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Old 28th October 2020, 12:56 AM   #51
CharlieLaub is offline CharlieLaub  United States
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I wonder if you, bolserst, or David, can comment on the method I have been using for generating the free-field response of a dipole (e.g. H-frame). I first build it and then install the driver. I then take nearfield measurements at the front and rear, in the plane of the mouth of the H-frame tunnel. Next I import these responses as FRD files into a program for crossover modeling. For the rear response, I make sure that the phase is inverted (this may or may not be captured in the measurement depending on what type of phase response was saved). Then I delay the rear by some amount of time that represent how long it takes for sound to travel the front-to-rear pathlength difference. Finally I reduce the gain of the rear sound by the ratio of the distances of each source (the front and rear mouth openings) to the listening position for which I am generating the free field response. Finally, I add the two responses (these are complex quantities) and get the sum (magnitude, which is converted to SPL). I take this as the free field response at the position that I am simulating.

I like this approach because the measurements capture the tunnel resonances, which I don't like to model. It also correctly captures the driver's response when mounted in the H-frame, where it will experience some extra loading from the air mass in the tunnel. This can influence the F and Q of its response compared to e.g. free air.

This requires me to build the enclosure and take measurements and cannot be used to do a priori modeling. But it is enough for me to get data that I can then use as part of the system crossover design and I do not need to go outdoors to make the measurements so I can do it anytime.

Any thoughts on that approach?
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Old 28th October 2020, 06:21 AM   #52
David McBean is offline David McBean  Australia
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Quote:
Originally Posted by bolserst View Post
Adding a bit to what DMcBean and Bentoronto have been discussing:
Many thanks bolserst, a very valuable and interesting contribution to the discussion!
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Old 28th October 2020, 06:26 AM   #53
David McBean is offline David McBean  Australia
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Quote:
Originally Posted by CharlieLaub View Post
I wonder if you, bolserst, or David, can comment on the method I have been using for generating the free-field response of a dipole (e.g. H-frame).
Hi Charlie,

Definitely one for bolserst :-).

Kind regards,

David
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Old 29th October 2020, 05:25 AM   #54
bolserst is offline bolserst  United States
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Quote:
Originally Posted by CharlieLaub View Post
… can comment on the method I have been using for generating the free-field response of a dipole (e.g. H-frame)….This requires me to build the enclosure and take measurements and cannot be used to do a priori modeling. But it is enough for me to get data that I can then use as part of the system crossover design and I do not need to go outdoors to make the measurements so I can do it anytime.
A couple questions:
- How high in frequency are you using the response?
- How are you determining relative level compared to the rest of the loudspeaker when designing the crossover?
Perhaps it is a fully active system where you simply use DSP LP crossover and adjust level to taste?

As long as you are crossing over below ka=1 where the two sides of the H-frame are still behaving like point sources, your method will give you results that should match what you measure outside quite closely. Even the path difference you use will have only minor effect on the response shape below ka=1…only changing the level. As you approach ka=1, path difference selection will start having more and more effect on the response.

If it is important to have more accurate response above ka=1 where directivity of the H-frame sources is coming into play, you might consider using an approach similar to what JohnK recommended many years ago. Basically do the combined NF approach like you are currently doing, but supplement it with far-field measurement of the H-frame using a measurement window long enough that you can splice it in at ka=1. I would think an indoor ground plane measurement with mike 1-2m away should give you what you need. Later this weekend, I will have access to the data used for the outdoor example I posted and can use it to compare this splice approach with the "full-up" outdoor measurement.
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Old 29th October 2020, 02:29 PM   #55
CharlieLaub is offline CharlieLaub  United States
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Quote:
Originally Posted by bolserst View Post
A couple questions:
- How high in frequency are you using the response?
- How are you determining relative level compared to the rest of the loudspeaker when designing the crossover?
Perhaps it is a fully active system where you simply use DSP LP crossover and adjust level to taste?
I use this approach for H-frame subwoofers only. Therefore I do not need to have an accurate picture of the FR above, say, 200Hz. This nearfield-only method is not accurate for higher frequencies, of course.

Because I am generating the "free-field" (4Pi) response I usually have to adjust the level down by 3dB or so to match the rest of the system. I do that by ear, along with other minor tweaks to the crossover, after I have developed and implemented it, as a final "tuning". Also, one may want an elevated bass response (or not) and so I feel this can be left as an adjustable parameter to some extent. Yes, I use DSP and its a fully active system, so I can do this ad nauseum (but hope not to!).

Quote:
Originally Posted by bolserst View Post
As long as you are crossing over below ka=1 where the two sides of the H-frame are still behaving like point sources, your method will give you results that should match what you measure outside quite closely. Even the path difference you use will have only minor effect on the response shape below ka=1…only changing the level. As you approach ka=1, path difference selection will start having more and more effect on the response.

If it is important to have more accurate response above ka=1 where directivity of the H-frame sources is coming into play, you might consider using an approach similar to what JohnK recommended many years ago. Basically do the combined NF approach like you are currently doing, but supplement it with far-field measurement of the H-frame using a measurement window long enough that you can splice it in at ka=1. I would think an indoor ground plane measurement with mike 1-2m away should give you what you need. Later this weekend, I will have access to the data used for the outdoor example I posted and can use it to compare this splice approach with the "full-up" outdoor measurement.
Right, I am basically following Kreskovsky's method, but do not do the far field measurement for higher frequencies because they are out of band and I don't need that information. He described the method here:
measure a dipole
He uses the front mouth response also for the rear, but I actually measure the rear because it is often slightly different on account of the magnet/basket and minor position differences within the H-frame, or when using a U-frame.
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Last edited by CharlieLaub; 29th October 2020 at 02:36 PM.
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Old 30th October 2020, 06:00 PM   #56
audfrknaveen is offline audfrknaveen  India
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In terms of theory and technicalities, there is so much knowledge being shared in this thread, and I'm very thankful to everyone here !!!

I made the h frame (please see the attached image)
But I have to wait for a few days before I test it.
I'm occupied with some other activities.
Hopefully I will update listening impressions soon.
Once again thanks a lot guys

Regards,
Audfrknaveen
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Old 11th November 2020, 07:46 PM   #57
bolserst is offline bolserst  United States
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Very Nice
Looking forward to any measurements or listening impressions you can post.
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Old 11th November 2020, 07:54 PM   #58
bolserst is offline bolserst  United States
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Quote:
Originally Posted by CharlieLaub View Post
I use this approach for H-frame subwoofers only. Therefore I do not need to have an accurate picture of the FR above, say, 200Hz. This nearfield-only method is not accurate for higher frequencies, of course…I actually measure the rear because it is often slightly different on account of the magnet/basket and minor position differences within the H-frame, or when using a U-frame.
If your interest lie solely in response < 200Hz (which would fit the ka<1 threshold for all but the largest H-frames) then your method will give accurate response for use in crossover modeling. The accuracy of the SPL level will still be dependent on exactly where you take the NF measurements, and what source separation distance you use in your delay calculations. But as long you are setup to be able to easily adjust level up or down as needed, I wouldn’t think you need to fret too much about the exact value. I’m not sure what you use for separation distance, but the Linkwitz recommendation for Hframes [ d1 + d2 + 2*(w/2) ] is a good rule of thumb that is reasonably close to the results of the acoustic center calculations.
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