Rational Acoustics

September 17th, 2014, 07:53 PM
Hi all,

I've got a number of questions that I'd like to hear your thoughts on, not all that related, possibly better suited to individual threads, but here goes.

1) For those of you that use the IR module, what do you use it for specifically? Im especially keen to hear from anyone who uses it as part of the normal workflow of aligning a sound system in a live sound / reinforcement environment. I learnt to align systems from 606, and Arthur, and a few others, and I have never really found a use for it (or the Live Ir mini window), since I feel that I can do everything I need to do with the mag and phase traces, but perhaps there is a good learning opportunity for me... I'm aware IR traces have more use by people who deal in room acoustics, but that isn't my kettle of fish.

2) I've had some enquiries about tuning monitoring systems in recording/mix studios recently, which sounds both very easy and hard to me. Easy for obvious reasons, of fewer speakers, fewer parameters to worry about etc, harder for the level of detail perhaps required and differing solutions (acoustic treatments perhaps, which are rarely an option for live sound systems). So Im curious to hear from anyone who does work in these kinds of places, what is your approach. (Perhaps it does relate to the 1st q).

3) Not so long ago I tuned a 7.1 playback system in a planetarium. They were quite specific about several things they wanted, level calibration similar to a cinema standard, etc. One thing no one could tell me, though, is if there is a standard upper corner frequency for the subwoofer channel. The sub was passive, and without processing made useful (or not) sound up to 1kHz or so. In a live sound environment my setting of a LPF would be determined by how I want it to interact with the rest of the system. Here, though, its an independent channel. My solution was to pick something that sounded right to me (somewhere between 100 & 200hz, but I cant remember where) which the client was happy with, but Im curious to know if there is a clear guideline in the THX (or similar) spec.

Very best,

Phillip Ivan Pietruschka.

Alfredo Prada
September 17th, 2014, 11:57 PM
I use the IR to save the measurement as a wave file, at least for the first before any changes and the final IR after all adjustments are done. Then I will have a wave record and also be able to open the wave file in other post processing analysis programs. And yes,for all the acoustic parameters also.

Chris Tsanjoures
September 18th, 2014, 10:55 AM
1) I use IR mode for finding the time arrival of specific frequencies, especially sub frequencies. I'll enter the arrival time gathered from IR mode into a TF engine. This way I can time the measurement to energy at say, 80Hz, without having to 'find' it manually. Your application is going to drive your usage, if you need to spec the RT60 or intelligibility of an environment, IR mode is the tool.

In TF mode, the live IR window is an essential view IMNSHO. It will show you any reflections or late (or early) arrivals, and how strong they are. Often times, our first job is to diagnose issues. This view will make timing issues obvious that may have otherwise not been as apparent. If your measurement is timed, the impulse will show itself centered in the live IR plot, if it is outside of timing, it won't be. Others will have more input here I'm sure.

2) see above - part of a sweet stereo image is minimizing or removing reflections. The live IR plot will help you figure where to place absorption/diffusion to get the late arriving signals down to a minimum. Other than that - studio tuning is about getting your L/R to have the exact same response/level and time at the optimum position.

3) Built into Smaart are cinema x-curves (see attachment). These are used after the initial tuning of a system to voice it to the Dolby cinema standard. You must calibrate your input in order to use this feature, and set the banding to 1/3 octave to be consistent with the industry. I hope that helps you out. - C.

September 19th, 2014, 06:51 AM

The majority (but not all) of the sound systems I measure and tune are setup temporarily for concerts, theatre shows, etc. With that in mind, I'm not sure what the value of saving the IRs for analysis in some other tool at a later date is. Given that, by the time I do that, the system may already be packed up and be in a truck on its way somewhere else.


1) Whilst I remember Arthur Skudra talking about finding the time of arrival of the Sub at the X-Over frequency and setting the delay finder to that. I just use the same delay time I use for the Mid-Hi box in the system of interest, and accept that the phase in the sub region will be quite steep. As long as the phase of the two systems overlaps/runs parallel for as long as possible, I don't see that setting of the reference delay time matters all that much, which is to say I think the end result ought to be the same. So why take extra steps? Pretty much I don't need to measure RT60 or any intelligibility parameters, hence I was asking for people using for aligning sound systems in concert / theatrical applications.

My experience of the Live IR window to date, is that is doesn't tell me all that much that I can't read from the mag/phase/coherency trace. Whilst I can conjure in my imagination some situations with intense specular reflections for architectural features, where it might make it faster to diagnose, I don't really find that to a problem very much. Either the reflections are easy predict and design around, or they are an inevitable consequence of the mandated speaker locations - for instance for under-balcony delays where there are few options on rigging location.

3) I didn't realise the X-Curve included a secondary trace for the Subwoofer, thank you. (FWIW I wasn't asked to match the system to the cinema X curve, this was discussed, and the client deemed it unimportant. However, Im not sure what other reference point to use (other than cinema) when tuning an independant subwoofer channel).

Chris Tsanjoures
September 23rd, 2014, 03:30 PM
"Why Take the extra steps"? Well, why will be determined by what your operational goals are.

I'm in the same boat as you, where the bulk of my freelance work is temporary concert sound. For run-and-gun applications, we have a similar workflow - there are many roads that lead to Rome. I just find it easier to judge slope with the energy timed to X-Over, which is all that matters here. Take a look at the attached screen shots - same system shown two ways.

In temporary sound applications where end fired sub arrays or other types of cardioid configurations are desired, it is much easier to achieve the end result when the phase trace is timed to a specific frequency so that you can work towards maximum cancellation at that frequency. You may enter numbers into the delay offset until you get the desired phase wrap - I just prefer to run an IR measurement to find that number, or get in the ballpark - it saves me time. This may be the only occasion you use IR mode during an alignment gig. As you are implying, Impulse Response mode isn't often used for day-to-day alignment work. It was wholly omitted from Smaart DI for this reason, like you, most of our users just don't have a need to use IR Mode.

Because Live IR is another way to view time, if there is a reflection arriving 4 ms after the impulse and that correlates with a drop in coherence at 250Hz, I know that it is a reflection causing that drop and not some other issue. Is it used for alignment...sort of...not in a way that can make or break a gig. As a diagnostic tool, or idiot check, many (including myself) find it indispensable. YMMV.