GOAL:

To find a moderate cost

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,

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.

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

I use software to calculate H2 and H3 in relative terms, compared to the fundamental:

Comment: It appears that

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

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)

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

The third harmonic for 4 microphones:

Comment: There are

Now we take measurements taken with the microphones in the near field (1cm) to look at low frequency response:

Comment: There is a

Comment: There is a

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…

Here is the measured indoors at a mic distance of 1cm with Sennheiser dynamic mic MD42 (“S-mic”)

COMMENT: Measurements indoor with mic distance of 1cm give good correlation above 150Hz. Below 150Hz the effects of the enclosure of 1/2 cu ft, there is a difference compared to the infinite baffle measurement by Purifi. It's unclear whether this difference can be reconciled.

Now the second part of Purifi's datasheet for 600Hz to 6KHz…

Sennheiser omnidirectional dynamic mic @ 40cm:

Compared to an indoor measurement at 0.4m using the S-mic, above 600Hz there is indeed a similar correlation between the S-mic and Purifi's datasheet.

Appendix:

PART 1b:

When we take a measurement indoors, we are prone to all kind of reflections, because sound can act like a wave. If you've ever taken a measurement of your speakers are your listening position, you may have noticed how the frequency response looks nothing like the straight lines from the manufacturer's data. As a countermeasure, we measure at a closer distance, and "block" all those reflections from affecting our measurement. One way to do this is to use a "gate"- to stop the reflections from being interpreted.

With a "gate" of 7 milliseconds, we can block the first echo, which is usually the floor (or ceiling) from affecting our measurement. Perfect right? Well, not so fast. Since we've used a gate of 7ms, the measurement precision is limited to about 150Hz. (142Hz to be exact, since frequency = 1/Time period; T being 0.007 seconds)

This means that EVERYTHING below 142Hz is invalid.

In the octave between, 2-4 KHz, which spans 2000Hz, our resolution is 142 / 2000 = ~1/14th of an octave. Between 500Hz and 2000 Hz, which is 2 octaves and having a numerical span of 1500Hz, we have a measurement resolution of 142/1500= ~1/10 over 2 octaves. And between 142- 425Hz, we're stuck with 1/3 octave resolution. So even though the curve looks nice and smooth on the left side of the graph, there is less accuracy in those lower frequencies.

Let's go back to look at the original graph:

What's the immediate consequence of this? The measurement below 600Hz is affected by room reflections (floor bounce)

(NB. To get a complete frequency response measurement, one option is to merge or blend the two parts together. First we, take near field measurements, a technique just 50 years new, by Don B. Keele . In short, we move the microphone up again the cone and take measurements. This way we know what the woofer or port is doing. This method largely ignores what the room is doing. Later, we can even apply some correction factors later to see what it might look like at 1m, or 3m, or 10m, and then blend the low frequency and high frequency measurements together. Here the late Jeff Bagby explains his near and far field blending process)

We must have strategies to measure, even if we had a Klippel Near Field Scanner or anechoic chamber. This is what Wolfgang Klippel does need to take a measurement in his office, which doesn't have space for his Near Field Scanner. He uses a big table for a ground plane measurement of a small consumer speaker:

Reference: Acoustical Measurement of Sound System Equipment according IEC 60268-21, Session #4: Simulated standard condition at an evaluation point, slide 8.

So one method is to measure the high frequencies in the "far field", and the low frequencies in the "nearfield". Please note that this term is NOT the same as the same term as that used in pro-audio use eg.

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"

-Different microphones to come...

-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**.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:

Comment: It appears that

**H2 ~-60dB between 100Hz and 3 KHz**and**H3 ~-70dB between 200Hz and 3KHz**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 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)

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 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 up to 10dB between the microphones in measurement of the 2nd harmonic when observing 110-116 dB**(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)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…

Here is the measured indoors at a mic distance of 1cm with Sennheiser dynamic mic MD42 (“S-mic”)

COMMENT: Measurements indoor with mic distance of 1cm give good correlation above 150Hz. Below 150Hz the effects of the enclosure of 1/2 cu ft, there is a difference compared to the infinite baffle measurement by Purifi. It's unclear whether this difference can be reconciled.

Now the second part of Purifi's datasheet for 600Hz to 6KHz…

Sennheiser omnidirectional dynamic mic @ 40cm:

Compared to an indoor measurement at 0.4m using the S-mic, above 600Hz there is indeed a similar correlation between the S-mic and Purifi's datasheet.

Appendix:

PART 1b:

*Measurement imprecision*When we take a measurement indoors, we are prone to all kind of reflections, because sound can act like a wave. If you've ever taken a measurement of your speakers are your listening position, you may have noticed how the frequency response looks nothing like the straight lines from the manufacturer's data. As a countermeasure, we measure at a closer distance, and "block" all those reflections from affecting our measurement. One way to do this is to use a "gate"- to stop the reflections from being interpreted.

With a "gate" of 7 milliseconds, we can block the first echo, which is usually the floor (or ceiling) from affecting our measurement. Perfect right? Well, not so fast. Since we've used a gate of 7ms, the measurement precision is limited to about 150Hz. (142Hz to be exact, since frequency = 1/Time period; T being 0.007 seconds)

This means that EVERYTHING below 142Hz is invalid.

In the octave between, 2-4 KHz, which spans 2000Hz, our resolution is 142 / 2000 = ~1/14th of an octave. Between 500Hz and 2000 Hz, which is 2 octaves and having a numerical span of 1500Hz, we have a measurement resolution of 142/1500= ~1/10 over 2 octaves. And between 142- 425Hz, we're stuck with 1/3 octave resolution. So even though the curve looks nice and smooth on the left side of the graph, there is less accuracy in those lower frequencies.

Let's go back to look at the original graph:

What's the immediate consequence of this? The measurement below 600Hz is affected by room reflections (floor bounce)

(NB. To get a complete frequency response measurement, one option is to merge or blend the two parts together. First we, take near field measurements, a technique just 50 years new, by Don B. Keele . In short, we move the microphone up again the cone and take measurements. This way we know what the woofer or port is doing. This method largely ignores what the room is doing. Later, we can even apply some correction factors later to see what it might look like at 1m, or 3m, or 10m, and then blend the low frequency and high frequency measurements together. Here the late Jeff Bagby explains his near and far field blending process)

We must have strategies to measure, even if we had a Klippel Near Field Scanner or anechoic chamber. This is what Wolfgang Klippel does need to take a measurement in his office, which doesn't have space for his Near Field Scanner. He uses a big table for a ground plane measurement of a small consumer speaker:

Reference: Acoustical Measurement of Sound System Equipment according IEC 60268-21, Session #4: Simulated standard condition at an evaluation point, slide 8.

So one method is to measure the high frequencies in the "far field", and the low frequencies in the "nearfield". Please note that this term is NOT the same as the same term as that used in pro-audio use eg.

*nearfield*OR*midfield monitors*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"

*To Be Continued… (live article)*-Different microphones to come...

-Implications...

Last edited: