What Did My Microphone Pick Up?

[IRL303]

Hi gang.

I wanted to see what would happen if I placed a cassette tape in-front of a speaker then a microphone behind the cassette which was hooked up to a oscilloscope. The mic was set to record when it detected sound. So I turned the mic on and ran a frequency from 30Hz to 200Hz right around 80Hz the mic activated and the oscilloscope starts to move. Around 100Hz its really moving. After it was done I looked at what it had recored in a program called wavepad and sure enough there was a waveform. In some places of the waveform there was a very crude looking sine wave that was distorted but, in other places it was very level and the up and down parts of the wave matched perfect.

So I was wondering if someone might be able to tell me what I recorded. I thought maybe it was just the test tone from the speaker the mic was picking up but the oscillator would of picked it up straight away as soon as I pressed play as would of the mic. I would be very greatful if someone could tell me what is going on... If anything?

Thanks guys.

 

[schip666!]

I'm not entirely clear on your experimental setup, but...

If what you got was a reasonable sound/shape that increased in volume as you raised the frequency, I think what you recorded was the frequency response of the speaker and mic. If you got something that ramped up and then back down in volume as the frequency increased I'd say the peak would be at the resonance of the system.

Can you explain what you are doing and why a bit more?

 

[sophiecentaur]

Your average room is full of resonances. They are often quite 'broad' and your ears / brain edit out the frequency response when listening to stuff. At the same time, you get an impression of what the room is like - i.e. size, quantity of soft furnishings etc. from what your ears pick up (colouration). A loudspeaker and microphone combination include so such 'intelligence' and the microphone just picks up the sound, warts and all.

If you pay loadsa money and buy yourself a hugely expensive hi-fi system and play it in a room with no acoustic treatment (your bathroom?), you have, in many ways, wasted your money. The frequency response will be rubbish but your ears may hear an OK sound.