## 4 Non-Contact Measurement of CPF via Cough Sounds
### Non-Contact CPF Measurement Device  
Figure 5 shows a non-contact cough strength measurement evaluation method based on cough sounds. Cough sounds can easily be measured using various microphones [56]. The device incorporates the CPF estimation model for mobile devices that enables cough sound recording and immediate CPF estimation and provides a user interface allowing CPS history management [58]. This device can facilitate a screening test to detect decreased cough strength in home medical care; however, the reliability and validity of CPS have yet to be fully determined. Previous studies have not fully considered the effects of disease and airway mucous because the participants were healthy volunteers [55; 56; 58]. Furthermore, the cough sounds were measured in a quiet ordinary room. A major problem with this method was the influence of sounds reflected from the walls and/or sound attenuation. Therefore, it is necessary to verify the reliability and validity of CPS and improve the accuracy of CPS estimation in patients for further clinical application. Evaluation of Cough Strength Using a Smartphone in the Elderly and in a Patient with Spinal Muscular Atrophy  
Figure 6 shows examples of elderly participants' cough sound patterns measured using a smartphone [58]. In the case shown in Figure 6a, although the respiratory function test showed vital capacity (VC) and forced expiratory volume in one second/forced vital capacity (FEV\({}_{1}\)/FVC) values exceeding the reference value, the cough strength values, such as CPS and CPF, were below the reference value of 270 L/min. In the case in Figure 6b, both respiratory function and cough strength exceeded the reference value. In these cases, the CPS was almost the same value as the CPF and could be used to estimate cough strength. The evaluation of cough strength using a smartphone demonstrated a median relative error of 13.55% in elderly participants and 6.19% in young participants [58]. Figure 5: Non-contact cough strength evaluation method. The smartphone (iPhone 6 A1586; Apple, Inc., Cupertino, CA, USA) was held in the left hand while the participant bent the elbow to 90 degrees and the shoulder to 0 degrees and then rotated the arm internally to 45 degrees. Figure 7 shows changes in unassisted and volume-assisted cough sounds in a 20-year-old man with spinal muscular atrophy. The low cough strength of this patient warranted access to mechanically assisted coughing, such as mechanical insufflation-exsufflation, during intercurrent respiratory infections. In this case, we attempted evaluate the cough strength using a smartphone (iPhone 6) with reference to a method reported in a previous study [58]. With the assisted cough, the cough sound amplitude could be clearly detected from the measured sound; however, the unassisted cough yielded a very weak cough sound, which could result in difficulty detecting the cough sound amplitude against noise. The previous study did not fully consider cough sound frequencies, although breathing sounds can be influenced by sound frequencies [21; 22]. The estimation accuracy could thus be further improved by considering the frequency domain. In addition, because the non-contact CPF measurement method was aimed at the daily evaluation of coughing ability and risk screening, it is implicitly assumed to be applicable to healthy or close to healthy users; however, application in users with a disease or airway mucous is also expected. Figure 6: Examples of cough sound patterns in elderly participants [58]. The grey line represents the measured cough sound signal. The solid light blue line represents the bandpass-filtered and rectified sound signal. The solid purple line represents the moving average with a time window of 20 ms. The red circle represents the maximum point of the moving average. (**a**) Example of a 77-year-old woman with a measured CPF below the reference value of 270 L/min. The respiratory function test showed slightly low values of %Vital Capacity (VC) = 83.5% and one second/forced vital capacity (FEV1/FVC) = 83.0%.