Originally posted by sensibull
Forgive me for asking another idiot newbie question, but in your help file you specify to use the AV processor's speaker trim to adjust L, R, or C speaker output until it matches target Cal Level dB, but to adjust the volume during the Measurement Level setting.
While I understand the difference between trim and volume, I'm a little confused about how to hit 75dB using the trim adjustments alone (which on my HK 635, only go from -10db to +10db). Is the assumption that I have previously calibrated my system and already know what volume level should hit 75dB on the test tone, and that I set the volume to that before I begin the SPL calibration, using the trim settings only to fine tune it? Or does it matter whether I use volume alone, or volume + trim during the SPL calibration phase?
While I'm already embarassing myself, I have two further clarification questions:
1. The point of the two calibrations is to match the RadioShack SPL with the Room EQ SPL (as dictated by recording level), and then to determine at what volume your soundcard creates a 75db signal, correct?
2. Does it skew the measurements if you use a lower target SPL? I have twin babies and never run my system very loud. I ran a preliminary test last night and even 80dB left my ears ringing a bit...
I'll tweak the help text for those sections to make it a little more clear, it is not the most intuitive part of the process.
The input calibration is used to give the Wizard an absolute SPL reference. A signal needs to be generated at a known level (as read off your SPL meter) and the Wizard needs to be told what that level is (via the Cal Level dB control). It will then adjust the input level control so that the signal captured by the soundcard has enough headroom (aims for 18dB) and adjust its own SPL meter reading to show the same figure as your SPL meter.
The process is based on the normal calibration routine for your AV processor, as carried out when you set up your system, using the internal calibration tones on your processor. The level of the calibration tone is usually not affected by your volume control, only by the speaker trims (the processor typically applies an internal volume setting that is appropriate for the calibration process). If the volume control does affect the calibration tone level, set it to whatever level is recommended for calibrating your processor. You will only need to alter the speaker trim if the level on your SPL meter is not at your calibration level - alternatively, just change the Cal Level dB setting on the wizard so that it is the same as the reading on your SPL meter.
Usually the cal level is 75dB, you can use a lower figure as long as you can trim your cal tone output to hit that lower level during calibration.
Setting the measurement level, which does involve adjusting your processor's volume control, is to take account of the output level of the soundcard and the input sensitivity of your processor and set a level for taking measurements that is high enough for accurate results but has enough headroom to allow for the effects of resonances. The target it aims for is to achieve the same level as used for the input calibration. First it sets its own output sig gen level by measuring the level on the local loopback connection, then it generates a speaker cal signal and waits for you to adjust the AV processor's volume to give the desired level measured off the SPL meter.
Regarding keeping the noise down, if you select the "Limit SPL when measuring" box below the Set Target Level button it will automatically reduce the signal levels when resonances are encountered to keep the overall level close to the target level for the speaker - it cuts the excess above the target to 1/4 of what it would otherwise be, e.g. a 12dB resonance would only give a level 3dB above the target. The graph will show the figures that would have resulted without the limiting, i.e. it would still show a 12dB peak. This does extend the measurement time though, as the limiter needs additional settling time. You can even set an artificially low Target Level during the measurement (e.g. 65dB) to reduce levels even further, as long as you remember to set the correct target level before searching for peaks or adjusting filters. However
I have just found a bug in the measurement figures that are recorded when the limiter is on
so wait for tonight's release if you want to use that feature.