MTW & MTW+ vs. 24 frequencies per octave & each has its own time window confusion – or – what where on earth is the connection between Leif Erikson discovering America and FFT windowing?
Question:
I am looking at OSM (Open Sound Meter), which to my understanding does not have MTW or MTW+ like smart, however, in the OSM Manual it is stated – 24 frequencies per octave • each has its own time window. Isn’t that almost like MTW?
Answer:
Ah, I see exactly what you’re noticing, and this is a subtle but important distinction. Let’s unpack it carefully.
In Smaart, you have:
MTW | MTW+ (Multiple Time Window):
Multi Time Window is Smaart’s way of maximizing the frequency range that contains a useful number of data points.
Introduced in Smaart v8.5.0, MTW+ is a supercharged version of MTW that uses more FFTs across the measured frequency range and reduces gaps in the MTW calculation pipeline that may result in missing rapid transient events. In Smaart v9, this feature is retained simply as “MTW”. This replaces the non-“plus” version of MTW from Smaart v7 and v8.
Now, in OSM:
When they say “24 frequencies per octave, each has its own time window”, it sounds like MTW, but there’s a nuance:
The “time window” in OSM may not be fully independent in the same way as Smaart. It often refers to the window applied for FFT or averaging for that specific frequency, but it might still be constrained by the main analysis parameters of the system.
Smaart’s MTW allows fully asynchronous windows, optimised for temporal resolution vs. frequency resolution per band, while some systems (including OSM) approximate this by allowing different pre-calculated windows per band, but within a global analysis framework.
So yes: functionally it is “MTW-like”, but it might not have the full flexibility and independence you get in Smaart. For most practical purposes—especially with high-resolution measurements—this can behave very similarly.
TZ´s comment:
Freakish academic language here. So what does his actually mean IRL when tuning a system on a gig? A lot. I personally am using SIM since 1992 and later Smaart, which from practical work approach is very similar. I always measure full range. In laymen terms – I can see the whole picture. I got very used to that, and in my mind necessary. Here´s why.
The term “picture” might also be a good analogy.
Say you are in the National Museum in Oslo and look at Leiv Eirikson Discovering America by Christian Krohg.
Leiv Eirikson Discovering America is painted in oil on canvas with the dimensions 313 cm × 470 cm (123 in × 185 in). The painting presents a view from the deck of Leif Erikson’s ship, looking out over the waves with land visible in the distance to the left. To the right in the picture is the title figure, holding the rudder and wearing a mustard-coloured tunic. He stands straight and points toward the land in the horizon. A few other men on deck are hunching and look seasick. (wikipedia)
OK, so when you want to see details on this massive paining and are not blessed with hawks vision, you need to zoom in by walking closer. As closer you are, as more overview you are missing. Not much of an issue as this is a static image, which hasn´t changed much supposedly since the 1890´s.
So you can safely zoom out again and move to another section to zoom in there and so on. You catch my drift?
Well, here’s the problem – we sound people are not looking at static picture. We are looking at an ever changing object, or a video, if you want to stay within that mental image we just established.
Now imagine that the boat is rocking, the waves are splashing, seabirds fly by, a whale appears, and one of the seamen is bent over the rail “feeding” the fish, etc. — okay, you see (pun intended) – there is rather a lot going on. The scene is ever-changing. You wouldn’t notice much of what is happening starboard if you were forced to zoom in on port. You couldn’t see what is going on the right-hand side, could you?
And that is the difference between MTW/MTW+ and fixed FFT windows. You do not get the whole picture at once with the latter. And the whole picture is precisely what it is all about at a live gig. When you have a bit more time at hand, and you do not need to fuss too much about perfect HF information, and are merely interested in making sure everything is working “OK”, you might just get away with using OSM.
It’s a brilliant tool, and I salute Pavel for having developed it. It’s very solid code, and supporting folks like Pavel is most important. So get the tool, even if you already have Smaart, and make sure to at least donate something equivalent to a proper dinner at a good restaurant to the developer. He more than deserves it—honestly, one cannot overstate that.
Concise technical verdict:
OSM’s per-band time windows give frequency-dependent measurements, but unlike Smaart’s MTW/MTW+, they do not provide fully independent, multi-FFT windows optimised for real-time, dynamic system tuning—so OSM can give a useful snapshot, but it can’t capture the “whole picture” simultaneously in a live music scenario.
Fun fact:
The National Museum of Art in Oslo does, in fact, host live events. The engineers of the museum have rather sensibly booked their spots at the upcoming Smaart Z2H Seminar in Oslo.
So, what on earth are you waiting for? Come to Oslo in May, learn “Smaart”, take a jaunt to the museum, and feast your eyes on this rather impressive painting. FFT windowing suddenly takes on a very different meaning, hopefully. And, armed with the knowledge you acquire at TZ’s Z2H Seminars, you’ll be quite capable of making proper use of pretty much all FFT measurement tools out there.
Smaart is excellent on its own, but most users find they get much more out of it after some structured training. That’s where our seminars come in. At TZ Audio we run practical seminars, both online and in-venue. We offer seminar-only or full “all you need packages” including software & hardware. It’s simply the fastest way to become comfortable and confident with the measuring a sound system.
If you’re in Norway, Sweden, Denmark or Iceland – or elsewhere – we offer is online seminars and traveling to Norway is a valid option too of course. We’re here if you have any questions about the software or upcoming seminars.
Thanks for reading!