Parkinson's disease (PD) is a neurological illness associated with a variety of motor deficits and non-motor deficits involving areas such as speech, mood, behaviour, thinking, and sensation. The PD-related vocal impairment results from the involvement of various speech subsystems including respiration, phonation, articulation, and prosody. The abnormalities in these speech subsystems are traditionally assessed using several acoustic measurements. Signal processing algorithms, in turn, offer an objective method for precise evaluation of speech performance from recorded signals. In this paper, we study the disordered speech of people with early PD who have not undergone pharmacotherapy treatment. Here, we demonstrate that acoustic measurements can reveal subtle changes in speech and thus significantly separate healthy persons from patients with PD. The various speech data were recorded from 23 people with recently diagnosed PD and 23 healthy control (HC) speakers. PD patients were scored according to the Hoehn and Yahr stages and the Unified Parkinson's Disease Rating Scale. We have found 19 representative measurements that are able significantly to separate both groups of speakers. Compared to HC, the PD speakers show abnormalities in measures of jitter, shimmer, noise-to-harmonics ratios, diadochokinetic rate, clarity and accuracy of articulation, sound pressure level, intensity of speech, melody of speech, number of pauses, and ability to reproduce perceived rhythm. Our findings show that (a) 78% of PD subjects indicate symptoms of vocal impairment that differ from the speech performances of the wider norm of healthy speakers, (b) lowered loudness of speech are significantly correlated with overall severity of disease.