Earlier this year, I dedicated columns to compact audio system tools from TerraSonde and SIA Software. With Meyer Sound's SIM 3 now shipping, it's a good time to take a peek at this highly respected diagnostic instrument. SIM, or Source Independent Measurement, is a three-channel technique that can use either live music or test signals to compute the transfer function — the system's affect on the signal — of a room or speaker system. To generate a transfer function, SIM takes two reference points — such as mixer out, processor out, or EQ out — and compares that to what arrives at another point in the system. The measurement being relative means that any excitation signal can be used, especially the program material itself.

First unveiled in 1986, SIM offered an approach where anomalies introduced by the interaction of a sound system's components, as well as the shifting acoustics of the listening environment, could be precisely measured and corrected by an engineer. Acoustic signatures of spaces shift as venues fill up with people. Before a show, a system's EQ can be tweaked to correct issues such as room resonance and echoes. However, that's in an empty space. Not only does an audience change a room's acoustics, but the gradual rise in temperature and humidity can affect reverb qualities and color the overall performance of the system.

SIM provides an efficient means to gather data to help engineers adjust system components for distortion, phase, polarity, dynamic range, and coverage. With those elements properly configured, an engineer could use SIM to properly set crossovers, EQ, delay, and speaker position. Most important, the analyzer allowed the operator to make system adjustments during a concert without intruding on the event.

With SIM 3, Meyer Sound has upped the ante. It's faster than the SIM 2 and only a fraction of the price. It has the ability to perform three concurrent transfer functions, which increases the flexibility of routing, and uses an embedded form of PCI (PC104+) that was initially developed for missiles and torpedoes (passing extreme shock and vibration tests in offiste labs). The SIM 3 comes with self-documenting user macros and preset procedures to assist smooth operation.

Tracing the way

The SIM-3022 audio analyzer, a 2U rack-space unit with two mic inputs and two line inputs, comprises the core of the SIM 3. Optional SIM-3088 multi-channel line switchers and SIM-3081 mic switchers complete the package. Data is stored on a Fast Flash Disk for quick access. The SIM 3 measures at more than 2,000 FFTsps and includes a CD-RW drive to export data. The combo of 24-bit conversion with the ability to make measurements with over 110dB of dynamic range permits THD measurements down to .001 percent. When gathering frequency response data, the SIM 3 displays info above 1kHz in less than 1 second and from 250Hz to 1kHz in less than 3 seconds. Real fast, in other words.

“Our rule of thumb for good data is 16 averages,” says Roger Schwenke, staff scientist at Meyer Sound who headed the SIM 3 development team. “We're able to update the screen every 300ms and can perform 16 averages at 2kHz and above on the very first update. This is a big boon in the field for when there's wind, or even just working in realtime. While moving a microphone around or changing the positioning of a loudspeaker, you can observe changes as they happen without having to sit and wait. In SIM 2, it averaged all the frequencies at the same rate, so it would take you 16 seconds to get a good room trace at all frequencies.”

Along with phase and frequency response measurements SIM 3 produces data on spectral content at all points in a system, as well as line spectrum readout for distortion analysis. There is a delay finder that identifies propagation delays between components in a system. In addition to helping handle latency issues, the delay finder internally synchronizes signals in time to permit accurate calculation of transfer functions.

Part of SIM 3's versatility is that it lets you quickly switch between different grouped measurement points that are relative to one another, or branches, without having to repatch. There's no limit to the number of branches, and you can have 10 switchers of each type (10 mic switchers, 10 line switchers) in a given setup. SIM 3 is equipped to handle the complexities of highly demanding multiple zone analysis.

Not a laptop

“There's been a continuum of projects using modified SIM 2 systems that led to the design of SIM 3,” says Schwenke. “Operators in the field wanted something more compatible with existing PCs, so now it's easier to move data and take screenshots. There's now a mouse, as well as the shortcuts that people wanted.”

“We also concluded that SIM 3 needed to be a dedicated hardware box; not a piece of software on a PC,” says Meyer Sound's communication manager, who is known as Larry the O. “Dedicated hardware provided greater processor power, and attention wouldn't be divided between other PC tasks. Plus, we could build it so it was roadworthy. And having a closed operating system, as opposed to a generalized open architecture, ensured stability. We also could address the demand for better interfacing than what was available with SIM 2.”

Consistency was another factor. “If you need to rent a SIM 3 machine, you know it will perform like any other,” Larry the O. says. “It's not like having to borrow someone's laptop and being forced to navigate through their system configuration to get your work done.”

However, the door isn't completely closed on a portable interface. “Some people asked for laptop control,” says Schwenke. “You'll notice there is an Ethernet port on the back of this machine. We've left that possibility for the future if there's enough demand for it and it seems a reasonable approach.”

Many Possibilities

While sound system alignment is a major application for SIM 3, it's by no means the only one. It can be any signal chain where you choose a reference and have two other points you want to measure relative to it. Also, there's nothing here that's tied to Meyer Sound speakers.

“We even had some discussion about SIM being its own brand,” says Larry the O. Source Independent Measurement is applicable to a wide variety of areas and is not simply wedded to the live-sound market.

“With SIM 3, you can characterize any piece of equipment, look at what the latency is going through it, look at what kind of phase shift is happening, and you can store that.” says Larry the O. For that matter, rental companies can perform a baseline test for all their gear when it's brand new. And every time a unit is back in the shop, they can measure and make sure it's up to standard.

“You can move through a large sound system in realtime, measure the parts you want to measure, and focus on your job as an engineer — which is making it sound good,” Schwenke says.


Alex Artaud is a musician living in Oakland, Calif. Reach him at alexartaud@earthlink.net