## 4 Discussion  
The sound pressure levels show in Figure 4 identify the peak level to be 72 dB, occurring at 4000 RPM with the diffuser open. This was the case for either half or full recirc. The sound pressure level with the diffuser open is only slightly higher than the other configurations and is to be expected, as the diffuser vent is closest to and directed towards the microphones. Full and half recirc had little variation between them; the only difference being full had a slightly higher level at lower RPMs. Most of the data fitted a relatively linear relationship (between RPM and sound pressure level) however all the data at 2000 RPM were slightly lower. Loudness showed similar findings to the sound pressure level. This non-linearity is likely a result of noise sources that are not dependent on RPM, such as the motor, whose noise will have greater influence at lower fan speeds, however no evidence was found that these had any significant effect.  
Figure 16: Sound pressure map of front face test at 1500 RPM, frequency between 280 Hz and 330 Hz.  
Figure 15: FFT of front face test at 1500 RPM, FFT up to 25 kHz (**a**) and up to 5 kHz (**b**).  
The metric sharpness (shown in Figure 10) did not reveal any problems with the data, the only interesting finding being that the fan speed 1500 RPM was shown to drop in sharpness. During testing the engineers noted that this speed produced what they deemed to be an 'annoying' sound, combined with the drop, shows that sharpness was not the reason for its annoyance. The drop in sharpness at 1500 RPM is explained by Figure 15, where the FFT shows that there is a high lower frequency component not present in the 2000 RPM FFT shown in Figure 11. Given sharpness is a measure of the proportion of high-frequency wave, a drop would be expected if either less high frequency was present or an increase in lower frequency was present. The specific loudness plots in Figure 8 show that at 1500 RPM a spike at around 3 Bark (\(\sim\)300 Hz) was present, which could not be seen at other frequencies. Figure 9 confirmed that this was present for each test configuration. The acoustic camera was then used to isolate the location of this frequency, which can be seen in Figure 16 to be emitted from near the bottom of the unit with a sound pressure of almost 49 dB. This is a sound wave with an overall sound pressure level of 52 dB (shown in Figure 4) is very significant. The FFT in Figure 15 confirms that this is much higher than any other frequency in the wave. At 2000 RPM a much lower magnitude frequency peak occurs in a similar location at 720 Hz, shown in Figure 14. The presence of this indicates that what is causing the noise is present in other tests, but at a different frequency and significantly lower magnitude. The reason for this noise is not fully clear and would require further analysis and potentially testing to understand. The use of acoustic camera to isolate the location will speed up any further testing. It is likely due to vibrational resonance of part of the unit being achieved, however this is only speculative at this point. The use of the acoustic camera to locate this source enables the redesign of the component to avoid this resonance or alternatively avoiding this RPM once the HVAC unit is in use.