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How to make quasi-anechoic speaker measurements/spinoramas with REW and VituixCAD

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napilopez

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Unfortunately I have no experience measuring big heavy speakers in spinorama form and I'm not in a hurry to start!

That said, maybe something like this setup from audioholics if you can measure outdoors?

image


@hardisj also had a similar setup with a rolling scaffolding thingamajig at one point before the NFS saved his back:

index.php


(How coincidental that both of these images are for the same speaker too!)

Seems like getting it up there is still going to be a pain, but at least such a setup should provide a stable enough surface.
 

morpheusX

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Thank you, the scaffolding is a great idea, i'm going to try to borrow one for this :)

If i can't, i just discovered that these type of scaffold can be rented, around 5 to 10€ a day.
One of these would be perfect:
5OkV6Zu.png
 
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morpheusX

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I have two questions regarding Measurements:
- How high is good enough? Is 1.60m good enough, or it should at least be a bit higher?
- Is it possible to invert the speaker in the horizontal measurements? i.e., instead of having the speaker upright, have it upside down? This would help immensely in the vertical measurements, to guarantee that the Maximum Height variation between H and V measurements is within 5cm ...

Thank you!
 

fluid

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I have two questions regarding Measurements:
- How high is good enough? Is 1.60m good enough, or it should at least be a bit higher?
You can see in previous pages the discussion on the effect of the distance between the speaker and the first reflection point which will determine the gate time. You can put the figures into the Vituix calculator and vary the height and measuring distance to see the effect on the resolution of the measurements as frequency goes down. 1.6 is better than 1 but worse than 3 or 5 or 10 :)

Outside higher is better but it becomes harder to go higher with a big speaker. Inside it is a balance to get the longest gate time.

Do the best you can check the result and see if you need to use other measurement techniques to fill in where the resolution of the gate makes the measurements ambiguous.
- Is it possible to invert the speaker in the horizontal measurements? i.e., instead of having the speaker upright, have it upside down? This would help immensely in the vertical measurements, to guarantee that the Maximum Height variation between H and V measurements is within 5cm ...

Thank you!
Horizontally yes if you measure at the same axis point the relative offsets of the drivers and phase relationships will be the same as the rotation point doesn't change. What might change is effects from the platform being measured on if it is now close to the tweeter. Indoors the reflection pattern might change depending on the size of the speaker relative to the room size. To measure vertically with the speaker horizontal the measurement point has to be coincident with the point of rotation which means there is only one position it will work. There is a good diagram in the CTA 2034 standard that shows the orientations for measuring.
 

witwald

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I have two questions regarding Measurements:
- How high is good enough? Is 1.60m good enough, or it should at least be a bit higher?
- Is it possible to invert the speaker in the horizontal measurements? i.e., instead of having the speaker upright, have it upside down? This would help immensely in the vertical measurements, to guarantee that the Maximum Height variation between H and V measurements is within 5cm ...

Thank you!
The 1.6m height can be used with gated measurement methods. However, it's not ideal, as the first reflection from the ground plane will come quite early. What would be preferred is a measuring height of about 4m. After that the benefits of a higher height become less noticeable.

Mounting the typical loudspeaker upside down for all the measurements is potentially a good idea. That puts the omnidirectional woofer output further away from the ground plane, making the reflections as weak as possible.

If taking these measurements outside, keep in mind that having the test platform mounted on soil versus concrete might be useful. The soil would have some low-frequency absorption properties that would attenuate the reflected sound sound waves.

It's not clear to me what you mean when you say "to guarantee that the Maximum Height variation between H and V measurements is within 5cm". Can you clarify what you meant?
 

morpheusX

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The 1.6m height can be used with gated measurement methods. However, it's not ideal, as the first reflection from the ground plane will come quite early. What would be preferred is a measuring height of about 4m. After that the benefits of a higher height become less noticeable.

I've received some quotes for renting the scaffold, and although the cost per day is cheap (15€/day), the shipping costs are very high (around 150€). As such, i will probably buy one, i should be able to get one (around 140€) that allows the speaker at around 2m Height.

If taking these measurements outside, keep in mind that having the test platform mounted on soil versus concrete might be useful. The soil would have some low-frequency absorption properties that would attenuate the reflected sound sound waves.
No way i could have the scaffold in unstable ground, these speakers are composed by 3 separate boxes with a total weight of 50kg :)
I'm actually frightened by the idea of rotating the scaffold, and a box fell on the ground :(

It's not clear to me what you mean when you say "to guarantee that the Maximum Height variation between H and V measurements is within 5cm". Can you clarify what you meant?

As mentioned, i will buy/rent, a scaffold to place the Speaker boxes, which i will use the maximum allowed height to place the speaker.
As can be seen in the CTA-2034A diagram, the height of the turntable for Horizontal measures (1) is lower than the height for the Vertical measures (2):
eB9b7Fi.png


But, as i want to use the maximum height provided by the scaffold, and given that the tweeter is the axis where the mic is pointing, i would only be able to do it if i increase the height for the Vertical measures.

Now, assuming that it would be possible to turn the speaker upside down, the tweeter would be almost in the same position for both the Vertical and the Horizontal measurements, thus allowing to use the maximum height provided by the scaffold.

I hope i was able to provide a clear response (i'm not a native speaker) :)
 

witwald

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@morpheusX Thank you for the detailed and clear description of the measurement setup that you intend to use. I'm thinking that using a scaffold instead of a turntable is going to be quite challenging when it comes to maintaining constant the location of the rotation axis during multiple measurements. You may need to place some sort of upright rod in a fixed position so that you can always position the centre of rotation that you have defined so that it will be at the same location in 3D space each time you need to move the scaffold.

I must admit that it's not clear to me the intent behind the ANSI/CEA-2034-A for that 5cm maximum offset distance between horizontal and vertical measurements. Maybe it is just so that the microphone does not need to be moved?

When performing the measurements, will you also be placing some acoustic absorption wedges on the ground between the speaker and the microphone? That could be helpful in attenuating the sound reflection from the floor and thus reducing its effect on the measurements.
 

fluid

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No way i could have the scaffold in unstable ground, these speakers are composed by 3 separate boxes with a total weight of 50kg :)
I'm actually frightened by the idea of rotating the scaffold, and a box fell on the ground :(
The ground under platform needs to be firm and level but the ground under where the speaker fires could be anything which is perhaps the arrangement witwald imagined.
But, as i want to use the maximum height provided by the scaffold, and given that the tweeter is the axis where the mic is pointing, i would only be able to do it if i increase the height for the Vertical measures.
I think those diagrams are confusing in the sense that they are trying to demonstrate that the centres of rotation and axis of measurement need to be no more than 5cm apart otherwise when combining the horizontal together they won't sum correctly as points on a sphere. I think the arrangement would be as witwald says below for a situation where the mic position is fixed and the platform moves. Moving the mic fixes that problem and if you use a laser pointer or some other method to ensure alignment there should be no issue. Alignment to within millimetres rather than inches should be possible.

You may need to place some sort of upright rod in a fixed position so that you can always position the centre of rotation that you have defined so that it will be at the same location in 3D space each time you need to move the scaffold.
I think you may have missed that in the image of Erin's setup above

Turntable fixed leg.png


 

ernestcarl

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I want.
better: send it to Amir lol

I’ve been too lazy to do it since I already have found the official measurements (on and off-axis) provided by Fostex. Their’s nothing very surprising. The main weakness of the monitor is the roll-off in bass and HF since it is just a small single driver and not a coax. I don’t even think it’s actually 4-inches — maybe smaller.

But I’ll measure it… remind me by the end if the week if I still haven’t. This monitor is used daily by me for other duties, and no way will I have it sent away for god knows how long.
 

Tangband

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Unfortunately I have no experience measuring big heavy speakers in spinorama form and I'm not in a hurry to start!

That said, maybe something like this setup from audioholics if you can measure outdoors?

image


@hardisj also had a similar setup with a rolling scaffolding thingamajig at one point before the NFS saved his back:

index.php


(How coincidental that both of these images are for the same speaker too!)

Seems like getting it up there is still going to be a pain, but at least such a setup should provide a stable enough surface.
Thank you for a very informative thread. :)
Here is some more details for beginners, that you probably already told:

1. Its good to see that a 50 dB window is used for the measurements, you often see people measuring with 80-100 dB that makes everything look flat. The professional standard is 50 dB.
If using smoothing, 1/6 oct is about the same as the ear will hear. Always save unsmoothed measurements.

2. The mic-holder can change the measurements result - one can use a long 1,5 metre plastic tube instead with the mic and cable attached. This is also the cause why Dayton imm6 shows innacurate results about 4 kHz - both the iPhone and the mics shape is causing diffractions with a bump at around 4 kHz .

3. When starting measurements as a beginner, its also good to read about the basics : psycoacoustics and the different laws of acoustics. Meaning:

*The precedence effect. The brains starts to select sounds at certain distances while the mic monitors all the sound.
*The HAAS effect and the law of the first wavefront.
*Critical distance.
*The speed of sound and wavelenghts of frequencies and the connection in time. This will help you measuring the right thing.

4. Read and learn about the differences between a rooms fundamental resonances and reflections.

5. Always measure only one loudspeaker at the time , the other turned off- read about the reason why.

6. Also read ”measuring loudspeakers ” by John Atkinsson at Stereophile. Its free to read on the net.

7. Read Dr. Tooles book ”Sound reproduction”.

Without this basic knowledge about acoustics a newbie can get seriously lost in the measurements.
I myself did those steps while measuring at the same time , the best thing to learn for real.
 
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pei

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The room certainly dominates the in room low frequency response but as a method for evaluating one speaker against another or checking that a design performs as intended these techniques still have value.

napilopez I am wondering if you have tried the ARTA method, as in the application note it is claimed that their simplified baffle step equalizer is more accurate.

"Note: ARTA and STEPS uses the previous expression for the estimation of the diffraction for spherical or rectangular baffled boxes. Some CAD and simulation programs are using a high-frequency geometrical model for the estimation of box diffraction at low frequencies. Such models can give larger errors on low frequencies than the simple model that is presented here"

https://www.artalabs.hr/AppNotes/AN4-FreeField-Rev03eng.pdf


You have a lot of data to compare against NFS and anechoic graphs so I wonder what the difference is.
I often make myself the same question. Which of the diffraction simulators is more accurate?

These are the results for ARTA (red), Soundeasy (green), VituixCAD (blue) and Jeff Bagby BDS (orange) applied on near field measurement of a Seas U16 (baffle 18 (w) x 33 cm (h), placed 9 cm axis X and 10.5 cm axis Y).

As you can see, near 2 dB difference between ARTA and the other three. ARTA does not take into account position of driver on baffle.


VituixCAD SPL.png
 

dc655321

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Is it possible to rotate the Mic into the required angles, instead of rotating the speaker?

One could rotate the mic around the speaker, but that is seldom practical.

Mics are directionally sensitive, but not like a laser beam. Some are even omnidirectional, so not very sensitive to changes in direction wrt source.
 
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napilopez

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Is it possible to rotate the Mic into the required angles, instead of rotating the speaker?

Yes, but as noted above it usually isn't practical. It adds extra variables, especially if measuring indoors. The two main issues are that 1) if measuring indoors, the distance to walls changes as you move the mic, and 2) you would have to be very careful to not change the distance from the microphone to the speaker as you move around the room, as that will affect the SPL level and though the shape of each off axis angle is unlikely to change much, it could misrepresent directivity (matters more if you are measuring at 1m than 2m, but still not ideal).

That said, I have moved the micr instead of the speaker on a couple of occasions where it was inconvenient to turn the speaker on its side for vertical measurements (side-firing woofers, for instance).
 

JRS

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So I am curious about measuring an active speaker--l suspect that the bass bin would be measured nearfield and spliced together just as a regular woofer and the approach for the restwould be to measure the midrange and tweeter separately with sufficient overlap. For example, if one anticipates a XO at 2000 Hz one would measure the mid out to say 4K or beyond, and the tweeter starting at some safe cutoff, say 1500 Hz and adjust the levels based on the manufacturers sensitivity data and figure out any additional offset based on the results.

Mic placing: should the mic be placed say midpoint between the acoustical centers, and left there for both measurements or placed on axis for each. I've seen both methods advocated, and in the past used the acoustic centers of each in accordance with the manufacturer's instructions. I suspect that for an asymmetric layout such as the one below, one mic placement, but where? BTW, I chose the layout for aesthetics, and wanting to stagger the dimensions to the baffle edge, as recommended by multiple sources. The planform for the baffle is curved, so this is just an approximation.
 

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Arash

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Does anyone know a practical method of measuring loudspeakers 3D-balloon (CLF) without an anechoic chamber using the near+far field measurements? Something similar to what Klippel NFS does but with spending more time measuring manually. Any idea?

I wish sound filed separation and 3D scanning technology will be more affordable, so everyone would be able to utilize it.
 

fluid

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So I am curious about measuring an active speaker--l suspect that the bass bin would be measured nearfield and spliced together just as a regular woofer and the approach for the restwould be to measure the midrange and tweeter separately with sufficient overlap. For example, if one anticipates a XO at 2000 Hz one would measure the mid out to say 4K or beyond, and the tweeter starting at some safe cutoff, say 1500 Hz and adjust the levels based on the manufacturers sensitivity data and figure out any additional offset based on the results.
For speaker design you want to measure as much of the response as possible of both drivers, for the tweeter measuring above free air resonance at lower volumes or with a protection capacitor should help to work out how far you can go before trouble starts.
Mic placing: should the mic be placed say midpoint between the acoustical centers, and left there for both measurements or placed on axis for each. I've seen both methods advocated, and in the past used the acoustic centers of each in accordance with the manufacturer's instructions. I suspect that for an asymmetric layout such as the one below, one mic placement, but where? BTW, I chose the layout for aesthetics, and wanting to stagger the dimensions to the baffle edge, as recommended by multiple sources. The planform for the baffle is curved, so this is just an approximation.
For a far field measurement you would place the mic at the point you intended to be the design reference axis, whether that be tweeter, mid or somewhere in between, including any horizontal offset, the phase offsets would be included in the measurement data so long as you don't move the mic. The point you pick will then represent the on axis data.

For near field measurements the mic is on the centre of the speaker very close. The mic location and the reference axis are no longer the same so the design software needs to add the baffle diffraction response and calculate the effect of the offsets on the response.

Asymmetric positioning seems like the wrong approach to dealing with baffle diffraction to me.
Something similar to what Klippel NFS does but with spending more time measuring manually. Any idea?
Right now there is no other technique freely available that removes the need to put a lot of work in manually or pay a lot of money to Klippel.
 
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JRS

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For speaker design you want to measure as much of the response as possible of both drivers, for the tweeter measuring above free air resonance at lower volumes or with a protection capacitor should help to work out how far you can go before trouble starts.

For a far field measurement you would place the mic at the point you intended to be the design reference axis, whether that be tweeter, mid or somewhere in between, including any horizontal offset, the phase offsets would be included in the measurement data so long as you don't move the mic. The point you pick will then represent the on axis data.

For near field measurements the mic is on the centre of the speaker very close. The mic location and the reference axis are no longer the same so the design software needs to add the baffle diffraction response and calculate the effect of the offsets on the response.

Asymmetric positioning seems like the wrong approach to dealing with baffle diffraction to me.

Right now is no other technique freely available that removes the need to put a lot of work in manually or pay a lot of money to Klippel.
Thank you, I am thinking at 1-2 meters, and so will aim at a midway spot on the line connecting.

The diffraction idea is to avoid hitting the same reinforcing frequencies in each direction. Whether it makes a significant difference is debatable, but it sims better, and I can't see where it does any harm to reduce the pass band ripple. I have heard that the major objection on the part of manufacturers has been having to produce and keep track of left/right pairs. I can't say, it may be about rotational symmetry for all I know. I've used it to reduce C2C spacing in D'Apolito configurations. Here it's mostly about the aesthetic, and I'm not crazy about the 7 inch distance from acoustic centers, but I am crazy about these particular ribbons, and side by side just seems odd.
 

fluid

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The diffraction idea is to avoid hitting the same reinforcing frequencies in each direction. Whether it makes a significant difference is debatable, but it sims better, and I can't see where it does any harm to reduce the pass band ripple.
You might want to consider this

"One thing that is important is to ensure that the tweeter is directly above the midrange (not offset). This can often be at odds with keeping drivers at different distances from cabinet edges, especially with very narrow enclosures. If there is an offset, you will get uneven dispersion around the crossover region, with the radiation pattern tilted [ 5 ]"

https://sound-au.com/articles/enclosures.htm#s9

With reference 5 being this

https://www.prosoundtraining.com/2013/01/25/tweeter-placement-two-way-loudspeakers/
 
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