0-20 kHz miniature acoustic transducer - Page 1
Forum section: electronics / projects

author: dzabakh
author_rank: Newbie
timestamp: February 07, 2019, 05:03:01 pm
content: Hello everyone!I have a long question and I will tell the whole story so that the community could help to correct any of its stages.Recently, on my PhD I've made an acoustic experiment: I irradiated a cone with a wide-band signal (1 kHz - 10 kHz) and measured the diffracted signal (1 kHz - 10 kHz). You can read the article at the link below if interested (hope I will not be sewed by the publisher :-D). I used Knowles RAB-32257 transducer and Audix TM-1 microphone. My sound card supports up to 48 kHz sampling frequency, so it can receive and transmit up to ~20 kHz signal.Now I need to raise the frequency so I need a small wide-angle transducer similar to this Knowles one but with 0-20 kHz band. A non-flat frequency characteristic doesn't worry me - I can just divide the signal spectrum by the characteristic in the end. Also, I need a 0-20 kHz precise mic because Audix TM-1 is not really good at above 10 kHz. Also, maybe I need something different to receive it.Maybe, I even would like to go to 20-200 kHz but, again, I don't know which transducer and receiver can give me that. And I also don't know how to record such a signal.It would be great if the community could give any suggestions.Link:https://drive.google.com/file/d/0B0YBXG8F7d28enZfUmVYVjNyNHRNQnlBNG9vX2xfMnRWZlZr/view?usp=sharing

author: SiliconWizard
author_rank: Super Contributor
timestamp: February 07, 2019, 05:38:01 pm
content: Hi and welcome. Sounds interesting.The Knowles ED-27230-000 transducer has a range of 20Hz-35kHz. There is currently some stock at Mouser. Finding something similar with a lower range below 20Hz will be very challenging, but do you really need that? (Took that from your "0-20 kHz band" requirement.)As to the microphone, as I just saw, the Audix TM-1 is rated for a frequency response of 20Hz-25kHz +/-2dB (https://audixusa.com/docs_12/units/TM1_-_TM1_PLUS_.shtml)So either it's crap and not up to specs (I don't know the Audix brand), either there could be a setup issue rather than a microphone one?If you're looking for something very good, you can take a look at GRAS stuff:https://www.gras.dk/products/measurement-microphone-sets(it's expensive)

author: dzabakh
author_rank: Newbie
timestamp: February 07, 2019, 07:53:34 pm
content: Hi, SiliconWizard!QuoteThe datasheet says the upper frequency is 35 kHz, but the graph says it goes only up to 8 kHz. Well, probably, the graph is lying.QuoteOh, no! My bad, it's just a misprint. Of course I need audible frequencies and a little above those.QuoteWell, I believe it is more about calibration - I didn't do it accurately enough:)QuoteYeah, I worked with those in another lab. They are really good, but due to Russian specifics I have to buy all the stuff on my own, so I'm on a budget

author: DaJMasta
author_rank: Super Contributor
timestamp: February 07, 2019, 08:22:39 pm
content: The normal audio band is generally 20Hz to 20kHz, so virtually any audio equipment should suit the bill. If you aim for something that claims to be sort of high fidelity, you should get at least a somewhat "flat" frequency response, and if you want low frequency content to be present in any real amount, you want a physically larger driver. For normal listening levels in open air, I'd start at around 4" wide speaker cone to get at least a little bit of actual response at the bottom of the audio band (though, if you actually want good performance at 20Hz or even below, you're going to need at least double that usually).As for higher than audio band stuff, many measurement mics will go to 30kHz or more, but if you want to go much higher, you're looking at ultrasonic transducers, and as with most things, getting several narrower band transducers to cover the ranges you're interested in is going to be a lot cheaper than trying to find a transducer with reasonable response over a very broad frequency range. Consumer grade microphones can probably do a good job in many cases, but will virtually never come with a measured characteristic response. I've used some Rode NT1 mics for measurement purposes, as they are very low noise, have a flat response, and are a bit cheaper than full blown measurement mics - but they don't come characterized, they are not fully omnidirectional (cardioid response), and they require phantom power.If you only need relative measurements, I'd focus on getting a fairly flat speaker situation figured out, then test a few decent mics that seem like good choices and just use the one with the flattest response normalized against your source. Getting a very flat response speaker set is going to be expensive, especially if you want good low frequency performance, and even inexpensive mics can have a pretty good response over most of the audio band, though stuff like housings and even mic grills can interfere with this.

author: SiliconWizard
author_rank: Super Contributor
timestamp: February 07, 2019, 10:04:14 pm
content: Quote from: dzabakh on February 07, 2019, 07:53:34 pmWell, I don't know. This reference seems not to be current at Knowles anymore, I just looked (and the 35kHz may have been misleading...)The ED series is there:https://www.knowles.com/series/dpt-ba-receivers/subdpt-hearing-instruments-receivers/series-ed-fedLooking at some of them, there doesn't seem to be anything really better in the higher range. The main application is hearing aids so... the 35kHz figure was probably erroneous or at least very misleading, not the frequency response probably. False hope maybe!This one may be usable though: Knowles TWFK-30017-000: can be found here:https://www.knowles.com/subdepartment/dpt-balanced-armature-receivers-and-speakers/subdpt-radio-communicationsits freq. response is definitely not flat but given the right amplification/calibration, it should be usable up to 20kHz (and probably above.)Although the response given in the datasheet is not the same than the one in the tech. specs pdf, so it's quite confusing too.Maybe you should contact Knowles directly to get more details.Another manufacturer is Sonion, you could also take a look, but their products are probably pretty much in the same ballpark.

author: JohnPen
author_rank: Regular Contributor
timestamp: February 08, 2019, 09:33:03 am
content: You mention that you might consider going up to 200Khz at some time in the future. To cover some of that range the Knowles microphone SPU0410LR5H-QB Rev H specification includes ultrasonics up 80Khz. Also there is an ultrasonic speaker available, Kemo Electronics Gmbh LM010, which covers from 2Khz to 60Khz. Calibration would require some effort however but is not insurmountable.

author: dzabakh
author_rank: Newbie
timestamp: February 08, 2019, 10:42:50 am
content: Hmm, this is very interesting, thank you!Have you used these? Are there any plug-and-play devices including these?I mean, my sound card can do only up to 48 kHz sampling, but if I use this Knowles microphone, I don't need any phantom power any more, right? Nor I need the sound card itself. So I can attach the mic and the source to some evaluation board and just record my signals to computer. Do you know any examples of such boards?

author: JohnPen
author_rank: Regular Contributor
timestamp: February 08, 2019, 06:59:42 pm
content: Sorry I don't know of any plug and play or evaluation boards. My interest was to explore any higher harmonics that perhaps were generated by a Chladni plate running at normal multiple audio frequencies. To this end a modified speaker was attached to a Velleman audio power amp, provided as a kit by Quasar, to the Chaldni plate. The frequency response of the amplifier goes from 3Hz to 200Khz. Normally the amp was driving the Chladni plate with audio frequencies to the modified loudspeaker however I needed to prove the microphone really did pickup the ultrasonics. Attaching the LM010 to the amp provided a wideband ultrasonic source to test the microphone. I used the microphone with a low voltage (3 volts) bias and coupled it to a high pass filter followed by a gain stage all on a small PCB. The circuit used an LM833 low noise dual op amp. The high pass was to avoid too much audible noise pickup in microphone interfering with the frequencies I was interested in. I built 4 of them which were than fed into a mixer the output then being fed to a scope and/or a spectrum analyser. It all worked OK but I have yet to carry on further due too many digressions. Really the problem is to house the Chladni plate plus microphones in an acoustic box which is more tricky in limited space. I will get back to it.