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Multi-Way Conventional loudspeakers with crossovers

Generating driver FRD setup questions for passive xovers
Generating driver FRD setup questions for passive xovers
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Old 27th June 2018, 01:44 AM   #11
artoff is online now artoff
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Quote:
Originally Posted by CharlieLaub View Post

2. Perform measurements:
Thank you for detailed information. Most of the answers was exactly what i searched, however i have one question.

How i can get 2 way bookshelf speaker correct FRD file which has front PORT on it.

Should i need to measure the port separately, and if yes, how i should do that?

Thank you guys this is very very helpful for me
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Old 27th June 2018, 03:41 AM   #12
CharlieLaub is offline CharlieLaub  United States
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If the speaker is not large and the port and driver cone are right next to each other, I think you can get away with positioning the mic in between the two for the nearfield measurement. It is not as good as being right next to the driver's dustcap (the valid upper limit of the nearfield data will be reduced somewhat), but it should work out fine on a small bookshelf monitor type speaker.

If the port is on the rear of the enclosure, you have to measure the port and driver outputs separately and must scale the port output by the ratio of the areas of port and cone and then account for the relatively longer distance from port to listener. There is an online tech note about nearfield measurements of ported system by Klippel here:
Nearfield Measurement with multiple Drivers and Port
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Old 27th June 2018, 05:26 AM   #13
kimmosto is offline kimmosto  Finland
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For information:

Quick manual for measurements and data preparation
Includes recommended measurement gear, method and data processing.
Dual channel measurement gear and mode is recommended to support simulation of off-axis responses, power response and DI.

Home of VituixCAD
Feature lists of VituixCAD and tools.
Links to youtube video lessons.
Download of test projects.
Download of setup, versions 1.1 and 2.0.
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Old 27th June 2018, 04:42 PM   #14
keithostertag is offline keithostertag
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@CharlieLaub- in reading Bagby's white paper, which describes using FRDBlender, he always says he's using a baffle, not a completed box. I want to confirm with you that is OK, since I have some drivers which I don't yet know what type or size box to put them in. My thought is that as long as I use a baffle that's very close to the final size (to get the step response) that I don't need to have the drivers in a completed box. Correct?
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Old 27th June 2018, 05:00 PM   #15
CharlieLaub is offline CharlieLaub  United States
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DO NOT USE ONLY A BAFFLE. caps intended for emphasis! Really.

Using only the front baffle will result in a dipole unless the baffle is very large (e.g. 1x1m or larger). Even then, the response will be quite different than when the driver is put in an enclosure eg. "box".

The whole point of measuring the driver in its final enclosure is to "measure" the exact diffraction and baffle step response that will occur for that driver, in that enclosure, at that location in the baffle. When you change those things, the response will also change.

If you are "not sure" about your enclosure, then I suggest doing some modeling of the low end from TS parameters (which you should also measure), modeling of the baffle step as a function of the baffle shape, and then look at the manufacturer's SPL plots to get an idea about the driver's response in the breakup region (at higher frequencies). I use all of these approaches to figure out approximately where I will cross over a driver, and what I might expect when I put it in the box. At some point you just have to jump in and try it. You can make up a test box out of particleboard if you want to experiment with different sizes/volumes and shapes.

Also, regarding Jeff mentioning the baffle: it's essentially the dimensions of the front baffle, and the location of the driver in that baffle, that determine the "baffle step" effect on the driver's response. The "depth" of the box does not really contribute. That is why Jeff talks about the baffle (the front baffle) because that is what is influencing the response. It's not because he is only using the front baffle and the rest (sides and/or back) is open.
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Last edited by CharlieLaub; 27th June 2018 at 05:13 PM.
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Old 27th June 2018, 07:19 PM   #16
keithostertag is offline keithostertag
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OK, thank you for the clarification, I really appreciate it.
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Old 2nd July 2018, 06:00 AM   #17
artoff is online now artoff
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Quote:
Originally Posted by CharlieLaub View Post
If the speaker is not large and the port and driver cone are right next to each other, I think you can get away with positioning the mic in between the two for the nearfield measurement. It is not as good as being right next to the driver's dustcap (the valid upper limit of the nearfield data will be reduced somewhat), but it should work out fine on a small bookshelf monitor type speaker.

If the port is on the rear of the enclosure, you have to measure the port and driver outputs separately and must scale the port output by the ratio of the areas of port and cone and then account for the relatively longer distance from port to listener. There is an online tech note about nearfield measurements of ported system by Klippel here:
Nearfield Measurement with multiple Drivers and Port
Thank you very much!
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Old 9th July 2018, 11:48 PM   #18
SyncTronX is offline SyncTronX  United States
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@Charlie or others,

After reading through the thread, am I right and assuming there are two nearfield and two far field measurement taking place?
  • A. I would also assume the first set if near field and far field you are describing are between the speaker driver and it's resonance within it's speaker cabinet?
  • B. Ed Simon had described the method to determine near field and far field by using and SPL meter and walking forward from the back of the room and when the SPL meter reads +3dB from the original reading, then at that point moving froward to the speakers I would be moving in the near field.
B method would be for speakers in a room/venue etc.
So, A, the bagby method is for the drivers and the cabinet. And B, the speakers and the room.

If so or if not, then please clarify and add to the discussion.

Cheers,
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Last edited by SyncTronX; 9th July 2018 at 11:50 PM.
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Old 10th July 2018, 01:24 AM   #19
CharlieLaub is offline CharlieLaub  United States
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I couldn't quite follow what you are saying RE "two measurements" etc.

The goal is to build a "wideband" and quasi anechoic measurement of the driver-in-box. Wideband means for all frequencies, DC to infinity. Quasi-anechoic means without the influence of room reflections and room resonances.

You cannot make a wideband measurement directly. Instead you use a combination of measurements, modeling, and merging of data sets. The measurements will typically include farfield and nearfield types.

A nearfield measurement is done very close (e.g. 1cm away from dustcap) of a driver. At this distance the room reflections and resonances are much lower in intensity than the sound coming directly from the driver diaphragm. The advantage is that you can measure to a very low frequency (but still not DC!) e.g. maybe 1-10Hz. The disadvantage is that a nearfield measurement also does not capture any interactions of the sound wave with the cabinet, so the baffle step does not show up in the nearfield data. But because you can directly measure the low frequency response of a driver independent of the environment, it's very useful since few other techniques can do that (that you can do at home).

A far-field measurement, on the other hand, does capture the "baffle step" that is created from the diffraction from cabinet edges, etc. Remember that measurements are a time record of what the microphone picks up. At some point in time, the microphone will also start to gather sound that has reflected off of a room boundary first before reaching the mic. The closest boundary will show up first in the time record, then the next, then multiple reflections, etc. When you try to use all the data to generate the frequency response, you get a lot of interference and the frequency response coming only from the speaker is difficult to determine. The goal is to take a measurement that does NOT include these interferences so that you see only the sound produced by the microphone.

In the far field measurement, there are a few milliseconds of time after the sound first reaches the microphone that do not contain reflections - it's only the sound coming from the driver and the cabinet/diffraction. It's possible to use only this portion of the microphone's measurement, that is to say the first 5-10 milliseconds. The DIYer sets a "gate" after which all the microphone data is set to zero. Then this "gated" data is converted from the time domain into the frequency domain. The result is a frequency measurement where no room interaction is present, just like if the speaker could magically be suspended in free space far away from any boundary. But there is a penalty for setting the data points to zero: you cannot determine the response at "low" frequencies. How low depends on how much data you are able to keep after the microphone first picks up the sound and when the first reflection appears in the time record from the microphone. It turns out that the lowest frequency that you can practically resolve is about 200-300Hz indoors. Since the baffle step is not always completely finished by this frequency and may extend down to 100Hz or so, we don't have all that we need.

The current practice of many DIYers is to try and merge the nearfield and farfield measurements. But this can only be done if the nearfield measurement is "corrected" so that is includes the baffle step. Once that is done the DIYer can overlay the two frequency data sets and combine them. One convenient way to correct the nearfield measurement data for the baffle step is to add to it a modeled baffle step response that takes into account the dimensions of the front baffle (with and height) and the driver diameter and its location on the baffle. Once the nearfield has been corrected, you are able to merge a data set that has good low frequency info (the corrected nearfield) and good high frequency info (the gated farfield) to get a merged frequency response. To extend that to the "wideband" response you assume that the behavior at the low and high frequency extremes, above and below the merged frequency response respectively, has a constant slope. For instance a driver in a closed box has an ultimate rolloff of 12dB/oct. Determining the high frequency rolloff takes a little guesstimation but it's likely between 18 and 30dB/oct. These extra "constant slope" parts of the frequency response are known as "tails" because they tail off to infinitely high and infinitely low (towards DC) frequencies. Adding the tails gives you the wideband response.

NOTE: for a driver like a tweeter, the far-field data already captures the response down to 200Hz or so. This includes below-resonance where the response is asymptotically reaching it's ultimate rolloff rate of 12dB/oct. Most people do not bother to make a near-field measurement on a tweeter for this reason. You just don not really need that data. Likewise, a port is really only operating at low frequencies, so you do not typically make a far-field measurement of a port, and it would be mixed up with the woofer sound anyway. A nearfield measurement on a port is all you need. For other drivers like woofers and midranges, or any driver that has a response passband extending below about 500Hz, both a nearfield and farfield measurement are a good idea.
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Last edited by CharlieLaub; 10th July 2018 at 01:34 AM.
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Old 10th July 2018, 01:58 AM   #20
SyncTronX is offline SyncTronX  United States
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Hi Charlie,

I just read your response after I added to the thread the following questions. Thank's for clarifying.

Not to hijack Keiths thread, but when I read the link of the Kimmosto's "quick manual for measurement..." file I noticed the speaker tower shares similarities to the speakers I inherited and in process of having them live again. I have new drivers for them that will need to measure. Where would place the mic to measure the two side firing woofers for this cabinet? For the Driver,

and then for the listening position: In the near field. In the far field.

Where to place the mic for rear bass port?

I guess flush with panel in middle of port?

Cheers,
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Last edited by SyncTronX; 10th July 2018 at 02:10 AM. Reason: Update after CL's response.
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