A smooth handset receiver frequency-response curve does not guarantee a clean voice call. A particular tone may reveal a scrape, a rattle or a short buzz even when the main acoustic output looks normal. Separating design-related nonlinear behavior from an intermittent defect requires more than one number: engineers need to examine the fundamental, harmonic distortion and abnormal sound under controlled drive and coupling conditions. This article works through a numerical example and a practical fault-isolation sequence.

• 1 kHzExample fundamental
• 2.24%Example H2–H3 THD
• 2THD + defects

1. Define the Device and Measurement Chain

In a moving-coil receiver, current in the coil interacts with a magnetic field to move a diaphragm and displace the air in front of it. A measurement microphone records the combined response of the receiver, its acoustic coupling and the measurement system. Changing the seal, contact pressure or front-cavity geometry can change both fundamental and harmonic response. Use the agreed ear simulator or coupler mounting method and do not attribute differences between dissimilar fixtures automatically to the receiver.

For a meaningful comparison, keep the source, amplifier, voltage at the receiver terminals, frequency, coupler, sealing parts and microphone position consistent. Measure voltage at the device, not just at the generator. When testing an assembled phone, also document the volume setting and signal processing: automatic gain control, limiting and equalization can alter the signal reaching the transducer. A system result therefore cannot automatically be treated as an intrinsic bare-receiver specification.

2. What Harmonic Distortion Measures

With a single-frequency sinusoidal input at f, an ideal linear system produces a steady-state output at that frequency only. A nonlinear system can also generate components at integer multiples such as 2f and 3f. For an acoustic result, determine the RMS sound pressure of the fundamental and each harmonic; combine the harmonic components by their squared values instead of adding percentages directly:

Suppose a 1 kHz test gives p₁ = 0.100 Pa, p₂ = 0.002 Pa and p₃ = 0.001 Pa. Including only the second and third harmonics gives THD₃ = 100% × √(0.002² + 0.001²)/0.100 ≈ 2.24%. The two harmonic pressures are respectively about 34 dB and 40 dB below the fundamental. Higher orders are excluded from this illustration; these are example values, not a product measurement.

A useful comparison specifies harmonic order, frequency, terminal voltage, fixture and measurement bandwidth. THD+N additionally includes noise and other nonfundamental components within a stated bandwidth; it is not interchangeable with THD. High background noise makes small harmonics harder to resolve. Near a deep fundamental-response minimum, a ratio may rise even if the absolute harmonic pressure has not risen appreciably, so inspect absolute component levels as well.

3. Regular Nonlinearity and Rub and Buzz

The force factor can change as the coil moves, and suspension stiffness can depend on displacement. These nonlinearities can generate harmonics whose levels vary with frequency and drive amplitude. Comparing second- and third-harmonic curves at low and higher permitted drive levels helps characterize that change. A single harmonic order, however, does not uniquely identify which physical parameter is responsible.

A different group of problems includes coil rubbing, loose particles, a lead touching another part or vibration at an assembly joint. Such faults can create short impulsive events, with energy spread across higher-order and nonharmonic components. They may be audible while an ordinary RMS THD result remains dominated by regular low-order harmonics. Conversely, high-frequency energy alone does not prove coil rubbing: room noise, fixture vibration and an electrical fault in the measurement chain can also affect the result.

4. Make the Measurement Repeatable

For development work, stepped-sine measurements can hold a critical frequency long enough for the response to settle before the components are analyzed. A sweep covers a broader range and lends itself to automation, but sweep rate, analysis window and algorithm affect the result. When investigating a narrow resonance or intermittent defect, narrow the frequency range, adjust dwell time and repeat. A shorter cycle is useful only if the chosen test still detects the defects of interest.

The sample rate and usable bandwidth must cover the intended harmonics. If the analysis extends to Nf, that highest frequency needs to lie within the useful range of the microphone, front end and anti-alias filter, below half the sampling rate with allowance for the filter transition. For stepped-sine spectra, select an acquisition time and window that control spectral leakage. Extra displayed decimal places cannot compensate for a noise floor close to the harmonic being measured.

5. Move From an Anomaly to a Reproducible Cause

1. Fix the conditions and repeat

   Record the frequency, terminal voltage, orientation and mounting of the suspect unit. Check whether the event recurs, and retain the raw trace, waveform and test configuration.

2. Rule out the measurement system

   Inspect the supply and amplifier output; substitute cables or microphone where appropriate. Check the fixture with a reference unit, then investigate background noise, contacts, coupling leaks and fixture vibration separately.

3. Separate frequency from level effects

   Within the permitted safe operating range, vary drive level and compare the fundamental, individual harmonics and defect signal. Revisit a narrower band to see whether the event depends on a specific frequency or amplitude.

4. Locate and verify

   Use inspection and controlled disassembly to investigate particles, leads, clearances and evidence of rubbing. A repeat measurement after a repair or substitution should support the hypothesis. Listening or a single curve alone is not a root-cause analysis.

For an intermittent event, retain independent runs and individual waveforms: excessive averaging can hide a brief occurrence. Production limits should be based on the distribution of good units, known defective samples and fixture repeatability, then reviewed against maintenance and field feedback. A detailed development test and a fast production test may differ, but the relationship between them should be traceable.