Is there any benefit from enclosing the rear of a sealed back mid-range driver? I am thinking mainly of horn loaded compression drivers and sealed mid-range domes. I can think of reasons for no rear enclosure, just a supporting pod/frame provided the driver is flush mounted together with effective edge diffraction control. Is there any evidence that there is significant acoustic rear output from the sealed back driver that can disrupt the forwards radiation?
@GM. I am not thinking of the effect of leaking air pressure but more the effect of chassis/frame/horn flare resonances that communicate directly to the air at the rear. So I am assuming the sealing is effective.
There are plenty of horn designs with free standing horn flares, as well as others with enclosed horn flares. Are the latter for aesthetic reasons or is there a benefit from enclosing the horn rear?
There are plenty of horn designs with free standing horn flares, as well as others with enclosed horn flares. Are the latter for aesthetic reasons or is there a benefit from enclosing the horn rear?
You don't want the horn to move so ensuring it is stiff would be more directly the goal. If it were to vibrate it could be an issue at the front as well as the back.
@AllenB. I am assessing my recent build. The mid-range and treble horn frames are 40mm thick laminated mdf with additional 30mm hardwood 1/4 round peripheral bracing. It should be stiff enough. The horn flare is bolted to the frame with 10x6mm bolts but the rear compression driver is unsupported. I see lots of pro audio horns similarly mounted. Is horn flare rear resonance a concern?
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If it is then it would be for the front as well.. or in general. You don't want sound being heard to originate from anywhere but the source.
Before you make assumptions though I'd test whether there actually is a problem. For example, I'd try wedging something between the driver and the box below, see if a difference can be heard.
Before you make assumptions though I'd test whether there actually is a problem. For example, I'd try wedging something between the driver and the box below, see if a difference can be heard.
Ah! As Allen noted, 'enclosing' a horn is all about increasing rigidity, damping + smoothing polar response or just for stacking purposes and/or weather protection. IME based on the pioneer's supporting the horn driver in HIFI/HT/prosound apps is 'de rigueur' and obviously including public address (PA) where the horn is unsupported.
That's one way to increase damping and then there's the stone age way: I remarked one time about how massive it would require to truly damp horn systems in general and Altec's poorly braced 825 cab in particular, so one owner took me to task and boy was he (pleasantly) surprised!
Cab is ~160 lbs, but required ~440 lbs to damp it!
not in a serious pro audio application that's for sure, unsupported and naked is a style/fashion/status thing as in "look I'm using a snazzy horn and compression driver aren't you impressed?!
certainly any audible rear resonance will also affect forward radiation/response.
sorry i tend to think that way because as an old soundman used to running things at war volume inerting things is a must in my books...
at home listening levels it may not even be a concern.
i'd be more on about time aligning that horn and tweeter!
I am strongly in favor of mass + stiffness + damping applied sensibly in a traditional enclosure design and in this case the woofer box is fully in compliance with these requirements. I am using the SB34NRXL75-8 up to 650 Hz. However the mid and tweeter are vintage JBL horns and do they really require mass damping to the same degree?
I can understand why extreme mass loading is effective for bass frequencies in typical vintage cabs which have poor build quality.That's one way to increase damping and then there's the stone age way: I remarked one time about how massive it would require to truly damp horn systems in general and Altec's poorly braced 825 cab in particular, so one owner took me to task and boy was he (pleasantly) surprised!
Cab is ~160 lbs, but required ~440 lbs to damp it!
My reasoning is, why have an enclosure if it is not necessary to contain rear radiation and rigid support can be achieved in other ways without the mass and size penalties. This is what I am trying to achieve. I can't say that I have noticed any audible issues I can pin on horn rear resonances, but I will take notice of the comments and arrange some sort of clamp support for the rear of the horn. It does occur to me that modelling the horn as a beam, a clamped-clamped beam still has resonances, just at different frequencies to a clamped-free beam.
The system is active 3-way using a 192/24 dsp crossover. The tweeter and horn are accurately time aligned. The time delay of the tweeter relative to the mid-horn is determined by reverse phase nulling at crossover frequency and is in close agreement with the tweeter/mid-horn physical offset.
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Right; MJK used a flexible ruler clamped to a table top to describe/promote his TQWT designs and I still have a dust mop with different distance taped points on the handle from a local Altec speaker system designer to get me in the right frame of mind when it became obvious he wasn't going to do it with math.
Some assume that the horn satisfies the radiation requirements for sound it will produce. Often however, much will spill out of the mouth uncontrolled. Some assume that the radiation efficiency with a narrow throat is all that matters and then let it radiate everywhere where it can be worse than being wasted, ie can come back to interfere with the frontal radiation.
Every horn is different. Polar measurements may be needed to assess the low frequency limit. It's often well above the cutoff anyway and the mouth size is often a further limitation. Sometimes baffling is the least you can do and careful horn extension can be even better.
You were wise to confirm this by measuring the distance and cross-checking. The null method can give false positives, and it won't tell you whether you match the slopes.
It's not necessary to use the null method if you can take separate (low/high) time locked measurements.
An enclosure does not necessarily solve diffraction issues, which are principally caused by sharp edges and surface mounted drivers. I give cabinet diffraction control a high priority, as can be seen from the photos in #5 above. The horn/tweeter baffle has a 45 degree front chamfer and the rear a 30mm radius roundover. The drivers are flush mounted.
In the interest of constructive discussion I could point to four areas that I'd be interested in confirming.
1. The size, location of and benefit from roundovers or champhers depends on a few things.
What is getting diffracted there?
how audible is this on a frequency by frequency basis?
How is the smoothness overall in the band and in context of the system it's a part of, as well as including the impact of directivity limitations?
2 & 3. The tweeter and midrange horn are clearly designed on diffraction slots.
4. The midrange vertical dimension is: 1. Based on a narrow angle and 2. relatively short. These represent a two faceted limitation on diffraction control, and the lower frequency support the horn can offer. An asymmetrical horn typically isn't optimal with a flat mouth.
1. The size, location of and benefit from roundovers or champhers depends on a few things.
What is getting diffracted there?
how audible is this on a frequency by frequency basis?
How is the smoothness overall in the band and in context of the system it's a part of, as well as including the impact of directivity limitations?
2 & 3. The tweeter and midrange horn are clearly designed on diffraction slots.
4. The midrange vertical dimension is: 1. Based on a narrow angle and 2. relatively short. These represent a two faceted limitation on diffraction control, and the lower frequency support the horn can offer. An asymmetrical horn typically isn't optimal with a flat mouth.
In the spirit of constructive discussion, here are some of my thoughts on the general issue of enclosure diffraction, not limited to horn speakers.
I generally follow the results of Olson's pioneering AES paper. In particular the box shape Fig 2L and corresponding frequency response measurement Fig 17. The methodology of Olson's measurements exposes the effects of edge diffraction on axial frequency response for different shaped enclosures.
For horn systems, the results may differ, since the directivity of a horn is not the same as typical cone or dome speaker, resulting in a different acoustic output encountering an edge. Nevertheless there are some investigations of horn systems which show axial frequency response unevenness attributed to edge diffraction. I am no expert in horn theory/design but I am extrapolating from my experience with cone/dome speakers on the basis that edge treatment should do no harm and could be of benefit.
I generally follow the results of Olson's pioneering AES paper. In particular the box shape Fig 2L and corresponding frequency response measurement Fig 17. The methodology of Olson's measurements exposes the effects of edge diffraction on axial frequency response for different shaped enclosures.
For horn systems, the results may differ, since the directivity of a horn is not the same as typical cone or dome speaker, resulting in a different acoustic output encountering an edge. Nevertheless there are some investigations of horn systems which show axial frequency response unevenness attributed to edge diffraction. I am no expert in horn theory/design but I am extrapolating from my experience with cone/dome speakers on the basis that edge treatment should do no harm and could be of benefit.