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ULTRA  LOW  NOISE  ACOUSTICAL  ANECHOIC  CHAMBER.                           HOME

Audio Scientific developed test techniques for evaluating faint sound produced by electronic components and modules like chokes / coils in DC/DC converters, capacitors on PCB, displays and more. Comparison, Coupler and Close Distance techniques present advantages and challenges. Customer test procedure or techniques developed by Audio Scientific are used, depending on a requirement or application.

Audio Scientific uses dual ambient noise attenuation for tasks requiring ultra low acoustical noise level. First attenuation of noise is performed by  Ultra Quiet Anechoic Chamber with dual walls. It consists of a very heavy steel wall combined with traditional noise attenuation materials and acoustical wedges. Then Ultra Quiet Anechoic Chamber stays in a Quiet Room, where ambient noise is already very low. All low level acoustical tasks are performed from another room, with setup connections going to bench through well isolated wall.

Entire setup is battery powered, well shielded  and isolated from power line. During special tasks requiring ultra low level acoustical noise, all remaining equipment in Audio Scientific Lab, lights, utilities and all electrical gadgets are turned off and even unplugged from power outlet, where applicable.

Before, during and after low level tasks, acoustical noise is checked for both: Ultra Quiet Anechoic Chamber and Quiet Room. FFT Spectrum and Scope View are analyzed for smallest traces of foreign source. DC and wideband AC electromagnetic fields are tested near tested object. Ultra sensitive Hearing Aid Compatible HAC probes are used to determine magnetic fields in audio band 50Hz to 50kHz. Such magnetic fields may be produced by tested object and be present inside of anechoic chamber with metal walls. All precautions are taken to ensure that Reference Microphone and  Tested Object are free from external influence.

Our Reference Microphone is a 1" microphone, with specified ultra low noise level of -2.0dB SPL "A". This is a flagship among ultra low noise microphones. It is very likely the best commercial ultra low noise microphone in the world presently. For less demanding tasks we use 1/2" microphone, with noise spec of +15dB SPL "A".

Audio Analyzer noise is estimated as follows, after calibration for 1" microphone:

          -15dB SPL "A" on +94dB SPL range.
          -32dB SPL "A" on +74dB SPL range.
          -60dB SPL "A" on +44dB SPL range.

How quiet is our anechoic chamber?  Presently we obtain acoustical noise level reading as follows:

          -2.0dB SPL "A" = ON,   in band 150Hz - 10kHz.              +0.4  dB SPL "A" = ON,  wideband.
          -1.3dB SPL "A" = OFF, in band 150Hz - 10kHz.              +41.2dB SPL "A" = OFF  wideband.

It seems Reference Microphone noise is much greater than Anechoic Chamber ambient noise. Numbers above apply to Reference Microphone, not Anechoic Chamber. Based on measurements explained later, anechoic chamber noise is predicted to be much lower, as shown here:

          -58dB SPL "A" = ON,   in band 150Hz - 10kHz.                  -55dB SPL "A" = ON,  wideband.
          -49dB SPL "A" = OFF, in band 150Hz - 10kHz.                     TBD SPL "A" = OFF  wideband.

This is surely ultra quiet acoustical anechoic chamber. We used a few techniques to determine predicted noise level in anechoic chamber. For example various large level acoustical signals applied in certain manner for obtaining chamber ISOLATION or SOUND ATTENUATION characteristic. At first DELTA between OPEN / CLOSED chamber door is obtained for large signals. Then OPEN DOOR ambient noise combined with DELTA provides predicted CLOSED DOOR ambient noise. After taking extensive sets of data we found a pattern that repeats, plots presented here match that pattern closely.

All data here is shown with FFT = 32k. Audio Analyzer has potential up to FFT = 1000k, allowing detection and measurement of extremely small signals.
 



ADDITIONAL  INFORMATION  ABOUT  NOISE  FROM  ELECTRONIC  COMPONENTS

Capacitor Noise   Technical Article 7/27/2004 by Kemet, The Capacitance Company.   http://www.kemet.com/kemet/web/homepage/kfbk3.nsf/vaFeedbackFAQ/242F5F2E69DCEC7485256EDF004CA495

Description:
Are your military ceramic capacitors subject to the piezoelectric effect?

Answer:
Certain classes of ceramic capacitors exhibit a normal characteristic, called piezoelectricity, than can cause unexpected effects in certain circuits. In some cases, the piezoelectric effect may result in the appearance of electrical noise, while in other cases, an acoustic sound may be heard, coming from the capacitor itself. Ceramic piezo effects are well known, and were even the basis for the ceramic phono cartridges used in the past.
Piezoelectricity is a common characteristic of many ceramic chip capacitors and occurs in those classes of dielectric which are classified as ferroelectric. Piezoelectric effects can result in noise for ferroelectric ceramic chips, such as those used for military BX & BR, as well as commercial EIA Class 2 and Class 3 dielectric, such as X7R, X5R, X8R, Y5V, Y5U, Z5U, etc. Piezoelectricity occurs in all ferroelectric dielectrics, regardless of manufacturer. Note that there are essentially no piezoelectric effects in Class 1 capacitors, such as C0G, NP0, or military BP - none of which are ferroelectric.

Piezoelectric noise is only occasionally an issue, since it is low level. However, it can show up in specialized applications subject to mechanical stress of the ceramic during shock, vibration, compression, and torsion. Examples include high gain pre-amps, hand-held microphones at rock concerts, and monitoring equipment subjected to sudden shock or heavy vibration. When it occurs, most piezoelectric noise is in the 3 KHz to 30 KHz ranges, although detailed studies have not been done over a wider range.

The piezoelectric effect is tied to the crystal structure of the dielectric. In ferroelectric materials, the crystal structure tends toward the tetragonal, with the Ti cation located at a non-centered position in the crystal. This results in an electric dipole structure. When this structure is mechanically deformed, the charge center of the crystal shifts, producing a dipole moment and polarization. This results in the appearance of a voltage at the capacitor terminals. That voltage increases as the mechanical deformation increases. This can be a design issue in high gain amplifier circuits subject to mechanical vibration or sudden impact, since these piezoelectric voltages could be coupled into the circuit, introducing errors.

The complementary effect also occurs, in that electrical stimulation of ferroelectric compounds can result in mechanical deformation. In circuits which operate at acoustic frequencies, the capacitors will tend to respond and may emit acoustic noise. As the frequency goes up, the capacitor can no longer respond, and the acoustic noise will be damped out

Remedies depend upon the operating constraints of the designs. Use of a different capacitor type is one obvious approach, and may be the only solution for low frequencies. Other possibilities include (must be evaluated by the customer, based on circuit requirements):
 

New capacitor from Murata cuts acoustic noise by 30dB    Info at EETimes 3/5/2009 http://www.eetimes.eu/products/passives/215800711

Murata’s GJ8 series of multi-layer ceramic chip capacitors (MLCCs) has been specifically developed to reduce acoustic noise in consumer and industrial electronics applications. Sound can be generated by the MLCCs at the input to the DC-DC converter in a notebook PC, or by capacitors in the control circuit of the LCD module in a mobile phone. This problem is caused by the expansion and contraction of the dielectric element in some MLCCs, which causes the PCB to vibrate at the amplitude of the voltage applied. When the frequency of the voltage applied approaches audio frequencies, a noise can be heard.

Murata’s GJ8 series has been specially designed to reduce this problem and is available with capacitance between 1 and 10µF. The acoustic noise generated by 4.7 to 10µF GJ8 series capacitors represents an improvement of at least 10dB over ordinary MLCCs; for 1µF models the improvement is up to 30dB. The series is available in low-profile models with thicknesses of just 1.25, 1.60 or 2.50 mm depending on capacitance. Rated voltage is up to 50V.

Aya Tonooka,
Murata Electronics (UK) Ltd.,
Oak House, Ancells Road, Ancells Business Park, Fleet, Hampshire GU51 2QW , United Kingdom
Tel: +44 (0) 1252 811666
Fax: +44 (0) 1252 811777
E-mail: ayatonooka@murata.com
Web: www.murata.eu



Reduce acoustic noise from capacitors.  Adding parts or cutting PCB slots can make a difference.  EDN Article, Damian Bonicatto, Landis+Gyr, Pequot Lakes, MN; Edited by Martin Rowe and Fran Granville -- EDN, February 17, 2011
http://www.edn.com/article/512775-Reduce_acoustic_noise_from_capacitors.php

Some surface-mount capacitors exhibit acoustic noise when operated at frequencies in the audio range. A recent design uses 10-μF, 35V X5R 1206 ceramic capacitors that produce noticeable acoustic noise. To quiet such a board, you can use acoustically quiet capacitors from manufacturers such as Murata and Kemet. Unfortunately, they tend to cost more than standard parts. Another option is to use capacitors with a higher voltage rating, which could reduce the noise. Those parts may also be more expensive than standard capacitors. A third path is to make a physical change to the PCB (printed-circuit board).

A ceramic capacitor expands when you apply a voltage and contracts when you reduce the voltage. The PCB flexes as the capacitor changes size because the ends of the capacitor mechanically couple to the PCB through solder (
Reference 1).

Reduce acoustic noise from capacitors figure 1Figure 1a shows a capacitor with no applied voltage, and Figure 1b shows an exaggerated condition of PCB flexing when you apply voltage to a capacitor. Applying the voltage makes the PCB operate as a speaker. Keeping that fact in mind, consider two methods for improving the situation. The first technique is relatively simple: If your circuit uses one capacitor, replace it with two in parallel, each with half the capacitance of the noisy capacitor. This approach lets you place a capacitor on top of the board and the other on the bottom of the board; the capacitors lie directly above each other, and their orientations are the same. As the upper capacitor tries to flex the board down, the lower capacitor tries to flex the board up. These two stresses tend to cancel each other, and the PCB generates little sound.
 


 

Reduce acoustic noise from capacitors figure 2Adding a second capacitor increases cost but not as much as replacing the noisy capacitor with one that might not create noise. A ceramic capacitor from Digi-Key sells for approximately 27 cents (1000). A quieter KPS-series part from Kemet costs approximately $1.50. The second method involves making a slot in the PCB near each end of the capacitor (Figure 2). When the capacitor expands and contracts, it flexes only a small portion of the PCB, which should reduce the noise.
 


A test with five 10-μF, 25V ceramic capacitors connected in parallel showed that putting three capacitors on top of the PCB and two on the bottom reduces the noise by 14 dBA (acoustic decibels). Routing a slot on both sides of the five capacitors reduces the noise by 15 dBA. Both are substantial noise reductions. A Murata JG8-series capacitor reduces the noise by 9.5 dBA. Combining these techniques should further reduce the noise.

 


 

Reference
 
  1. Laps, Mark; Roy Grace, Bill Sloka, John Prymak, Xilin Xu, Pascal Pinceloup, Abhijit Gurav, Michael Randall, Philip Lessner, and Aziz Tajuddin, “Capacitors for reduced microphonics and sound emission,” Electronic Components, Assemblies, and Materials Association, Capacitor and Resistor Technology Symposium Proceedings, 2007.
 
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