Investigate the relationship between sound pressure level (SPL) and signal amplitude.

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Joe Benn 13:2                   A Level Physics – ‘Sound Level & Power’                       14/11/02

Sound Pressure Level & Amplitude

Aim

Investigate the relationship between sound pressure level (SPL) and signal amplitude.

Summery

I constructed the apparatus as shown below and collected results.  The results do not show what I expected and do not adhere to the rules and physical theory I have researched.

The investigation has not shown what I had intended it to but I can suggest several reasons as to why.

NB: Large copies of all the graphs used are included at the end of the document

Planning

As a sound engineer I have wondered for some time as to the relationship between increases in power output of a sound system and changes in sound pressure level (SPL).  It seems to me that there is not a direct relationship between the two.

I plan to simplify the idea of a sound system into a signal generator/amplifier and a single speaker.  The SPL will be measured using a sound pressure level meter (Figure 2).

Variables and constants; the SPL meter will be placed a fixed distance away from the speaker, the frequency emitted from the speaker with remain constant, the amplitude of the signal will be varied to change the power  output of the speaker.

                             

Figure 1

Figure 2

SPL meter as it was used in the application

I performed some preliminary experiments to determine:

  • The distance between the SPL meter and the speaker.
  • The frequency of the signal the speaker should emit.
  • The lowest amplitude that could be sensed by the SPL meter.
  • The highest amplitude that could be produced by the signal amplifier.

I found:

  • For the SPL meter to register at low amplitudes the furthest distance the speaker can be from the sensor is 3.5cm.
  • If the SPL meter is closer to than 2.5cm from the speaker the speaker driver causes interference and abnormally low SPL readings.
  • Due to the design of the speaker I was using, unusual buzzes and hums were audible at frequencies between 200 – 400Hz and 600 – 700Hz.  In order that these frequency problems do not cause anomalies in my results, the speaker will emit a constant 500Hz signal.
  • Signals with amplitude less than 10mV are not detectable at a distance of 3.5cm with the SPL meter I was using.
  • The signal amplifier I was using could produce a signal with amplitude of up to 800mV.
  • The scale on the amplitude control of the signal amplifier was inaccurate; to ensure accurate results I used an oscilloscope (CRT) to guarantee correct amplitudes.
  • The display on the SPL meter was small and difficult to read, to ensure accurate readings I connected the output terminals to a voltmeter with a Vu dial.
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After the necessary alterations were made, apparatus appeared as it does below.

Figure 3

                                                        Figure 4

Safety

The results are to be taken in a laboratory environment; the apparatus of the experiment pose no serious danger.  When I was assembling the equipment I found the single retort stand to be a little unstable.  I clamped masses to one side of the cross member in an effort to counterbalance the weight of the SPL meter, this worked, the equipment was stable.

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