Overview
SIM System II is a powerful, instrument optimized for making audio- frequency measurements of an acoustical system and applying precise electronic corrections to adjust the system 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.)

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 and multi-segment level meters for each measurement input.

Physical Model

SIM 3 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).

Frequency Response Mode

In Frequency Response mode, the SIM 3 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 1/48th 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.

Frequency Response Mode Display

Measurement of three parametric filter bands with equal Bandwidth and Gain settings.

Frequency Response Mode Display
Measurement of an analog audio tape recorder illustrating S/N+D by frequency (red trace, upper window).

Signal-to-Noise

SIM System 3 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.

Frequency Response Mode Display
Amplitude Response and Signal-to-Noise (top window) and Phase Response (bottom window).

Delay Finder

In Delay Finder mode, the SIM 3 Sound Analyzer displays a windowed impulse response of the system under test, permitting accurate identification of reflections. The SIM 3 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.

Delay Finder Mode Display

Loudspeaker impulse response with boundary reflection; ±70 msec. (upper trace) and ±7 msec. (lower trace).

Spectrum

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.

Spectrum Mode Display With Flat top window shape and Peak Finder cursor.

SIM (Source Independent Measurement) 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. It enables 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 3 overcomes these limitations with new algorithms that substantially eliminate errors.

Signal Thresholding

The SIM 3 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 Blanking

Coherence is the output power of the system under test that is due to the excitation signal, divided by the system s total output power. Its value varies between 1 (no contamination) and 0 (100% contamination).

For each frequency data bin, the SIM 3 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 3 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.

Constant-Q Transform

The SIM 3 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

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.

At maximum capacity, SIM 3 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 3 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 3 affords prodigious power for the most complex acoustical analysis tasks.