Audio Expertise for
ULTRA LOW NOISE ACOUSTICAL ANECHOIC CHAMBER.
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
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
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
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"
-15dB SPL "A" on +94dB
-32dB SPL "A" on +74dB
-60dB SPL "A" on +44dB
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
Capacitor Noise Technical Article 7/27/2004 by Kemet, The
Are your military ceramic capacitors subject to the piezoelectric effect?
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
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):
- Use a different dielectric - C0G can replace X7R for low cap values, if
the package size increase is acceptable.
- Use a different type of capacitor, such as tantalum (the 1206 0.1 uF 50
volt ceramic can sometimes be directly replaced by a tantalum equivalent,
- Use a leaded part, rather than SMT - the leads tend to decouple the
mechanical stress from the chip
- Use a smaller footprint SMT part, to minimize the span on the board,
which can help to isolate the chip from flex and vibration effects
- Minimize vibration on the board by changing the board mount system,
adding dampening materials near the chip, or by relocating the chip.
- Use a part with thicker dielectric, usually corresponding to a higher
voltage rating. This reduces the voltage gradient, which reduces
piezoelectric noise, if the package size increase is acceptable.
- Use a chip with greater overall thickness, which helps to prevent
physical distortion and stress.
New capacitor from Murata cuts acoustic noise by 30dB
at EETimes 3/5/2009
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.
Murata Electronics (UK) Ltd.,
Oak House, Ancells Road, Ancells Business Park, Fleet, Hampshire GU51 2QW ,
Tel: +44 (0) 1252 811666
Fax: +44 (0) 1252 811777
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
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
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
shows a capacitor with no applied voltage, and
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.
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
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
). 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.
- Laps, Mark; Roy Grace, Bill Sloka, John Prymak, Xilin Xu, Pascal
Pinceloup, Abhijit Gurav, Michael Randall, Philip Lessner, and Aziz
for reduced microphonics and sound emission,” Electronic Components,
Assemblies, and Materials Association, Capacitor and Resistor Technology
Symposium Proceedings, 2007.