Slight Problem

[TFlenker]

Hi, I'm using the the pcRTA software and card in a PAC IV box, with M53 microphones. I have been using the pcRTA package at work for a few months, and everything was going smoothly until the other day.

The problem I'm having occurs when I am using the SPL portion of the package. When I start testing, it will at first appear to capture the correct curve, but after a moment the lower frequencies (125Hz and less) suddenly jump about 20 decibels higher than they should be. The problem is not consistent; it seems to happen in mic inputs 1 and 2 more often than in 3 and 4 (although it does happen to all microphones). It's difficult to explain exactly what is happening, and nothing I've tried has seemed to work. All 4 mics are being subjected to approximately 80 dB SPL total of white noise.

Any idea of what's wrong or how to fix this?
I have tried recalibrating the card and microphones and this has not had an effect. Thanks for your help.

[Mike Frost]

Your problem stems from the use of white noise as the stimulus. There is a 31dB span from the highest level to the lowest in the white noise signal. The pcRTA is auto-ranging to give you the best possible signal-to-noise ratio based on the highest signal level.

You have two options:

1. Use pink noise. This is the stimulus that pcRTA is designed to measure. If you measure the internal pink noise source (without using microphones) you will see a flat line on the graph.

2. Use a fixed dynamic range. Lock the dynamic range to the correct setting for the acoustic input level.