The Measurement of Unsteady Surface Pressure Using a Remote Microphone Probe

The overall goal of this experimental method is to perform high spatial resolution measurement of the turbulence-induced pressure fluctuations over surfaces of bodies immersed in fluid flows. This method can help answer key questions in the field of fluid dynamics, aeroacoustics, and flow-induced structure vibrations, such as spatial and temporal characteristics of the unsteady surface pressure induced by turbulent motions with small-length scales. The remote microphone probe can provide detailed information of characteristics of the unsteady surface pressure fields, with relatively high spatial resolution over a wide range of frequencies.

Prepare for the experiments by constructing the remote microphone probe. This requires several components. Primary among these is a small microphone that is sensitive to the frequencies of interest.

The size of the remaining components depends on the desired sensitivity and frequency response of the probe. This probe uses a length of stainless steel tubing with a half millimeter inner diameter. It also requires a long piece of soft tubing, with a 1.25 millimeter inner diameter.

A custom cut and drilled piece of Plexiglass will serve as the microphone cradle. On one side of the cradle, is a hole to accommodate the stainless steel tube. On the opposite end of the cradle, is another hole.

It is along the same axis and accommodates the soft tube. These two holes are connected through the Plexiglass. On top, there are two concentric holes.

The larger outer hole will hold the microphone. The smaller inner hole accesses the cavity. This cross section provides a guide to the construction of the cradle.

Note that the taper needed to match the side hole diameters is near the entrance for the stainless steel tube. Have epoxy ready to attach the components to the cradle. Get the microphone and orient it with its diaphragm pointing toward the cradle, and seat it in its position.

When it is in position, apply epoxy to fix it in place. Next, insert the stainless steel tube into the opening made for it before securing with epoxy. Attach the soft tubing in its position in the same way.

At the other end of the soft tubing, use a plug to seal the tube. With all the elements attached to the cradle, this probe is ready to be used in an experiment. Next, turn attention to the model surface for the experiment.

This demonstration uses a flat plate with prepared measurement points. At a point of measurement, there should be a through hole perpendicular to the surface. The hole should have the outer diameter of the stainless steel tube in the probe.

For the experiment, mount the model surface in the wind tunnel. Next, get a microphone probe to attach to the model surface at the measurement hole. From the rear of the surface, thread the stainless steel tube of the microphone probe into the hole.

The tube opening should be flush with the measurement surface. Apply epoxy to the side opposite the measurement site to secure the tube in place. With the microphone probe in position, surround it with acoustic foam to prevent parasitic noise from contaminating the system.

Next, work with the soft anechoic tube from the probe. Move the closed end out of the test section of the wind tunnel. Surround it with acoustic panels to isolate it from its surroundings.

Back at the probe, connect the microphone to the inputs of a low-noise amplifier, which is outside the tech section of the wind tunnel. Next, move to the electronics outside of the tunnel. The low-noise amplifier is connected to the data acquisition system.

Set the gain of the amplifier to ten. Calibration of the remote microphone probe requires several pieces of equipment. For the first phase, use a high quality reference microphone with frequency independent response, along with the piston phone.

In addition, there should be an amplifier with its output connected to the data acquisition system. Connect the microphone to the input of the amplifier, and set the input and the output gain to ten decibels. Next, get the piston phone.

Insert the microphone into the piston phone for the measurement. When ready, turn on the piston phone. Turn to the data acquisition system to set the data acquisition frequency, and number of samples.

Begin acquiring and saving the voltage from the reference microphone to calculate the calibration constant. Once the calibration constant is determined, remove the microphone from the piston phone. Now, move the microphone to the model’s surface, use a holder to position the microphone perpendicular to the surface over the probe tube, where the pressure fluctuation will be measured.

Align the microphone with the center of the probe tap, and one millimeter above it. The next steps require a loud speaker with an internal amplifier connected to a function generator. Mount the loud speaker near the model’s surface and 2.5 meters from the microphone.

The speaker should be off and the cone should be directed toward the model’s surface. Outside the wind tunnel, work with the function generator. Turn it on and use the white noise option to provide the acoustic signal.

Return to the loud speaker and ensure the amplification is at minimum. Then, turn the loud speaker on and adjust its volume as high as possible without causing damage. Return to the data acquisition system.

Acquire and save time series data from the voltage outputs of the probe and reference microphones. Calibration is the most critical step of this procedure. To ensure success, use a model to predict the transfer function and compare it with the experimental results.

In addition, use high quality devices, including the reference microphone and loud speaker. These are plots of the coherence between the pressure fluctuations as measured in the remote microphone probe and the reference microphone as a function of frequency. This probe system has a 5.35 centimeter long primary tube.

For comparison, this probe has a 10.4 centimeter long primary tube. The coherence is above 0.97 in the frequency range of interest. These plots are of the magnitude of the transfer functions from the same two probes.

The blue curves are experiment results. The green curves are theoretical predictions. The predictions of the dynamic response are accurate across most of the frequency range.

Small aberrations in the probe may explain disagreements at middle and high frequencies. Comparison of the data and the predictions for the slope of the face shift reveal the predictions are slight overestimates. This may be due to small errors in measurements of tube lengths, or by temperature variations.

In this plot, the light grey band represents the range of previous surface pressure spectra data at various Reynolds numbers. The data points represent measurements using remote microphone probes. The measurements are within the spread of measurements observed in the published literature.

Once mastered, calibration of a single remote microphone probe can be done in five minutes if it’s performed properly. While attempting this procedure, it’s important to repeat the calibration with various loud speaker volumes to obtain the highest coherence for the remote microphone probe. After this development, this technique paved the way for researchers to explore the characteristics of the pressure fluctuations over surfaces with large curvature and limited spacing under complex flow conditions.

Working with loud speakers generating high volume white noise can be extremely hazardous. Don’t forget precautions, such as using noise-muting headphones while you’re performing this procedure. After watching this video, you should have a good understanding of how to build, set up, and calibrate a remote microphone probe.