forum.zelfbouwaudio.nl

MartyF schreef:

Naar mijn idee zitten de tweeters te ver uit elkaar om onderling te koppelen. De afstand tussen (het hart van) de bronnen moet kleiner zijn dan de halve golflengte over het hele frequentiebereik.

1. Far Field (...)
Spacing less than one wavelength creates a constant phase front but comb lines
start to form beyond one wavelength separation. At two wavelengths separation
the first cancellation occurs. Directivity continues to decrease with more severe
comb line effects as the spacing increases beyond two wavelengths.
2. Near field. Urban, et al [1] derives a more restrictive criterion of no more than a
half wavelength separation between drivers at their highest operating frequency.

Recommended Spacing
is less than One Wavelength

For the tweeter line very close center-to-center spacing is difficult to attain (...)
Hence, with any mounting flange allowance at all, the one or half wavelength c-t-c criteria are very difficult—if not impossible--to satisfy at 20 kHz. But, if we relax the c-t-c criterion, more secondary lobes would appear in the 10 to 20 kHz frequency range.
Fortunately, in this octave the ear is less sensitive (per Fletcher-Munson curves) so any secondary lobes likely would be less audible to the listener.
Thus, if one wavelength spacing at 10 kHz is adopted as a compromise, then tweeter spacing would need to be 34.4 mm (1.35”) c-t-c apart.

Briefly, the coupling conditions can be summarized as follows:
An assembly of individual sound sources arrayed following a regular step
distance on a planar or curved continuous surface is equivalent to a single sound
source having the same dimensions as the total assembly if one or both of the
following two conditions are fulfilled :
1) Frequency: The step distance (distance between the acoustic centers of
individual sources) is smaller than half the wavelength over the operating
bandwidth.
2) Shape: The wavefronts generated by individual sources are planar and
together fill at least 80 percent of the total radiating surface area.

Additional conditions were published in the Audio Engineering Society journal
paper ''Wavefront Sculpture Technology'', JAES Vol. 51, No. 10, October 2003.
The first two WST conditions were re-derived (based on an intuitive approach
using Fresnel analysis) and in addition it was shown that:
3) Deviation from the ideal, target wavefront (flat or curved) must be less than
a quarter wavelength at the highest operating frequency (this corresponds to
less than 5 mm curvature at 16 kHz)
4) For curved arrays, enclosure tilt angles should vary in inverse proportion
to the listener distance (geometrically this is equivalent to shaping variable
curvature arrays to provide equal spacing of individual enclosure impacts on
the audience listening plane)
5) Limits exist concerning the size of each enclosure, the minimum
allowed listener distance and the relative angles that are allowed between
enclosures.
L-ACOUSTICS defines the practical implications of these conditions as
Wavefront Sculpture Technology. The first WST condition dictates the
design constraints for achieving single sound source performance at lower
frequencies. By loading high-frequency compression drivers with the
L-ACOUSTICS "DOSC" waveguide it is possible to meet the second WST
condition at higher frequencies. Since WST conditions are satisfied over the
entire audio bandwidth, the sound engineer or designer is provided with a
"single" loudspeaker with well-defined coverage and wavefront shape, thus
allowing the geometrical distribution of energy to be precisely installed to
match the geometry of the audience seating area.

happystation.nl schreef:

Het nadeel is dan dat ze niet mooi doorlopen in het sprankelende hoog.

b_force schreef:

Off-axis (directivity) storten er velen echter nogal in (3inch en groter wat mij betreft).

happystation.nl schreef:

Ik moet zeggen dat ik niet echt weet wat off-axis gedrag betekend.