GOAL:
To find a moderate cost microphone that's capable of measuring the lowest distortion drivers, in an indoor environment.
PROBLEM:
Measuring loudspeaker drivers in an indoor environment can be a challenge, with limited space and tools that diyAudio'ers have at our disposal. In this small study I will endeavour to see how well one can measure drivers, in box, not a large IEC baffle, without an anechoic chamber, or laboratory grade instrumentation.
SUMMARY:
Scroll to the bottom of this post.
Interested? Let's proceed. As usual, this a collaboration with other diyAudio users. Thanks in advance to @Hörnli,@5th element and @DcibeL for their direction and feedback. Special thanks to @IamJF for your patient replies and professional advice. As usual, all errors are mine and mine alone. I welcome any queries, feedback and contribution.
METHOD:
To start I chose to look at a 6.5” midwoofer, the PTT6.5W04 the first driver released from Purifi's catalogue. As a 2021 release, there may be a few in the field, so it can serve as a kind of reference for a recently released and available driver.
Here's the frequency response, along with H2 and H3, from Purifi's datasheet
Comment: Being a 4 ohm driver, the SPL is between 87-91dB through much of the range. As usual, because the driver is not mounted in an enclosure (standard procedure for manufacturers) there's a roll off below 100Hz. In practice, the bass response is determined by the T/S parameters and the enclosure design.
In the first instance, we trace these curves with an auto tracing program, and export into a measurement viewer, so we can work directly with the numbers:
In the original datasheet, the 2nd and 3rd harmonics are obscured by the Text Box in the lower left, and the 3rd harmonic (H3) falls below 0dB at 6KHz. Hence, the harmonic data is accurate only between ~60Hz and ~6KHz.
I use software to calculate H2 and H3 in relative terms, compared to the fundamental, between 60Hz and 6KHz:
Comment: Between 100H to 3.5KHz, H2 is <-60dB, between 200Hz and 3 KHz and H3 ~-70dB.
There are numerous 3rd parties who have reviewed/taken measurements of this driver. Some have been more successful in measuring H2 and H3 eg. Erin's Audio Corner and HifiCompass. Others have not been as successful*
*In this measurement, the microphone is 10cm from the driver, and thus tasked with observing up to 121dB. The microphone's own distortion is 1% at 126dB.
With thanks to @IamJF, it has been shown that a microphones have their own distortion profile . With such a wide variance in maximum SPL and distortion ratings by different manufacturer's, DIYers and enthusiasts might be left wondering "What microphone can give me an honest harmonic distortion assessment of a driver?"
For my study, I measure the PTT6.5W04 mounted in a 14L enclosure of dimensions ^40x20x30cm (LxWxD), with a drive level of 2.83V.
For far field measurements, I place the microphone 40cm from the baffle. Using gating of 10ms, reasonably accurate frequency response information can be taken from ~ 600Hz and above. To calculate the 1m SPL, subtract 8dB.
The graph below shows the readings from 4 microphones:
Comment: All microphones show similar responses up to about 5KHz, with larger differences starting to show up above, notably Umik-1.
Here is the 2nd harmonic measurement, for each microphone.
Comment: There are differences of up to 10dB between microphones in measurement of the 2nd harmonic when observing 98-101dB
The leading contender is the S-mic
The third harmonic for 4 microphones:
Comment: There are minimal differences between the microphones in measurement of the 3rd harmonic when observing 98-101 dB
Now we take measurements taken with the microphones in the near field (1cm) to look at low frequency response:
Comment: There is a difference of 5-10dB between the microphones in measurement of the 2nd harmonic when observing 110-116 dB in the region 50Hz to 300Hz.
Comment: There is a difference of up to 6dB in between the microphones in measurement of the 3rd harmonic when observing 110-116dB (between 50Hz and 200Hz) The leading contender is the S-mic (cyan)
For a comparison to Purifi's datasheet, I split the graph into 2 parts- the near field for a look at the bass to lower mids…
Now I measured with the microphone that shows the most promise between my samples of MiniDSP Umik-1, Sonarworks Xref20, Earthworks M23 (2024 model), and Sennheiser MD42.
Here is the measured indoors at a mic distance of 1cm with a Sennheiser MD42 (“S-mic”)
Above 200Hz, there is closer agreement with Purifi's datasheet: H2 is around ~-60dB. But H3 is still ~-60dB.
Above 200Hz, the effects of the woofer being in a box (with a passive radiator) the differences are still apparent. It's unclear whether this difference can be reconciled.
Addit:
Here is my sample of the PTT6.5X measured at distance of 1cm with a Line Audio Omni1 microphone:
Unfortunately the Omni1 is NOT able to characterise the low distortion of the PTT6.5XNFA when measuring at 1cm. It's observed H2 and H3 is 10dB higher
Now the second part of Purifi's datasheet for 600Hz to 6KHz…
as measured by Sennheiser MD42 omnidirectional dynamic mic, "S-mic" @ 40cm:
as observed by Line Audio Omni1 electret condenser microphone @ 40cm:
I was NOT able to resolve H2 and H3 as clearly as Purifi with my sample of the Earthworks M23 (2024 release), Sonarworks Xref20v4 (2015 release), miniDSP UMIK-1 (2013 release).
EDIT 04/05/24 : Big clean up, improved precision in tracing Purifi's datasheet (previously 1/12 smoothing, now 1/48 parts per octave without smoothing), separated near field and far field measurements, corrected erroneous graphs, moved explanations to Appendix, added 4th microphone- Sennheiser MD42, nicknamed “s-mic"
EDIT 22/05/24 : Added a video about how to take measurements by@1audio Demian Martin. A nice little primer in video form, suitable for people relatively new to taking measurements. If you've been struggling or stuck with taking measurements, this is a great introductory lecture. Only 1hr 30 mins!:
EDIT 29/05/24: measurements by Line Audio Omni1 microphone added.
Moved appendix to post #56 (This post has limit 20 images)
To Be Continued… (live article)
-Reference microphone to come (Bruel & Kjaer)
-Measurement tips
-Implications...
To find a moderate cost microphone that's capable of measuring the lowest distortion drivers, in an indoor environment.
PROBLEM:
Measuring loudspeaker drivers in an indoor environment can be a challenge, with limited space and tools that diyAudio'ers have at our disposal. In this small study I will endeavour to see how well one can measure drivers, in box, not a large IEC baffle, without an anechoic chamber, or laboratory grade instrumentation.
SUMMARY:
Scroll to the bottom of this post.
Interested? Let's proceed. As usual, this a collaboration with other diyAudio users. Thanks in advance to @Hörnli,@5th element and @DcibeL for their direction and feedback. Special thanks to @IamJF for your patient replies and professional advice. As usual, all errors are mine and mine alone. I welcome any queries, feedback and contribution.
METHOD:
To start I chose to look at a 6.5” midwoofer, the PTT6.5W04 the first driver released from Purifi's catalogue. As a 2021 release, there may be a few in the field, so it can serve as a kind of reference for a recently released and available driver.
Here's the frequency response, along with H2 and H3, from Purifi's datasheet
Comment: Being a 4 ohm driver, the SPL is between 87-91dB through much of the range. As usual, because the driver is not mounted in an enclosure (standard procedure for manufacturers) there's a roll off below 100Hz. In practice, the bass response is determined by the T/S parameters and the enclosure design.
In the first instance, we trace these curves with an auto tracing program, and export into a measurement viewer, so we can work directly with the numbers:
In the original datasheet, the 2nd and 3rd harmonics are obscured by the Text Box in the lower left, and the 3rd harmonic (H3) falls below 0dB at 6KHz. Hence, the harmonic data is accurate only between ~60Hz and ~6KHz.
I use software to calculate H2 and H3 in relative terms, compared to the fundamental, between 60Hz and 6KHz:
Comment: Between 100H to 3.5KHz, H2 is <-60dB, between 200Hz and 3 KHz and H3 ~-70dB.
There are numerous 3rd parties who have reviewed/taken measurements of this driver. Some have been more successful in measuring H2 and H3 eg. Erin's Audio Corner and HifiCompass. Others have not been as successful*
*In this measurement, the microphone is 10cm from the driver, and thus tasked with observing up to 121dB. The microphone's own distortion is 1% at 126dB.
With thanks to @IamJF, it has been shown that a microphones have their own distortion profile . With such a wide variance in maximum SPL and distortion ratings by different manufacturer's, DIYers and enthusiasts might be left wondering "What microphone can give me an honest harmonic distortion assessment of a driver?"
For my study, I measure the PTT6.5W04 mounted in a 14L enclosure of dimensions ^40x20x30cm (LxWxD), with a drive level of 2.83V.
For far field measurements, I place the microphone 40cm from the baffle. Using gating of 10ms, reasonably accurate frequency response information can be taken from ~ 600Hz and above. To calculate the 1m SPL, subtract 8dB.
The graph below shows the readings from 4 microphones:
MiniDSP Umik-1 (2013 model)
Sonarworks Xref20 (2015 model)
Earthworks M23 (2024 model)
Sennheiser MD42 (?model year- purchased second hand)
Sonarworks Xref20 (2015 model)
Earthworks M23 (2024 model)
Sennheiser MD42 (?model year- purchased second hand)
Comment: All microphones show similar responses up to about 5KHz, with larger differences starting to show up above, notably Umik-1.
Here is the 2nd harmonic measurement, for each microphone.
Comment: There are differences of up to 10dB between microphones in measurement of the 2nd harmonic when observing 98-101dB
The leading contender is the S-mic
The third harmonic for 4 microphones:
Comment: There are minimal differences between the microphones in measurement of the 3rd harmonic when observing 98-101 dB
Now we take measurements taken with the microphones in the near field (1cm) to look at low frequency response:
Comment: There is a difference of 5-10dB between the microphones in measurement of the 2nd harmonic when observing 110-116 dB in the region 50Hz to 300Hz.
Comment: There is a difference of up to 6dB in between the microphones in measurement of the 3rd harmonic when observing 110-116dB (between 50Hz and 200Hz) The leading contender is the S-mic (cyan)
For a comparison to Purifi's datasheet, I split the graph into 2 parts- the near field for a look at the bass to lower mids…
Now I measured with the microphone that shows the most promise between my samples of MiniDSP Umik-1, Sonarworks Xref20, Earthworks M23 (2024 model), and Sennheiser MD42.
Here is the measured indoors at a mic distance of 1cm with a Sennheiser MD42 (“S-mic”)
Above 200Hz, there is closer agreement with Purifi's datasheet: H2 is around ~-60dB. But H3 is still ~-60dB.
Above 200Hz, the effects of the woofer being in a box (with a passive radiator) the differences are still apparent. It's unclear whether this difference can be reconciled.
Addit:
Here is my sample of the PTT6.5X measured at distance of 1cm with a Line Audio Omni1 microphone:
Unfortunately the Omni1 is NOT able to characterise the low distortion of the PTT6.5XNFA when measuring at 1cm. It's observed H2 and H3 is 10dB higher
Now the second part of Purifi's datasheet for 600Hz to 6KHz…
as measured by Sennheiser MD42 omnidirectional dynamic mic, "S-mic" @ 40cm:
as observed by Line Audio Omni1 electret condenser microphone @ 40cm:
(partial) SUCESSS!
For frequencies between 600Hz and 6KHz, there is a close correlation between the harmonic measurement of a Purifi 6.5" woofer (PTT6.5X04NFA) mounted in a speaker box^, taken in an ordinary room with a Sennheiser MD42 and Line Audio Omni1 to Purifi's own measurements, which were taken in a custom made hemi-anechoic chamber with laboratory grade precision instrumentation. I was NOT able to resolve H2 and H3 as clearly as Purifi with my sample of the Earthworks M23 (2024 release), Sonarworks Xref20v4 (2015 release), miniDSP UMIK-1 (2013 release).
EDIT 04/05/24 : Big clean up, improved precision in tracing Purifi's datasheet (previously 1/12 smoothing, now 1/48 parts per octave without smoothing), separated near field and far field measurements, corrected erroneous graphs, moved explanations to Appendix, added 4th microphone- Sennheiser MD42, nicknamed “s-mic"
EDIT 22/05/24 : Added a video about how to take measurements by@1audio Demian Martin. A nice little primer in video form, suitable for people relatively new to taking measurements. If you've been struggling or stuck with taking measurements, this is a great introductory lecture. Only 1hr 30 mins!:
EDIT 29/05/24: measurements by Line Audio Omni1 microphone added.
Moved appendix to post #56 (This post has limit 20 images)
To Be Continued… (live article)
-Reference microphone to come (Bruel & Kjaer)
-Measurement tips
-Implications...
Last edited:
Thanks for continuing with this topic and digging deeper.
With an average level in your home of 50dBSpl(A) ... you will not be very happy ... and I would for sure not be happy with 30dBSpl(A) in my bedroom.
About the topic - I'm still wondering about some of the M23 THD, e.g. H2 at 500Hz is higer as with all the other mics. But it's way lower around 300Hz. It does not fit.
That list is pretty wonky. No chainsaw is 110dBSpl in 1m, that's simply forbidden in Europe. And a PA speaker in the disco is way louder - it's at least 100dBSpl(A) in the audience. Low level noise (and noise measurements for ruleas and goverment cause they don't know better) is mostly A weighted. "20dB" is a nice number but not a SPL level
With an average level in your home of 50dBSpl(A) ... you will not be very happy ... and I would for sure not be happy with 30dBSpl(A) in my bedroom.
About the topic - I'm still wondering about some of the M23 THD, e.g. H2 at 500Hz is higer as with all the other mics. But it's way lower around 300Hz. It does not fit.
You make a good point about the distortion measurement at 500Hz.
Astute readers like yourself are aware of the limitations of taking frequency response (and distortion) measurements in the farfield in a non-anechoic environment.
In fact, the distortion measurements below 800Hz are highly suspect. Furthermore, frequency response below 2KHz, as measured by the (Farina) log sine sweep needs carefully interpretation.
Give me some time to amend the original post to explain why this is the case, in plain language and minimal jargon.
I think most of us measure distortion at about 30cm from the driver. Measuring close range does dismiss cone breakup and surround radiation contribution, present in the far field. You could argue about 'nonlinear vs. linear distortion' of course, but imho that is nitpicking. I'd rather know the whole story.
This being not completely relevant to the topic subject of course, but we don't need mikes that do distortion well at 1cm from the cone, unless one wants to examine specific parts of distortion. These are quite irrelevant to those who don't build their own drivers though.
This being not completely relevant to the topic subject of course, but we don't need mikes that do distortion well at 1cm from the cone, unless one wants to examine specific parts of distortion. These are quite irrelevant to those who don't build their own drivers though.
Just quickly about noise. A measurement in my living room during the day but with kids in school.
I'm living in the countryside but 400m to the motorway. Normal windows, nothing special soundproove. Selfnoise of mic + instrument is 19dBSpl(A) - so there is significant influence!
As you see with the C rating there is low frequency rumble from the road - but you can't hear that! We are not sensitive for lf at low levels. Therefore the A weighting makes sense in this case.
So noise in a quiet room is quickly as low as most measurement microphone noises.
I'm living in the countryside but 400m to the motorway. Normal windows, nothing special soundproove. Selfnoise of mic + instrument is 19dBSpl(A) - so there is significant influence!
As you see with the C rating there is low frequency rumble from the road - but you can't hear that! We are not sensitive for lf at low levels. Therefore the A weighting makes sense in this case.
So noise in a quiet room is quickly as low as most measurement microphone noises.
Near field measurements have the advantage of better SNR; ambient noise as well as room reflexions are drowned out by the sound pressure of the DUT. However, as even low far field SPL values are high SPL values in the VNF (very near field), microphone distortion will increasingly become a factor.
In general, realize that not only microphones produce their own distortion and noise, but also your mic preamp, your power amplifier, etc., have their own distortion and noisefloor.
Especially microphone pre-amp noise will become a big issue. You will always want to set the gain of the pre-amp so that it stays just below the clipping threshhold of the pre-amp, as to maximize signal to noise. Afair this can be even done with the Umik, as the Umik has switchable gain settings.
In general, realize that not only microphones produce their own distortion and noise, but also your mic preamp, your power amplifier, etc., have their own distortion and noisefloor.
Especially microphone pre-amp noise will become a big issue. You will always want to set the gain of the pre-amp so that it stays just below the clipping threshhold of the pre-amp, as to maximize signal to noise. Afair this can be even done with the Umik, as the Umik has switchable gain settings.
The A weighting is relevant wrt speaker measurements. 125Hz is more than 15 dB down, so actual SPL isn't 20 but 35dB. Thus you need well over 95dB level to measure -60dB (0,1%) HD at low frequencies. If 0,1% at low frequencies would be relevant that is, I think that anything below 0,5% is quite OK for sub 100Hz. Furthermore the A-curve descends at high frequencies too, so all of those wanting to measure 3d to 7th order HD have to check their background levels too. I think A-weighting is pretty useless for us.
Attachments
You can't calculate from the 20-20kHz integrated value to the 125Hz narrow band value! The spectrum values are way lower as the 20-20kHz values.
My quick measurement was just referenced to the noise level table and that real numbers can be very different.
Analysing surrounding noise level for S/N calculations is a total different thing - every full band noise level (A, C, Z, Leq, ...) is pretty useless for that.
No I know, did my acoustics and noise courses, be it long ago. Afaik one 3d octave band can be louder than the overall mean level if the rest is hardly contributing. We don’t have to argue about such things, I only wanted to point out the relative uselessness of broadband noise measurements when evaluating conditions for acoustic measurements. Certainly if A weighting is involved.
Well it appears to me that whenever a driver/loudspeaker is being measured, what we are measuring is in fact our complete measurement chain.
From the audio output to the amplifier to the speaker to the microphone back to the input.
So whilst a USB mic is very convenient, they were probably never intended for distortion measurements because they are limited to the built-in interface. And the electret condenser mics are limited by their own self noise eg. 32dB for Panasonic WM61A or distortion limits.
Of course we would all love to have an industrial grade measurement setup like an Audio Precsion or Klippel Near Field Scanner. Maybe there are necessary to measure drivers like Purifi or Bliesma… but that opens a new can of worms (if the drivers are lower distortion than the microphones…)
On the other hand a lot of microphones for the “prosumer” market seem to be for music/content creation/recording where anything goes.
A microphone for DIY distortion measurement would need to be
1) Omindirectional
4) Flat from 20 to 20 KHz( (+/- ? dB)
3) low self distortion of 0.01% when observing-
x dB @1m
Hence max SPL of
(x+34) dB with 0.5% THD
(x+40) dB at 1% THD
(x+50) dB at 3% THD
4) self noise of 20dB? 15dB? Other?
5) Phantom powered
6) closer to $200 instead of $2000
Is this even possible?
Then we’ll have a winner
Of course there are some tricks that may or may not help. Eg. use of repetitions to lower the noise, or stepped sign measurements.
Moving the mic closer will minimise room reflections for clearer measurements of bass frequencies that increases the observed SPL (and distortion) by the mic by 10-25dB.
Is there a way we can “subtract” the microphone distortion, the way we can subtract the microphones non-flat frequency response?
From the audio output to the amplifier to the speaker to the microphone back to the input.
So whilst a USB mic is very convenient, they were probably never intended for distortion measurements because they are limited to the built-in interface. And the electret condenser mics are limited by their own self noise eg. 32dB for Panasonic WM61A or distortion limits.
Of course we would all love to have an industrial grade measurement setup like an Audio Precsion or Klippel Near Field Scanner. Maybe there are necessary to measure drivers like Purifi or Bliesma… but that opens a new can of worms (if the drivers are lower distortion than the microphones…)
On the other hand a lot of microphones for the “prosumer” market seem to be for music/content creation/recording where anything goes.
A microphone for DIY distortion measurement would need to be
1) Omindirectional
4) Flat from 20 to 20 KHz( (+/- ? dB)
3) low self distortion of 0.01% when observing-
x dB @1m
Hence max SPL of
(x+34) dB with 0.5% THD
(x+40) dB at 1% THD
(x+50) dB at 3% THD
4) self noise of 20dB? 15dB? Other?
5) Phantom powered
6) closer to $200 instead of $2000
Is this even possible?
Then we’ll have a winner
Of course there are some tricks that may or may not help. Eg. use of repetitions to lower the noise, or stepped sign measurements.
Moving the mic closer will minimise room reflections for clearer measurements of bass frequencies that increases the observed SPL (and distortion) by the mic by 10-25dB.
Is there a way we can “subtract” the microphone distortion, the way we can subtract the microphones non-flat frequency response?
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Not that I know of. But do we have to? Loads of research papers show that our ‘hearing chain’ is quite indifferent to nonlinear distortions and measured by the popularity of tube amps, fullrange speakers and studio gear adding harmonic products to the original signal I’d say a lot of us even like them. So is that what we have access to not already more than good enough?
Yes you make a good point.
If our measurement imprecision means that all our drivers are much better (by 6-10dB; an order of magnitude) than we previously thought they are, then do we rejoice?
That there's no need for brand B or P drivers, because model K or J has already been available for a long time (decades)
No need for Scan-Speak Ellipticor, Illuminator or even Revelator. In fact the good ol’ classic with the copper sleeve, nylon or polypropylene cones and good old classic like JBL, Altec Lansing, Beyma, RCF and Fane of the last century were already GREAT!
The only thing this century has done has made things smaller. And more affordable. by making them more affordable (great sound for all) it has become also more popular and more disposable (Fast electronics)
OR does it mean that harmonic distortion, intermodulation distortion (two tone or multi-tone), higher order or non-harmonic distortion measurements, amplitude distortion taken by DIYers to bediscarded interpreted with a high degree of caution due to poor less than ideal methodology?
eg. microphone already contributing significant distortion, microphone not sensitive enough, measurement affected by reflections, affected by poor signal to noise ratio
Reference:
https://www.klippel.de/fileadmin/kl...aningful Tolerances for Signal Distortion.pdf
If our measurement imprecision means that all our drivers are much better (by 6-10dB; an order of magnitude) than we previously thought they are, then do we rejoice?
That there's no need for brand B or P drivers, because model K or J has already been available for a long time (decades)
No need for Scan-Speak Ellipticor, Illuminator or even Revelator. In fact the good ol’ classic with the copper sleeve, nylon or polypropylene cones and good old classic like JBL, Altec Lansing, Beyma, RCF and Fane of the last century were already GREAT!
The only thing this century has done has made things smaller. And more affordable. by making them more affordable (great sound for all) it has become also more popular and more disposable (Fast electronics)
OR does it mean that harmonic distortion, intermodulation distortion (two tone or multi-tone), higher order or non-harmonic distortion measurements, amplitude distortion taken by DIYers to be
eg. microphone already contributing significant distortion, microphone not sensitive enough, measurement affected by reflections, affected by poor signal to noise ratio
Reference:
https://www.klippel.de/fileadmin/kl...aningful Tolerances for Signal Distortion.pdf
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Omni1 is a leading contender based on my brief survey...
All figures are from manufacturer spec
italic denotes independent measurement
? denotes derived calculation
bold denotes leading contenders based on above requirements
* denotes MEMS based microphone
All figures are from manufacturer spec
italic denotes independent measurement
? denotes derived calculation
bold denotes leading contenders based on above requirements
* denotes MEMS based microphone
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That Omni1 is becoming unobtanium when you proceed like this .
But you could consider those suppliers too:
.But you could consider those suppliers too:
- Roga (lots of more or less affordable IEPE based solutions, but wait! an interesting IEPE to XLR-48V converter too!)
- Mipro (MM100)
- Dr. Jordan (quite a few competitive mikes)