Contents - Index

FAQ
Index
Some answers to questions that may arise...

There is a message with a yellow background telling me "Check for Correct Signal Track!".
Most tests with OmniMic (other than "SPL/Spectrum" and "Oscilloscope") require particular test signal tracks to be played on the system that you are measuring.  The tracks contain precise test signals necessary for the OmniMic software to do its analysis.  Do not substitute other test signals, as any results would have no meaning!  For example, although the "Pseudo Noise" (CD Track 1) signal sounds like pink noise, it is actually very different and use of pink noise in its place will not produce a useful result.  The proper signal type names to play for any OmniMic test is displayed at the top of the OmniMic screen. 

Played test signals seem to have gone crazy!
In rare cases, this can happen when the computer's recording and playback processes become very corrupted due to unusual processing interference or hardware problems (it's best to not have the computer doing unrelated complex operations while trying to make measurements!) or intermittent connection of a USB connector.  In the Config menu, there is a menu item for Reset System. If you use this, there will be a several seconds delay while the system attempts to get itself aligned again, then the problem should be fixed.  If that still doesn't fix it, try shutting down the OmniMic software, unplug the OmniMic, then restart the software and plug OmniMic in again.

Sometimes, the sweep tones may seem to "hiccup" or restart part way through a sweep.  This is the software trying to position the output timing with respect to the OmniMic's acquisition process, and will usually self correct after a few sweeps.  If it occurs often, there may be a difficulty in the setup, such as excessive noise levels or a bad connection.

What's the switch on the side of the Omnimic40k do?
The Omnimic40k is capable of handling signal levels beyond 140dBSPL (which is very loud) with low distortion.  When you are working with levels above about 125dBSPL, you should put the switch toward the back (wider end) of the microphone.  This inserts an internal 24dB attenuator into the analog signal path so that the A/D converter inside the Omnimic is not overdriven by the high signal levels.  When this switch is in, be sure to put a check mark in the box, at the top right of the main form, that is labeled "Attenuator used" so that the software knows to correct the displayed levels to match the inserted attenuation.

What's the silver and black bump on the side of the Omnimic40k?
That's the sensor that picks up the timing signal from the PhotoLink box when you need to include true time of flight in measurements.  See Using the OmniMic Photolink device

I get messages saying that my "Control Panel" settings are not correct for OmniMic (original version).
This can happen in modern Windows operating systems.  For proper operation of OmniMic at higher sound levels, you must make sure that the system recognizes the microphone as a 2 channel microphone (the second channel is used for handling higher signal levels).  The message gives specific, easy, directions for making the proper settings.  The exact procedure may change in later Windows versions, so you may have to manually open the Windows Control Panel and locate the pages for making the needed settings.  The settings may need to be done for each of the computer's USB ports that might be used with OmniMic.

The top of the OmniMic window shows the message "OmniMic not found".
This indicates that the presence of the OmniMic is not detected.  OmniMic hardware must be attached to a working USB port on the computer for the program to be able to make measurements.  Try using a different USB port on the computer and make certain that the blue indicator light (at top or back of the OmniMic) is illuminated.  Unplug any other USB-based audio hardware from the computer, and if necessary try rebooting the computer. Please note that OmniMic cannot typically be used reliably with Windows versions earlier than Windows 7, with Macintosh OSes or under Linux or Linux/Wine. Windows 10 or 11 is advised.

The graphs of frequency response (or other measurements) is varying at lower frequencies.
This is probably because the test signal is not being played loud enough to overcome the background noise.  Try playing the test signal louder or moving the microphone closer to the speaker(s).  For many systems, the response at lower frequencies may be insufficient to overcome the noise.

Impulse response and phase traces seem to jitter left and right during measurements.
This is normal to a small degree.  It should not happen when using OmniMic40k with its PlotoLink deveice.  But because OmniMic without a Photolink can't know when the test signal was emitted at a loudspeaker, it must refer phase and time displays to the largest peak of the recovered impulse response shape, which can vary in level and polarity with responses that have multiple peaks of similar level.  If you know the first peak is positive (or negative), then you can select the synch polarity with the [+] (or [-]) buttons at bottom left of the Impulse Response display.

Harmonic Distortion plots don't seem to make sense at frequencies where response dB values are low.
This again results from insufficient test signal level arriving at the OmniMic relative to noise.  High levels of background noise or weak signal levels will show as distortion.  You can find the distortion "threshold" of a measurement due to noise by simply silencing the test signal.  Only distortion levels that appear significantly higher (at frequencies of interest) than this threshold when the test signal is playing should be assumed to be valid.

I can't measure harmonic distortion at higher frequencies -- the curve drops off the plot
Original OmniMic can "see" sound only in the range of human hearing (20kHz and below), so it cannot see harmonic energy that falls at frequencies higher than this.  So 2nd harmonic cannot be measured above 10kHz; 3rd harmonic cannot be measured above about 6.7kHz, etc. (Double the frequency limits mentioned here, when using an OmniMic40k and an appropriate 96ksps test signal).

Why do square waves played through my speakers look weird (using the Oscilloscope)?
Most loudspeakers cannot reproduce square waves well.  This may be due to imperfections in frequency response or phase response (the Frequency Response must be both flat and with phase near 0 degrees -- "linear phase").  The reproduction of a square wave that looks like the ideal typically requires the speaker (and OmniMic) to have flat, linear phase response from about 1/10th to 10 times the frequency of the square wave you are looking at.  It will not be possible to show a square-looking waveform outside of the range of about 50Hz to 2kHz because the audio range of the microphone is 5Hz to 20kHz.  Distorted looking waveforms could also be because of sound reflections off of surfaces within the room.  Try moving the microphone closer to the speakers to see if the shape is better.

The frequency response at very high frequencies seems to drift around over time.
This is likely to be a side effect of slightly different sample clock rates used in OmniMic and your digital signal source.  To minimize this effect, choose the "sine sweep" signal rather than the "pseudo noise" signal for measuring frequency response for your more critical measurements.  Room positioning of loudspeakers and setting of equalizers at lower frequencies can generally be done using the more pleasant pseudo noise signal, if desired.

The impulse response of my speaker shows ringing before the impulse happens!  That's impossible.
This happens when your speaker has strong response out to and beyond 20kHz (or 40kHz for OmniMic40k).  OmniMic can only "see" out to about 23(46)kHz (displays to 20(40)kHz) because of its 48(96)kHz sample rate.  It accurately calculates both magnitude and phase response right up to the highest audio frequencies, but as far as it can tell, the response drops completely beyond 23(46)kHz, and data above that is unknown.  The correct impulse response that would be equivalent to a response that is strong with smooth phase to some frequency, and then its magnitude drops away above that frequency, will have "pre-ring" in the shown response.  This doesn't mean that the speaker has output before it sees input -- it only means that an actual full band time response corresponding to a seemingly abruptly ending frequency response would have to have such a characteristic. If higher frequencies were known for the device being measured, and could be included, they would cancel the pre-ring (notice that the pre-ringing wave appears to have a period of approximately 1/23(1/46)kHz, well beyond audibility, where the data stops).  Or, if the response rolled off naturally before 20(40)kHz (like human ears or most speakers do), that would result in a phase response, delay, and HF attenuation which would prevent the apparent pre-ring.  But a perfect impulse response measurement of a broadband response that is strong up to a frequency where it suddenly disappears should have the pre-ring, it is not an error but is how the system must be represented in a limited bandwidth system.  

I get almost no response above 20kHz:
With the original OmniMic hardware, response data is limited to 20kHz.
With OmniMic40k hardware, response up to 40kHz can be measured, BUT the test signal source and any following amplification, must be configured for 96kHz sampling rate or performan to at least 40kHz.  Note that many class D amplifiers do not reproduce signals above 20kHz well or at all, so they are probably not idea for this purpose.  If you are getting the test signal from your computer in Windows, make sure that its playback sample rate is set for 96ksps or higher. 

My measurements show a rolled-off high end.  I know my speakers are flat and my system is all very High-End.
First, make sure all system tone controls or equalizers are disabled (just in case).  But check your signal source (file player, CD or DVD player, or even some computer sound cards) for "creative" signal handling -- some very expensive CD players are doctored to "fix" a misunderstood "pre-ring" time-domain problem  and in doing so mess up the frequency response of their playback.  Usually, inexpensive players don't have this problem.  You may have to spend less money to get a more accurate test signal source! 

For PC-based playback systems used to provide the test signals, make certain that your computer is adjusted to play those in DVD quality (for original OmniMic Hardware) or "Studio Quality" for use with OmniMic40k hardware.  That is, 48ksps rate for original OmniMic and 96ksps rate with OmniMIc40k hardware.  PCs can and often do use different quality settings for different audio output devices (DACs, etc.) so it is best to check each one in the Windows Control Panel.  You can use the "Config->Soundcard Settings" to get to the controls for this.
(Under recording interfaces, however, the orignal OmniMic hardware should always be set for 2-channel DVD quality)!!

If using a receiver or integrated amplifier for driving test speakers (when testing speakers, rather than the speaker and amplifier system), be certain that any tone controls for it are set to flat, and any EQ or loudness controls are turned off.  Many class-D or class-T amplifiers (but not Hypex UcD or nCore, or IcePower) are not optimal for loudspeaker testing, as their frequency responses at high frequencies can be affected somewhat by loudspeaker impedances.  A class-AB type amplifier or a class-D type mentioned above is usually preferable for most accurate testing purposes.