Bass Waveguides

[Taterworks]

For a long time it has been taken as gospel that low-frequency horns must be of sufficient length to generate a highly-damped quarter-wavelength resonant mode, which generally defines the low corner of the horn's bandwidth. This requires that low frequency horns must be many feet in length, which leads to large folded designs which are both expensive and complex to build. Compromises are generally made in the area of mouth size, either requiring a large number of cabinets to achieve the desired mouth area or maximizing the amount of quarter-wave action that occurs and limiting the upper bandwidth of the horn to compensate for any degradation of transient accuracy through the resonant means.

However, I have identified some low-frequency horns that seem to defy this rule. A couple of low-frequency horn-loaded designs made by Community Professional Loudspeakers lack the path length necessary to form a quarter-wave resonant mode at their low corner frequency, the first being the XLT415 4x15" horn-loaded subwoofer (+/-3dB to 45 Hz), and the R6-Basshorn, having a sextet of 12" low-frequency drivers and a low corner of -3dB at 60 Hz. Both horns have a 'straight' configuration.

I inquired about this when I worked for a summer at Community, and it was explained to me that the two critical parameters for horn-loading were flare rate and mouth size. If those two conditions are met, quarter-wave resonance is superfluous, and can actually be detrimental to the accuracy of the sound. (No one that I interviewed spoke favorably of the sound quality of 'folded' bass horns.)

The two Community bass horns have the added benefit of being usable to relatively high frequencies, with the upper end of the bandwidth at around 500 Hz for the R6-basshorn and 300 Hz for the XLT415. However, it does appear that the tradeoff here is cone area -- the XLT415 uses a quad of 15" drivers, and the R6-Basshorn uses a sextet of 12" drivers. Also, there is no 'compression chamber' to speak of in either horn. The S1 of the waveguide is about 0.75 Sd in both cases, which would seem to alleviate the issue of throat reactance that necessitates small rear chambers.

Community R6-BASSHORN Spec Sheet

Community XLT415 Spec Sheet



Within the greater body of knowledge of low-frequency acoustic horn design, these Community designs have always remained a curiosity, as well as the "Planar Waveguide Horn" designs by Dynacord and a similar (improved) design built by Stage Accompany as the XL-Bin. All of the above would seem to comprise the low-frequency equivalent of the high-frequency 'waveguide' horns that are becoming popular for their low coloration, and are identified by their short path length, hence the title of this thread, "Bass Waveguides".

Just how compromised can the path length of an acoustic horn be, as long as the flare rate and desired mouth size are preserved?

Stage Accompany XL-Bin Data Sheet

Dynacord V 28 PWH Data Sheet

[MaVo]

The horn is more than a meter long and has quite the high throat area due to the lots of drivers used, which cuts the length down compared to one driver. there is no magic going on here, just a small midbass horn. If you had entered those rough values into hornresp, you could have spared yourself of writing such a long text

[AndrewT]

Hi,
if you maintain mouth area and maintain flare rate, to suit the lowest passband frequency of your straight horn, you automatically achieve path length.

There are two exceptions that come to mind.

Multiple drivers allow larger throat area and the consequence is reduced length.

Corner loading (Pi/2) and 2plane loading (Pi) allow a reduction of mouth area, again the consequence is reduced length.

If the reduced length turns out less than wavelength/2 then you are truncating the horn.
The more you truncate the worse the ripple and the further up the frequency range that the ripple extends.

It's compromise.

Set the required passband frequencies and see what is required, then apply the various compromises. Build each and find which are acceptable and which sound plain bad. That is what a reputable manufacturer would do. Have you the resources to find all the answers?