In the US, there is more reliance on the evidence to be introduced compared to the credentials of the person testifying. The most widely-adopted evidentiary standard is the Daubert Standard. This translates into a series of rules in Articles I and VII of the Federal Rules of Evidence, adopted by most states, which requires the trial judge to determine if the testimony is relevant and rests on a reliable foundation. Rule 104(b) says that
When the relevance of evidence depends on whether a fact exists, proof
must be introduced sufficient to support a finding that the fact does
exist. The court may admit the proposed evidence on the condition that
the proof be introduced later.
In other words, it could be a legally relevant question whether a substance is cocaine, or baking powder. The party wishing to exploit a particular fact must establish to the court's satisfaction the relevance of the purported fact. Then FRE 702 governs qualifications:
A witness who is qualified as an expert by knowledge, skill,
experience, training, or education may testify in the form of an
opinion or otherwise if:
(a) the expert’s scientific, technical, or other specialized knowledge
will help the trier of fact to understand the evidence or to determine
a fact in issue;
(b) the testimony is based on sufficient facts or data;
(c) the testimony is the product of reliable principles and methods;
(d) the expert has reliably applied the principles and methods to the
facts of the case.
More emphasis is placed on objective properties of the purported knowledge, compared to the training of the expert. Indeed, the requirement for the expert is not about training, it is more broadly about knowledge, skill, experience, training, or education.
The scientific bar that has to be cleared for admissibility is not particularly onerous. So-called "voice prints" are admissible in many jurisdictions, though in the domain of academic acoustic experts there is a significant concern that voice prints have not been "calibrated" i.e. objectively proven to be reliable (see United States v. Addison, 498 F.2d 741). The primary question, then, is whether this kind of evidence is relevant and reliable, and the qualifications of the person doing the measurement is less important (therefore, an FBI lab technician can testify to one fact, and an internationally-recognized professor of speech acoustics can testify to an opposing fact – the jury has to sort it out).
Credentials were a central issue in Kumho Tire Co. v. Carmichael, 526 U.S. 137, where one of the holdings was that "The Daubert factors may apply to the testimony of engineers and other experts who are not scientists", "It is the Rule's word "knowledge," not the words (like "scientific") that modify that word, that establishes a standard of evidentiary reliability". The chiropractor could therefore be allowed to testify to the back-pain question, and a sports-medicine MD could likewise testify to the opposite fact.