I haven't measured a full set of polars from horns for quite a while. I think the last time I did measure I rotated the horn at its mouth (point B in the image) for the measurements.
Over the years I have read some people rotating at the throat end of the horn.
Which is correct ? (where do you rotate if you have a speaker with direct radiator bass units and a horn loaded mid/tweeter ?)
Thanks,
Rob.
Over the years I have read some people rotating at the throat end of the horn.
Which is correct ? (where do you rotate if you have a speaker with direct radiator bass units and a horn loaded mid/tweeter ?)
Thanks,
Rob.
Attachments
Hi Rob, definitely from the throat end.
Ideally, the acoustic center will be right over the axis of rotation, and TOF's will measure the same when rotated.
Ideally, the acoustic center will be right over the axis of rotation, and TOF's will measure the same when rotated.
Same with me, the baffle being the pivot point. Have you got something exciting you will be sharing?
In the end it shouldn't matter which point you use as long as you measure well.
You might enable more information if you choose the acoustic origin but this varies with frequency on a practical horn and can be a challenge to understand and find.
You might enable more information if you choose the acoustic origin but this varies with frequency on a practical horn and can be a challenge to understand and find.
Must disagree.
It's very advantageous ime, to properly locate acoustic centers of each driver. When they stay stable under rotation, you know you have.
Spot on timing between driver sections, that holds up under rotation, improves both on and off axis measurements.
Also, acoustic centers vary so little with frequency, the common wisdom about them varying by frequency almost goes into the myth bucket imo.
The apparent variation, different TOF vs loopback etc, of driver's acoustic centers..... is usually more due to their bandpass changing, or more precisely their high-pass frequency varying off-axis, than anything else...ime. Iow, it's more about FFT math than true varience.
I wasn't actually talking about that. In any case you get there either way.
Again I've maybe not communicated well. I chose a certain way of saying that it is a virtual point, that you can't just put your finger on the throat itself.
Also, mentioning the fact that they can vary is meant as nothing more that a rhetoric indicator to self that one has grasped the concept of the virtual origin.
The time of flight to the measurement microphone determining the acoustic center for a conical or straight sided horn as depicted in the OP will remain the same when rotated from the throat axis, assuming the microphone has “line of sight” to the phase plug/throat entrance.
The time of flight to the mic from the phase plug throat entrance of a horn with a vertical diffraction slot becomes longer measured more off axis in the horizontal plane compared to the vertical, as a longer distance is traveled going “around the bend” than in the vertical path which has a straight path at any angle confined within the horn wall boundary.
Art
What an interesting subject...
Note the Charlie Hughes article, linked just above, assumes that the measurements are in an anechoic environment, and that the application is cinema or PA--not home hi-fi. Everyone I know in home hi-fi is listening at 3-4 m from the loudspeakers (mostly 3m--from the front baffle of the loudspeakers), so the advice given in Hughes' article isn't of much use to the home hi-fi world, who always listen in the "near field" below some frequency--usually around the Schroeder frequency of their listening room. For headphones, just about everything is nearfield.
However, you can see the difference in transfer function using nearfield vs. farfield when you open up the EASE viewer and calculate the "balloon" for the loudspeaker that you're looking at. There is a 1,2, 5, etc. metre distance that you can select. If you're worried about the differences measuring in the "nearfield"--inside one wavelength--you can quantify that "measurement error" if you use 1m (which I always recommend except at subwoofer frequencies).
Note that the question raised by the OP...
If you, however, time align the multiple apertures using digital delay based on the mouths of said horns--at least in the horizontal direction assuming vertical stacking of the horn apertures--and place the horns with their mouths in the same physical vertical plane in space, the off-axis time alignment also stays the same as the on-axis measurement,
So for the question raised, the real question is, are you using multiple ways employing multiple horn or driver apertures, or are you using a single horn--a multiple entry horn? That is the real question once you start to integrate the loudspeaker ways together.
Chris
Note the Charlie Hughes article, linked just above, assumes that the measurements are in an anechoic environment, and that the application is cinema or PA--not home hi-fi. Everyone I know in home hi-fi is listening at 3-4 m from the loudspeakers (mostly 3m--from the front baffle of the loudspeakers), so the advice given in Hughes' article isn't of much use to the home hi-fi world, who always listen in the "near field" below some frequency--usually around the Schroeder frequency of their listening room. For headphones, just about everything is nearfield.
However, you can see the difference in transfer function using nearfield vs. farfield when you open up the EASE viewer and calculate the "balloon" for the loudspeaker that you're looking at. There is a 1,2, 5, etc. metre distance that you can select. If you're worried about the differences measuring in the "nearfield"--inside one wavelength--you can quantify that "measurement error" if you use 1m (which I always recommend except at subwoofer frequencies).
Note that the question raised by the OP...
...opens a bit of a can of worms for those people that believe in "physical time alignment" of their multi-horn loudspeakers (using multiple horn apertures). Note that as you move off-axis from a "time aligned" set of horns with physical (not digital) time alignment, the more out of time alignment you get from the different length apertures. This can be quite large in terms of relative phase or time-of-arrival. As you move around to the side of a "time aligned" set of multiple horns, each horn foreshortens at a different rate based on the difference of their horn lengths.
If you, however, time align the multiple apertures using digital delay based on the mouths of said horns--at least in the horizontal direction assuming vertical stacking of the horn apertures--and place the horns with their mouths in the same physical vertical plane in space, the off-axis time alignment also stays the same as the on-axis measurement,
So for the question raised, the real question is, are you using multiple ways employing multiple horn or driver apertures, or are you using a single horn--a multiple entry horn? That is the real question once you start to integrate the loudspeaker ways together.
Chris
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Maybe there is a difference if you are doing driver integration (design!) or a system performance declaration for the end user (listener).
How about this one: https://inlowsound.weebly.com/uploads/1/9/8/5/1985965/4-26-2018-007_orig.jpg
I would argue that for na off axis measurement for a complete product intended to be descriptive for an end user, the baffle would be more useful. But as someone said, in this case it wouldn't differ much...
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How about this one: https://inlowsound.weebly.com/uploads/1/9/8/5/1985965/4-26-2018-007_orig.jpg
I would argue that for na off axis measurement for a complete product intended to be descriptive for an end user, the baffle would be more useful. But as someone said, in this case it wouldn't differ much...
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Thanks for the replies.
Basically I'm waiting for a decent dry weekend to re-measure my MEH project and try a different crossover / EQ. (I've already set it up in room but want to get the speaker outside to get a better measurement)
I asked the question as my horns are 40 degree vertical, and I can easily notice the treble drop off if I stand up and walk towards them. This made me wonder whether the measurements would be noticeably different depending on whether they were taken from the throat or mouth. I've attached a pic of my horn 'side on' with the green lines representing 15 and 30 degrees off axis, from the throat, and from the mouth. I'd expect the 30 degree off axis measurements to look quite different, even though they could both be described as '30 degree off axis, 1 metre'
Rob.
Basically I'm waiting for a decent dry weekend to re-measure my MEH project and try a different crossover / EQ. (I've already set it up in room but want to get the speaker outside to get a better measurement)
I asked the question as my horns are 40 degree vertical, and I can easily notice the treble drop off if I stand up and walk towards them. This made me wonder whether the measurements would be noticeably different depending on whether they were taken from the throat or mouth. I've attached a pic of my horn 'side on' with the green lines representing 15 and 30 degrees off axis, from the throat, and from the mouth. I'd expect the 30 degree off axis measurements to look quite different, even though they could both be described as '30 degree off axis, 1 metre'
Rob.
Attachments
One could say that there is a black box view and withe box view of a speaker. The white (transparent really) view is when you know whats inside. The black box view is one where only the box external interface is visible - all implementation aspects are hidden. Its through the external interface that the box (product) value can be consumed. For a user, the black box view is relevant. The black box ends in its external interface and where the sound emitted is no longer effected by the box. The surface where this happens is the reference point for the user and its here one should describe the product from a user perspective e.g. its directivity, sound pressure capability etc.
Its not possible to measure 30 deg form the throat as the walls in the horn will obstruct the sound waves. No one could listen there anyways.
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Its not possible to measure 30 deg form the throat as the walls in the horn will obstruct the sound waves. No one could listen there anyways.
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That's right, a point in space does not know about the 'other' measurements. A point in space is all that's needed to get phase (and by phase I mean delay as well). The axis about which you rotate, has nothing to do with time alignment.
It could be said to complicate the manual analysis of higher order horn modes, but if you know this then it doesn't need to be said. It shouldn't technically affect power calculations but there's always room for a little error if the procedure is not sufficient.
It could be said to complicate the manual analysis of higher order horn modes, but if you know this then it doesn't need to be said. It shouldn't technically affect power calculations but there's always room for a little error if the procedure is not sufficient.
Thanks for the clarification.
Totally agree you can't just put a finger on the throat.
That said, the throat will be much closer to a horn's acoustic center than the mouth.
I strongly believe that POR's should be directly under acoustic centers to whatever extent possible. (Charlie Hughes 'point of rotation')
That has worked very well for me, in achieving multi-way phase alignments, that hold up under rotation.
If someone is only interested in magnitude measurements, i guess rotating with the mouth or baffle being over the POR, works OK.
But since the measurements time distance to acoustic centers, and not the distance to POR's, phase info is obviously invalid.
Personally, i think phase is the key to good xovers, so i can't see how/why we would ever want to rotate over anything other than acoustic centers (as best we can...not always easy, huh?!)
By phase, I mean time and phase....such to phase traces overlay.
So in contrast to your last post, I'd have to say the axis of rotation has everything to do with time alignment.
In the near field, standing up greatly increases the off axis angle.
Pat Brown's article "Far-field Criteria For Loudspeaker Balloon Data is helpful to understand the criteria determining the difference between near field and far field.
https://www.prosoundtraining.com/2010/06/28/far-field-criteria-for-loudspeaker-balloon-data/
A measurement taken at one meter would not provide polar information that could be accurately extrapolated to greater distance.
At a distance practical for outdoor high frequency polars, using a minimum measurement distance the longest dimension of the loudspeaker multiplied by 3, the high frequency pattern will appear slightly wider if the POR (point of rotation) is at the front of the cabinet rather than the horn throat.