Music in the Round #51 Page 2
Dayton Audio OmniMic Precision Measurement System
It find it interesting that the rise of multichannel systems for audio and home theater seems to have spawned a number of useful and affordable systems for taking acoustical measurements. I suspect that while stereo guys are just as persnickety about system setup, the inclusion of DSP-based room equalization in many AVRs and pre-pros has sensitized many more to the need for close attention to room acoustics. In addition, some users, motivated by confusing results from the various automatic EQ programs, have sought out an independent measurement system to confirm those systems and/or to inform themselves. Strangely, few of these measurement systems are specialized for use with multichannel systems (more on that below), and they work equally well (or better) with two-channel systems.
Dayton Audio's OmniMic Precision Measurement System ($349.99) is remarkably compact: all the works are contained in the microphone itself. The mike is powered by and connects to a PC with a USB cable and is mounted on a standard microphone stand with a supplied clip. Also included are a pair of discs: a CD-ROM containing the OmniMic software for loading onto your computer, and a CD with test signals to play in your CD player. That's itno external mike preamps, soundcards, power pods, or cable links running along the floor between PC and OmniMic. In fact, once the software is installed on your computer, you can pull the OmniMic out of the closet and be ready to test and measure in minutes.
The first time you open the OmniMic program, it checks for the mike's presence and prompts you to enter its ID number, so that the proper calibration curve is used. The first screen (fig.1) shows one of six modes of operation, selectable via tabs across the top. In a window above the tabs, you are prompted to play the CD track with the necessary test signals for the chosen mode. The screen for each mode also includes display windows, and buttons or menus for the control of various options. Almost all graphs are accompanied by sliders with which the user can scale and displace the x and y axes. There is no printed manual, but an extensive Help menu is included.
The six basic modes:
1. Frequency Response: Displays frequency and impulse responses based on pseudo-noise sweeps (CD track 1), or repetitive short sinewave sweeps (CD track 2). Options include time-gating the response and smoothing the display (from 1 to 1/96 octave or none).
2. FFT (per hertz) or Real-Time Analyzer (RTA) (per octave): Can be used with any signal source, including pink noise. Integration/update time and response weighting (A, B, C, or none) are adjustable. Also on this screen is a simulated analog dB meter with selectable weighting. OmniMic provides a calibration option for users with a reference source or sound-level meter.
3. Oscilloscope: Simulates an analog oscilloscope for the display of audio-frequency waveforms and transients. Can be set to trigger from the rising or falling edge of a transient. An optional 10Hz high-pass filter is provided.
4. Harmonic Distortion Analyzer: Based on long sinewave sweeps (CD track 3), this screen displays harmonic distortion vs frequency and permits independent display of the second, third, fourth, and fifth harmonics, as well as their sum (fig.2). Smoothing options (from 1 to 1/96 octave or none) are provided.
5. Reverberation Decay: Based, again, on long sinewave sweeps (CD track 3), this displays signal decay and permits assessing RT60. The integration time limit can be controlled, as well as the analysis's upper and lower bounds.
6. Bass Decay: Using the sub-500Hz slow sweep (CD track 4), the upper window shows the bass-frequency response, the lower window the spectral decay (time vs frequency), for four amplitude levels from 0 to 20dB. In addition, the top window can be switched to one of two waterfall plots, to graphically display delayed energy contributions from reflections (fig.3).
This is a powerful set of tools. Data results can be stored, recalled for display, arranged to compare multiple results on the same graph, and treated with math operations such as Filter and Normalize. A Snapshot feature quickly grabs any screen. Another really useful feature is that most functions run continuously, permitting the user to observe, almost in real time, the effects of such manipulations as repositioning a subwoofer or adding acoustical treatments.
If you're wondering "What will I do with this?," the answer will depend on your needs and skills. Most audiophiles can see the value of frequency response (1) and bass decay/response (6)and, of course, an RTA/FFT (2) is a great way to explore, in real time, the spectral properties of whatever signals interest you, as well as how your system transforms them. The values of the oscilloscope (3) and the display of reverberation decay (5) are less apparent, and unless you're a speaker modifier or builder, the distortion tool (4) will give you information you probably can't do anything about. However, as you delve deeper into the OmniMic, you'll learn more and morethe system's power is in its layering and combining of tools and data.
The OmniMic is only one of several reasonably priced measurement systems, each of which has its strengths and weaknesses. RoomEQ Wizard focuses more on listening-room acoustics, will develop room-mode correction filters for a parametric equalizer, and is free. However, the user must procure a suitable mike (including its calibration data), as well as a preamp and soundcard. Hooking up all of this also requires multiple interconnects, which in my room are hazards to navigation. The learning curve isn't too steep, but the more you use it, the more you'll get from it. RoomEQ Wizard is supported by its developer on the website, where you'll also find a lively community of users.
I reviewed a version of AcoustiSoft's ETF in 1998, and since then it has been subsumed into a more sophisticated program, RPlusD Modal Analyzer. At $129.95 or $199.95 (the price depends on the version), this is a set of acoustical measurement tools that will suit even professional users, but, as with RoomEQ Wizard, those users must buy a mike and preamp/soundcard combinationand ETF's use is not intuitive, at least at the beginning.
The XTZ Room Analyzer (see my columns in the November 2008 and November 2009 issues) is much more of a single-purpose application. It measures and displays system/room acoustic response in the amplitude and time domains, as well as RT60, and from those measurements develops corrective EQ filters that can be inserted in a parametric EQ. The complete package ($299.99) includes a USB mike and stand and sufficient cabling for most rooms. The displays are colorful and informative, but XTZ doesn't run continuously or work in real time.
I found OmniMic as easy to set up and use as XTZ, more powerful in its test suite, and comparably pricedbut unlike XTZ, it won't generate correction filters. It's not as powerful in its feature set as either RoomEQ Wizard or AcoustiSoft's RPlusD, lacks the correction filter calculation abilities of the former. What distinguishes OmniMic from the rest is that they generate all the test signals in the PC and feed them to an analog input of your system, while OmniMic relies on a CD with four signal tracks and avoids the dangerous clutter of cables in the room. The price paid is that you must now juggle the CD player's remote control as well as OmniMic's user interface, but this is a worthwhile tradeoff for OmniMic's continuous measurement and display options.
What all of these systems lack is adequate attention to the needs of multichannel fans. The output of each is, quite properly, a monophonic signal, although the OmniMic CD has two channels of that signal. As a result, all require multichannel users to repeatedly unplug and then reinsert the signal cable in the system's various analog input channels, in order to test each individually. Moreover, unless your AVR or pre-pro digitizes the analog input, you can't assess what built-in room EQ software is doing. Even with redigitization and feeding the test signal to all the channels by setting the AVR or pre-pro to All Mono, you still have to disconnect the output connections to all the channels not being measured. This problem could be resolved if Dayton Audio offered, instead of a CD, a multichannel Blu-ray disc with each of its four tracks repeated once for each of the 7.1 channels. That would make OmniMic even more attractive to me.
Overall, I think that Dayton Audio's OmniMic is a useful and easy-to-learn general tool that most audiophiles will enjoy and not outgrow for a long time, if ever. I found it easy to use and a snap to set up. Consequently, it's the first tool I grab when I want to know what's upand I'm still learning.