A Pneumatic Loudspeaker Experiment Traveling Through Time: How Would You Measure It?

[NTK]

I still have no idea what you are talking about. When I see the term "pneumatic", these are the images I have in my brain:

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[DeltaSignum]

these are the images

Not exactly. It's more similar to pneumatic (air) engine, with the loudspeaker diaphragm instead of a piston.

 

[olieb]

If so, what would you expect to change when the pneumatic path is opened versus blocked?

I was just judging from the frequency responses of your measurement. That mimics a high pass with the mentioned parameters.
I would expect “something“ to change, but the output in both cases is identical. So I figured that the “pneumatics“, whatever it is, is not doing anything to the output (in the range shown).

 

[DeltaSignum]

is not doing anything to the output

Me too, perhaps except that the response appears to be shifted down by about 20 Hz from 160.

At 20 Hz (example), the wavelength is about 17 m, so a microphone at 2 cm is only about 0.0012 λ from the source. At that distance it is deep in the acoustic near field, and the result should not be interpreted as a conventional far-field SPL response of the complete system.

 

[Vladimir Filevski]

Acoustically, I have nearfield measurements taken about 2 cm from the cone

2cm is too big distance, should be less than 1cm, preferably 0.5cm.

At 20 Hz (example), the wavelength is about 17 m, so a microphone at 2 cm is only about 0.0012 λ from the source. At that distance it is deep in the acoustic near field, and the result should not be interpreted as a conventional far-field SPL response of the complete system.

Nearfield measurement is hemispherical (2-π), far-field is full sphere (4-π). Nearfield measurements boost frequency response by up to +6dB SPL, depending on the frequency and the enclosure dimensions. It is necessary to perform Baffle Step Compensation (BSC) on nearfield measurements to get accurate far-field frequency response.
Because your 2-inch loudspeaker enclosure baffle is very small, there is no need for BSC - both nearfield and far-field frequency responses below 150 Hz are identical (sans +6dB SPL difference), i.e. nearfield measurement can be interpreted as far-field frequency response.

 

[DeltaSignum]

t is, is not doing anything

I made another measurement in my advertising workshop. For this latest one I connected two modules in series to increase SPL. 5 cm is already too far to avoid room effects, and still too close to see clean interference between modules, so everything here is approximate.

I don't want to describe subjective listening impressions, what I hear with a signal generator or music. Anyone can try that - STL files, printing and assembly instructions are free at https://deltasignum.org/.

What interests me is how to measure this properly without having to sell the workshop to buy measurement equipment

What I found interesting is the drop from ∼160 Hz to ∼40 Hz - two octaves with average slope about 8 dB/octave (sealed box 12 db/oct). So, this "pneumatic engine" is just a product of my quarter-century-old imagination, or not - that my question for myself

P.S. On GitHub I tried to explain how it works, but I ran out of suitable acoustic terminology, so I had to improvise a little.

P.S.2 Adding one possible analogy to make the basic idea easier to understand.

 

[Vladimir Filevski]

For this latest one I connected two modules in series to increase SPL. 5 cm is already too far to avoid room effects, and still too close to see clean interference between modules, so everything here is approximate.
---
What interests me is how to measure this properly without having to sell the workshop to buy measurement equipment

Connecting loudspeakers in series doesn't increase SPL, providing the input signal is the same. Connect only one loudspeaker and measure it.
5 cm distance is too big, it leads to errors in measurement because of room modes and interference from the second loudspeaker. Set microphone distance at 1cm or less, and measure again.
You don't need to buy new measurement equipment, your REW is just fine.

 

[DeltaSignum]

Connect only one loudspeaker and measure it.

The previous measurements were already made with one module at 0.5-2 cm. This 5 cm measurement was an extra - using two modules in series (8 ohms total) with 8 V input, because one 2" module does not provide enough SPL at that distance compared with 1 cm. In the "pneumatic"logic the two modules in series should balance each other. Maybe you didn't enlarge the screenshot - the measurement conditions are noted on the graphs.

 

[olieb]

What I found interesting is the drop from ∼160 Hz to ∼40 Hz - two octaves with average slope about 8 dB/octave (sealed box 12 db/oct). So, this "pneumatic engine" is just a product of my quarter-century-old imagination, or not - that my question for myself

You should look for the slope in the range where it is approaching a constant value (that is below ≈100Hz) and not from the peak down, as that will give you a wrong value (typically too small as in this case).

5 cm is already too far to avoid room effects, and still too close to see clean interference between modules, so everything here is approximate.

Why don't you measure outside outdoors, on the floor (speaker and microphone) or on a big table?
EDIT: But I do not think there is a need, IME your near field (1-2cm) measurements show clearly what is going on.

Just from the looks I would say this is some kind of Helmholtz resonator with a VERY low tuning. The small volume (2) and the tube probably do not create much of extra resonances (Though a bit of that might be seen in the range ≥200Hz), so I am not so surprised that this behaves in all things practical like a closed box.

 

[DeltaSignum]

I would say this is some kind of Helmholtz resonator

Resonances exist everywhere, including in combustion engines. They can be part of the system without being the operating principle of the whole system.
Nobody calls an entire combustion engine a Helmholtz resonator just because resonances exist in its intake or exhaust system. Combustion happens in the cylinder, gas flow continues through the exhaust path, and resonant behaviour may appear in particular parts or frequency ranges.

My thinking here is similar: resonance may exist in "DZ / pneumatic acoustics", but I treat it as a parasitic condition, not as the engine of the system.

One of my goals was to find a way to make the air moved by the other side of the diaphragm behave as part of the diaphragm's moving mass, with the effective mass increasing as frequency decrease. Theoretically, under perfect conditions, the low-frequency response should be unlimited.

p.s. The compensator is usually silent. This can be determined from the OPEN/CLOSED behavior, or you can 3D-print it and check for yourself.

 

[Hayabusa]

2cm is too big distance, s

No it's good enough for low frequencies, even 10cm is useful

 

[Vladimir Filevski]

The previous measurements were already made with one module at 0.5-2 cm. This 5 cm measurement was an extra - using two modules in series (8 ohms total) with 8 V input, because one 2" module does not provide enough SPL at that distance compared with 1 cm. In the "pneumatic"logic the two modules in series should balance each other. Maybe you didn't enlarge the screenshot - the measurement conditions are noted on the graphs.

Oh, yes - I can see it now, when enlarged.
Measured at 1cm and with 1.5V signal, "closed" and "open" curves are identical. 1.5V should be into a linear regime for a 2" loudspeaker - I would trust these two measurements only.
I don't recommend signal higher than 2V, because there is a possibility for an overload - 3V seems to me too high for a 4 ohms 2" loudspeaker when exposed to low frequencies. Even worse is 8V for two loudspeakers in series - each one receives 4V, which is way too high for a 2" loudspeaker, both loudspeakers are certainly overloaded and measurement doesn't represent the real frequency response.
Two identical loudspeaker in series will have the same frequency response as one loudspeaker. Even total SPL stays the same with the same input signal.
Measurement at 5cm distance will receive unwanted output from the port and from the other loudspeaker - it is wrong to measure nearfield at such big distance.

 

[Vladimir Filevski]

No it's good enough for low frequencies, even 10cm is useful

It is bad. Measurement at 5cm or 10cm distance will receive unwanted output from the port (it is very close) and from the other loudspeaker (it is very close) - that is wrong way to measure nearfield frequency response of cone of the driver at such big distance.

 

[NTK]

No it's good enough for low frequencies, even 10cm is useful

For a 2 inch driver? (See post #1).

 

[Vladimir Filevski]

Theoretically, under perfect conditions, the low-frequency response should be unlimited.

Theoretically, that is perpetuum mobile! Not possible, even in perfect conditions.
Measured frequency response of your loudspeaker is typical for a 2" loudspeaker in sealed box.

 

[DeltaSignum]

Theoretically, that is perpetuum mobile! Not possible, even in perfect conditions.

Then this should also be a perpetum mobile, but it exists. Same 2" driver, x4. Just to understand it in 1/0 logic - does the response exist or not.

So, I see only one way: extend the system to 16 speakers and do the measurements in the far field. Or sell my workshop, cats and used cars and buy professional measurement equipment.

p.s. Not unlimited SPL, but unlimited extension in the sense that effective moving mass Mms(f) grows as frequency drops, so Fs(f) = 1/(2π√(Cms·Mms(f))) also drops. ((ff is the excitation frequency, and Mms(f)M_{ms}(f) is the effective moving mass at that excitation frequency.)). No energy is created, just mass loaded by the air moved by the other side of the diaphragm. Not a perpetum mobile, just a variable-mass system.

 

[Vladimir Filevski]

Then this should also be a perpetuum mobile, but it exists. Same 2" driver, x4. Just to understand it in 1/0 logic — does the response exist or not.
So, I see only one way: extend the system to 16 speakers and do the measurements in the far field. Or sell my workshop, cats and used cars and buy professional measurement equipment.

Put four of your 2" drivers in a simple closed box (serial-parallel configuration) - their frequency response will be the same as your measurement here, under the same conditions. Enlarging your system to 16 units will not bring anything new.
Your REW is just fine, you don't need to buy professional measurement equipment. The main problem is your inexperience in measuring loudspeakers.

p.s. Not unlimited SPL, but unlimited extension in the sense that effective moving mass Mms(f) grows as frequency drops, so Fs(f) = 1/(2π√(Cms·Mms(f))) also drops. ((ff is the excitation frequency, and Mms(f)M_{ms}(f) is the effective moving mass at that excitation frequency.)). No energy is created, just mass loaded by the air moved by the other side of the diaphragm. Not a perpetum mobile, just a variable-mass system.

Previously you wrote "unlimited low frequency response", now it is "unlimited extension" - both are impossible, in practical sense. Strictly speaking, any 2" driver mounted in any type of enclosure has "unlimited low frequency response", but his SPL will be microscopically small at low frequencies, without any practical use.

 

[DeltaSignum]

reviously you wrote "unlimited low frequency response", now it is "unlimited extension

Sorry for my mistake. I didn't mean that I was going to destroy Tokyo with a 2" loudspeaker, so I clarified that I was not talking about SPL.

But what about the ~8 dB/octave average slope from ~160 Hz to ~40 Hz, and the repeatedly appearing structure in the infrasonic region below ~10 Hz? Doesn't that remind you of an engine?

 

[Vladimir Filevski]

But what about the ~8 dB/octave average slope from ~160 Hz to ~40 Hz, and the repeatedly appearing structure in the infrasonic region below ~10 Hz? Doesn't that remind you of an engine?

No, no engine at all.
Use the same 2" driver, put it in a simple closed box and measure again. Measurements will be the same, indistinguishable from your present ones.

 

[DeltaSignum]

Measurements will be the same, indistinguishable from your present ones.

Same 2" driver, sealed box, 1 cm, 1.5V as suggested. Looks different.
Infra looks similar - apparently a UMIK-1 noise artifact.