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Best sound absorbing material: Rockwool, Audimute, or Furniture?

By Nathan Lively

rockwoo-audimute

My wife and I recently bought a house (well, she bought a house and she’s letting me rent a room) and for the first time in my life I have the opportunity to do whatever I want with my office. I installed a few different sound absorbers, taking measurements along the way, and here are my results.

The Room

The first time I walked in I knew it would need some help. When I clapped my hands I heard something ringing and it was more difficult than it should have been to understand someone talking a few feet away.

fresh paint

Let’s estimate where the room starts to transition from modal behavior to diffusion by dividing the three times the speed of sound by the room’s smallest dimension (3c/RSD).

3390 / 9.02 = 376Hz

Let’s look at the Room Simulation in REW.

REW

Since the length and width are almost the same we can see two axial modes lining up nicely around 50, 100, and 150Hz. I hope some absorption will help with that.

I took four sets of measurements:

  1. Empty
  2. With ceiling mounted absorption panels
  3. With wall mounted Audimute sheets (and the ceiling panels)
  4. With furniture (and ceiling panels and Audimute)

Let’s get a global view of the overall changes with a spectrogram.

spectrogram all

And now the RT60 over 1/3 octave frequency bands.

RT60 all

I’m looking at the value in REW called Topt RT60 because:

The start point for the classical T20 and T30 measures of RT60 is where the Schroeder curve has dropped 5 dB below its peak. That works well in the large spaces for which RT60 is most applicable, particularly if the source used for the measurement is omnidirectional. In domestically sized rooms using normal, directional loudspeakers as sources the initial drop of the Schroeder curve is quite sharp (the Early Decay Time is quite short) meaning the -5 dB point lies within the early decay region rather than the diffuse field region. That in turn means the T20 and T30 figures underestimate the RT60 time. Where the EDT is much shorter than the T30 RT60 figure REW’s Topt RT60 calculation uses a start point based on the intersection of the EDT and T30 regression lines, to determine a point that lies within the diffuse field region. It then tests each possible end point in 1 dB steps and picks the one that gives a regression line with the best linear fit. That produces a more reliable RT60 figure.

REW user manual

I think it’s pretty clear that the biggest improvement here is around 400Hz. There’s a bunch of energy in the midrange that just made it nasty to try to have a conversation in here. And it’s no surprise that the ceiling absorbers made the biggest difference. They were also the biggest pain to install, but totally worth it. Thanks to Matt Bombich here in Minneapolis for those.

Also keep in mind that the measurements are not made with each material installed independently. I didn’t take down the Rockwool ceiling panels so I could measure the Audimute sheets by themselves. The Audimute measurement shows the improvement on top of the ceiling panels and then the furniture on top of all of that.

Those axial modes I hoped to tackle did not see such a significant improvement. I’m not an expert on absorbers, but I assume I would need some kind of low-frequency monsters.

Did you notice the 60Hz hum in every measurement except for the final one? I’m not sure what finally killed it.

Empty

You can see the room modes predicted around 50, 100, and 150Hz.

With 3 Ceiling Absorbers

With 2 Audimute Panels

The Audimute panels gave a nice improvement to the low-mids.

After I purchased them I discovered that they are also available in green. Instead of painting the wall I could have simply hung green panels and covered them up with the black curtain when they weren’t being used. 🤷🏻

With All Furniture

The only thing you can’t see in this photo is a twin mattress I put on top of that book shelf for even more absorption.

Overall, I’m really happy with the way the office sounds. The lighting is a problem, but I can figure out a better layout later.

Here’s a link to all of the impulse responses I recorded. Maybe you can do your own analysis and find some things I missed.

SPECIAL REPORT: Sound System Design for Small Venues with Bob McCarthy

By Nathan Lively

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Subscribe on iTunes, SoundCloud, or Stitcher.

Support Sound Design Live on Patreon.

Are you ready for over an hour of on-the-job training on sound system design for small venues with one of the most experienced sound system technicians in the world? Yeah you are!

In this episode we cover:

  1. How to measure sound systems in heavy wind.
  2. Dealing with highly reverberant spaces.
  3. The precedence effect.
  4. When to subdivide your system.
  5. The advantages and disadvantages to coupling your subs.
  6. How to setup a basic cardioid subwoofer array.
  7. How to aim a pair of L/R mains.
  8. The next update to SIM3.
  9. Combining speakers and microphones with shape and scale in mind.
  10. The best way to potty train your dog.

Details from the podcast:

  1. All music in this podcast by Steve Knots
  2. McCarthy’s book: Sound Systems: Design and Optimization
  3. Sound Design Live eBook
  4. Meyer Sound
  5. USITT
  6. Community hashtag: #XOVR
  7. McCarthy’s blog posts about subwoofer arrays
  8. 6 Smart, Proven Methods To Control Feedback Onstage (Without EQ)
  9. Quotes:
    1. Outdoors you take picture after picture and then lay them over and see where the trends are. You have to throw out certain ones that are not plausible.
    2. Nobody goes into acoustics because they think I’m going to get really good at putting fiberglass on the walls to make dead rooms, but in the world of small spaces that are going to do loud music that is exactly what you need.
    3. The overflow that you have potentially on a wall is no worse than you have, as a certainty, into the other speaker that’s coming from the other side.
    4. The HASS effect or precedence effect (or whatever you want to call it) is vastly over simplified in people’s brains.
    5. System design for small rooms is much more challenging and difficult than large spaces.

Goran O.

Q: What’s the approach in sound design for venues with bad acoustics that are very reverberant? I have mixed in these places and I must say that I’m trying hard to forget the feeling. It is very often the case that the owner of the club has spent money buying a decent sound reinforcement system, but then doesn’t care about acoustics. Or, there are cases when a concert is produced in some kind of multipurpose venue (which is normally a warehouse) and you are expected to create great sound. Would it help to try a more “distributive” approach.

Q: What about using the Haas (or precedence) effect in sound distribution? Bob says that delaying sound systems further then the first arrival of direct sound will mess with the phase, but what about spatial feeling?

Q: I mix a lot of great bands at a student club. Capacity is about 300-400 people. It has a balcony and under-balcony space. It would be great to have Bob’s opinion on how to distribute the speakers because this venue is part of the Faculty of Electrical Engineering and Computing and some students (volunteers) in the club are studying in the Department of Electroacoustocs so they are hungry for knowledge (as I am).

  • Left side of venue is a wall of mirrors
  • One EV bass speaker (tour-x TX2181) per side
  • Four bass speakers in first raw are just barrier for audience.
  • Mid/high boxes (EV XI-1152 – Rotatable 60° x 40°) are 2.4m from floor to bottom of speaker.
  • You’ll see also some JBL speakers behind EV (on middle of stage), but those are not in use.

sound-design-live-sound-system-design-myths-tricks-bob-mccarthy-goran-kset

sound-design-live-sound-system-design-myths-tricks-bob-mccarthy-goran-kset-sketchup

Download Google SketchUp Design

Interactive Panorama
(download for best results)

  1. Goran KSET Venue
  2. Goran KSET Stage
  3. Goran KSET Balcony

Simon E.

Sound system design for a rock show with six actors on close-mic’d wireless, and a band of piano, bass, drums, violin, and sampler in the stage-right wing.

I really wanted to fly speakers over the stage, but in this space it’s not going to happen (no roof infrastructure at all, not even for lighting) so I am resigned to two speakers on tall stands at the sides, angled in and tilted down in an attempt to get an even coverage. Based on the basic rule from Bob (summarized in your earlier article) of “back row on axis at distance 2D, front row at bottom edge of coverage at distance D”, I think it’s close enough. In this case, with 90×60 speakers, it works out to ~9′ up, 15 degrees down, and about 25-30 degrees in to avoid the hard side walls.

If all goes well, I will be borrowing a pair of Nexo PS15s (original version – 100ºx55°) and a pair of PS8s (100ºx55°) which will go on the floor in the middle as front-fill.

sound-design-live-sound-system-design-myths-tricks-bob-mccarthy-simon-theatre1

 

Mark N.

I am in the process of redoing a PA in a 650 seat worship room and have been trying to find the best placement for the subs and mains. The stage is in one corner with mains directly above the steps currently aiming at each aisle.

The center speaker has moved slightly off center and the x-over between the mains is in the middle of seats. I have stressed the concern of rearranging the seats so the x-over is in the aisles.

I can rehang, reposition, and the speakers can be visible. My thought was to flip the mains over so the horn is at bottom to get past the projectors. EV has seen the room and suggested that, too.

Here is the info of our current set up, along with photos and drawings of the room.

Room Info:

  • 70’x70′ square, ceiling height is 20′ from floor, 18′ from stage.
  • Walls are just drywall, 2 being outside walls with brick veneer on outside.
  • The ceiling is gypsum, suspended. Above it is empty with metal roof. This part of the building is actually shaped like a pyramid so roof peaks near center of audience about 45′.

PA:

  • Main Cluster- (3x) EV 1122s/66 (60ºx60º)
  • Amps: (1x) 2ch EV cps2.9
  • Subs: (2x)

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Thorsten B.

Q: When is the next update to SIM III coming out? It’s been a while.

Q: I have a theory that if I use the same model of microphone on every input and the same model of loudspeaker on every output, the sound will be more even and smooth because in the interaction between microphone and speaker there is only one frequency response instead of 12 or more if I use the typical 4-6 types of microphones and speakers. Do you agree? Does this work in the real world?

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Copyright © 2022 Nathan Lively

 

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