Seeking Mr. Sayers advice.

[RICK FITZPATRICK]

Hello Mr. Sayer. I finally found some time to REALLY study your SAE site, and I have to admit, first of all, your knowledge on this stuff is very profound. And offering it free online is nothing short of fantastic. AND, I owe you an apology, for my posts that probably made you laugh, but if they in any way offended you, again, my apology. Confusion and frustration are my WORST enemys. Knowledge is one thing, experience is an animal of a different color. You have both and I AM JEALOUS All I have is enough info to put my foot in my mouth everytime I open it. That being said, I have one question(to start). Would you mind explaining for me why a room needs to be large, for bass frequencys to develop, and why I can still hear them in a small room? If I play a bass in my little studio, I hear the all the notes. And if I record them, they are all there, clear down to open E. And I don't hear any difference in the volume of different notes, in regards to standing waves cancelling and adding.(is that correct?) I made a recording of three and a half octaves of the chromatic scale, on bass and I heard no difference in the volume of each note.(played them back in another room)I am totally confused on this issue. I know your busy, and I don't mean to write a book.
fitz.

 

[John Sayers]

good question

Hi Rick – Interesting question and one that I’ve had a problem with myself.

As I see it it goes something like this.

Sound travels at 330m per second therefore a wave of 100Hz is 3.3m (11ft) long. And 10Hz is 33m (100ft) long. As you can see the low frequencies from 10 – 100 hz are dirty big waves which if they were water you could surf them . 50hz is around 6.6m (18 –20ft).
Now you hear these frequencies because sound is pressure difference OK? If you are out in an open paddock you will still hear them cos it’s pressure dif.
When a wave hits a wall it reflects back starting from ZERO no matter where it is in the waveform. So if you have a room that’s 10 x 10ft a full wave of 50hz is never able to fully form as it keeps starting from zero at each reflection, whereas a room that has a 20ft dimension will allow a full 50hz wave to develop AND reflect thus creating a true resonance at that frequency.

That’s my take on it but perhaps barefoot (He’s the physicist around here ) may have another version ?? I'm happy to stand corrected

Cheers
John

 

[RICK FITZPATRICK]

Appreciation day

Thanks for the enlightenment John. I'm just a curious person. And busy people who take thier time to satisfy my curiosity, have my sincere appreciation and gratitude. Hope I can return the generosity. BTW, you have your heros, but youv'e become one of mine. Mainly because you are a gentleman.
fitz

 

[John Sayers]

You're welcome you are not a Gemini are you?? or an Aries

cheers
John

 

[rambo]

Hi Rick....
If a low freq is played it will also create a spectra of overtones spaced by definitions. Now if your hearing is exposed to a set of overtones from a lo freq. but with the fundamental freq. removed. You will still hear the low freq. ..... that's one reason why you still can get som lo end- out of a small radio physcly not capabel to play back at those frequencies. So yes you will still hear the low's even if they are not existing, due to what John explained.


rgds
Olle

 

[frederic]

You're welcome you are not a Gemini are you?? or an Aries

cheers
John

I'm a gemini! And so am I!

 

[Beathoven]

* ... Coming out of no where *

Hey I'm a gemini too!

* Returning no where... *

 

[cordura21]

There is a plugin called MaxxBass that add these harmonics to add more bass to the sound, even in small speakers. I guess it's based on these principles.

In fact, I read that one common mistake is to add eq in the fundamental frequency of an instrument, when it will be smoother to add it in one of the harmonic frequencies.

 

[RICK FITZPATRICK]

Lo Hz ghost?

Hello all, and thanks. John, I'm a Sagitarius. Figures huh! Born on a Capricorn cusp too! And Rambo, is this correct? In a small space, you are hearing the overtones, not the fundimental. Therefore, the mind must be putting 2 and 2 togeather? Is this "fundimentaly" what your saying? If it is, then that implys a WHOLE lot And if you record and playback in a small room, then the same thing happens? Boy, then that tells me something, like in a big room, where the fundimental really exists, what does the mind interprete any different than in a small room? It would be interesting to make a recording of the same note, in a small room and a large room, to hear the difference. And actually, to me, this WOULD prove to me, if or if not the mind can actually hear the difference. And to furthermore illustrate the difference, then record say a low E in a small empty room with no acoustical treatment, and then the same note with say diffusers, then absorbers. And THEN,in a room large enough for the fundimental to develop, do the same thing. Of course, then it would seem you would have to play it back in an anechoic room, to hear the difference in the recording Boy, acoustics is such a wierd science, and I AM NOT a scientist. Just a musician. But my ears are no different than thiers, although, mine suck. TOOOOOOO many years playing loud electric guitars That was long before anyone talked about ear damage!! But as to the explaination, Thanks much. I'm going to investigate this further, and see if I can do some kind of test recordings. This has got my curiosity TOTALLY out of control now But now that I think about it, if I don't have my brain circuits burned out, this DOES imply a lot of things. Like-if thats the case, then a mic diapham must have a resonant frequency also? But the mic, has no brain, so how does it interpret overtones from the fundimentals-shit, this is getting confusing. Hey, the gap in the recording head is only so wide, and how does the eardrum...........nevermind....ah OVERLOAD-BLINK, BLINK BLINK
Mind crash, mind;;;;;;;;;;......crash;;;(&^#(*&*)(............................................?Where did I put
that quad cup of expresso ? This is going to take a while!!!

 

[RICK FITZPATRICK]

What happened?

GEEEEEEEEEEzzzzzzzzzz,what happened to the field, shit, sorry about that. I don't know what I did

 

[frederic]

Re: What happened?

GEEEEEEEEEEzzzzzzzzzz,what happened to the field, shit, sorry about that. I don't know what I did

Either go back to sleep or have a quad-shot cappucino <big grin>

 

[Beathoven]

At the end of your post there's a lot of "."

Just remove half of them and everything will be back in order...

Beathoven

(P.S.: If you don't do so, we're gonna straight up kick yo ass )

 

[RICK FITZPATRICK]

Whew!

Almost got my ass kicked clear down to open E(My butts resonant frequency!)

 

[barefoot]

So if you have a room that’s 10 x 10ft a full wave of 50hz is never able to fully form as it keeps starting from zero at each reflection, whereas a room that has a 20ft dimension will allow a full 50hz wave to develop AND reflect thus creating a true resonance at that frequency.

That’s my take on it but perhaps barefoot (He’s the physicist around here ) may have another version ?? I'm happy to stand corrected

There seem to be some misconceptions here about the effect of small rooms on bass response.

First of all (offering the correction that John welcomed), and to first approximation, low bass frequencies are actually dramatically louder in small rooms. The equations describing even the simple physical model of a piston radiating into a finite leaky volume of air clearly show why this occurs. I won't go into the physics, but suffice it to say that when the wavelengths are large compared to the dimensions of the room the math works out differently and these frequencies are louder than they would be if the room dimensions were much larger. This is the reason why those little 4" speakers in your car or 1" speakers in your headphones can put out such deep bass. It’s also the reason why all that bass disappears when you wind down the car windows or slightly pull the headphones away from your ears.

"So a smaller room gives deeper bass. Let’s set up our studio in the closet!"

Not so fast…..

A bigger room is still better because it's nearly impossible to make the bass response of a small room accurate. The bass response of a small room will have very large peaks and dips, and the smaller the room the higher the frequency these response anomalies will occur. Why? The reason for this is often referred to as the "Allison Effect" because it was first formally described in a paper by Roy Allison of Allison Acoustics (smart guy but I don't like his speakers). I just got done paraphrasing this effect in this thread https://homerecording.com/bbs/showthread.php?threadid=62201

This is the effect where the pressure fluctuations of bass waves reflected off the walls cause frequency dependent changes in the acoustic loading on the woofer cone. So basically, the woofer puts out some bass, a pressure peak or dip is reflected back which in turn effects how well the woofer can transmit the new bass wave it’s trying to produce.

This is a very tricky problem and the only practical solution is use a large room enhanced with bass traps which will in turn push this effect down to lower frequencies where the ear is not so sensitive. [note: The Allison effect even occurs in headphones but the dimensions are so small it happens in the midrange!]

....Now if your hearing is exposed to a set of overtones from a lo freq. but with the fundamental freq. removed. You will still hear the low freq. ..... that's one reason why you still can get som lo end- out of a small radio physcly not capabel to play back at those frequencies. So yes you will still hear the low's even if they are not existing, due to what John explained....

This is indeed a real psychoacoustical effect and cordura is right about it being the way MaxxBass works, but it has nothing to do with room size and bass loading.

Hope this helps clear things up.

barefoot

 

[John Sayers]

Ahh - thanks for that barefoot - I knew you'd come good on this one.

It also makes a lot of sense from my experience of trying to mix in small rooms.

cheers
John

 

[RICK FITZPATRICK]

Thanks gents. Your generosity is appreciated. I wish I had a way to return it. I'm going to shoot for the largest room I can afford to build. Hopefully one that doesn't sound like s__t. Although, I think with the knowledge you provide here, I might even end up with one that sounds half way decent. And half way is better than I've ever had before. So as john says, cheers!
fitz

 

[cordura21]

Question about the Allison Effect

great info about the Allison Effect.

On question about it: dos it also affect other sound sources or just speakers or similars? For example, playing an acoustic bass in a small room can have an accurate bass response? Thanks, Andrés

EDIT:
the purpose of this question is to know if small rooms, while not good for monitoring bass frequencies, could be good for recording them. My guess is that if that for a bass drum the Allison Effect will affect the sound, since it's a device similar to a speaker. But what about the body of an acoustic bass, or one of those bassy saxophones or a tuba?

 

[barefoot]

corduroy,

A bass source is a bass source, doesn't matter if it's a speaker, a tuba, or your flatulent mother-in-law.

So no, small rooms are not good for accurately recording any bass source. Of course, you can record any sound you want, if you happen to like the effect

Oh, and I forgot to mention before that the normal standing wave problems most everyone is already familiar with are also still convoluted into the whole room response issue.

barefoot

 

[cordura21]

thanks a lot for the knowledge. Cheers, Andrés