Where a music copyright case is filed affects how it is analyzed, how expert testimony is used, and what the forensic musicologist’s report must accomplish. The two circuits that handle the majority of music copyright disputes — the Ninth Circuit, which covers California, and the Second Circuit, which covers New York — apply different tests for substantial similarity. The difference is not academic. It shapes the expert’s work from the outset.
The Ninth Circuit uses a two-part test for substantial similarity. Both parts must be satisfied for a finding of infringement.
The extrinsic test is an objective comparison of the protectable elements of the two works. It requires analytical dissection — filtering out unprotectable material such as common chord progressions, standard rhythmic patterns, and other musical building blocks, then comparing what remains. Expert testimony is not only permitted on the extrinsic test but is often essential. This is where the forensic musicologist’s report does its primary work: identifying what is protectable, documenting prior art, and presenting the comparison through forensic notation, parallel transcriptions, and other exhibits designed for a non-musical audience. The extrinsic test is the gatekeeper — it is what allows a case to survive or be dismissed at summary judgment. If the extrinsic test does not hold up, the intrinsic test should never get its chance.
The intrinsic test is subjective. It asks whether an ordinary reasonable listener would find the two works substantially similar in their “total concept and feel.” Expert testimony is not permitted on the intrinsic test — the determination belongs to the jury. The intrinsic test has drawn criticism from within the Ninth Circuit itself, with judges arguing that “total concept and feel” risks extending copyright protection from expression into ideas. In practice, laypersons are often persuaded by musical elements — instrumentation, key, tempo — that the law specifically seeks to exclude from the analysis. This tension between what a jury hears and what the law protects is one of the most persistent challenges in music copyright litigation.
While the law treats the extrinsic and intrinsic tests as sequential steps, the forensic reality is more intertwined. Substantial similarity is often a matter of originality, which in turn informs whether copying occurred. The forensic musicologist works within both dimensions simultaneously, even though the court evaluates them separately.
The Second Circuit applies what is known as the “ordinary observer” test, refined into a “more discerning ordinary observer” standard when the works at issue contain both protectable and unprotectable elements. The question is whether a lay observer — not an expert — would find the works substantially similar, focusing on the protectable elements rather than the work as a whole.
Expert testimony plays a smaller role in the Second Circuit. Courts have held that a judge’s “good eyes and common sense” are sufficient to evaluate substantial similarity, and dismissals for lack of substantial similarity at early stages of litigation are common. The forensic musicologist’s analysis may still be valuable — particularly in complex matters where the protectable and unprotectable elements are difficult to separate without musical expertise — but the procedural framework gives it less formal weight than the Ninth Circuit does.
The circuit determines the architecture of the report.
In the Ninth Circuit, the report is built to survive the extrinsic test at summary judgment. That means rigorous filtration, thoroughly documented prior art, and exhibits that present the objective comparison with precision and clarity. The expert’s analytical dissection is the foundation on which the case stands or falls at a critical procedural stage.
In the Second Circuit, the report supports a different argument — that the similarities a lay observer perceives are or are not grounded in protectable expression. The analysis may be equally rigorous, but the procedural role is different. The expert informs the court’s evaluation rather than providing the framework for it.
In practice, the sequential logic of these tests — extrinsic first, then intrinsic — is cleaner in the case law than it is in the work itself. Forensic evaluation is highly intertwined. Substantial similarity is often a question of originality, which informs whether copying occurred, which circles back to what is protectable. A forensic musicologist working within either circuit’s framework navigates these interdependencies constantly.
One practical tension deserves mention. The intrinsic test asks lay listeners to evaluate “total concept and feel” — but laypersons are often drawn to musical elements the law specifically does not protect: instrumentation, tempo, key, production style. A forensic musicologist’s work includes making the distinction between what sounds similar and what is actually similar in protectable terms clear enough that the finder of fact can apply it. That clarity is what separates a well-informed verdict from one driven by impression.
In either circuit, the quality of the forensic musicologist’s work determines whether the musical evidence is understood clearly enough for the finder of fact to reach a well-informed and just decision. The procedural path differs. The professional standard does not.
Until 2020, the Ninth Circuit applied the inverse ratio rule — a principle holding that the more access a defendant had to the plaintiff’s work, the less similarity was needed to establish copying. In Skidmore v. Led Zeppelin, the Ninth Circuit abrogated the rule, finding that it “defies logic” and “creates uncertainty for courts and parties.” Access and similarity are now evaluated independently.
The Second Circuit never adopted the inverse ratio rule. The abrogation in the Ninth Circuit brought the two circuits closer together on this point, though significant differences in their overall approaches to substantial similarity remain.
Expert reports are built to the standard the matter requires, including the procedural demands of the circuit in which the case is filed. The initial call to assess scope and fit is free and carries no obligation. For information on report structure and engagement costs, see What Is a Forensic Musicology Expert Report? and How Much Does a Forensic Musicologist Cost?.
