DIY Acoustic Anechoic Test Chamber - Page 1
Forum section: electronics / projects

author: EEVblog
author_rank: Unknown
timestamp: September 08, 2023, 10:35:55 am
content: I've alway wanted to build a proper acoustic anechoic test chamber.So this thread is just a starter.Haven't researched anything yet, but obviously I need a 1m long chamber (standard test length, so maybe 1.2m total at least)Big enough for test gear like a big scope, so maybe 0.5m x 0.5m square profile.Inside would be acoustic foam or acoustic wool of some description around the outside.Next I'd need a sound meter that can do really low level measurement. Most are 30dB minimum IIRC, but I want to go down to say 20dB, or at least 25dB which is the low of of really low noise fans.Any links or thoughts appreciated.

author: EEVblog
author_rank: Unknown
timestamp: September 08, 2023, 10:41:00 am
content: Hemi hard floor chambers are a thing for mics, speakers and other things.I have a hard concrete floor in my dungeon, so I probably only need a box to sit over the mic and the gear.My Dungeon does have a constant air flow coming in which I can't turn off, so there is some base level hum there, I'll have the test how much.https://www.iac-australia.com.au/anechoic-chambers/hemi-anechoic-chamber/LF wedges:https://www.iac-australia.com.au/wp-content/uploads/2020/12/Global-Test-Brochure.pdf

author: EEVblog
author_rank: Unknown
timestamp: September 08, 2023, 10:44:29 am
content: Wow.

author: EEVblog
author_rank: Unknown
timestamp: September 08, 2023, 10:46:10 am
content: https://www.ikoustic.co.uk/case-studies/how-make-anechoic-chamber/

author: Marco
author_rank: Super Contributor
timestamp: September 08, 2023, 10:51:39 am
content: If you are diying any way and keeping it small, making an OSB+drywall(cheap mass) floor on some springs for the box is little extra effort.The "proper" research institute chambers are room in room, massive concrete cubes on springs.

author: EEVblog
author_rank: Unknown
timestamp: September 08, 2023, 10:58:12 am
content: Mics:18dB noise floor:https://www.bqc.be/en/measuring-devices/sound-level-meter/sound-level-meter-class-1-20-134-dba-0-01-20-khz/a-19409-1000012513dB noice floor:http://www.tenmars.com/m/2001-1682-704317,c19586-1.php?Lang=en

author: EEVblog
author_rank: Unknown
timestamp: September 08, 2023, 11:01:32 am
content: Quote from: Marco on September 08, 2023, 10:51:39 amMy dungeon is a concree slab foor in the basement carpark of the lab building.

author: Marco
author_rank: Super Contributor
timestamp: September 08, 2023, 11:11:42 am
content: So acoustically coupled to a carpark and the neighbouring road. I don't know to what extent it matters, but the research institute I interned/lab-rabbited at had a foundation with less noise sources nearby than that and it was still an isolated concrete box.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: September 08, 2023, 04:23:40 pm
content: For small items we usehttps://www.bksv.com/en/transducers/simulators/electroacoustic-accessories/anechoic-test-box-type-4232but it's not cheap. You certainly don't want any test equipment inside, other than mics and exciters. IMO, a chamber has to be darn near air-tight to eliminate low frequency stuff from air handlers and such. As it gets larger, the walls have to be rigid and non-resonant.

author: switchabl
author_rank: Frequent Contributor
timestamp: September 08, 2023, 04:28:15 pm
content: Quote from: EEVblog on September 08, 2023, 10:35:55 am1m distance sounds about right (rule of thumb is > 2*source size and >wavelength/4) but I think you'd want that in all directions so you don't have nearfield coupling with the absorbers.Quote from: EEVblog on September 08, 2023, 10:58:12 amConsidering that you need something like 20-40 measurement points for a good sound power measurement... repositioning the mic per hand will get old real quick. A discrete mic + preamp + audio interface setup might be more flexible and easier to integrate with a motorized scanning system.EDIT: That is assuming you want to do ISO 3745-style measurements. If this is just about SPL at a single position, then one of those meters should be fine. But do you really need an anechoic chamber for that?

author: coppice
author_rank: Super Contributor
timestamp: September 08, 2023, 04:37:15 pm
content: Quote from: Conrad Hoffman on September 08, 2023, 04:23:40 pmAnechoic chambers are not just acoustically isolated. They are really well thermally isolated, too. They can heat up pretty quickly, with even small amounts of dissipation inside. That usually means you need a considerably bigger chamber than your equpment needs, just to increase its thermal capacity, and reduce the rate of temperature rise. I've never used a chamber without getting very sweaty, and while I've been in them the door has usually been open most of the time.

author: coppice
author_rank: Super Contributor
timestamp: September 08, 2023, 04:42:00 pm
content: Quote from: EEVblog on September 08, 2023, 11:01:32 amWhile that is a disadvantage, its not insurmountable. Does the car park see traffic 24 hours a day? Can you test outside office hours? Is the car park itself the only source of vibration exciting that concrete slab? E.g. are there any train lines within a kilometre or so? However bad those excitations get, there is always an isolation technique to beat it. Its just a matter of how deep your wallet might be.

author: nctnico
author_rank: Super Contributor
timestamp: September 08, 2023, 11:02:42 pm
content: Quote from: Marco on September 08, 2023, 10:51:39 amI don't think that is necessary at all. I suspect Dave's measurements are A weighted so any noise below -say- 20Hz is not relevant.What will probably work is a big box with pyramid shaped foam elements poiting inwards and a suspended (steel) net to put the DUT onto. That way you have sound dampening from all sides. The net will work as a dampening system to keep low frequency vibrations out. The anechoic chamber they have at a research institute I used to work for, is constructed like this. Should be easy enough to construct on a smaller scale.

author: EEVblog
author_rank: Unknown
timestamp: September 09, 2023, 10:42:26 am
content: Quote from: nctnico on September 08, 2023, 11:02:42 pmYes, AFAIk all fan and instrument noise measurements are A weighted.QuoteThe concrete floor will be adequate, and possibly even desirable. As I posted above, this is a technique called a Hemi anechoic chamber.I figure just a 1.5m long box with no bottom on it and filled with sound dampning material that I then lower over the mic on a small tripod and the instrument on the concrete floor 1m apart should do an adequate job.To get really low frequency you need massively long acoustic wedges of course, that would be impractical.e.g. 50Hz wedges are 1.5m long on their own, and you'd need them 5 sides.

author: EEVblog
author_rank: Unknown
timestamp: September 09, 2023, 10:44:21 am
content: Quote from: coppice on September 08, 2023, 04:42:00 pmLow frequency vibrations won't be a problem. I'll have to turn off stuff stuff I have running in there, but apart from that it should be very quiet. I can test at any hour I like.

author: EEVblog
author_rank: Unknown
timestamp: September 09, 2023, 10:49:45 am
content: Quote from: switchabl on September 08, 2023, 04:28:15 pmNot really. Yes it's just about getting a reasonable setup so I can kinda-sorta measure low noise fan instruments.e.g. it sucks in my latest video saying "it's a bit noisy".Hownoisy compared to another product people can use as a benchmark.I can just get a good low noise mic (say 20dBA) and simply try it in my dungeon with everything switched off and see what I get. If it's low enough I could put up some regular acoustic panels on the walls to help prevent reflections (or have some heavy drop drapes from the roof) and call it day. I have a bunch of acoustic those panels spare.So my first port of call should be acquire a good quality 20dB capable measurement mic.

author: EEVblog
author_rank: Unknown
timestamp: September 09, 2023, 11:15:06 am
content: 15db *drool*https://www.bksv.com/en/instruments/handheld/sound-level-meters/2255-series

author: switchabl
author_rank: Frequent Contributor
timestamp: September 09, 2023, 11:24:02 am
content: Quote from: EEVblog on September 09, 2023, 10:49:45 amI think that makes sense.The thing with (hemi-)anechoic SPL measurement is that you are basically going to a lot of trouble to very accurately exclude all the noise radiated to the back and to the top. On the plus side, this is a highly reproducable measurement. And for a machine that is going to be placed on a machine floor with lots of open space and a high ceiling, it makes sense to specifiy noise like this to a degree. For a bench-top instrument that's probably going into a smallish lab, maybe close to the wall, IMHO not so much.For comparison purposes, measurements done in a reasonably quiet, reproducable lab-like space (+ some room treatment if necessary) might actually be more meaningful.That's purely from a technical point of view, of course. Building an anechoic chamber might be fun and make for good content.

author: EEVblog
author_rank: Unknown
timestamp: September 09, 2023, 12:19:44 pm
content: Quote from: switchabl on September 09, 2023, 11:24:02 amAgreed. But the lab unfortunately contains significant aircon and other noises noise from next door (open roof space across the entire floor). But I'd have to actually measure it to know what it's like.Would likely have to do measurements late at night or on weekends.I have to turn my own aircon off when I'm shooting videos otherwsie you can hear it.My shipping cubicle might be a bit quieter.QuoteYes, it at least makes it a bit more interesting rather than just open in my lab.Step 1, get a kick-arse mic.

author: EEVblog
author_rank: Unknown
timestamp: September 09, 2023, 12:25:29 pm
content: 12dBAhttps://svantek.com/products/svan-979-class-1-sound-vibration-level-meter/Asian one that does 20dBAhttps://www.hzaihua.com/products_cont.html?id=78#d3

author: EEVblog
author_rank: Unknown
timestamp: September 09, 2023, 12:53:30 pm
content: Just remembered that Flir are in my business park, and they own Extech now who have a very suitable 20dBA meter:https://www.flir.com.au/products/407790a/?vertical=condition%20monitoring&segment=solutionsI've given them a ping.

author: jonpaul
author_rank: Super Contributor
timestamp: September 11, 2023, 04:36:34 am
content: Dave Bravo for the initiative and ideas.1/Noise measurement: Noise floor depnds on mic sensitivity and mic preamp. Laboratory mics soecif the noise floor.1960s..1980s Gen Rad handbook on noise measurementhttps://worldradiohistory.com/BOOKSHELF-ARH/Technology/Technology-Radio/Handbook-of-Noise-Management-Peterson-9th-1980.pdfVintage classic sound meters with calibrated mics like Gen Rad 1982 are on ebay USA and ham fleas ...~$100..250 US.Includes weighting filters, 1/2" GenRad mic and preamp.https://www.ietlabs.com/pdf/Manuals/1982_im.pdfWe calibrate with General Radio 1562A Sound Level Calibrators2/ Anechoic Chamber volume depends on equipment DUT, walls ~ 30 cm..100 cm thick are common.A cheap test can be with cardboard egg crates or various sizes/shapes toothed soft packing foam or similar.3/ In EMI testing ...similar issues ,IF you have a quite open space away from the City, do it outdoors..."free field" testing alterniative to test chambers: (image is for EMI test)Setup a sturdy test table 3..10m frim any road or structuer,Run mains or batter/inverter mains supply, test equipoment in the field,Setup DUT supported above ground or tableUse sound level meter at correct distance and 90 deg to axis.use time and space averaging not a single point measurement.Bon chance!Just the ramblings of an old retired EEHAVE AN ABSOULTELY FANTASTIC DAYJon