Help Needed University Acoustics Assignment

Hello This forum looks incredible, I have been doing much searching and unfortunatley havent found what I need however. My acoustics lecturer has recommended tuning all our treatment for an assignment and doesnt like the use of broadband or off the shelf solutions. We have some nasty modes at approx 50Hz, 80Hz and 125Hz measured in FuzzMeasure. The rest of the room seems relatively ok. We need to tune either bass traps or helmholtz resonators for these three modes. I would really appreciate the following help / answers: 1. where to find the most professional guide possible to build a helmholtz resonator, also any companies that sell them? 2. how to tune bass traps to specific frequencies. 3. why do some experts in this forum favour broadband solutions over tuned? - my other research suggests this too : / 4. I would love opinions on how to attack these frequencies, the rest of the room is actually not too bad due to book cases. Its literally only these 3 modes we would like to address. Please see this link to our overall ave measurement results: http://www.freezabox.com/AllFreqResponseAve.pdf Floor Plan: http://www.freezabox.com/floorplan.jpg Thank you very much in advance. Steve London UK

stevesdata wrote:

1. where to find the most professional guide possible to build a helmholtz resonator, also any companies that sell them?

Helmholtz resonator design is straightforward physics. The best document with design AND test verification is BBC RD 1992-10. Of particular importance is section 7.2. As you design for the problem frequencies, you can use a single unpartitioned rear chamber. Depending on your prof's personality, he will readily accept it, or jump down your throat.

2. how to tune bass traps to specific frequencies.

You build them, install them, and then measure the effect of them in the room, and modify. Repeat.

3. why do some experts in this forum favour broadband solutions over tuned? - my other research suggests this too : /

Somewhat vague. Most advice follows: Treat mf and hf issues with hf absorption Use bass traps for general LF control Use tuned absorbers for modal peaks/nulls. Where it normally falls apart is when writing to people who are good carpenters, but have no desire to do calculations. Cut and build makes them happy.

4. I would love opinions on how to attack these frequencies, the rest of the room is actually not too bad due to book cases. Its literally only these 3 modes we would like to address.

My suggestion is a common cavity Helmholtz resonator with two frequencies absorbed by cavities and the third by membrane absorption in the absorber. RD 1992-10 gives the core idea from studying section 7. Enjoy and let us know how it turns out! Andre

stevesdata wrote:

1. where to find the most professional guide possible to build a helmholtz resonator,

I would suggest the textbook for the class you're taking.

also any companies that sell them?

Coca-Cola? :D :D :D

4. I would love opinions on how to attack these frequencies, the rest of the room is actually not too bad due to book cases. Its literally only these 3 modes we would like to address.

I think if you consult the text for your class and/or brainstorm with your classmates, you'll come up with something good. Good luck,

AVare wrote:

stevesdata wrote:

1. where to find the most professional guide possible to build a helmholtz resonator, also any companies that sell them?

Helmholtz resonator design is straightforward physics. The best document with design AND test verification is BBC RD 1992-10. Of particular importance is section 7.2. As you design for the problem frequencies, you can use a single unpartitioned rear chamber. Depending on your prof's personality, he will readily accept it, or jump down your throat.

2. how to tune bass traps to specific frequencies.

You build them, install them, and then measure the effect of them in the room, and modify. Repeat.

3. why do some experts in this forum favour broadband solutions over tuned? - my other research suggests this too : /

Somewhat vague. Most advice follows: Treat mf and hf issues with hf absorption Use bass traps for general LF control Use tuned absorbers for modal peaks/nulls. Where it normally falls apart is when writing to people who are good carpenters, but have no desire to do calculations. Cut and build makes them happy.

4. I would love opinions on how to attack these frequencies, the rest of the room is actually not too bad due to book cases. Its literally only these 3 modes we would like to address.

My suggestion is a common cavity Helmholtz resonator with two frequencies absorbed by cavities and the third by membrane absorption in the absorber. RD 1992-10 gives the core idea from studying section 7. Enjoy and let us know how it turns out! Andre

Many many thanks. I spent a long time trying to find something like the BBC paper. I had decided a long time ago that tuned helmholtz resonators would be ideal, but then my research all pointed at broadband designs with advice also to not bother trying to tune treatment in this way. I think this advice was aimed at consumers with only a basic knowledge of acoustics. I will let you know how it goes much is hypothetical, however if you want I will send a copy of the report. 8)

lovecow wrote:

stevesdata wrote:

1. where to find the most professional guide possible to build a helmholtz resonator,

I would suggest the textbook for the class you're taking.

also any companies that sell them?

Coca-Cola? :D :D :D

4. I would love opinions on how to attack these frequencies, the rest of the room is actually not too bad due to book cases. Its literally only these 3 modes we would like to address.

I think if you consult the text for your class and/or brainstorm with your classmates, you'll come up with something good. Good luck,

Our core text was The Master Handbook of Acoustics which although excellent didnt provide enough detail to actually go out and build something. I love the text but find very theoretical at times. I think my main problem was conflicting research and very poor advice offered from many resources less good than this site. The main issue was finding some solid research on building the helmholtz resonator, so I should be fine now. Many thanks 8)

I was also thinking about following some of the BBC Research and making a skyline diffusor. http://www.johnlsayers.com/phpBB2/viewtopic.php?t=326 I found a calculator and found out I could tune to around the same frequencies as my highest mode, so I was assuming this could be a good tactic for a mode? Can I confirm these are good for targetting low end as well as higher frequencies? :?:

stevesdata wrote:

Many many thanks. I spent a long time trying to find something like the BBC paper. I had decided a long time ago that tuned helmholtz resonators would be ideal, but then my research all pointed at broadband designs with advice also to not bother trying to tune treatment in this way. I think this advice was aimed at consumers with only a basic knowledge of acoustics. I will let you know how it goes much is hypothetical, however if you want I will send a copy of the report.8)

Your welcome. The BBC RD reports are a goldmine of information. FYI here is the link to the BBC RD index. Agreed with using Helmholtz resonators. Like you wrote, many people are afraid of applying detailed acoustic knowledge. I would love a copy of your report. A link to here would be great so that al the people here can share your knowledge.

I found a calculator and found out I could tune to around the same frequencies as my highest mode, so I was assuming this could be a good tactic for a mode? Can I confirm these are good for targetting low end as well as higher frequencies?

Aaa, uhm, ooh, yes it will work at low frequencies if designed for it. Schroeder type diffusors are effective down to frequencies where the well depth is one quarter of the wavelength. So for for your lowest mode of 50 Hz you just need a diffusor depth of 1.5 m. If all this seems confusing, don't worry. I have studied acoustics for years, and the single constant is that the more I know, the more I know how much I don't know. If the trend continues, once I know all that I can know, I will know nothing. :lol: What is your major? I think another phrase is "your first." Knowing nothing; Andre

I am studying Music Technology Specialist which means 2/3 recording, sequencing etc. and 1/3 modules which will make me more emplyable inc. web design, video, codecs, acoustics, business, broadcasting. Minor / majors are the right terms but many UK courses are specialist designed to have a broad range of complementry skills for the minor part of the course. For example if I did minor / major with radio broadcasting 1/3 would be radio broadcasting and I would loose business, web design modules etc. First is usually a grade. Minors also get called electives - I think this is a US term. Thanks a lot for your help. Some of my work is crude in comparison to some of the things I have found on this site - but will post. We are trying to forge some solid treatment concepts and post ASAP - will have a look at Schroeder diffusors, I have a feeling they will probably be better than Skyline because of the low freq probs. See you soon - will try to post proposed treatment solutions later today. :wink:

stevesdata wrote:

- will have a look at Schroeder diffusors, I have a feeling they will probably be better than Skyline because of the low freq probs.

I'm llooking forward to your reports. Skyline diffusors are Schroeder diffusors. Good luck! Andre

Unless you use a huge diffuser it works as a reflector at low frequencies..