Hello.
I'm setting up auditory fear conditioning, and I wonder how I can measure decibel of a tone for conditioned stimulus. I want a 75-dB tone and have a decibel meter.
I am not sure where I need to place the decibel meter in the context
to adjust a 75-dB tone. near the speaker? on the bottom? in the middle? The speaker is on the right wall of a square shaped context and if I want to proceed fear extinction in a different octagon-shaped context, I need to adjust the tone again for the new context, right? In this case, where do I place the decibel meter ?
Thanks for reading and I'll be waiting for your tips.
In general if subjects are exposed to sound, sound levels should be measured near the subject, if possible. If not, take your measurement position such that it allows you to calculate the sound level at the subject's position.
It would help to know more about your equipment, e.g., whether your microphone system has a probe tube on it. In essence you want to put the microphone at the position of the eardrum, which can be done with a probe tube and an anesthetized animal. Otherwise measure with the microphone at the position where the animal's eardrum(s) will be during the procedure. You also need to be sure that you are using a free-field calibrated microphone. If you change the setup you need to re-check the calibration. Make sure the space around the animal is free of hard, sound-reflecting surfaces or cover such things with soft cloth.
I'm assuming the tone you are using is like a click or fairly broad-band noise AND your contexts are of similar volume-wall height-etc. Then the sound level (measured where the animal's head would be within the context) should be similar. Personally, I'd make both measurements to be sure -it shouldn't take much time. The sound source matters in this. The multi-frequency characteristics of a click or broad band noise reduces the effect of things like altered context or changes in materials within the context. Single frequency tones or narrow band noise are affected by these changes in different ways depending on the frequency range (high or low).
Remember also that your term "75dB" can mean many things, depending on the filter settings (e.g. dBA, etc.) for mixed-frequency bands and "SPL" for pure tones. For example ou will need to know the difference between a signal level given in dB SPL and one given in dB.Hz-1 The differnce is not trivial and will be essential if you want to have publishable data.
To avoid reflections from the walls and other surfaces in the room which can cause errors in the actual sound level at the animal's position, I would recommend that you cover all free surfaces with acoustic foam. There are many varieties of commercially available foam, but you can also use egg cartons in a pinch. These are not as effective as foam, but are better than nothing. Good luck!
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