SIM System II : Acoustic Test and Measurement System
Features & Benefits
SIM System II is a powerful, compact instrumentation product line comprising the SIM- 2201 Sound Analyzer, SIM-2403 Interface Network, and a selection of software options, microphones and accessory cables. The instrument is optimized for making audio-frequency measurements of an acoustical system and presenting analysis data that allows the system operator to apply precise electronic corrections to adjust its response.
SIM System II implements Meyer Sound's Source Independent Measurement technique, a dual-channel method which accommodates statistically unpredictable excitation signals. Any excitation signal which encompasses the frequency range of interest (even intermittently) may be employed to obtain highly accurate measurements of acoustical or electronic systems. (For example, concert halls and loudspeaker systems may be characterized during a musical performance, using the program as the test signal.)
Housed in a four-space rack-mountable industrial chassis, the SIM-2201 Sound Analyzer performs 32-bit floating-point audio signal measurements with >100 dB dynamic range (actual input signal range is greater because of selectable gain). The instrument permits two-port measurements between any two of three front-panel inputs (one microphone with switchable phantom power, two isolated line level), and incorporates a rear-panel multi-pin interface for automated measurements of two-channel systems. Optional hardware and software upgrades permit up to sixty-four analysis channel capacity.
Measurement data may be displayed as amplitude vs. time (Impulse Response), or amplitude and phase vs. frequency (Frequency Response). A single-channel Spectrum mode is provided, and frequency domain data are displayed with a logarithmic frequency axis. A Delay Finder function determines and internally compensates for propagation delays.
The SIM-2201 incorporates a front panel-controllable precision signal generator with low-distortion sine wave, pink noise and modulated, weighted pulse outputs; multi-segment level meters for each measurement input; a removable hard disk and DOS format 1.44 MB floppy disk drive for data storage; high-resolution color monitor output; and dedicated rear-panel multi-pin system interface connectors. A headphone output is provided for aural monitoring of the measurement inputs. User-friendly software with pull-down menus streamlines operation, and measurement data may be exported to disk in ASCII format for post-processing or laser printed for presentation.
SIM System II is designed for not only characterizing, but also electronically correcting, acoustical systems. Its architecture and nomenclature follow a physical model consisting of a loudspeaker in a room (object of measurement) with a measurement microphone, and a parametric equalizer (correction network) connected in series with the input signal. The excitation is assumed to be neither totally random nor predictable, though known test signals such as noise or stepped sine waves may be used.
The instrument's three input ports are connected respectively to the correction network input (A), the network output (B), and the measurement microphone (C).
Measuring a loudspeaker in a room and correcting its response with parametric equalization.
In Frequency Response mode, the SIM-2201 displays amplitude, phase and coherence (or signal-to-noise) data for transfer function computations between any two of the three measurement inputs. The frequency response (amplitude and phase) transfer function is computed by dividing the cross-power spectrum by the auto-power spectrum of the reference channel.
Three different frequency response measurements may be selected:
- Room + Speaker - the unequalized system response, measured by comparing the equalizer output and microphone.
- EQ - the equalizer response, measured across the equalizer from input to output.
- Result - the corrected system response, measured by comparing the equalizer input and microphone.
Frequency Response data are displayed in two windows, with amplitude and coherence (or signal-to-noise) in the upper window and phase in the lower. A selectable Group View displays all three of the above frequency responses at once (v. 2.3 only), and an inverse of the equalizer response is available for adjusting correction networks to match system response aberrations. A Zoom feature reveals fine details of the system response.
The maximum resolution in Frequency Response mode is 1/24th octave (1/30th octave in v. 2.0 Lab Zoom mode). 1/6th or 1/3rd octave smoothing may also be selected to facilitate observing general trends; when smoothing is employed, missing data points in the response curve are interpolated if they fall within the smoothing interval.
Click the thumbnail for a full size view of the Frequency Response
(Images are approximately 100k)
SIM System II provides for display of dynamic signal-to-noise + distortion at each frequency, as an alternative to coherence.
The signal-to-noise of a measurement may be calculated as the coherence divided by one minus the coherence. Where the coherence is 1 (no contamination), signal-to-noise is theoretically infinite. (>100 dB, the dynamic range of the instrument). At 0.5 coherence, where the noise equals the signal, signal-to-noise is 0 dB.
Click the thumbnail for a full size view of the Signal-to-Noise
in Frequency Response mode.
(Image is approximately 100k)
In Delay Finder mode, the SIM-2201 Sound Analyzer displays a windowed impulse response of the system under test, permitting accurate identification of reflections. The SIM-2201 calculates the propagation delay between the reference and measurement channels, and sets an internal delay to compensate for any time offset and synchronize the channels. The function is accurate to within approximately 1/8" (for sound in air at STP) with propagation distances up to 1000 feet.
An associated External Delays Procedure determines and displays the time offset between two measured systems and may be used in setting delay lines to synchronize physically separated systems.
Click the thumbnail for a full size view of the Delay Finder
(Image is approximately 100k)
Spectrum mode employs single-channel computation to display the spectral content of a measurement input with 1/6th octave resolution. Amplitude flatness is ±0.1 dB.
Optimized for sine wave stimulus, Spectrum mode is used primarily for distortion analysis and can resolve up to nine harmonics. Distortion components are summed and displayed as a percentage relative to the amplitude at the cursor position.
A Peak Hold funtion, in which the cursor finds and remains on the spectral line with the highest instantaneous amplitude, is provided.
Click the thumbnail for a full size view of the Spectrum mode.
(Image is approximately 100k)
SIM (Source Independent Measurement)
SIM is a two-channel acoustical analysis method in which the excitation signal may be independent of - that is, not generated or determined by - the measurement system. Itİenables highly accurate frequency response measurements where it is inconvenient to employ calibrated test signals, or where an excitation already exists.
Conventional FFT analyzers have been used to make such two-port measurements, but they are subject to significant errors when the excitation is statistically unpredictable, especially when the output is contaminated with noise. Averaging can help, but with conventional methods, some error term always remains in the result.
SIM System II overcomes these limitations with new algorithms that substantially eliminate errors.
The SIM-2201 Analyzer permits establishing an amplitude threshold value for the measurement input signal. When the signal exceeds the threshold at a given frequency, the transfer function is computed for that spectral line. When the signal is below threshold, it is ignored. Signals that overload the instrument's inputs are automatically rejected.
In this fashion, the instrument builds a valid frequency response over time. If two or more samples are acquired at a given frequency during the measurement period, they may be averaged.
Signal thresholding enhances the signal-to-noise of the measurement. It is useful when the excitation contains a given frequency only sporadically, is otherwise sparse in content, or varies widely in level.
Coherence is the output power of the system under test that is due to the excitation signal, divided by the system¼s total output power. Its value varies between 1İ(no contamination) and 0 (100% contamination).
For each frequency data bin, the SIM-2201 Analyzer establishes a preset coherence threshold that is tied to the number of averages employed. Where the coherence drops below the threshold, the frequency response traces for that bin are blanked from the display. The SIM-2201 therefore does not display data of questionable accuracy, simplifying interpretation of the data.
Coherence blanking enhances measurement accuracy in the presence of substantial output signal contamination. It is useful for rejecting output noise, reverberation, or other effects that may bias the measurement.
The SIM-2201 utilizes a short measurement window at high frequencies and progressively longer windows for each successive lower octave. Since the excitation is not correlated to the measurement window, reflections that extend from one window into the next are averaged out due to uncorrelated arrival.
The use of this near constant-Q transform allows measurement and correction of near-field reflections while properly rejecting reverberation, and displays frequency-domain data with equal resolution per octave.
Vector averaging is the statistically correct way to remove both periodic and random output noise contamination. It is employed whenever a time-invariant system is to be tested in a noisy environment.
Often viewed as magnitude and phase, audio signals may also be converted into a vector containing real and imaginary components at each frequency. A contaminated signal thus may be visualized as the actual vector with noise vectors surrounding it. In vector averaging, the real and imaginary parts are linearly averaged simultaneously. Since the noise vectors are uncorrelated to the excitation, they statistically reach a value of zero, yielding the actual magnitude and phase value for that frequency.
Vector averaging yields the most accurate estimate of the system response as long as the frequency response is stable with time. RMS averaging is also available for occasions where the system frequency response is time-variant (for example, when making outdoor measurements under windy conditions).
Vector representation of an audio test signal with noise contamination.
SIM System II is designed to accommodate varying levels of complexity in measurement and equalization requirements. Three versions are available:
SIM System II v. 2.0 comprises the SIM-2201 Sound Analyzer with single DSP engine, and system software v.İ2.0. Optimized for laboratory operation and automated measurements of two-channel systems, v. 2.0 is upwardly expandable to v.İ2.3s or 2.3m.
SIM System II v. 2.3s employs two additional DSP engines, v. 2.3s software and an accessory Stereo Interface Snake. It allows for simultaneous display of three live frequency response functions. This version is optimized for speed in analyzing systems with up to two channels.
SIM System II v. 2.3m employs system software v.İ2.3, two additional DSP engines, interface card, accessory cables and up to eight SIM-2403 Interfaces.
At maximum capacity, SIM System II v.İ2.3m accommodates sixty-four microphones and displays simultaneous real-time measurements of the room, equalizer and equalized system response for any of sixty-four separate measurement branches. Switching and selective muting of branches is controlled from the SIM-2201 Analyzer. With the ability to utilize multiple measurement microphones, accurately determine and compensate for multiple propagation delays, and efficiently manage large amounts of data, SIM System II v. 2.3m affords prodigious power for the most complex acoustical analysis tasks.
SIM-2403 Interface Network
The SIM-2403 Interface Network is a switching unit that may be controlled remotely from the SIM-2201 Sound Analyzer (v. 2.3m only). Incorporating internally calibratable microphone preamplifiers with phantom power and dB SPL multiple-segment meters, each SIM-2403 provides access and control of up to eight distinct microphones and correction networks. The unit may be connected in Mute mode, allowing muting and level control of individual channels, or a Safe mode when muting is not desirable. Front- panel LED's indicate muting functions and status.