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PostPosted: Sun Nov 25, 2018 9:57 pm 
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My question is about which frequencies to aim at for bass trap design for a room used for vocals only, more particularly male vocals.

This theory might be completely off, but I'm trying to get through the haze of acoustic material to work out what you specifically do. Thanks for any feedback.

I am thinking there is no reason to make bass traps for below the frequencies of the male voice, because if you're not making sounds lower than that then there will be no lower resonances That's what I'm thinking. Is that correct?

So since an adult male voice starts from about 85Hz (or a bit lower if you have a very deep voice), I figure I need to design bass traps to broadly trap frequencies between about 80 Hz and 160 Hz. Does this make sense? After 160 Hz broadband diffussion will be effective.

If I enter my room dimentions in the Amroc calculator https://amcoustics.com/tools/amroc?l=360&w=300&h=240&r60=0.6 I see some frequencies below 80Hz (47.64 Hz, 57.17 Hz, 71.46 Hz, 74.41 Hz). Why worry about them? Won't they only come into effect if you make sound with those frequencies?

Now on the Aroc page there are a whole bunch of frequences between my 80 Hz and 160 Hz target. Why not just aim for the bottom three Axial mode ones? (95.28 Hz, 114.33 Hz, 142.92 Hz) because aren't all the other mode frequencies, echos, or reliant on the axials? whether those or the axials under 80hz?

85.88 Hz F2 1-0-1 tan
6 91.51 Hz F2# 0-1-1 tan
7 95.28 Hz G2 2-0-0 ax
8 103.17 Hz G2# 1-1-1 obl
9 111.11 Hz A2 2-1-0 tan
10 114.33 Hz A2# 0-2-0 ax
11 119.1 Hz A2# 2-0-1 tan
12 123.86 Hz B2 1-2-0 tan
13 132.11 Hz C3 2-1-1 obl
14 134.83 Hz C3# 0-2-1 tan
15 142.92 Hz D3 0-0-2 ax

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PostPosted: Tue Nov 27, 2018 1:04 am 
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I am thinking there is no reason to make bass traps for below the frequencies of the male voice, because if you're not making sounds lower than that then there will be no lower resonances That's what I'm thinking. Is that correct?
It's not just about the frequencies in the sound source that you will be tracking (in this case, vocals), but about the room acoustics in general. The low end response of the room defines the overall room "signature", which is why very small rooms ("booths") always sound "boxy", regardless of treatment.

Also, there really are low frequencies in human speech, way down to the 50 Hz range: they just aren't very useful for the speech itself, but they are still there.

So you need treatment that covers the entire spectrum, even if the room is only used for vocals.

Think of it this way: For tracking those vocals, would you actively search for a mic that ONLY has sensitivity down to 120 Hz and up to 4 kHz, and doesn't pick up sound outside that range? Of course not: such a mic would sound terrible. You'll still use a mic that covers the entire spectrum, even though it might roll off on the low end and high end.

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frequencies between about 80 Hz and 160 Hz
Why that range alone? Why stop at 160 Hz?

Quote:
After 160 Hz broadband diffussion will be effective.
In a vocal booth? Diffusion? Ummm.... nope. I don't think so!

Most vocal booths are not large enough to use diffusion, even if it was possible, and there's no reason to use diffusion for vocals in any case!

Besides, what diffuser would you choose that is able to diffuse down to 160 Hz? It would have to be HUGE! Assuming you want a Schroeder or Skyline, the wells would have to be 6 1/2 inches wide (nearly 17cm) and over 42 inches deep (more than 1 meter)... :shock: You'd have to make it from 8 x 8 lumber, and you'd be using full studs turned sideways, poking over 3 feet (one meter) out of the wall... :)

If you wanted to use a poly-cylindrical diffuser instead (probably better for vocals: smoother), then it would have to be at least 7 feet wide and three and a half feet deep in the middle, to get down to 160 Hz...

You should probably take a look at how diffusers work, and how they are designed and built, to understand why it is out of the question to have a diffuser tuned to 160 Hz, and why you wouldn't want such a diffuser in a vocal booth anyway.

For a vocal booth, just orient the layout correctly, set up the mic and talent correctly, facing the right way, keep the rear absorptive and the front and sides partly reflective and partly absorptive, and you'll be fine.

Quote:
If I enter my room dimentions in the Amroc calculator https://amcoustics.com/tools/amroc?l=36 ... 40&r60=0.6 I see some frequencies below 80Hz (47.64 Hz, 57.17 Hz, 71.46 Hz, 74.41 Hz). Why worry about them? Won't they only come into effect if you make sound with those frequencies?
If you look at your console, DAW or soundcard, it shows specs that go down to 20 Hz and up to 20 kHz, and a dynamic range of about 100 decibels. Why bother using that for recording vocals, when you don't need such large ranges? How come you didn't get an 8 bit sound card that only covers 100 Hz to 4 kHz, and a dynamic range of 48 dB? After all, that's all you need for recording vocals... :) Why use a 16 bit or 24 bit card with full-spectrum capability, when you don't need that much? ... :)

I think you see the point.

(Perhaps even better: Why live in a house when a tent is plenty good enough?)

Quote:
between my 80 Hz and 160 Hz target.
I'm still not understanding your choice of frequency range there. I get why you think you don't need below 80, but why would you stop treating bass above 160? The vast majority of the major issues in a vocal booth will be above 160 Hz...

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PostPosted: Tue Nov 27, 2018 10:08 pm 
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Location: Brisbane, Australia
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Also, there really are low frequencies in human speech, way down to the 50 Hz range: they just aren't very useful for the speech itself, but they are still there.


Ok, like Um, or Ah-hah, or a grunt maybe. Okay, so I go down to 50. Can I avoid going down to 40hz? I get that a sound at 50Hz has harmonics that repeat up at higher frequencies, but you still have to make that lower direct sound, dont you? Are you saying once a rooms reverberation starts up that reverberation itself reaches down, if a low mode on the amroc scale is there for the room dimentions?

Quote:
frequencies between about 80 Hz and 160 Hz. Why that range alone? Why stop at 160 Hz?

I figured I could use a combination of superchunk corner traps, which I calculated* at 60% effectiveness at 160hz, if average 12" average thickness - not a disaster. Plus, make at least one (depending on room size) of Ethan Winers low bass traps design ([url]Ethan Winers article at http://ethanwiner.com/basstrap.html[/url]), which he says cover 150 Hz and 300 Hz. There is one referenced there that goes lower, but I didn't want to make that because I would end up with too much reflection going on, because that design uses plywood facing. So, I thought I would look at other bass trap ideas like using MLV and slats where I target whatever I have to target below about 80 Hz.

* Using http://www.acousticmodelling.com/porous.php, if I have an average 12 inch (~300mm) thickness of Owens Ccorning 705 (6.0 pcf = 96 kg/m3 = 30.000 MKS Rayls/m = 30.000 K Pa.s/m2) (lets say 17” deep to the corner) with no air gap, it shows over 60% efficiency at 160Hz - not a disaster, and then can suppliment the Ethan Winers low bass traps design.

Quote:
After 160 Hz broadband diffussion will be effective.In a vocal booth? Diffusion? Ummm.... nope. I don't think so! . . . Most vocal booths are not large enough to use diffusion, even if it was possible, and there's no reason to use diffusion for vocals in any case!

So sorry I meant absorption, not diffussion, actually. and even that is not accurate.

Quote:
Besides, what diffuser would you choose that is able to diffuse down to 160 Hz? It would have to be HUGE! Assuming you want a Schroeder or Skyline, the wells would have to be 6 1/2 inches wide (nearly 17cm) and over 42 inches deep (more than 1 meter)... :shock: You'd have to make it from 8 x 8 lumber, and you'd be using full studs turned sideways, poking over 3 feet (one meter) out of the wall... :) . . . If you wanted to use a poly-cylindrical diffuser instead (probably better for vocals: smoother), then it would have to be at least 7 feet wide and three and a half feet deep in the middle, to get down to 160 Hz...You should probably take a look at how diffusers work, and how they are designed and built, to understand why it is out of the question to have a diffuser tuned to 160 Hz, and why you wouldn't want such a diffuser in a vocal booth anyway...For a vocal booth, just orient the layout correctly, set up the mic and talent correctly, facing the right way, keep the rear absorptive and the front and sides partly reflective and partly absorptive, and you'll be fine.


I did buy some diffuser plans fro Acoustic fields, though not built anything yet. For example, there is one called the QD-13 that has its deepest well as 26.5 cm. As I understand it, that design is based on a 1/4 wavelength approach, and I did some wavelegth comparison from http://www.procato.com/calculator-wavelength-frequency/ finding that 26.5 x 4 is a wavelength somewhere close to 100hz. The placement of those are supposed to be at least 4-5 feet from the recording position. this is how I am understanding those things. So, I was figuring if I had a 10 foot long room i could use these at the back.


I had said, "If I enter my room dimentions in the Amroc calculator https://amcoustics.com/tools/amroc?l=36 ... 40&r60=0.6 I see some frequencies below 80Hz (47.64 Hz, 57.17 Hz, 71.46 Hz, 74.41 Hz). Why worry about them? Won't they only come into effect if you make sound with those frequencies?"
Quote:
If you look at your console, DAW or soundcard, it shows specs that go down to 20 Hz and up to 20 kHz, and a dynamic range of about 100 decibels. Why bother using that for recording vocals, when you don't need such large ranges? How come you didn't get an 8 bit sound card that only covers 100 Hz to 4 kHz, and a dynamic range of 48 dB? After all, that's all you need for recording vocals... :) Why use a 16 bit or 24 bit card with full-spectrum capability, when you don't need that much? ... :)

Sort of, I get you need a range beyond what you need in most things, because of design effectiveness, but I use a Low cut filter, I think it's called, to cut out the low base sounds that might be in a room when I record - oh, hold on :-) there it is, why would I do that if I am imagining there are no low frequencies to cut out. ANd, if I watch an EQ spectogram, I can see the bass frequency, low as it is fade way when I press the low cut filter on my preamp, yes, ok, that low bass sound is there.
All right I get your idea that a rooms 'signature' incudes low frequency reverberation, I just dont understand it? is it because outside sound waves that are so low you cannot hear them, get into your room from outside? inspite of what you try and do to isolate your recording space (the ,low ones would be the ones to get through) - that would nake sense to me, that they could propogate at 40hz, 50 hz, whatever. the initial sound at those frequencies, I wouldn't hear with my headphones, but they could be there, even in low amounts. What about electric hum at levels that wouldn't be picked up as a hum issue, but nevertheless seepoing in as low frequency, actaully direct sound?
Okay, about the spaces resonating with low frequency modes anyway. Yes, I can understand whne you blow air at the lip of a bottle it make a fequency sound, that has nothign to do with the zero or very little sound in the breath passing over the lip (Talking myself into it, here:-)) so then it's just simply to resonating signature of a room, like a bottle, that according to amrock includes some of those low frequencies in its resonating signautere! Really, yes I suppose so. Is that it?

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PostPosted: Wed Nov 28, 2018 2:36 am 
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Quote:
I did buy some diffuser plans fro Acoustic fields, ...
Well, there's your problem! That's it, right there! :)

Your money would have been far better spent on correct information about diffusers, by buying the book "Acoustic Absorbers and Diffusers" by Cox and D'Antonio. Those are the guys who did decades of actual research on diffusers, in real acoustic laboratories, to discover the principles by which they work. And they don't work on fantasy. It would be nice if they did, but they don't. They work on phase changes, and that requires dealing with entire wavelengths, or half-waves in some cases. The guys who wrote that book derived the equations that describe how real diffusers work in the real world.

Buy the book: it's expensive, yes, and maybe the math is a bit intense, but the principles are easy to understand, and well laid out.

Quote:
The placement of those are supposed to be at least 4-5 feet from the recording position. this is how I am understanding those things. So, I was figuring if I had a 10 foot long room i could use these at the back.
If you read the book, instead of the Archie comic version (!), you'll find that ALL numeric-based diffusers need to be at least ten feet from the listener in order for the artifacts to have smoothed over sufficiently, but that IN ADDITION the distance must be at least three full wavelengths. So if the low cut-off frequency of the diffuser has a wavelength of greater than about 3.3 feet (39 inches or 1m), then ten feet is not enough: you have to get further away than three times the lowest WAVELENGTH (not quarter wave: FULL wave). If you understand the principles of waves you'll get why it MUST be like this.

The basic issue is that all numeric sequence diffusers produce some form of lobing patterns and interference. Here's what they look like for a QRD sequence:

Attachment:
QRD-Diffusion-lobing--pattern-graph-SML-ENH.PNG


They MUST do this because they change the timing and therefore the phase of the sound wave, in addition to changing the direction and intensity. The incoming wave front hits different parts of the diffuser, and is firstly broken up like that, with each part of the wave going down the well that it happened to arrive at, hitting the bottom, and coming back up again. If the wavelength happened to match the depth dimension of the well, then it will come back out the top exactly out of phase with the way it went in. The wave that is four times the frequency will do the same, but not the wave that is one quarter the frequency. Nope. Sorry. It just doesn't. The cut-off frequency is set by the depth of the deepest well. The device will still SCATTER down to half of that frequency, but at a quarter of that frequency, it simply reflects the wave, without doing anything to it. Because according to basic acoustic theory, an object has to be at least comparable in size to the wavelength in order to have any effect on it. Waves that are considerably longer than the dimensions of an object will NOT be affected by it. Only ones that are shorter, or similar in size.

Quote:
... has its deepest well as 26.5 cm. ... finding that 26.5 x 4 is a wavelength somewhere close to 100hz.
Ummm... Nope! The wavelength at 100 Hz is 11.3 feet, which is 135.6 inches, or 344 cm! Certainly not 26.5cm. A quarter of 100 Hz is 2.825 feet, or 33.9 inches, 86cm. You made a big mistake in your calculations.

In fact, the deepest well in a numeric-sequence diffuser needs to be a half wave deep (in order to get phase cancellation at that frequency). At 160 Hz, the wavelength is 84 inches, so the half wave is 42 inches. Therefore the device needs to be 42 inches deep.

If you use a QRD diffuser calculator (which are FREE!), you will find that for a typical N=13 diffuser for 160 Hz will need a maximum block height of 39 inches, and a minimum block width of 6.5 inches. If you made that with 8x8 lumber, which measures 7 1/4 inches on each side, the resulting diffuser would diffuse down to 160 Hz, it would scatter down to one octave lower, at 80 Hz, and the upper cut-off frequency would be 934 Hz. The minimum distance to the listener position would be 254 inches, which is a little over 21 feet...

Here are the actual results that I calculated for you, using a real diffuser calculator, and showing the cross-section of the 1D version (normal Schroeder diffuser), with the diagram that shows the lobing directions:
Attachment:
QRD-calc-for-160Hz-diffuser.jpg


Here's the actual design for the 2D version (Skyline style) of such a device, :
Attachment:
QRD-calc-for-160Hz-diffuser-2.jpg


And here's the diagram of block heights for those 8x8 lumber blocks that you would need to build the 2d version:
Attachment:
QRD-calc-for-160Hz-diffuser-3.jpg


You do NOT need to pay somebody to make the plans for you: you can do it yourself using this same program, called "QRDude", which is FREE. You can get it here : https://www.subwoofer-builder.com/qrdude.htm If somebody made you pay for doing the same thing, then perhaps you've been scammed.

Quote:
The placement of those are supposed to be at least 4-5 feet from the recording position.
21 feet, according to this ACTUAL calculator that uses the REAL equations. Take a close look at the calculate real results, above. Your room would have to be about 30 feet long to use such a device, since it is 3 feet deep itself, and you do need a bit of space to stand in. A 40 foot room would be better...

Quote:
I had said, "If I enter my room dimentions in the Amroc calculator https://amcoustics.com/tools/amroc?l=36 ... 40&r60=0.6 I see some frequencies below 80Hz (47.64 Hz, 57.17 Hz, 71.46 Hz, 74.41 Hz). Why worry about them? Won't they only come into effect if you make sound with those frequencies?"
Because the modal response of the room shapes the entire frequency response curve for the room, regardless of whether nor not you actually make any sounds at those frequencies.

Think of it this way: If you are standing on a hillside that is steeply sloped, do you have difficulty walking UP the hill or ALONG the hill? Even though you have no intention of going DOWN into the valley below you, does that valley still affect you? Yes, it does, because the valley is part of the shape of the hill. Even if it is night time, in total darkness, you can still feel that the valley is there, even if you can't see it, just by walking around. The slope cause by the valley is always there, and affects your life, even if you never see it, and never plan to walk down into it: it is still there.

Same with the hills and valleys in the frequency response of the room: they are sill there, all of them, and the "valleys" way down low still cause a slope in the response for the frequencies where you will be "walking around". Even if you never play any tone that goes that low, the valley is still there, and will still affect the tones that you DO produce in that room.

If you are walking around that hill, and get tired of limping off to one side all the time, then you can just fill in the valley! Dump a lot of dirt in it, to level it out, and it won't affect you any more: you can walk on level ground. Ditto with your room acoustics: if you want a level frequency response then "fill in the valley" with suitable treatment.

Quote:
but I use a Low cut filter, I think it's called, to cut out the low base sounds that might be in a room when I record - oh, hold on :-) there it is, why would I do that if I am imagining there are no low frequencies to cut out.
Yup! If there were no lows in the room, you would not need to use EQ to filter them out! :)

Quote:
All right I get your idea trhat a rooms 'signature' incudes low frequency reverberation, I just dont understand it? is it because outside sound waves that are so low you cannot hear them, get into your room from outside?
Very likely there ARE lows in the room, coming from outside sources, or even from the person in the room just being there. But even if there were NO lows at all (if you had perfect isolation), the acoustic response curve for the room is still shaped by the dimensions and treatment of the room. The ground you are walking on is shaped by the valley below and the hillside above, even if you never plan to go there. Those still affect the slope of the ground WHERE YOU ARE WALKING, as well as the parts where you are NOT walking.

Quote:
What about electric hum at levels that wouldn't be picked up as a hum issue, but nevertheless seepoing in as low frequency, actaully direct sound?
It's a hard concept to get your head around, but there doesn't actually need to be any sound at those frequencies in order to affect the shape of the acoustic response of the room. The response curve is already shaped, sound or no sound. The tree in the forest still does make a sound even if you weren't there to hear it. The room response still has a shape, even if you don't make any sounds that would reveal it. It is still there. The hill and valley are still there, even if you don't shine any light on them to see them, or walk over them to feel their shape...

Quote:
Ok, like Um, or Ah-hah, or a grunt maybe. Okay, so I go down to 50. Can I avoid going down to 40hz?
Nope! If you want to walk on level ground, you have to level out the ENTIRE valley, not just the bottom part. You have to fill it up to your own level, not just drop a few stones in the riverbed.

Quote:
but you still have to make that lower direct sound, dont you?
Do you have to walk in the riverbed to known that the ground on the hillside slopes down? :)

Quote:
I figured I could use a combination of superchunk corner traps, which I calculated* at 60% effectiveness if average 12" average thickness - not a disaster.
I'm not sure where you are getting your information from, but it is highly suspect. Insulation 12" deep might have a coefficeint of absorption of 0.6 at 160 Hz, yes, but that does NOT mean that it is 60% efficient! All that it means is that it can absorb 60% of the random incident sound in a purely diffusive and reverberant room. As long as your room meets those conditions, then you could expect it to absorb 60% of the energy that would otherwise be reflected by the wall behind it. Let's assume that it is 50%, to make the math easy. A drop of 50% is a reduction of 6 dB, so your device would be able to reduce the modal peak at 160 Hz by about 6 dB. If that peak happened to be 15 dB high, then it would come down by 6dB, and would now "only" by 9 dB high. If your mic happened to be in a null which was 30 dB deep, then that null would now be only 24 dB deep.

You need to put the numbers in perspective, to understand what they actually mean for you. To get more absorption, you would need to find a lighter weight (lower density) insulation and make it thicker. A deep superchunk could have a coefficient of over 0.8 a 160 Hz, and over 0.7 at 60Hz, for example.

Quote:
And make at least one (depending on room size) of Ethan Winers low bass traps design ([url]Ethan Winers article at http://ethanwiner.com/basstrap.html[/url]), which he says cover 150 Hz and 300 Hz.
If you have only one, where would you put it? Modes happen in all three dimensions, on all three axes of the room. So if you put your trap at a location where some of those modes never reach, it won't so anything to them. If you fill in the valley around the other side of the hill, it wont do anything to make the ground flatter on YOUR side of the hill...

Quote:
There is one referenced there that goes lower, but I didn't want to make that because I would end up with too much reflection going on, because that design uses plywood facing.
That's a TUNED trap, and only absorbs at a specific frequency. Those are very god for dealing with stubborn modal issues, but not what you need for a vocal booth.

Quote:
Using http://www.acousticmodelling.com/porous.php, if I have an average 12 inch (~300mm) thickness of Owens Ccorning 705 (6.0 pcf = 96 kg/m3 = 30.000 MKS Rayls/m = 30.000 K Pa.s/m2) (lets say 17” deep to the corner) with no air gap, it shows over 60% efficiency at 160Hz - not a disaster.
:shock: Way too dense! You want LOW frequency absorption, not HIGH frequency absorption. Use 701, not 705. Even 703 would be better than 705! You need something around 7000 rayls, or less, to get down low effectively.



- Stuart -


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PostPosted: Sun Dec 02, 2018 12:16 am 
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That book is on my purchase list, there is an ebook version, cheaper than the hard copy. I will digest everything you have said. Will take me a little time. Yes, I checked, I see, my calculations were way off.

That diffuser example, I got the plans for must be reaching at a much higher frequency range. I'll have to work it out again. It's interesting there is a lot talked about bass traps but diffusion does bring life into the room.

At the moment in my room I have some plastic hedge leafage hanging on my back wall. That seemed like a good cheap man's version, until I got to the exact correct way of doing it. Is that a reasonable approach?

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PostPosted: Mon Dec 03, 2018 9:14 am 
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I am not unusual and can hit 65Hz quite easily.

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