A sound pressure level meter for ultrasound

laboratory instrument for the measurement of ultrasonic power output in the frequency range 10 kHz to 100 kHz

by

dr.Godfried-Willem Raes

postdoctoral researcher
Ghent University, Orpheus Institute & Logos Foundation

2015

When it comes to sound pressure measurement, sound level meters nowadays can be found on the market at very decent prices. However, if you want to measure sound pressure in the ultrasonic range, these meters cannot be used. (1). First of all, the dbA measurement scale is without object as we are in the area of inaudible sounds. But, even the linear unfiltered scale (if dBC is available on the meter) is pretty useless as the frequency response curve rarely extends beyond 20 kHz. Of course such measurements can be carried out using Broel&Kjaer calibrated measurement microphones, but at a cost of over 12.000 Euro, this seems rather prohibitive. This project describes a simple and cheap analogue meter allowing measurement of sound pressure levels in the range 10kHz up to 100kHz. The sensitivity is not as good as when using Broel&Kjaer devices, but within the range 76 dB SPL and 140 dB SPL, it performs quite well and with reasonable linearity (+/- 3dB). The circuit makes use of a MEMS microphone by Knowles Acoustics, type SPH064HT5H-1. This microphone, measuring only 3.50 x 2.65 mm and 1.0 mm thick, is omnidirectional. The acoustic port has a size of merely 0.2 mm! The datasheet is given in the notes below (2).

Circuit drawing:

The first stage, build around a precision opamp, is an amplifier with variable gain: 46 dB (200x), 26 dB(20x) or 6 dB (2x). There is a roll off of -3 dB at 100 kHz, mainly due to the limited gain-bandwidth product of the opamp. Sound pressure level at the acoustic port should not exceed 124 dB. If very high sound levels are observed, increase the distance between source and instrument. The BNC connector carries the amplified signal and is provided to connect an oscilloscope to observe clipping and waveform. The common distance for measurements in the ultrasonic range is 30 cm. At 1 m distance (the normal value for acoustic measurement) , the SPL should be expected to be ca. -12 dB, provided there are no reflective walls or surfaces nearby. Ideally measurements should be performed either in open space or in an anechoic chamber.

After the amplifier the signal is sent to a true-rms conversion circuit (Analog Devices AD736) (3). The DC output of this circuit feeds the large analog VU panel meter. The optimum output voltage range for this chip is 0 to 4V, yielding a good match with the panel meter we selected. Sound pressure levels corresponding to the values read on the meter, can be deducted from the following table.

switch position gain VU meter value SPL
0 46 dB 0dB (100%) 96 dB
0   +3dB 99 dB
0   -20dB (10%) 76 dB
-20 dB 26 dB 0dB (100%) 116 dB
-20 dB   +3dB 119 dB
-20 dB   -20dB (10%) 99 dB
-40 dB 6 dB 0dB (100%) 136 dB
-40 dB   +3dB 139 dB
-4 0dB   -20dB (10%) 119 dB

The table is valid for a distance between sound source and microphone of 30 cm. A a reminder, note that sound pressure goes down with the square of distance. Thus at 15 cm distance the values will be 6dB higher and and 60 cm 6 dB lower. Although the instrument is fairly insensitive to lower audio frequencies, it should go without saying that measurements ought to take place in a silent environment.

 

Hand drawn PCB-board for the circuit above (at 200% scale):

The MEMS microphone, an SMD component, is soldered on the component side of the board. Soldering little wire wrap wires to this component is a really tedious job that can only be accomplished under binoculars and using appropriate tools . The acoustic port of the microphone is on the component side as well. Here is a picture of the assembled board: The microphone can be seen on the right side of the picture. An acoustical pipe mounted on the cabinet allows the sound to come in and protects the microphone. It renders the instrument more directional. This is a view on the inside of the front panel before assembly: All components are secured to the chassis with some two component epoxy glue. The figure-8 mains power connector is on the top side of the enclosure. The instrument has no fuses as power consumption is very low and the small 2 x 15 V transformer is short circuit proof.

The entire circuit as well as the panel meter fit nicely into an cast aluminium BIM-box. The result looks like:

For calibration, we used a sinewave oscillator set to 10kHz connected to an audio amp and loudspeakers. We turned up the volume for a reading of 90 dBC on our normal acoustic sound level meter and thus determined the reading on our own instrument. Next we fed a 10 Vrms signal at 40 kHz to a Murata transducer for the same frequency that after the datasheet gives 90 dB SPL under these conditions and checked our own meter for conformity. Of course the resulting precision cannot exceed that of the reference meter at the one side and the precision of the Murata datasheet at the other. However, at least now we dispose of an instrument capable of doing reliable comparative measurements on ultrasonic output levels obtainable with just about any transducer as well as determining frequency response curves of transducers in the ultrasonic range. (5)

Final critical remarks:

The performance of the preamp could be improved by selecting a precision opamp with a higher gain-bandwidth product. For the OP27 this is only 8 MHz. (4). However, in this design, the improvement will be small as the MEMS microphone has a much steeper roll-off at 100 kHz.

The sensitivity for lower ultrasonic SPL levels could be improved by using 2-stage preamping: a first fixed stage with 30 dB gain (OP27), followed by an adjustable stage for another 20, 40 or 60 dB gain (OP07).

 

dr.Godfried-Willem Raes

Ghent, 30.09.2015


Notes:

(1) This project is part of the ongoing research of the author in gesture controlled devices over the last 35 years. Systems, based on Sonar, Radar, infrared pyrodetection, accelerometers and other technologies are fully described in "Gesture controlled virtual musical instrument" (1999), in "Quadrada" (2003), "picradar" (2004) as well as in his doctoral dissertation 'An Invisible Instrument' (1993). Artistic productions and compositions using these interfaces and devices have been: <Standing Waves>, <Holosound>, <A Book of Moves>, <Virtual Jews Harp>, <Songbook>, <Slow Sham Rising>, <Gestrobo>, <Technofaustus> , "PicRadar Studies", <Namuda Studies> etc.

(2) Knowles Acoustics, datasheet for the SPH0642HT5H-1

(3) Analog Devices, datasheet for the AD736

(4) Analog Devices, datasheet for the OP27

(5) Our <Tinti> robot in particular made this instrument essential, as frequency modulated ultrasound is an intrinsic part of the design.

 

 

Bibliographical references:

RAES, Godfried-Willem "Een onzichtbaar muziekinstrument" (Gent, 1993)

RAES, Godfried-Willem "Gesture controlled virtual musical instrument" (Ghent, 1999)

SINCLAIR, Ian Robertson., "Sensors and Transducers" (London, 1992) , ISBN 0 7506 0415 8


First published on the web: 29.09.2015 by dr.Godfried-Willem Raes

Last update:2016-04-02