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Room Modeling Workflow

Practical_Audio_2

by Pat Brown

An alternative title for this blog could be “How a Balloon Pop Can Save the Day” for a sound system designer. When building room models for acoustics simulation, the order of workflow is important. The intuitive order is

  • 1. Build wireframe
  • 2. Assign absorption (ABS)
  • 3. Calculate reverb time (RT)

Logical, but it can lead to some huge errors. If a surface material has a low alpha, such as concrete, and it covers a large area, a small error in the coefficient will lead to a huge error in the RT.

An Example
On a recent project, a parking garage, the calculated reverb time of the space was over 9 seconds, using the “default” concrete coefficients and the Sabine equation. Measurements in the space revealed the actual RT to be just over 2 seconds. This illustrates some of the problems with statistical reverberation equations, which is that they are fickle and conditional. The parking garage, with its low ceiling, does not have a “mixing” geometry. The ABS coefficients for concrete are not correct for this scenario, and since there is so much of it a small discrepancy produces a very large error. Read more »

Take a Step Back in time to the Brussels Worlds Fair 1958

Take a step back in time to 1958.  Don & Carolyn Davis (SynAudCon founders), William H. Bell and George Petry were selected to exhibit their HiFi system at Brussels World’s Fair.   For 10 days, they staged demonstrations every 30 minutes drawing crowds which probably totaled 50,000.

What a neat article.

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Pat’s Blog – The Middle Seat

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I don’t see movies in theaters often, but when I do, I want a good seat. This means getting there 30 minutes early to get a seat along the center line of the theater, usually about half-way back. This provides the best stereo image and mix of the surround channels.

Last week we were in south Florida for a short vacation, and decided to see a movie. It was an impulsive decision and we walked into the theater without my requisite 30-minute lead time. theaterI was certain that the “sweet spot” was already taken. To my amazement, the center seats were the only seats left. The aisles were completely lined with senior citizens, there for the matinee. I approached an elderly lady at the end of the aisle, and said “Excuse me, but can we get through?” She replied, “Sure. It’s a shame that someone has to take those middle seats!”

So, while the middle seat may be a curse on an airplane, it’s the best spot in a movie theater. Just don’t tell the seniors.  pb

 

DSP Comparison Revisited

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It’s been about 5 years since I published a couple of articles on the differences between Digital Signal Processors (DSP). You can find those articles on the SynAudCon website. I had to revisit the topic on a recent loudspeaker measurement project.

The back story is that a loudspeaker manufacturer sent me a two-way loudspeaker for testing, along with the required settings (IIR filters) for the DSP. The resultant response did not look as expected, so we investigated and found the DSP to be the culprit. I had the manufacturer send me a measured IR of their DSP, and I compared the  frequency response magnitude to two DSPs that I have in the measurement rack. Figure one shows the comparison.

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Figure 1 – A comparison of three popular DSPs with the same settings. Click to enlarge.

So, DSPs aren’t any closer now than they were when I did the comparison articles 5-years ago. A few thoughts:

  • No one is necessarily right or wrong regarding the response shape of an IIR filter. There is no universally accepted definition for the relationship between the filter Q and bandwidth.
  • The differences are not a problem when the loudspeaker response is actually being measured, since the same target curve can be reached with any DSP. An example is when equalizing a sound system.
  • The differences are a problem when filters are numerically dialed into a DSP and the resultant response is not verified, or when one make/model DSP is used to prototype the loudspeaker and a different make/model DSP is used to deploy the filters. I run into this quite often when producing loudspeaker data files for room modeling programs.
  • If a loudspeaker really, really needs a specific DSP curve (not unusual), the quick way to get it is to use an FIR filter. These are supported by a growing number of DSPs. The response of an FIR is not dependent on the DSP, so long as the required tap length is supported. I can load an FIR into my measurement program to process the stimulus. No DSP needed.
  • This loudspeaker would have “worked” with any of these DSPs, but the results would sound slightly different. Such differences are often attributed to the wrong cause, such as the digital resolution, the “quality” of the product, or the oxygen content of the copper cables.

Technological advancements in audio have not eliminated the need to measure.   pb

Why Reverberation Formulas Don’t Work

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It’s been over 100 years since Wallace Clement Sabine sculpted his famous reverberation formula for estimating the room reverberation time from a handful of variables that include the room volume, surface area, and the “alpha” of the material covering each surface area – the so-called absorption coefficient. Multiplying the alpha by the surface area yields the Sabins of absorption. His formula worked well for the space in which he derived it – the Fogg Lecture Hall at Harvard University.

T = (V/Sa)0.161

where

T = Reverberation time for 60 dB of decay

V = Volume

S = Surface Area

a = absorption coefficient

While the formula established Sabine as the “father of architectural acoustics,” the formula was quickly found to be quite conditional. The accuracy varies wildly with rooms of different volumes, shapes, and Sabin content. Modified versions were produced by Eyring, Fitzroy, and others, but the take-away is that RT formulas provide estimates, at best. Recommended further reading is Leo Baranek’s scholarly paper on the subject.

Some Conditions

Four major conditions for reasonable accuracy of the Sabine equation include:

  • 1. Low absorption (less than 10% on average)
  • 2. Uniform absorption distribution (no concentrated absorption areas)
  • 3. “Mixing” room geometry (major room dimensions not grossly different)
  • 4. Large internal volume (small volumes are modal over much of the audible spectrum). This condition makes it more likely that the other conditions are realized.

Very few rooms meet the criteria for even reasonable accuracy of Sabine’s equation. That fact does not keep many modern practitioners from banking on the formula and using it to prescribe room treatment for difficult spaces. A recent thread in the SynAudCon Member’s Forum reinforced the problems with using the Sabine equation in small spaces, such as control rooms, studios, etc. It is also important to remember that absorption coefficients have similar accuracy issues for the same reasons. The absorption of a material depends on the sample size, sample shape, room size, placement, and other variables. Published coefficients are estimates at best.

Even with the conditions and inaccuracies, the Sabine and Eyring equations provide a very useful “first look” at the acoustics of a large space. They provide a benchmark that the results of more sophisticated approaches, such as ray tracing, can be compared to when formulating the approach to a system design.

A Sabine Space Example

Rather than write more about where the Sabine equation doesn’t work, I’ll present an example of a room where it can be expected to work reasonably well. Techny Towers is our preferred location for direct-to-reverberant ratio studies. Three “Techny Sessions” have been conducted in the space, including a line-up of beam-steered line arrays, a distributed mode loudspeaker, and a line-up of passive sound columns. SynAudCon Member’s can login and search for “Techny” to view these and other resources.

Techny could be referred to as a “Sabine space.” It serves as a reminder of the large errors that can occur when using Sabine’s formula with other room types.   pb

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Techny Towers Chapel of the Holy Spirit

Pat’s Blog – New GratisVolver Version

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The freeware GratisVolver™ continues to be one of the most useful applications that I own. The ability to convolve impulse Zoom_H1responses with anechoic program material let’s me sit at the mic position and listen to what a room measurement sounds like in the comfort of my office. It is also a powerful tool for producing the room impulse response (RIR) from a sweep recording. I used it to produce the RIRs for our “Techny sessions,” which are among our top downloads ever. I’ve written much about this technique in the past. Here’s a link for our members.

The Need for Proper Relative Levels

A great tool just got better. The previous version always normalized the RIR to full scale. That’s pretty standard practice and not a problem when looking at a single measurement. But, I usually make log-spaced measurements that I want to compare later. At Techny I use 20, 40, and 80 ft. When these are normalized to 0 dBFS, the reverberant field level gets louder with increasing distance from the source and the direct field stays the same level. That’s just the opposite of what happens in a real room. I’ve always been able to mentally ignore this, but I no longer have to.

Calibration Support Added

The new GV adds calibrated RIR support. You can select the closest measurement as the reference, and the more distant measurements will be the correct relative level – great for showing how the direct field becomes swamped by reverberation with increasing distance from a source. The new version also adds some formatting options.

Thanks to Bengt-Inge Dalenback, author of CATT-Acoustic™ for this powerful freeware tool. You can download it from www.catt.se.

Lastly, there’s a new Techny session coming out soon. It will include RIRs of some small-format line arrays and Don Keele’s powerful CBT array. All of the RIRs were produced using – wait for it – GratisVolver™. pb

GVolverFigure 1 – The new GratisVolver. Yes, that’s 80 dB+ of dynamic range for an RIR made from a recorded sweep!

Pat’s Blog – Prepping for Fall Specialty Seminars

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Things are buzzing at SynAudCon this week, and it’s not a ground loop. Preparations are under way for two upcoming specialty seminars – SynAudCon Digital and Making Wireless Work.

SynAudCon Digital – Better Than Ever

Steve Macatee and Brad Benn flew in on Monday for a couple of days of prep.

We devoted the first day to manual revisions. Topics were added. Topics were removed. We re-balance the content of this seminar each time we have it, as dictated by new developments in the marketplace and feedback from the previous event. New demos were added for dither and noise-shaping. We capped the day with some Thai food in a local country town, talking digital the whole time.

On day two, we went through the hands-on workstations, with about half the day spent on firmware updates. The workstations include our new Cisco managed switches for computer exercises. Since last time we’ve added some fiber links using mini-GBICs on the switches. The Audio-over-Ethernet exercises include Dante-enabled products from eight different manufacturers. They’re apples, oranges, and pears, and attendees will get a good exposure to a variety of interfaces as they cycle through the workstations. We’re going to make it work, break it, and then make it work again, merging the eight workstations into a single system as the final exercise of the seminar.

As the sun set, Brad and I dropped Steve at the airport, picked up the gear from last week’s Denver seminar, and hit a sports bar to talk more digital audio.

SynAudCon Digital – Nov. 16-18, 2015 in Washington DC

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Making Wireless Work – New Staff Member and Some Tweaks

I’m loving this event, because I get to sit back and watch the presenters mold the presentations through conference calls and emails. This time around Eric Reese (Sennheiser) will join Tim Vear (Shure), Karl Winkler (Lectrosonics), and James Stoffo (Radio Active Designs) to round out the staff. James is bringing spectrum analyzers ranging from $300 – $30,000, and Tim Vear is bringing his faithful HP along with the new Tektronics RSA306. We’re going with two 12 ft projection screens and a roving wireless camera to give the attendees “eyes on” all of the demos.

These guys talk antennas like fishermen talk lures, and from listening in on their conference calls I can confirm that they really like this stuff. Someone needs to warn Las Vegas that the Tuscany Resort will be radio active in early December.   pb

Making Wireless Work – Losing the Wires Without losing your Mind
December 3-4, 2015 – Las Vegas, NV

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System Integration Asia Covers SynAudCon Training

SynAudCon’s Training in Singapore received a nice write-up in Systems Integration Asia.

Check out page 24.

Pat’s Blog – Keele Tone-Burst Test

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The Chicago, IL Sound Reinforcement for Technicians seminar has come and gone. One of the highlights was an evening presentation by DB Keele, Jr. Many of you know Don from his work at Electro-voice and Harman, and for his patents on Constant Directivity (CD) horns and Constant Beamwidth Transducer (CBT) arrays. One of his numerous other accomplishments is the development of a test for the short-term power handling of loudspeakers. This was the subject of his evening presentation.

The “boink” test uses a 6.5 cycle wavelet to pulse a loudspeaker. It is performed at 1/3-octave intervals. The stimulus itself is 1/3-oct in bandwidth due to its length. There is an article with more detail in the SynAudCon Member Library. The WAV files were included on the SynAudCon Test CD for Sound Reinforcement Systems. It is long out of production, but you can download the files below. The files are stereo, with the bursts repeating at 1-second intervals in the left track and 2-second intervals in the right track.

Just download (2 MB) and unzip to a directory of your choice.   pb

Keele Tone-Burst WAV Files

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DB Keele, Jr. tests my “snowman” demo 3-way loudspeaker. 

Pat’s Blog – A Forgotten Skill Set

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These days we all spend a lot of time looking at computers. In today’s connected, app-driven world, there can be little time for anything else. A skill set that is fading into the sunset is the ability to work with tools.

The previous generation had a different experience. Sound companies once carved their place in the market by their ability to fabricate. Visit a loudspeaker designer and you would find a fully-equipped wood and metal shop for hammering out ideas. Without the ability to fabricate, a sound contractor was at a great disadvantage. If they couldn’t source a part, they had to make it – sometimes on-site. tools

When my son went off to college in the early 2000’s, he was immediately designated the handyman for his frat house because “None of the other guys knew how to use tools.”

I recently performed some polar measurements on a loudspeaker array prototype. As part of the project I needed to isolate one of the small array transducers into a box of its own, a box which didn’t exist. It had to have exact external dimensions for the edge diffraction to be right, and it had to rotate about an exact point in space for the polar measurements. By the time an hour had passed, I had used a table saw, jig saw, electric drill (with Forstner bit), pneumatic nail gun, and band saw, along with a MIG welder, on some scrap materials that I harvested from the wood and metal shop bone yards. It was ugly and cost nothing, but it was exactly what I needed.

Modern audio practitioners have to know a lot of stuff – much more than previous generations. The time that could once be devoted solely to audio must now be divided between multiple technical fields, each of which could take a full-time effort to master. There’s only so much time, and becoming proficient with tools can get crowded out. There’s just no time. But is there?

Time is a resource, and it can be reallocated. My advice to the up-and-comers? Put down the smart phone, turn off the social media and the fantasy sports, and learn how to use tools. Browse the tool department at Home Depot and ask “What’s this for?” Buy some tools and start tinkering. Fix, repair, modify, destroy. Learn what makes stuff “tick.” Can you solder? Why not? Over time you will develop a skill set that will make you indispensable to your employer, or set you apart from your competition.

That app that we are staring at will soon be obsolete. The ability to use tools will never be.  pb

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