortion measurement.

With this conventional distortion measurement, you can see the second and third harmonics plotted at their actual measured frequencies, along with the fundamental.

Frequency Normalized distortion measurement compares the harmonic levels to the fundamental level at their measured frequency before their ratio is plotted, rather than the fundamental level at the excitation frequency. This removes the effect of the non-flat frequency response from the distortion and makes it easier to see the peaks in the distortion response independent of the peaks and dips in the fundamental response. Here, you can see both regular THD, the orange line, and normalized THD, the blue line. And as you can see, you have a high Q here at resonance, but apart from that there is very little distortion, so you can focus your efforts on planning around this peak. If you were going by conventional distortion, you could be wasting your time trying to solve resonances you don’t have with this second bump on the graph here.

So that’s piezo-MEMS speaker measurements in a nutshell. Check out our website for more information on testing MEMS speakers, or if you want to learn more about normalized distortion measurement.

 

 

100 Things #96: Laser Displacement Measurement of a Loudspeaker

Laser displacement measurement is a technique for measuring the peak displacement of a loudspeaker diaphragm at various power levels, frequencies or both. Did you know that SoundCheck can easily be configured to include a laser signal path? This makes it easy to correlate diaphragm displacement with electrical impedance and audio artifacts. In this short video, we demonstrate laser displacement measurements of a loudspeaker.

Laser Displacement Measurement

Get Our Free Laser Displacement Measurement Test Sequence

Ready to try it for yourself? You can read more and download this laser displacement measurement sequence here.

More information on configuring SoundCheck for use with lasers is also available in the  SoundCheck Manual.

 

Video Script: Laser Displacement Measurement of a Loudspeaker

Displacement lasers can be used to measure the peak displacement of a loudspeaker diaphragm at various power levels, frequencies or both. Did you know that SoundCheck can easily be configured to include a laser signal path? This makes it easy to correlate diaphragm displacement with electrical impedance and audio artifacts. Let’s take a look.

First, we create a Laser Signal Path in Calibration and once that’s done, a new calibrated device file for the instrument.  The sensitivity of most lasers is expressed in Volts per Millimeter and in this case, our laser’s sensitivity is 100 volts per millimeter.  After creating custom units, we can enter the sensitivity value, select a hardware channel and we’re ready to measure!

In this sequence, we’re using a stepped sine sweep starting at 1 kHz and ending at 20 Hz, and  we’re also simultaneously measuring the impedance and frequency response of our speaker under test.  The recorded time waveform from the laser can be analyzed just like any other waveform but there’s one additional post processing step required after analysis, converting the displacement level from RMS to peak.

As you can see, configuring SoundCheck for laser measurements couldn’t be easier. The resulting data can be used to study the displacement of the speaker under test and can even be used in conjunction with other SoundCheck measurements to calculate more advanced metrics such as Thiele-Small parameters. You can learn more about advanced speaker measurements on our website, www.listeninc.com.

 

100 Things #78: Evaluating Stereo Soundfields using Time Selective Response

Time selective response in SoundCheck can be used to evaluate a stereo sound system’s localization and envelopment characteristics. By calculating the interaural level differences using a head and torso simulator, SoundCheck can measure and display the sound localization. SoundCheck can measure both the interaural level difference, and interaural cross correlation, all in a single sequence.

Evaluating Stereo Soundfields using Time Selective Response

Check out our pre-written Stereo Soundfield Parameters sequence

Try time selective response for yourself! This complete sequence measures parameters such as Interaural level difference (ILD) and interaural cross-correlation (IACC) using a Head and Torso Simulator (HATS). The spectrum is measured at each ear and calculates interaural level differences (ILD), spectral balance, delay differences, and interaural cross-correlation (IACC). The measurements are made using different time windows, so that the direct, early, late or total parts are separated.

Video Script:

Time Selective Response (TSR) has been part of SoundCheck since 2001. It’s frequently used to make reflection-free measurements of loudspeakers in ordinary rooms.

But did you know it can also be used to evaluate localization and envelopment of stereo sound systems? Let’s listen to a full stereo music excerpt in which the locations of the instruments are easily heard. For the best listening, use either stereo headphones or a pair of speakers spaced apart.

(Audio: Full stereo example.) 

The sound of the band is well spread from left to right, and the soloist is clearly in the center. Next, listen to the same music, where the sound of the band is concentrated in the middle:

(Audio: Narrow stereo example.) 

Using TSR and simple post-processing steps, we can measure the Interaural Level Differences (ILDs), which are strongly related to stereo spread and localization.

Now listen to the same music as it might sound in a concert hall where you are enveloped by the reflections and reverberation of the hall:

(Audio: Full stereo with reverb example.) 

Using TSR and some advanced post-processing steps, we can measure the Interaural Cross Correlation (IACC), which is strongly related to envelopment or a sense or spaciousness.

The SoundCheck measurements start with a log sweep played first to the left channel, (Audio: TSR sweep L , under)

…then to the right channel, (Audio: TSR sweep R , under)

…and finally, to both channels. (Audio: TSR sweep L and R , under)

For each sweep, the frequency responses from the HATS ears are analyzed using Time Selective Response. These frequency responses are used to calculate the Interaural Level Differences, or ILDs.

The uncorrelated ILDs indicate the ability of the sound system to produce directional effects. ILD’s are mostly related to the perceived speaker locations.

The direct ILDs without reflections, shown in the dotted curves, are the most important. The large difference indicates very good localization. The solid lines, including all reflections, indicate that the room tends to reduce the direct localization effect a little. 

The frequency responses are again used to calculate the Interaural Cross Correlation (IACC).

IACC is most important above 500Hz.

The yellow curve is based on the direct sound when both speakers are playing the same signal. It is close to 1, indicating central location and little sense of envelopment.

The green curve is based on the direct responses, without reflections, when the speakers are playing different sounds. It is between 0 and 1, indicating some envelopment.

The blue curve is based on the total responses, including reflections. It is between 0 and 1, but a bit lower, indicating additional envelopment provided by the room.

Using TSR analysis, SoundCheck can measure Interaural Cross Correlation (IACC) and Interaural Level Difference (ILD). When interpreted together, they objectively describe many of the spatial characteristics of sound systems. The pre-written sequence “Stereo Soundfield Parameters,” available from Listen, puts all this and more together in an easy-to-use package. Check out the Listen website to learn more.

100 Things #71: Calculating Thiele-Small Parameters of a Loudspeaker

SoundCheck can be used for loudspeaker design, performing measurements of loudspeaker performance in an enclosure, known as Thiele-Small parameters. SoundCheck includes three different methods for calculating Thiele-Small parameters: known mass, added mass, and known volume. Depending on what driver measurements are known and unknown, these three sequences provide options for all testing scenarios. Operator messages steps are included in all sequences, making testing a wide variety of devices easy with one sequence.

Calculating Thiele-Small Parameters of a Loudspeaker

Try it Yourself

You can try these sequences for yourself! Check out all three Thiele-Small Parameter test sequences in our sequence library, where you can download them free of charge and try them on your own SoundCheck system.

Learn more about Thiele-Small Parameters

Listen founder and president Steve Temme, and support manager Steve Tatarunis, co-wrote a technical article on practical impedance measurement. The paper dives into both single channel and dual channel measurement methods, considerations, techniques, measuring thiele-small parameters, and more.

Video Script:

Thiele-Small (T/S) Parameters define the mechanical, electrical and electromechanical properties of a loudspeaker and predict how a driver will work in an enclosure. It’s a common measurement for loudspeaker designers, and it’s easy to do in SoundCheck. 

SoundCheck offers 3 built-in methods for calculating the Thiele-Small parameters – known mass, added mass, and known volume. You can also use a laser, but that’s a subject for another video. Let’s take a look.

The ‘known mass’ method is when the mass of the driver is known along with the effective cone diameter and coil resistance. The sequence begins with user prompts to enter all three of these. Next a stepped sine sweep is used to calculate the impedance of the loudspeaker.  A series of post processing steps uses this curve and the data entered by the user to generate the Thiele-Small Parameters, which are displayed on two tables.

Thiele-Small parameters can also be measured when the mass of the driver is not known. In this case, we use the ‘added mass’ method. First the user is prompted to enter the driver diameter and a stepped sweep is run to calculate the impedance curve without the added mass attached.

Next the user can either enter the DC Resistance of the loudspeaker or use the minimum impedance method.  Using minimum impedance involves searching for the lowest value of the impedance curve.  Now, we enter the value of the added mass in grams and attach it to the loudspeaker. This added mass can be modeling clay or mounting putty stuck to the cone close to the voice coil to change the resonant frequency of the loudspeaker. Another stepped sweep is run and a second impedance curve is calculated.  A series of post processing steps uses the data entered and measured to generate the Thiele-Small Parameters in two tables.

The third method, known volume, assumes we know the total volume of the enclosure. In this case, we enter the driver diameter and we make an impedance measurement using a stepped stweep with the driver mounted vertically in free air. Like before, we can enter the DC Resistance of the loudspeaker or use the minimum impedance method.The second impedance measurement is made with the driver mounted in the known volume and a series of post processing steps uses the measured and user-input data to generate the Thiele-Small Parameters.

If you already have SoundCheck, you can download test sequences for all 3 methods from our website – check it out!

100 Things #62: Make Directional Measurements with Polar Plots in SoundCheck

SoundCheck’s Polar Plots make directional measurements simple. No matter what device you are testing, whether it be a VR headset, spatial audio, or headset sound leakage, SoundCheck can automate the measurement and turntable control. This means long high resolution measurements become as simple as starting a sequence! We even have a free, prewritten directional measurement test sequence using the new Portland Tool & Die MDT-4000 turntable, available here.

Make Directional Measurements with Polar Plots in SoundCheck

Try our loudspeaker polar plot measurement sequence for yourself!

Our polar plot sequence measuring a loudspeaker is pre-written and ready to use. This sequence measures the polar response of a loudspeaker in both the vertical and horizontal dimensions and displays the measurements on polar plots. This sequence is designed to work with the Portland Tool & Die MDT-4000 turntable.

Video Script:

Making directional measurements in SoundCheck is simple! SoundCheck supports turntables from a variety of manufacturers including Outline, Linear X, B&K and of course, Portland Tool and Die. These can all be controlled through a custom step in SoundCheck to be operated as part of an automated test sequence. Let’s take a look. 

Here I have a Portland Tool and Die MDT-4000 turntable – this a great turntable, by the way.  I’m going to use this to rotate a speaker, and I have a stationary measurement microphone to capture the recorded waveform. And of course I have SoundCheck on my laptop, along with an AmpConnect 621 Audio interface.

Here’s a  simple loudspeaker test sequence that plays a test signal through a loudspeaker and measures the response. Now let’s say I want to measure the response of the speaker every 10 degrees for a full 360 degree rotation. I just need to open up the test sequence….And I am going to add a custom step telling it to rotate the speaker by 10 degrees and re-measure, saving the results in the memory list and plotting them on a polar chart, now I’m going to loop that whole measurement procedure so we continue moving it and measuring until we do a full rotation. And now I’ll edit the display step to get the results output to a polar plot. Now let’s run the sequence…and there you are – fully automated directional measurements! 

There are many ways to use this functionality, for example directional measurements on smart devices with microphone and speaker arrays. You can also put headphones or VR headsets on a rotating head and torso simulator and measure the sound leakage that occurs when the noise inside the headphones leaks outside to the point where it’s audible to others around.  There are also many applications in spatial audio measurements. 

If you want a quick way of getting started with directional measurements, head on over to our website where you can download basic directional measurement sequences for speakers and microphones for a variety of different turntables.

100 Things #35: AES75 (M-Noise) Measurement of Max SPL for Loudspeakers

The AES75 standard details a method for measuring maximum linear sound levels of a loudspeaker system or driver using M-Noise, a test signal specifically developed to emulate the dynamic characteristics of music. Our pre-written test sequence automates this measurement to the new AES 75 standard for Maximum SPL, removing subjectivity, increasing reliability, and saving time.

Automated AES75 Measurements in SoundCheck

More Resources for Measurement to AES75

Would you like to try this yourself? If you already have SoundCheck, you can download the AES75 (M-Noise) test sequence. Please note that you will need the waveform filter (part # 2032) and transfer function (part # 2021) modules installed on your SoundCheck system.

Get a copy of the AES75 Standard.

 

Video Script: Automate Measurements Using M-Noise with SoundCheck

Our pre-written test sequence automates measurement to the new AES 75 standard for Maximum SPL, removing subjectivity, increasing reliability, and saving time.

This standard details a method for measuring maximum linear sound levels of a loudspeaker system or driver using M-Noise, a test signal specifically developed to emulate the dynamic characteristics of music. Clearly defined limits for linear frequency response and coherence determine the Max SPL level and remove any measurement ambiguity.

Implementing the standard manually relies on an operator’s subjective judgment of real time spectrum analyzer data. It also requires multiple iterations of a measurement, therefore is labor intensive.

Our test sequence is fully automated. We use the same test signal and calculations outlined in the standard, but automated analysis steps objectively calculate the measurements and drive the next steps in the procedure.

Let’s take a look.

Here, you can see we are using the freely available M-Noise test signal introduced by Meyer Sound. This stimulus features a relatively constant peak level as a function of frequency, but a diminishing RMS level with increasing frequency.

First, we use a test signal approximately 20dB below our expected Max SPL to obtain a provisional linear frequency response, linearity, coherence and signal to noise ratio. We then increase the test level by 3dB, and compare the results to the initial value. The results must be within +/- 1dB, have a coherence of at least 97% and a signal to noise ratio 15dB or higher so that we know we are operating in the speaker’s linear region and our signal to noise ratio is sufficient for accurate measurements.

Next we automatically increase the test level by 3dB and compare it to the initial results, normalized to the current test level. Multiple measurement iterations take place until one of the ‘stop’ conditions is reached. These conditions are either:

  • the live measurement differs from the linear frequency response by at least 2 dB over at least two octaves
  • the live measurement differs from the linear frequency response by at least 3 dB anywhere, or
  • the Coherence Reduction Target is met –  this means the signal to noise ratio is 10dB or less and/or the coherence is 91% or less.

When one of these limits is reached, the sequence then reduces the test level to the last level that passed and repeats the measurements in 1dB increments to find the precise Level at which the response deviates from the base level.

Once this level is established, the device enters a burn-in process, where the M Noise stimulus is played through the DUT for five minutes and fifty-three seconds and again compared to the initial result. This long duration measurement is also used to generate peak, rms and A weighted RMS sound levels. If the response curve remains consistent, this curve is the Max SPL curve according to the standard. If it is not within the acceptable limits, the device is cooled down and the tests repeated with a longer stimulus duration.

All the operator needs to do is enter any stimulus limits based on the operating range of the DUT into the sequence before starting, then return when the test is complete.

As well as saving time, SoundCheck mathematically calculates the data in analysis steps within the sequence, which avoids the subjectivity of relying on operator interpretation of real-time spectrum analyzer outputs. This increases repeatability and confidence in the results. This sequence is available free of charge on our website. Check it out!

 

 

 

100 Things #26: Production Line Polarity Measurement

SoundCheck offers a rapid production line polarity measurement that can identify correct wiring at the same time as other production line acoustic tests. This is valuable as the conventional method of polarity measurement –  determining if a diaphragm moves outward or inward when voltage is applied – is not practical on the production line.

Production Line Polarity Measurement in SoundCheck

Learn How to Measure Polarity in SoundCheck

Check out our knowledgebase article on Polarity Measurement.

More information is also available in the  SoundCheck Manual.

 

Video Script: Production Line Polarity Measurement

Having the proper polarity marking or wiring is a critical requirement for any transducer, particularly those that are used in conjunction with others.  If there are errors in wiring or terminal markings, the intended performance of the transducer and how it interacts with other transducers in a system can be compromised.

The simplest test for polarity might be applying a DC voltage to a speaker and determining if its diaphragm moves outward or inward as the voltage is applied.  If everything is wired or marked correctly, we would expect the diaphragm to respond with an outward motion when a positive voltage is applied.  While this method is simple and accurate, it is not practical for production line test.

For production line testing, SoundCheck has a couple of useful methods for polarity detection. The first option is to place limits on the phase response curve of the device under test. Since the phase response changes by 180 degrees when the polarity changes, this can be an accurate method for polarity detection. The other option is to enable the Polarity check box which can be found on the Time tab of most of SC’s analysis editors.  Once enabled, every time the Analysis step is applied, SC will calculate an impulse response of the current measurement. The impulse response is then analyzed to determine if the largest peak has a positive or negative value.  If positive, a value of 1 will be generated into the Memory List, if negative, a value of -1 will be generated into the Memory List. A limit can then be applied to this value to create a Pass/Fail verdict in the ML Results tab.

These polarity detection options have been available in SC since 2011 and provide the user with two accurate methods for measuring the polarity of your transducer.

 

 

 

100 Things #17: Simulated Free Field Measurements Without an Anechoic Chamber

With SoundCheck you can make simulated free field measurements in an ordinary room without an anechoic chamber – in a typical lab, office, or even your home. By performing near field and far field measurements, we are able to utilize the measurement strengths of each technique; near field measurements for low frequencies and room reflection immunity, and time-windowed far field measurements for high frequencies. Splice these measurements together, and the result is the free field response of the loudspeaker. All without an anechoic chamber, and all within a single SoundCheck sequence.

Simulated Free Field Measurements Without an Anechoic Chamber

Learn more about free field measurements with SoundCheck

In this online seminar, Steve Temme explains how to use simulated free field measurements to provide accurate free-field measurements across the entire audible frequency range without an anechoic chamber. This seminar talks about setup and equipment considerations, testing in different environments, and analysis of results.

Try simulated free field measurements for yourself with our Splice sequence, available in our sequence library.

Video Script:

Free field loudspeaker measurements are often included in loudspeaker specifications. These measurements are usually made in an anechoic chamber. However, anechoic chambers are not cheap, particularly those large enough to give accurate measurements at low frequencies. This means that many engineers don’t have easy access to a suitable chamber, even more so if they are working from home.

With SoundCheck you can make free field measurements in an ordinary room without an anechoic chamber – in a typical lab, office, or even your home. Here’s how it’s done.

First, measure the near field response of the loudspeaker using a stepped sine sweep or Stweep, and placing the microphone very close to the low frequency driver. If the loudspeaker is ported, you also need to make a measurement at the port or ports. This near field measurement is accurate at low frequencies as it’s unaffected by room reflections, but it does not accurately represent the free field response at high frequencies. 

Next, place the microphone in the far field and measure the time-windowed frequency response using a continuous log sweep with the Time Selective analysis algorithm. The far field, time-windowed measurement is unaffected by room reflections, but it is not accurate at low frequencies as the room size limits the width of the time window and therefore the frequency resolution.

Now, if you examine the two responses you can see an overlap range in the middle where the shapes of the curves align. Using this, you can select a precise frequency to splice the two halves of the measurement together and display a response over the entire frequency range. This is implemented in several post-processing steps which include calculating the impulse response, and automatically correcting for differences in amplitude and phase.

Here you can see the simulated free-field response measured using this method compared to the same speaker measured in an anechoic chamber. In fact, the simulated free field response is actually more accurate than the anechoic chamber measurements because of its small size and 120Hz cut-off frequency

We can also compare our measurements to the manufacturer’s published data. Here we see a tight correlation across the whole frequency range, even at low frequencies – likely because a much larger chamber was used.

If you want to know more, there’s a detailed seminar covering both the theory and practical aspects on our YouTube channel, and a pre-written test sequence for use with SoundCheck can be downloaded from our website for free!

100 Things #10: Low Cost Audio Measurement with SoundCheck ONE

SoundCheck’s low cost audio measurement software, SoundCheck One, is a budget-friendly solution for basic audio tests. It is simple to use with sequence templates to quickly test simple devices such as headphone, microphones and speakers. Using the same advanced algorithms as the regular version of SoundCheck, it is fully compatible with the regular version, and may be upgraded at any time.

Low Cost Audio Measurement with SoundCheck ONE

Learn more about SoundCheck ONE

SoundCheck Packages and Modules

 

Video Script: Low Cost Audio Measurement with SoundCheck ONE

SoundCheck One is an entry level SoundCheck package designed for basic transducer measurements. It is low-cost and easy to use, making it ideal for production, end-of-line testing and benchmarking applications.

Under the hood it features the same unique algorithms and advanced test methods as the full version of SoundCheck; the difference is in the user-interface.

SoundCheck ONE is supplied as a complete package including software and an audio interface – either an AmpConnect ISC or AudioConnect. Instead of SoundCheck’s sequence editor for test development, pre-configured test templates for speakers, headphones and microphones are provided.

Each template contains all the typical measurements for those devices. The tests themselves cannot be edited but individual steps can. For example frequency range, levels and limits can be modified and measurements toggled on or off to generate product-specific sequences. This makes it fast and easy to configure basic tests that can be deployed across multiple products.

Because SoundCheck ONE uses the same data format as full-featured versions, its curves, waveforms and results can be shared for deeper analysis with users who might have an advanced license.

At approximately half the price of a regular SoundCheck system, SoundCheck ONE is a great entry-level option. While it doesn’t offer the limitless flexibility of the full versions of SoundCheck, its template-based approach makes it extremely simple to configure and use.

Furthermore, because the SoundCheck platform is modular, your investment is not wasted if future testing demands more flexibility. SoundCheck ONE can be upgraded to a more advanced SoundCheck package with full sequence development capabilities at any time. The test sequences will continue to work on the upgraded system, where they can be expanded and developed to meet all your measurement challenges.