A loudspeaker’s design might be perfect but thorough and objective 100% end of production line testing is the only way to know that the speaker design approved is the design that’s being built and shipped to a customer. Whether they are speakers for voice applications, high-end pro audio, or pro-consumer products, the speaker performance must be quantitatively consistent unit-to-unit, batch-to-batch and in some long-life products, year-to-year.
How Does End-of-Line Testing Work in Speaker Production?
Some speaker manufacturers apply only a brief single tone test to their final products and hope their test operators are able to decide in an instant whether the speaker is able to be shipped. The “rub-and-buzz” test is the most common for measuring audio defects that occur due to an off-center speaker cone or loose particles floating within the speaker’s magnetic gap or excess glue somewhere in the build. While rub-and-buzz testing is critical and should be performed on each speaker unit before release, it only verifies that the speaker is not audibly defective-- a pretty low standard of performance. It does not completely assure that the speaker performs properly.
Seven Tests For Great Speakers
Once loudspeakers reach the end of the production line, they should be quantitatively tested to ensure they meet each important parameter of speaker quality and that the production batch in which they are built yields statistically desirable results. The seven Production Validation Tests should include:
Frequency Response Margin: Every speaker design has its own frequency response curve shape. In production, that shape should fit into a tolerance window that is consistent with the intended purpose of the driver. Unplanned peaks or dips in the response need to be identified and become a possible basis for test failure.
THD Margin: Total Harmonic Distortion, also known as the distortion factor, is a metric that is determined by the ratio of harmonic components compared to the fundamental frequency. High distortion can be difficult to hear on a short test but can be easily measured. If undetected it can cause an unpleasant listener experience or in voice applications harm critical intelligibility.
Rub-and-Buzz Test: Severe rub-and-buzz is easily detected in a quick test but some high-order harmonic noises, which are less easy to detect, can be measured. Catching them is important since they can actually be signs of reliability problems and eventual speaker failure.
Impedance Margin: The impedance curve is important to measure since it is one of the most primary performance parameters. Curves outside the margin can show issues with voice coil wire, speaker stiffness, moving mass issues, or inductance changes that may impact other components in the speaker system.
Average Sensitivity: Sensitivity ratings for loudspeakers measure the amount of acoustic output generated per unit of power. How loud it is. Measuring sensitivity is one of the most important measurements since it is the end result of motor strength, impedance, and moving mass. A speaker with low sensitivity can cause huge problems for the end-user who must compensate with high power which can, in-turn, result in loudspeaker damage.
Resonant Frequency: The resonant frequency is one of the most basic parameters of a speaker and often determines the use for the speaker, whether woofer, midrange, or tweeter. If resonant frequency is out of spec, it can throw off the tonal balance of the entire audio system. In a woofer, it can cause a tuning mismatch with the enclosure which was designed based on an expected resonant frequency.
Polarity Testing: Proper polarity ensures that wiring has been installed correctly. Multiple speakers which are out of phase with each other can cause low frequency phase cancellation and ruin the sound quality of the entire audio system. This is absolutely necessary in multi-driver audio systems.
Why End of Line Testing is Crucial
Speakers can be a significant investment and clearly play a critical role in the enjoyment and appreciation of a performance--or even just getting your order right at the drive-thru window. A production validation test that only covers a single parameter (or only a few of them) in a loudspeaker can cause an entire audio system to work improperly leading to a dissatisfied customer, product returns, and damaged brand image. Before purchasing any loudspeaker, make sure you understand the rigor of the manufacturer’s testing processes. You should even be able to request quality control records of the speaker quality test your individual speaker(s) underwent during the factory acceptance test.