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Natural Science Forum / Physics / Acoustics / October 2004Has anyone here ever built an impedance tube?
Hello to everyone, as a part of my masters thesis in Technical Acoustics I want to built a modern impedance tube for absorption measurements, using the two-microphone-technique described in ASTM E 1050-90 and EN ISO 10534 (in Europe) respectively. Being faced with some problems, mainly regarding the construction, I am looking for people who have already done this. So if anyone here has ever built an impedance tube or knows about people or work groups that did so, I would be grateful for any kind of hints. If further persons here are interested in this subject, exchange of experiences may of course take place in this newsgroup! Best regards, Hannes Krummheuer Technical University Berlin > as a part of my masters thesis in Technical Acoustics I want to built a > modern impedance tube for absorption measurements, using the > two-microphone-technique described in ASTM E 1050-90 and EN ISO 10534 > (in Europe) respectively. Being faced with some problems, mainly > regarding the construction, I am looking for people who have already > done this. I have built and still use a few conventional impedance tubes. I've begun a two-microphone version, but did not complete it since it offered little in addition to the conventional style. It is important to define the frequency range over which you want to make good measurements. The high frequency limit is limited by the microphone spacing (want's it to be narrow) and the tube diameter (wants it to be a small diameter). The low frequency end is determined by the signal to noise ratio, and the phase stability and the phase matching quality of the two microphones. The main advantage of the two-microphone tube is its speed of operation, which can be nearly instantaneous. Angelo Campanella > It is important to define the frequency range over which you want to > make good measurements. [...] The low frequency end is determined by the > signal to noise ratio, and the phase stability and the phase matching > quality of the two microphones. Hello, thanks for your answer! One of my main problems at the moment deals with the low frequency end. The ISO-standard says, microphone spacing must be at least 5% of the longest wavelength. Peng, Morrey and Sanders (1998)# did some calculation of errors and result in even 8,3%. If I use the ISO-value and calculate for a low frequency end of 50 Hz (which is required for my purposes), it leads me to a minimum microphone spacing of 34 cm! On the other hand, Bruel+Kjaer claims to go down to 50 Hz with their impedance tube (Type 4206). I haven't seen the tube in real, but from the pictures I am sure that the microphone spacing is much less than 30 cm. I comprehend the calculations of Peng et al., but I am at the same time convinced that the B+K-engineers know what they are doing and won't sell equipment that is not functional for thousands of dollars. So this is my dilemma at the moment. This issue is important for me, because I need reliable results down to 50 Hz. Certainly I could use a spacing of 34cm, but the resulting high frequency end is less than 500 Hz, which is quite unsatisfying. To avoid the calibration procedure and phase mismatching problems, I will be likely to use the one-microphone-method with a MLS-signal as source, first described by Chu (1986)##. Furthermore I will built two systems, one for low and one for high frequencies. Best regards, Hannes Krummheuer # Peng/Morrey/Sanders, The Measurement of Low Frequency Impedance Using an Impedance Tube, J Low Freq Noise 17, 1998. ## Chu, Transfer function technique for impedance and absorption measurements in an impedance tube using a single microphone, J Acous Soc Am 80(2), 1986. > This issue is important for me, because I need reliable results down to > 50 Hz. Certainly I could use a spacing of 34cm, but the resulting high > frequency end is less than 500 Hz, which is quite unsatisfying. My impression of the low frequency limit is that there are good matched microphones that B&K can supply so the the narrower spacing can be used. It is perhaps most likely that their commercial equipment for sale will already have the practical dimensions. > To avoid the calibration procedure and phase mismatching problems, I > will be likely to use the one-microphone-method with a MLS-signal as > source, first described by Chu (1986)##. Chu wrote a second paper on practical experience with that method. (JASA 83(6) June 1988. He included more than one microphone position (one microphone, several locations). > Furthermore I will built two systems, one for low and one for high > frequencies. Good idea. But read Cu's second paper first, again, to select carefully your parameters. Angelo Campanella I am also interested on this subject. We are building an apparatus with two different tube diameters. There is one with 30 millimeter and another one with 40 millimeters. Those tubes are being used with different microphone positions. They have about 700 millimeters long. We are following the ISO FDIS 10534-2 recommendations and both W. T. CHU papers: JASA 80(2), August 1986 and JASA 83(6), June 1988. We are getting the same frequency response at two different microphone positions, although the coherence is near to one on the frequency range of zero up to 5 kHz. But one question is about how much power should be driven by the speaker? Any suggestions? Thanks, Elias | : > > This issue is important for me, because I need reliable results down to | : > > 50 Hz. Certainly I could use a spacing of 34cm, but the resulting high [quoted text clipped - 20 lines] | : > | : > Angelo Campanella > But one question is about how much power should be driven by the > speaker? The sound level at any null position must be greater than the background noise form the environment that can penetrate to the same microphone position. For very deep nulls (no absorption) this become a limiting precision factor. Make an error analysis of the best (deepest) null you want to be able to measure accurately. The speaker sound level needs to be greater than the background noise that enters to the microphone. Angelo Campanella Thanks Angelo. One more question.... There is a big difference between pressure microphone and random microphone in the measurements in a impedance tube? Elias Teodoro | : > Elias Bitencourt Teodoro wrote: | : > > But one question is about how much power should be driven by the [quoted text clipped - 8 lines] | : > | : > Angelo Campanella > There is a big difference between pressure microphone and > random microphone in the measurements in a impedance tube? There should be no difference between random and normal incidence microphone. The Random Incidence type is made to have a relatively flat frequency response in a reverberation room where sound comes from all directions. At high frequencies (where the circumference of the microphone is greater than an acoustic wavelength) the microphone will have greater sensitivity for sound arriving normal to the diaphragm. The normal incidence microphone (also known as a free field microphone, and similar to a "pressure" microphone) is designed to have a flat frequency response for sound arriving on the normal to the diaphragm. For high frequency sound coming for other directions, it will have a lesser response. Far more important is the specific location of the pressure relief vent that allows the static pressure behind the diaphragm to equilibrate to atmospheric (barometric) pressure. All outdoor and casual use microphones now have that vent buried inside near the electrical contact. That is a very poor choice for wall mounted microphones such as used in the two microphone method, since it can be exposed to significant perturbations n static pressure, and influences the phase stability or phase accuracy at low frequencies, right where you will be attempting to extend low frequency impedance measurement accuracy. It may also be very poor for the one microphone Chu method (this vent choice needs to be investigated further). I believe that the old and original vent position was along and just outside the diaphragm perimeter, somewhere among the threads of the front grille cap. This needs to be clarified. Angelo Campanella Thank?s Angelo. Now let?s go to de experiments measurements. Elias Teodoro | : > > There is a big difference between pressure microphone and | : > > random microphone in the measurements in a impedance tube? [quoted text clipped - 34 lines] | : > | : > Angelo Campanella |
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