AES Section Meeting Reports

Los Angeles - August 29, 2017

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On August 29th, 2017, the Los Angeles chapter of the AES proudly welcomed acoustic architect, builder and designer Jerry Steckling (of JSX Audio and Jerry Steckling Designs) and his presentation on the challenges and compromises of acoustical design for today's small listening rooms. With his many years of experience designing every type of room from garage conversions to some of the most iconic rooms and theaters in the world (Skywalker Sound, Enzy Studios in Mumbai, India and Snoop Dogg's recording studio and many more), Jerry expertly discussed the realities to consider when creating a practical small room listening space in the modern world.
Jerry wanted us all to understand that the science of acoustics is a "very ancient set of knowledge", pointing out that the ancient Greeks and Egyptians had outdoor auditoriums constructed with perfectly shaped rock that enabled the audience to understand the spoken word without any loudspeakers or electronics. Acoustics have not changed since then; there is nothing new in how air moves, how we hear sound and manipulate sound passively with obstacles, absorption and diffusion.
To give us a better understanding of the science how we deal with small room acoustics, Jerry clarified what sound is: the vibration of the barometric pressure of air, a ripple, as if you tossed a pebble into a pond, causing the air pressure to wiggle and travel away from the source. He also stressed the importance of understanding certain measurements such as the speed of sound is 1130 ft/sec. and the wavelength of a 100Hz tone is 11 ft. Most sound instruments, equipment or loudspeakers will create a pressure wave at the source, like a piston, pushing some air in front of it which compresses that air and that pressure wave leaves the source at the speed of sound towards us, the listener. The pressure wave is not where the air particles are moving, that happens in front of that. At every boundary we also have air pressure. It is important to note that there is no particle movement of the air at these two points, it is somewhere in between. In small room acoustics we sometimes want to dampen these waves and we have to understand where to do that in the room. Putting sound dampening materials on a wall (boundary) does not correct the problem as there is no movement of air particles there to dampen.
Here is the basic concept: the loudspeaker is speaker box #1 and the room is speaker box #2 and those two things need to work together. In dividing up the air pressure movement in a room there are 3 small room frequency regions or zones:
1. Ray acoustics: Frequencies, like billiard balls, bounce off of walls, ceilings, consoles, etc.
2. The X Zone: due to the small size of the room, the wave length (a 100Hz tone is 11 ft.) is larger than the length of the room so the loudspeaker will not create any modes or bounces. It will literally be raising the barometric pressure in that room at that audio rate and it is virtually omni-directional.
3. The Eigenmode Zone: where we have resonances and reflections.
To know how to manipulate the sound in these zones, Jerry wanted to make sure that we understood how our hearing works. Our ears are located about eight inches from each other so we are able to hear stereo information down to a certain frequency to where it's not phased differently enough from ear to ear to tell where it's coming from. We must also be aware of the Haas, Precedence or "Cocktail Party" effect where, in a room full of lots of talking, our brain inherently helps us to focus on a specific sound or voice. As trained listeners, we can hear stereo down to about 250Hz. That means that our "Ray Zone" is 250Hz and above, where we study the room acoustics as billiard ball bounces. This is called, in Ray acoustics, the Initial Time Delay Gap.
Ultimately, what we want to experience is the stereo imagery of the loudspeaker system and not any of the room influencing that. Therefore, when we're tuning a room, we want to look at the ceiling band, the resonant sidewall band and/or the floor band where that early reflection could smear our imagery. We must respect the Initial Time Delay Gap, the room design and not have any hard reflections above 250Hz. Any hard reflections can be softened with soft acoustic materials and a hard back wall reflection can be broken up by diffusion.
So here's our ultimate goal: To create a small listening room that not only sounds good but looks good as well. To help create this proper small listening room the Eigenmode regions consider the mathematical relationships between the length, width and height of the room. With charts and graphs, Jerry demonstrated several examples of rooms he has analyzed and the complex measurements required to determine the many modes where energy is cancelled or built up by the room's reflections. One room was 20'Lx14'W with a 9' ceiling (typical size of a small suite) and each measurement from wall to wall, side to side and top to bottom had a reasonable distribution of multiples and resonances when the room was "excited". To calculate these measurements, Jerry uses special software designed to chart where these modes might happen.
The better room ratios are built on prime numbers, not multiples. A 9'x9'x9' room, or cube, is obviously not an ideal shape. When we inherit spaces that are not on ideal multiples, panel (or diaphragmic) absorbers are used to handle these modes, including bass build up. These typically, however, have a rather high Q, only absorbing in a narrow band of frequencies, so they aren't very practical in most rooms. A soft material behind the absorber can be used to broaden that response and lower the Q. A Helmholtz absorber has a cavity, or mouth, and the size of the cavity determines how low or high the frequency absorption is. The amount of the absorptive material put inside of the cavity will alter the size of the Q of the device and works as a resistor. Placing a panel where the maximum air particle movement exists in the room, often in the center, will dampen that wave. This isn't usually practical so the panel is moved away from the center of the room and towards the wall which puts it more in a pressure zone where there is less air particle movement but out of the way of the listener. (Jerry has the luxury of being able to order up all the materials ahead of time that he might want to use once the room is built. He can then set up his loudspeakers and mics and start putting the dampening materials in the walls and can actually see the modes move around in the room and where they are terminating.)
Jerry discussed a popular legacy room design back in the 70's and 80's, the LEDE (Live End, Dead End) design, meaning that the front of the room was mostly reflective and the back of the room mostly absorptive. The shape of the room (i.e. high, tilted ceilings) was protecting the early reflections in that Ray zone and was not reflected back towards the listener but back towards the back wall where they were absorbed or diffused. Jerry considers this a decent design that has stood the test of time.
Jerry pointed out that there are many acoustical mistakes that people have made and make, including putting a bass absorber next to a sub woofer which will absorb the wave too early in the listening experience, not allowing the wave to enter the room.
Even though all these measurements and graphs help in the room's design, every item that is put in the room will alter these measurements and make the tape measure and the speed of sound conflict. Carpeting, ceiling lamps, couches, even people, work as absorbers and diffusers. A typical sheet rock wall or ceiling moves and therefore is an absorber as well. When a room is excited by bass, the energy and heat generated are absorbed and also escapes, so when we hear the bass outside that means the energy is no longer there in the room. Therefore, the Eigenmode zone and Golden Ratios are not going to agree anymore. For that reason there is no predictive software available to tell what is going to happen. Most of us don't inherit Golden Ratio rooms to design so we have to compromise. Here the Initial Time Delay Gap is very specific; let's protect the billiard ball bounces from the loudspeaker to the listener in those early milliseconds. In handling the Eigenmodes, sometimes the figures need to be adjusted so we'll know what to change to achieve a good small listening environment.
Through the years Jerry has also worked on many active absorption rooms where the Eigenmodes are controlled actively. Jerry will put the speakers in the front of the room as well as in the back, a couple high and a couple low, to handle the modes. There are also diagonal modes, from the front lower corner to the back upper corner. There is a lot of processing and filter shaping to make this work and take that mode which would normally exist in the middle of the room and move it back so that the listener is in a really clean un-moded zone.
Being an acoustic designer, to Jerry, is like being a luthier, where changing materials will impact the sound of the room, like changing the strings on a guitar. That is where the craft is for Jerry. He gets handed so many different scenarios where he has to make compromises and he dares anyone to show him the perfect listening room because it doesn't exist. He will always work with the client to make sure they understand where the compromises will be. Sometimes it's monetarily driven.
Jerry discussed dealing with concrete block which, in its native form, is porous and you can hear right through it so it always needs another surface. He doesn't feel that enough material can be added to make it absolutely inert. What we are ultimately trying to achieve is a pleasant tone and all these different surfaces create a particular tone and texture.
One question asked of Jerry referred to floating floors. Concrete floors are great conductors of sound, transmitting audio 5 times faster than air and very efficient so we want to disconnect the final floor surface from the concrete. This can be done inexpensively with rubber isolators that cost a dollar a piece, isolating the floor from the concrete and remaining unattached from the walls.
Another question asked about creating a good listening room in an asymmetrical space. This is difficult and symmetry is basic as we are listening to a stereo signal.
Jerry also talked about his favorite TEF (Time Energy Frequency) software for testing. He does most of his measurements now with Studio 6 Digital software on his iPad and uses a calibrated microphone with an external pre-amp so he knows he is accurate and can adjust the difference between the left and right speakers to ¼ of a dB.
Other questions referred to pink noise measurements of rooms and his solution using the software Thumper below 250Hz and pink noise over 250Hz. It is harder to use Thumper in a small room as it can smear over into the X zone.
Jerry was also asked about the digital room correction systems such as Odyssey for personal listening, theater or screening rooms. According to Jerry they aren't great if there is an audience of more than one as these systems are all very location spe-cific.
We are grateful to Jerry Steckling for this extremely thorough and informative presentation.

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