Hi gang... this should be easy, but got stuck. How do I test my horns at 1M when my horns are 36 in" long?
That puts mic at 3.37" from mouth. Seems weird. Even 4' seems awfully close.
Zene
That puts mic at 3.37" from mouth. Seems weird. Even 4' seems awfully close.
Zene
Measuring all around is going to take in the power regardless, but the axis of rotation is up for discussion for other reasons.
Those are pretty cryptic responses, guys--like you're going out of your way to be opaque to impress someone (i.e., not the person asking the question). How about doing the OP a favor and explaining what you mean a bit more to a bunch of learning amateurs?
To the OP: the maximum distance from the device under test and the measurement microphone is a tradeoff between having nearfield acoustics issues (i.e., being too close to the source) and having the room's early reflections interfere and mar the direct arrivals from the driver/horn (i.e., the "minimum phase" response of the driver/horn).
In the case of a horn, you can take the horn's mouth as the zero datum instead of the throat, since early room reflections don't really enter into the picture until floor bounce or furniture or side-wall bounce become an issue.
If you had an anechoic chamber to measure in, you'd probably back off to 3 metres or so for the measurements.
But unless you live somewhere that's quiet outside and the ground is flat and uniform (like a smooth asphalt or concrete surface) to do ground plane measurements, you have to compromise and use 1m. Don't worry too much about it--most people do not consider that the human hearing system has resolution limits that make the information obtainable from from most anechoic measurements unnecessary, i.e. the psychoacoustics of the equation.
Chris
To the OP: the maximum distance from the device under test and the measurement microphone is a tradeoff between having nearfield acoustics issues (i.e., being too close to the source) and having the room's early reflections interfere and mar the direct arrivals from the driver/horn (i.e., the "minimum phase" response of the driver/horn).
In the case of a horn, you can take the horn's mouth as the zero datum instead of the throat, since early room reflections don't really enter into the picture until floor bounce or furniture or side-wall bounce become an issue.
If you had an anechoic chamber to measure in, you'd probably back off to 3 metres or so for the measurements.
But unless you live somewhere that's quiet outside and the ground is flat and uniform (like a smooth asphalt or concrete surface) to do ground plane measurements, you have to compromise and use 1m. Don't worry too much about it--most people do not consider that the human hearing system has resolution limits that make the information obtainable from from most anechoic measurements unnecessary, i.e. the psychoacoustics of the equation.
Chris
Zene,
The measurement distance starts at the horn mouth, not the throat.
If you are measuring a bass horn, the near field is shorter than for a high frequency horn.
Polar measurements should be done in the far field for accuracy, and the axis of rotation should be the center of the mouth, regardless of what AllenB or anyone else says 😉
SynAudCom
https://www.prosoundtraining.com/2010/06/28/far-field-criteria-for-loudspeaker-balloon-data/
I'll let Pat Brown do the explaining:
In regard to full range frequency measurements, Pat Brown wrote :
A working “rule-of-thumb” for determining the boundary between near-field and far-field is to make the minimum measurement distance the longest dimension of the loudspeaker multiplied by 3.
While this estimate is generally acceptable for field work, it ignores the frequency-dependency of the transition between the near and far fields.
It is often thought that a remote measurement position is necessary for low frequencies since their wavelengths are long. Actually the opposite is true. It is more difficult to get into the far-field of a device at high frequencies, since the shorter wavelengths make the criteria in Item 4 more difficult to satisfy.
Item 4:
4. The distance from the source where the path length difference for wave arrivals from points on the device on the surface plane perpendicular to the point of observation are within one-quarter wavelength at the highest frequency of interest .
(Figure 2)
Fig. 2- The figure shows the path length difference to the microphone position for a loudspeaker whose largest dimension is 2 feet. Note that even though the transducer is smaller than the cabinet face, the entire front baffle of the enclosure can radiate energy. Attenuation balloons for this loudspeaker can be measured up to 17kHz at 9 meters. The upper practical limit for loudspeaker modeling is 10kHz.
An important distinction between high frequency and low frequency measurement, criteria #4 can be satisfied up to 210Hz for a subwoofer with a one square meter face measured at one meter with a measurement mic on the ground plane.
Hope that helps explain some of the concepts.
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I didn't expect all this respose, thx. Before I go thru it all; mic at 1m or less from horn mouth puts the tweeter (mounted in line wth horn driver) almost 2m from mic. These measurements will also be used to set volume levels as well.
Did I step in it again?
Did I step in it again?
You have opened up a problem that can't be avoided when matching levels when relatively near to sources that are relatively far apart.
Direct sound drops at -6.01 dB per doubling of distance (once the far field has been reached..) so can only be matched at one specific distance when the sources are at different distances from the listening or measurement mic position.
http://www.sengpielaudio.com/calculator-SoundAndDistance.htm
From a level matching standpoint, placing the tweeter in the center or above the horn mouth may be the best approach.
On axis phase between the tweeter and horn can be aligned with the proper choice of crossover slopes and polarity.
Some diffraction of the midrange around the tweeter if placed in the horn center must be weighed against the relative time/phase differences at the listening locations.
From a time alignment standpoint, aligning the acoustic point of origin (which is seldom the exact position of the voice coil..) has merit (on axis..), but still requires the proper choice of crossover slopes and polarity. The negative sonic effects of the tweeter diffraction off the horn body are a problem, as well as SPL difference at different locations.
At any rate, solutions to one problem will expose other problems, and whether using passive crossovers or DSP (digital signal processing) the two separate locations of the sound sources can only correct those problems in one specific location.
Of course, none of that has stopped speaker designers from "picking their poison" 😎
Art
Can you describe or display a picture of what you've got (i.e., is it a “Frugel-Horn 3" with separate tweeter)?
Presently, it sounds like you've got a 3' bass/midrange horn+driver, but then you indicate that you're trying to time-align a tweeter to the horn. Much mystery here (or prior knowledge of your setup that's not being shared in this thread). Time alignment of separate horn/driver apertures is a different subject than time-aligning horn-loaded or direct radiating tweeters to a 3' horn/driver, and setting relative gains is a separate process altogether.
Chris
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