Introduction
Following on from part 1, where the design of fitting an ORTF pair into the diminutive Mini-ALTO was covered (using a pair of Sennheiser MKH 8040 mics with modified MZL connectors), this second part of the blog post concerns testing the compact rig. There are two main aspects that I wanted to test to see if such an ORTF rig is usable: transparency (i.e. how much does the windshield colour the sound) and wind protection.
Transparency
My main concern with colouration was not just the basket structure in general, but the thicker plastic rings that mark the join of the end caps to the cylindrical part of the windshield basket. The impact of such rings can be quite discernible with set ups in some windshields: others have noted the impact of the large rear plastic ring in the Rycote ORTF windshield, and I have noted and measured the impact of the still chunkier ring in the Rycote Cyclone on the sideward-facing lobes of the fig 8 mic when used for mid-side recording. The rings in the Mini-ALTO are much less substantial than in these examples, but, given the 80mm diameter of the basket, they are closer to the mics, so the key questions are: i) is the impact measurable?; ii) is there a difference between the impact in the two different pod sizes? and, iii) if there is a measurable impact, does this matter – i.e. does it translate to noticeable issues when making field recordings?
Without an anechoic chamber it is difficult to get an exact read on the transparency of any windshield, but, as I did with my original Mini-ALTO 115 test, for a reasonable quick and dirty test I placed a bare mic on the windshield ORTF shock-mount in front of a speaker (in my treated studio) playing pink noise, then carefully added the windshield pods without moving the mic for a second recording. I did this with the ORTF bar oriented square-on to the speaker (0 degrees), at 45 degrees and side-on to the speaker (90 degrees). I repeated the exercise for both the 90mm and 136mm pod sizes. The results for each pair of recording were compared using a spectrum analyzer and overlaid as follows:
In these tests with the ORTF bar square-on to the speaker the sound is arriving at the cardioid mic about 63 degrees off-axis. In the case of the 90mm pods the plastic ring for the end caps is directly between source and capsule, whereas with the longer 136mm pods the ring sits beyond the direct line. Given this, it is not surprising to see a greater impact when the 90mm pods are used, in addition to some generally increasing attenuation of high frequencies, although the significant colouration only kicks in at 15.2kHz and above.
In these tests with the ORTF bar at 45 degrees to the speaker the sound is arriving at the cardioid mic about 16 degrees off-axis. In the case of the 90mm pods the direct line between source and capsule is through the end cap of the windshield, whereas with the longer 136mm pods the direct line is through the cylindrical part of the basket and near to the end cap ring. It is interesting to see that, despite the seeming greater significance of the plastic ring to the 136mm pods at this angle, the colouration arising from the basket is a little less than with the 90mm pods.
In these tests with the ORTF bar at 90 degrees to the speaker the sound is arriving at the cardioid mic at 35 degrees off-axis. In the case of both the 90mm and 136mm pods the direct line between source and capsule is along the longitudinal axis of the windshield and, thus, through the centre of the end cap of the windshield: in short, the mic position relative to the sound source, apart from being angled, is very much in accordance with a mono directional mic in the windshield as per the original design intention. As a result, the impact of the basket is minimal (and comparable to my previous tests of the Mini-ALTO 115 with a supercardioid mic), although a couple of troughs at high frequencies – at 15.2kHz and 17kHz – are visible with the 90mm pods.
For the purposes of comparison, here is my previous test of an MKH 8030 with the Min-ALTO basket turned side-on to the speaker source so that the fig 8 mic is on axis to the sound source:
This is useful, since it shows that – within the limitations of this testing methodology – that the ORTF pair in the Mini-ALTO with 90mm and 136mm pods is less affected by the basket than the fig 8 mic in the Mini-ALTO 115. And for another comparison, here is the same fig 8 test repeated with a Rycote Cyclone.
The more significant impact of the Cyclone basket is doubtless largely due to the substantial plastic ring for its end cap.
The pink noise testing evidently shows some impact of the Mini-ALTO basket on the ORTF set up, much of which is a general and modest loss of high frequencies, with a few more obvious anomalies over 15kHz. The colouration, however, looks better than feared, not radically different between the two pod sizes, and, above all, less significant than that with a fig 8 in the Mini-ALTO 115 and far less than with a fig 8 in a Rycote Cyclone.
As I noted when I tested the Mini-ALTO 115, there is one thing demonstrating differences with this pink noise test or, indeed, more exhaustive and expert tests in an anechoic chamber, but how does the colouration actually sound? It is hard to come up with a perfect test, especially with limited resources, but I have settled on an approach that some at least may find informative. Eschewing the variability of successive live recordings, I placed the ORTF rig in front of a pair of speaker (Vivid S12s) in my studio and played back a short section of a recent recording of mine of a singer-guitarist (Luke Chapman), angling the ORTF bar square-on to the speaker, and with the mic aligned to the speaker so that it was 55 degrees off-axis. This positioning ensured that the plastic ring for the end cap was exactly on the line between the speaker and the mic with the 90mm pod, so very much the worse case positioning. Here are the resulting mono sound files:
If really keen, you can download the files and set them up in a DAW and flip between short repeated sections of each pair, which is what I have done. In both the recordings with the windshield you can hear the expected slight change to the high frequencies when the basket is added, as anticipated following the pink noise tests, but I would suggest that it is subtle and, therefore, unlikely to be of great significance to the majority of those making sound recordings in the field. Perhaps more importantly, there seems little practical reason to favour the longer 136mm pods over the more compact 90mm pods in terms of transparency: of course, there may be differences in wind protection, which is what we will explore next.
Wind protection
The laws of physics dictate that the small 80mm diameter of the Mini-ALTO cannot offer the wind protection of larger basket designs, and that was never the intention behind its compact design. That said, the small windshield performs reasonably well outdoors, and I was surprised in my original tests in breezy conditions to note that it outperformed the Rycote Nano Shield, which, although another smaller basket windshield design, has a larger cross-section (measuring internally 86mm high and, with its elliptical form, 107mm wide), although testing confirmed that larger windshields did provide better wind reduction. The questions I have for ORTF in a Mini-ALTO, therefore, are not how it compares to the same in much larger baskets, but the following: i) is there a noticeable difference in the wind reduction performance between the two different (i.e. 90mm and 136mm) pod sizes; and ii) how does the wind protection for an ORTF pair compare to that afforded to other rigs in a Mini-ALTO? With the last, I was particularly interested to see how the ORTF pair compares to a mid-side pair in a Mini-ALTO: if performance was similar to the last, then, given that I have been happy to use MS pairs in the Mini-ALTO where wind conditions allow and where compactness is paramount, this would mean that the ORTF design is similarly viable from a functional point of view.
First off, the matching front and rear pods provided by Radius Windshields for this whole experiment mean that the two Mini-ALTOs are non-standard lengths, so a couple of bespoke furs had to be made by the sewing department in Stroud. These are of the same, longer pile, grey fur that Radius provides as an option for the existing Mini-ALTOs and I have matching ones for my Mini-ALTO 115s. It might seem trivial or obvious, but it is important to have identical furs when testing the wind performance of the different rigs and pod sizes. Wind tests in the real world, where wind is turbulent and not laminar, are never quite as easy as you might think: simultaneous recordings are essential, of course, but the windshields have to be placed a bit apart to avoid one protecting or otherwise affecting the other, so wind gusts can vary a bit in terms of impact and timing. The other difficulty is matching mics: despite my growing mic locker I don’t have a stash of multiple MKH 8040 mics, but just one pair. My solution to this problem has been to record a single channel in each of the two Mini-ALTO ORTF rigs, which is fine: we are after a comparison of the wind performance of the two different sized pods not making beautiful stereo recordings, and this serves the purpose just as effectively. And for the comparison between ORTF and mid-side, where single-channel recording could be a bit misleading (i.e. comparing one channel of ORTF to, say, just the side mic of an MS pair seems like apples to oranges), I used the pair of MKH 8040s in ORTF in one of the test Mini-ALTOs and a MS pair of MKH 8030 and MKH 8090 in the Mini-ALTO 115 I use for MS: I’ve noted very little difference in wind performance with the wide cardioid vs the cardioid mics in MS.
Starting off with the windshields with no fur, here is an excerpt of a windy gust on a pretty breezy day, with the ORTF rig using 90mm pods compared to the Mini-ALTO 115 with an MS pair (MKH 8090 and MKH 8030).
And then compared using a spectrum analyzer and overlaid as follows:
Both listening to the sound file and viewing the spectrum analyzer show that the ORTF pair in a Mini-ALTO with 90mm pods more than holds its own against the MS pair in the Mini-ALTO 115, each with the bare basket. But it is equally clear from the audio that the the differences are not constant. Moreover, we need to think about the impact of wind direction on performance, given that the two stereo pairs sit at 90 degrees to each other in relation to their windshields: in this example both mic pairs were side on to the wind direction, so that the ORTF windshield presented its small end-on form to the wind while the MS pair’s windshield presented its side to the wind. Changing this around by rotating the mic stand 90 degrees, here is a second set of recordings with the mics facing directly into the wind, so that the windshield housing the MS pair was end-on and the windshield housing the ORTF pair was side on.
The impact of wind on the MS pair is less in this second test compared to that on the ORTF pair. What we can draw from these comparisons, however, is that overall the ORTF pair in the 90mm pods seems to hold its own compared to the the Mini-ALTO 115 with an MS pair, with both baskets being bare.
Moving onto tests with furs added, which, of course, reflects more typical usage of the Mini-ALTOs outdoors, here we have a pair of ORTF and MS recordings with the wind coming from the side:
And then with the mic stand rotated 90 degree so that the two pairs are aimed into the wind, albeit with the ORTF pair’s windshield side-on to the wind and the MS pair’s windshield end-on to the wind:
So, the fur (while, of course, reducing wind noise) does not change the situation: the broadside offered by the windshield when an ORTF pair faces into it and the broadside of the windshield offered when an MS pair faces at 90 degrees to the wind are what creates the most windnoise. With this caveat, which has implications for usage (in any given situation one array may outperform the other: equally it could be argued that wind direction and sound source direction – if not negotiable by moving the mic position – might influence choice of mic array), there is no obvious difference in the overall wind performance and, to some extent, this is predictable given the mic capsule locations and orientations in the two arrays.
So now we should turn to the matter of the two pod sizes for the prototype ORTF rig: does the increased volume of the 136mm pod windshield offer an advantage in wind reduction over its shorter counterpart with the 90mm pods? For this, of course, I was able to orient the two windshields identically, and recorded a single cardioid in the ORTF rig in each simultaneously.
First up we have the 90mm pods (with fur) facing so that the wind direction was end-on to windshield:
And then the simultaneous recording using a Mini-ALTO with 136mm pods (with fur):
And then the mic stand was rotated 90 degrees so that wind direction was side-on to the windshields:
There is again some gust to gust variability between the two windshields, doubtless reflecting the highly localized differences in the turbulent wind you get in the real world, but there is nothing in these short clips (or, indeed, the much longer recordings I made) to suggest that the longer version of the ORTF Mini-ALTO with its 136mm-long pods outperforms the shorter 90mm version. This applies in both orientations into and at 90 degrees to the wind.
Conclusions
So what’s the verdict? Is an ORTF pair viable in the diminutive Mini-ALTO? If so, is a short symmetrical Mini-ALTO with two 90mm pods as effective as a longer version? My short answer is, yes, an ORTF pair is viable in a Mini-ALTO and that the advantages of pods longer than 90mm are so small as to be insignificant: so you might as well use the more compact 90mm pod version. Moreover, I would suggest that an ORTF pair is just as viable in a Mini-ALTO as a mid-side pair, although the number of cardioid mics that are short enough to make use of the 80mm diameter windshield for ORTF are few and far between: so far I have identified the MKH 8040 used here, the Schoeps CMC1 KV + Mk4, the Nevaton MC59S(C) and the upcoming DPA MMP-GS with the existing 4011 capsule as suitable candidates, and, in a third blog post, will be testing at least some of these. And there well be other mics that would fit without resulting in capsules close to the windshield basket. There are other caveats to add to this, but these are very much the same as with the Mini-ALTO for use with a mono directional mic: above all, a small diameter windshield will – all other things being equal – perform less well at reducing wind noise than a larger windshield; and, second, the structure of a basket, especially with significantly chunky plastic components, will provide colouration of sound above and beyond the curtailing of high-frequencies that is inherent to any fabric covering of a mic. So, of course, a Cinela Albert is a better bet for ORTF in terms of acoustic transparency and wind performance (as, indeed, is my own TIG-welded Mega-Blimp), but that’s not really the point: the Mini-ALTO is designed – above all – to be compact, to offer modest wind protection, and to allow rapid changing between bare mics and full basket. Now the latter, which is so relevant to production sound recording with a supercardioid or shotgun mic, may not be quite so relevant to an ORTF pair, but compactness is relevant to many recording in the field. Many people do seem to love dinky little recording rigs with miniscule recorders and lightweight stands (if a stand at all). For them, moving from a pair of furry slip-on covers for an ORTF pair on a stereo bar to a Mini-ALTO containing an ORTF pair would offer better wind performance and a more practical, transportable and robust form for the setup. For those already using ORTF in larger blimps, then a Mini-ALTO ORTF rig offers more compactness for those times when (small) size really matters, just as is the case for using the Mini-ALTO instead of larger windshields for MS or even DMS. Given that many windshields – I am thinking especially of traditional cylinder types such as the Rode, Rycote Modular and Rycote ORTF windshields – don’t have noticeably less colouration than a Mini-ALTO, for many it just comes down to size vs wind reduction: just as it would for a mono supercardioid or shotgun mic. So, yes, ORTF in a Mini-ALTO is usable and will appeal to many. Whether or not that translates to commercial viability is beyond me, not least given the small number of mics that are short enough: that is one for Simon Davies and the team at Radius Windshields to ponder. If the two blog posts on this experiment have piqued your interest, do get in touch with them (they are eminently approachable and responsive) and let them know as feedback will doubtless influence where they go with this!
