Pensados Place #21 - Bob Hodas

[JoRillo]

A little bit of what I will say is going to be paraphrased - I'm watching and asking questions as they come.

18:15

"For 20 cycles to develop you need 30 feet."
"100Hz takes eleven feet to develop."

Does this include reflecting off a wall? For example 5 feet this way 6 feet that way = 11 feet?

21 Minutes

He talks about moving the foam/absorption off the wall so it can do more damage (my words not his). Is there any kind of rule of thumb for this? or just the farther off it can be the better?

25 Minutes

Q "Where do you like the speakers to sit (height wise)?" A "Depends on the phase measurement of the speaker"

Do I have to call Yamaha to get the answer on my HS80's phase measurement? or does anyone know where I find those online?

34 Minutes

Under 200 Hz is Omni directional - I knew this prior - but what makes it Omni at that point - better yet what makes it go from just plain directional to Omni at all?

Thanks in advance!

[JoRillo]

Were they such bad questions?

[Glenn Kuras]

He talks about moving the foam/absorption off the wall so it can do more damage (my words not his). Is there any kind of rule of thumb for this? or just the farther off it can be the better?

The further the panel is off the wall the lower it will absorb to a point, but keep in mind that if you are trying to deal with only, say 60hz coming off a wall you are better off to use something tuned to deal with it.

[John White]

Originally Posted by JoRillo

Were they such bad questions?

This has been posted multiple times on this forum. It seems to get longer and more boring each time.

[RJHollins]

Q "Where do you like the speakers to sit (height wise)?" A "Depends on the phase measurement of the speaker"

Do I have to call Yamaha to get the answer on my HS80's phase measurement? or does anyone know where do find those online (I haven't goggled yet, will do that soon as I finish watching - just wanted to type it in and not forget)

did you get an answer ???

[JoRillo]

Originally Posted by RJHollins

did you get an answer ???

Nop

[JoRillo]

Only 1/4 answers in two days? Come on guys... the video was just so you had a reference - and for those of us who have never seen it. No one else wants to help out with an answer or two?

[latestflavor]

Originally Posted by JoRillo

A little bit of what I will say is going to be paraphrased - I'm watching and asking questions as they come.

18:15

"For 20 cycles to develop you need 30 feet."
"100Hz takes eleven feet to develop."

Does this include reflecting off a wall? For example 5 feet this way 6 feet that way = 11 feet?

21 Minutes

He talks about moving the foam/absorption off the wall so it can do more damage (my words not his). Is there any kind of rule of thumb for this? or just the farther off it can be the better?

25 Minutes

Q "Where do you like the speakers to sit (height wise)?" A "Depends on the phase measurement of the speaker"

Do I have to call Yamaha to get the answer on my HS80's phase measurement? or does anyone know where I find those online?

34 Minutes

Under 200 Hz is Omni directional - I knew this prior - but what makes it Omni at that point - better yet what makes it go from just plain directional to Omni at all?

Thanks in advance!

the 20 hz 30ft principle is just pure sine tone, as it would be in an open field. it has nothing to do with reflections. it helps visualize concepts for us mortals.

rule of thumb on pourus absorption is 1/4 wavelengh away from wall is best (depending on your target absorption freq) - they work elsewhere less efficiently and as always cost and efficiency are a factor (if they were free who would care about efficiency)

height wise - im not looking at vid right now but is he talking about orientation? if so, vertical insures least phasing issues. if you never move an in and out of your mix position (mix with mouse, plugins - dont move around a desk or use outboard) it matters less. when you move horizontally the distance between tweeter, woofer, and ear change and create phase problems. i am an extreme sceptic to using speakers horizontally. if he is not referring to this, my apologies.

bass below 200hz doesnt automatically "become" omni. basically, starting there it becomes more directional (for example, at 300hz its still pretty omni) and of course increasing in directionality all the way up to your hearing limit, where it us highly directional. so it is a gradual increase.

[JoRillo]

Thanks man.

Originally Posted by latestflavor

height wise - im not looking at vid right now but is he talking about orientation? if so, vertical insures least phasing issues. if you never move an in and out of your mix position (mix with mouse, plugins - dont move around a desk or use outboard) it matters less. when you move horizontally the distance between tweeter, woofer, and ear change and create phase problems. i am an extreme sceptic to using speakers horizontally. if he is not referring to this, my apologies.

He was referring to the actual height of your monitors in their correct position (whether that is vertical or horizontal) and how high you should make your stands. I am about to begin construction on my second set of stands since I just got new speakers. Just trying to make sure I do it right the first time.

bass below 200hz doesnt automatically "become" omni. basically, starting there it becomes more directional (for example, at 300hz its still pretty omni) and of course increasing in directionality all the way up to your hearing limit, where it us highly directional. so it is a gradual increase.

My question is more of what makes a low frequency (larger wave) begin to leave the speaker in all directions. I know that it is a gradual thing that happens, but this is more of a why I guess. Like why is it that a high frequency wave isn't going to disperse in every direction but a low frequency wave will.

[BriHar]

I don't know if one can explain it without getting to mathematical. Simple experimentation shows that you can much more readily pinpoint the source of a higher pitched tone than a low pitched one (this is the reason the positioning of a sub is not so critical generally). Don't know if this really helps but if you take a long flexible rod and shake it vigorously at one end, the travel at the far end is the same or even less than the travel your hand movement makes. If you now shake it much slower, the travel at the end is quite a bit more than the travel of your hand. It's the nature of frequency that the higher becomes more directional. Light under normal circumstances doesn't "bend" around corners, LW communications are very omni as opposed to SW and esp. MW.

[JoRillo]

Originally Posted by BriHar

I don't know if one can explain it without getting to mathematical. Simple experimentation shows that you can much more readily pinpoint the source of a higher pitched tone than a low pitched one (this is the reason the positioning of a sub is not so critical generally). Don't know if this really helps but if you take a long flexible rod and shake it vigorously at one end, the travel at the far end is the same or even less than the travel your hand movement makes. If you now shake it much slower, the travel at the end is quite a bit more than the travel of your hand. It's the nature of frequency that the higher becomes more directional. Light under normal circumstances doesn't "bend" around corners, LW communications are very omni as opposed to SW and esp. MW.

I think I understand what you mean. Thanks for the responses everyone.

[latestflavor]

Originally Posted by JoRillo

Thanks man.



He was referring to the actual height of your monitors in their correct position (whether that is vertical or horizontal) and how high you should make your stands. I am about to begin construction on my second set of stands since I just got new speakers. Just trying to make sure I do it right the first time.



My question is more of what makes a low frequency (larger wave) begin to leave the speaker in all directions. I know that it is a gradual thing that happens, but this is more of a why I guess. Like why is it that a high frequency wave isn't going to disperse in every direction but a low frequency wave will.

ok, i see. the actual height of your speaker stands should be done through measurement - the prediction models arent relaible enough and there are too many other factors for such an important detail.

[JoRillo]

Originally Posted by latestflavor

ok, i see. the actual height of your speaker stands should be done through measurement - the prediction models arent relaible enough and there are too many other factors for such an important detail.

I think you are miss understanding me.

I will definitely be measuring the stands before building them, but in the video he said you should find out the phase measurement of the speaker. This will vary speaker to speaker, so I was wondering if anyone knew what/where the measurement on my Yamaha HS80s would be/would be found. I always was told to have the tweeters pointing about an inch over my ears at listening position, but when I heard him say that I questioned my previous statement.

[Ethan Winer]

Originally Posted by JoRillo

"For 20 cycles to develop you need 30 feet."
"100Hz takes eleven feet to develop."

That's simply not true. If it were, headphones wouldn't work, and you'd never hear anything below 400 Hz in your car.

--Ethan

The Acoustic Treatment Experts

[JoRillo]

Originally Posted by Ethan Winer

That's simply not true. If it were, headphones wouldn't work, and you'd never hear anything below 400 Hz in your car.

--Ethan

The Acoustic Treatment Experts

Hmm, that's a good point. Couldn't that include reflections then? In your car for example - I don't know quite how low a car sub can/could go - but could it be possible that what you are hearing is that wave develop after multiple reflections? Or that you are not truly hearing (for example) anything under 100 hz, only the people that are a distance away from you can hear the fully developed wave?

[johndykstra]

It's referring to modal reinforcement. Of course you can hear 20hz in a confined space, it's just that a smaller space doesn't boost frequencies below it's dimensions. And no, reflections do not count in the travel.

[John White]

Originally Posted by johndykstra

...it's just that a smaller space doesn't boost frequencies below it's dimensions...

John would you mind elaborating on that. I'm not able to visualize what you mean. Thanks-

[Ethan Winer]

Originally Posted by JoRillo

could it be possible that what you are hearing is that wave develop after multiple reflections?

The term you're looking for is called pressure mode. This means the waves are too long to fit into the enclosed space, so instead they just increase (and decrease) the ambient atmospheric pressure. But they're still waves, and they still impinge on your ear drums to create sound. As John explained, all that really happens when a room is too small is you don't get a ringing peak. Which IMO is a Good Thing.

--Ethan

[Rod Gervais]

Originally Posted by John White

John would you mind elaborating on that. I'm not able to visualize what you mean. Thanks-

John,

Picture it like this - let's suppose we were to suspend you 100' in the air (we'll use Sky Hooks just so nothing can interfere with the sound) and we set you 3' from a speaker.

What frequencies would you imagine you could not hear coming from the speaker?

There are no reflections - and not enough room (between you and the speaker) for anything below (roughly) 369 Hz to fully develop before reaching you............ yet you would hear every frequency below 369 Hz that the speaker was capable of transmitting.

this is because you will have the physical experience of the entire wave passing you regardless of your distance from the speaker.

This doesn't change simply because you're in a room smaller than the wavelength........

In either case (the room or hanging off that sky hook) you're stationary - the speaker is stationary - you are positioned in such a manner that certain sound waves never have a chance to fully develop in the space between you and the speaker - the only thing moving is the wave.......

Now - within enclosed spaces the room will introduce anomalies that affect what it is you hear simply because a resonance with in the space occurs before your brain has a chance to differentiate between the direct and reflected sounds. Thus a null is a null (to your ear) even though the original wave, in it's entirety, might have passed you before the null took place. However - even waves with lengths longer than room dimensions will still be heard within the space.

This is a good thing - otherwise we'd have to build some pretty huge rooms just to be able to hear transmissions at even 30 Hz (roughly 37')

Rod

[kasmira]

Originally Posted by Ethan Winer

That's simply not true. If it were, headphones wouldn't work, and you'd never hear anything below 400 Hz in your car.

--Ethan

The Acoustic Treatment Experts

Well, its true in the obvious sense that it is as long as the wavelength. I suppose its a matter of what you consider the term "develop" to mean; it doesn't mean you need 30ft of space to hear it. I also don't know the context it is talked about in the video though.

[boggy]

Originally Posted by johndykstra

It's referring to modal reinforcement. Of course you can hear 20hz in a confined space, it's just that a smaller space doesn't boost frequencies below it's dimensions. And no, reflections do not count in the travel.

I can't agree. In small rooms it is easier to literally boost low frequencies, because phenomenon called "room gain", an inherent acoustical low pass shelving filter which start below lowest room mode.

Also if small space can't "transmit" low frequencies, closed headphones never will play anything below 2-5kHz.

Actually, we obtain a pretty flat response from 4Hz to 25kHz in one room with dimensions 3.5mx3.5mx2.6m (dimensions are before treatment), at listening position. EDIT: (it is essential that we used sealed box design for woofer)

[johndykstra]

Originally Posted by boggy

I can't agree. In small rooms it is easier to literally boost low frequencies, because phenomenon called "room gain", an inherent acoustical low pass shelving filter which start below lowest room mode.

This is something I've never experienced, or even heard of. Links to studies?

Originally Posted by boggy

Also if small space can't "transmit" low frequencies, closed headphones never will play anything below 2-5kHz.

Right. This is what I was saying is it not?

[John White]

Hey Rod, What's up buddy.

Originally Posted by Rod Gervais

...this is because you will have the physical experience of the entire wave passing you regardless of your distance from the speaker.

Yes, yes and yes. Understood and great analogy.

Originally Posted by Rod Gervais

Thus a null is a null (to your ear) even though the original wave, in it's entirety, might have passed you before the null took place...

Very interesting and good fodder for DanDan's SLF....BIR study.
Still, I'm a bit confused as to why he wrote "doesn't boost frequencies below it's dimensions". Is that to say that although "a null is a null (to your ear)", a wave that is not fully developed can't form a peak? Surely that's not what was intended.

[John White]

You guys beat me to it. Now, though, I'm really confused.

Edit:
What do you mean by a small space can't "transmit" low frequencies?

[latestflavor]

Originally Posted by JoRillo

I think you are miss understanding me.

I will definitely be measuring the stands before building them, but in the video he said you should find out the phase measurement of the speaker. This will vary speaker to speaker, so I was wondering if anyone knew what/where the measurement on my Yamaha HS80s would be/would be found. I always was told to have the tweeters pointing about an inch over my ears at listening position, but when I heard him say that I questioned my previous statement.

Ah, right sorry. Its been a while since i saw the vid. yes, i was wondering this as well because why would it be different than the yamaha manual that says aim tweeter directly at ear.

[boggy]

Originally Posted by johndykstra

This is something I've never experienced, or even heard of. Links to studies?

This is a well known to car audio designers. I can't remember papers which analyze this in detail, possibly because very small number of acousticians analyze small rooms (in detail)... and it is really not that difficult to have a (bigger) room where room gain starts from 20-30Hz (lowest room mode), so it may become negligible...
This can be (carefully) measured... btw.
I learned it through the years, from loudspeaker designers...
If i find something more relevant than mere web forums discussions, I will post it here.
This phenomenon is mainly measured, and may vary from room to room, no precise modeling exist, for both slope and gain of this LP shelving filter (AFAIK).

Originally Posted by johndykstra

Right. This is what I was saying is it not?

I understand that you say that small volume space can't transmit low frequencies, and closed headphones are example of (extremely) small volume (space) which can transmit all frequencies down to 20Hz, to our ears, without help of any bigger room.

[Jens Eklund]

Yes, “Room Gain” below the first modal frequency is possible but as said earlier, you need a pretty small room (with very ridged boundaries) and a speaker capable of producing very low frequencies for this to become an issue. The effect can be simulated in the program SoundEasy if anyone has access to it.

[JoRillo]

Originally Posted by latestflavor

Ah, right sorry. Its been a while since i saw the vid. yes, i was wondering this as well because why would it be different than the yamaha manual that says aim tweeter directly at ear.

I could very well just need to point the tweeters at my ears, I havn't read the manual tbh, that's just the rule of thumb for speaker placement from what I have read around the forums. Now I am interested in just know what the hell he was talking about.

[johndykstra]

Originally Posted by John White

Still, I'm a bit confused as to why he wrote "doesn't boost frequencies below it's dimensions". Is that to say that although "a null is a null (to your ear)", a wave that is not fully developed can't form a peak? Surely that's not what was intended.

It's always been my understanding that while a room of say a 10' depth has the ability, as a room of any size, to reproduce 20hz, it's dimensions are not large enough to reinforce that frequency with resonance. The loudest this frequency can be heard is that of the intensity emitted from the speaker.... I.e. no ringing or modal peaks reinforced due to the room's size.

This room gain idea is new to me.

[BriHar]

So if a smaller room eliminates lower frequency reinforcement, helping eliminate modal peaks and ringing, why are small rooms considered bad for mixing in esp. in the LF spectrum. I realize we are focusing here on a single principle and it is probably some other that is the root of the problem. This is a grey area. I always thought that because the (small) room cannot support the lower frequencies, this is why they (small rooms) are a problem, but what is said here seems to show that this is good. I'm confused...