On average, in aging males, the prostate enlarges (benign prostatic enlargement or BPE) and may cause bladder outlet obstruction (BOO). The internationally standardized method for diagnosing BOO is based on measurements of urinary flow rate and bladder pressure, using a catheter inserted into the bladder via the urethra. This method is invasive, time-consuming, and uncomfortable for the patient. We are developing a novel diagnostic method based on perineal recording of sound during urinary flow. Although it is known that (some aspects of) the recorded sound are (among others) related to the degree of obstruction, an exact and unique relation allowing derivation of the degree of obstruction from the sound recording is not known. In a biophysical model of the urethra, we found that the weighted average frequency, the standard deviation, and the skewness of the power spectrum are monotonically related to the degree of obstruction. The standard deviation was the most significant predictor of BOO (89\% correct). Based on this model study, we are confident that a simple noninvasive acoustic method for diagnosing BOO caused by BPE can be developed. This would lower the threshold for urodynamic testing of patients with lower urinary tract symptoms resulting in higher therapeutic efficacy.