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Production Noise Immunity (PNI)

Module of the KLIPPEL QC SYSTEM

  • Reliable detection of invalid measurements
  • Automatic repetition of corrupted measurements
  • Valid results derived from corrupted measurements
  • Avoid wrong classification of good speakers
  • Fast testing in a noisy environment
  • Reduces costs for additional shielding 
Production noise immunity

A coil rubbing in the gap, buzzing parts, loose particles and other irregular loudspeaker defects cause symptoms of 80 dB below the sound pressure level of the fundamental component. Those symptoms can easily be corrupted by ambient noise generated in the production environment leading to invalid results and a "fail" classification of a good speaker.  

The Production Noise Immunity Module uses a second microphone located in the far field of the measurement setup or outside the test box to monitor the ambient noise. According to a patent protected technology, the ambient noise signal is analyzed in the same way as the signal measured by the test microphone close to the loudspeaker. If the level of noise equals the level of test signal, the particular part of the response is invalid and the measurement will be repeated automatically. Valid parts of each corrupted measurement are stored and merged together giving eventually a complete valid response, although the test is permanently subject to production noise bursts. Without merging technique all tests are invalid due to production noise.

Applications

Driver testing

It is recommended to place the measurement microphone in a test enclosure and to operate the drive unit in a test frame firing into the box. The attenuation of the ambient noise is limited by the cone of the loudspeaker system to 5 .. 20 dB rising to higher frequencies. This shielding is helpful but NOT sufficient to do reliable testing in a noisy production environment where high sound pressure peaks cannot be avoided. In conventional test systems ambient noise will be interpreted as a defect drive unit and will reduce the yield rate. Additional housing of the test station and transporting the drive units into a more silent test environment is expensive and not practical in most cases.

The NI option ensures that Rub&Buzz problems which occur at low levels (40 .. 80 dB below the fundamental) can reliably separated from ambient noise disturbances.

Exploiting the extremely short measurement time of Klippel QC, repetitions do not degrade the overall measurement time too much.

Drive unit measured in a test enclosure

Loudspeaker System in sealed test chamber

Completely sealed test enclosures are also very beneficial for testing smaller loudspeaker systems (e.g. for multimedia). Here the loudspeaker system and the test microphone are located inside the test enclosure. Although this shielding provides good attenuation (between 20-30  dB), high peak values of ambient noise corrupt conventional measurement techniques. The second ambient noise microphone and the algorithms used in Noise Immunity ensure full sensitivity for rub and buzz defects, loose parts and air leaks.

loudspeaker system measured in sealed test chamber

System Testing in a semi-open box

The measurement of large loudspeaker systems (e.g. professional loudspeakers) requires large test enclosures, which are expensive and require production space. Placing the test object into the enclosure requires special means and time.

The handling can be significantly simplified by using a semi-open test enclosure, which is placed close to the assembling line. This arrangement gives about 10..20  dB of attenuation at higher frequencies. Conventional measurement techniques without ambient noise immunity are prone to invalid results and require high limit values, which reduces the sensitivity of the measurement. 

 

Semi-open box system test on production belt

System testing in free air

If no large test enclosure and no means providing additional acoustical shielding are available the ambient noise immunity becomes more and more important to identify invalid measurements. It is important to place the test microphone as close as possible to the drive units. For testing large speakers using multiple drive units and for checking air leaks in the enclosure multiple microphones multiplexed by the measurement system are recommended. Performing a long test sequence required for complex systems (e.g. surround sound system) a repetition of a complete test is not efficient in case that a single test step failed due to ambient noise. The NI option only repeats the corrupted task and ensures a valid overall test.

Testing a 5.1 system under free air conditions

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