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This page describes products and general use; it is not medical advice.
A 128 Hz tuning fork is a metal fork that vibrates at 128 cycles per second and is used by clinicians to test vibration sense in the hands and feet. In US regulation it is a Class I device exempt from premarket notification. Clinicians, clinic buyers and students use it; the published accuracy figures below show it is better treated as a quick bedside screen than a measurement.
What a 128 Hz tuning fork is
The FDA classification regulation, 21 CFR 882.1525, defines a tuning fork as “a mechanical device which resonates at a given frequency” that “is used to diagnose hearing disorders and to test for vibratory sense.” The 128 in the name is the frequency in hertz. The regulation covers tuning forks as one device type; its text does not separate weighted from unweighted forks or medical from other forks, and it uses no label such as “C128”. This page therefore makes no claim about differences between weighted, unweighted or “C128” forks.
For the general instrument (types, parts, care), see what a tuning fork is. This page covers the 128 Hz fork specifically.
Regulatory class
| Item | What 21 CFR 882.1525 states |
|---|---|
| Class | Class I (general controls) |
| Premarket notification | Exempt from the 510(k) procedures in subpart E of part 807, subject to the limitations in 882.9 |
| Quality management system | Exempt from the part 820 requirements except records and complaint files (820.35) |
| Intended use | Diagnose hearing disorders; test for vibratory sense |
Source: 21 CFR 882.1525 (Cornell LII), checked 2026-10-30; the official text is at ecfr.gov.
What the 128 Hz fork is used to test
The fork is used to check vibratory sense, the ability to feel vibration on the skin over bone, typically at the toes and fingers. Results are usually recorded qualitatively (felt or not felt). Graduated forks such as the Rydel-Seiffer type are an alternative that gives a scaled reading.
Accuracy and limits
A 2014 study of 100 consecutive patients at a neurology EMG clinic (Lai et al., Journal of Clinical Neuromuscular Disease 15(3):96-101) compared forks against sensory nerve action potentials, with blinded examiners. The conventional 128 Hz fork, scored as abnormal when sensation was absent, had 21% sensitivity and 88% specificity. A graduated Rydel-Seiffer fork (64 Hz in that paper) had 29% sensitivity and 86% specificity, and the difference between the two was not significant. The authors noted there are no uniform criteria for what counts as reduced vibration.
| Fork | Sensitivity | Specificity | PPV | NPV |
|---|---|---|---|---|
| 128 Hz, qualitative | 21% | 88% | 49% | 67% |
| Rydel-Seiffer, graduated | 29% | 86% | 54% | 69% |
Source: Lai et al. 2014, PMC4957578, checked 2026-10-30. Interpretation of any test result belongs to the treating clinician.
Specifications and what the regulation covers
The regulation is the specification-level reference: a tuning fork is defined by its resonant frequency and its two stated uses (hearing disorders and vibratory sense). For testing other sensory modalities in the same exam, see Semmes-Weinstein monofilament sizes, and for the hearing use see audiometer types.
Clinics equipping a foot-screening exam can see how the fork fits among other items in the diabetes clinic supply checklist; related bedside instruments are covered in what a reflex hammer is. Instruments of this kind are listed under diagnostic, monitoring and imaging products.
Limits to keep in mind
- The qualitative 128 Hz fork is not a measurement: in the study above it had 21% sensitivity.
- Criteria for “reduced” vibration sense are not uniform, according to the study authors.
- Class I status describes the device type, not the accuracy of any test result.
FAQ
Is a 128 Hz tuning fork a medical device?
Yes. 21 CFR 882.1525 classifies tuning forks as Class I devices used to diagnose hearing disorders and test vibratory sense.
Does a tuning fork need FDA 510(k) clearance?
No. Under 21 CFR 882.1525 it is exempt from premarket notification, subject to the limitations in 882.9.
Is there a difference between a weighted and an unweighted 128 Hz fork?
The classification regulation does not distinguish them, so this page makes no claim on it.
How accurate is the 128 Hz fork for neuropathy?
In the Lai 2014 study, 21% sensitivity and 88% specificity against sensory nerve action potentials.
Sources
- 21 CFR 882.1525 Tuning fork, Cornell LII, checked 2026-10-30.
- Lai S et al. Diagnostic Accuracy of Qualitative vs. Quantitative Tuning Forks: Outcome Measure for Neuropathy. J Clin Neuromuscul Dis 2014;15(3):96-101, https://pmc.ncbi.nlm.nih.gov/articles/PMC4957578, checked 2026-10-30.
