Investigation of the response of a microphone / loudspeaker over a range of frequencies

Dependent Variable:        Amplitude (mV)

I will be using the signal generator to alter the frequency being produced – therefore this will be the independent variable. This will alter the amplitude of the wave being shown on the CRO so this is the independent variable. I have used the same equipment throughout the experiment in order to ensure a fair test.

Diagram

Method

The apparatus was set up as shown on the previous page. Firstly I did three ‘checks’ to ensure that (to check if they are properly calibrated.):

• The amplitude of the signal remains constant

• And that the period of the wave on the CRO corresponds to the frequency on the dial of the signal generator.

(See results 1,2 & 3)

This showed that the error increased with the increase in frequency but it was generally quite well calibrated.

I then linked up the microphone and loudspeaker and produced to take readings of at intervals of 50 Hz. However when the amplitude showed an obvious peak I then altered to every 10 or 20Hz to ensure a detailed peak on my graph.

Results

1.        100Hz                F=1 / period                4.2 x 2=8.4                F=1 / 8.4 x 10-3  = 119

2.        500Hz                F=1 / period                3.8 x 0.5=1.9                F=1 / 1.9 x 10-3  = 526

3.        1000Hz        F=1 / period                4.8 x 0.2=0.96        F=1 / 0.96 x 10-3  = 1041        

Analysis

My graph shows three peaks in the relationship between frequency from the signal generator and the CRO. (These are marked by a red  on the graph.)I appear to have one anomalous result (circled.) My graph shows that an increase in frequency results in an increase in amplitude up to a certain point and then it decreases to another peak.

Modifications

I did not have to make any changes to the plan for the experiment. I was told how to set up the apparatus and how to carry out the experiment so there were no problems.  However I found that the loudspeaker was being very sensitive and that any background noise in the classroom was affecting it, which meant that it was hard to take an accurate results.

Also the other person’s signal generator (who was using the same desk as me) was interfering with my CRO which meant that it was hard to get an accurate reading as I had two signals going to the loudspeaker. Therefore I ended up doing it alternately with her (she switched sig. gen. of whilst I took my reading.) But this meant that I ran out of time to do all the results that I had wanted to do. There appears to be another peak at about 850Hz but I could not do this in as much detail as I had the other peaks.

Evaluation

My experiment appears to have been quite accurate with only one anomalous result however the line is not very smooth, which is probably as a result of all the background noise affecting the CRO. However it still shows the relationship between the frequency and the amplitude effectively.

Conclusion

I have fulfilled my aims as I have become “proficient with using a signal generator and a CRO.” Also I have proved that there is a relationship between amplitude and frequency by investigating how the amplitude of a signal from a microphone varies as the frequency of a fixed amplitude signal applied to the loudspeaker varies. I varied that amplitude signal for a large number of intervals between 100Hz and 1000Hz and this has shown the relationship I described earlier (and is shown on my graph.)