Myth or Fiction. Sub waves and distance?

[Rolyasm]

On a couple of occassions and from people supposedly in the know, I have heard some statements and would like opinions/facts as to whether or not they are true.
1)In small rooms a person will not hear the lowest (20-@35Hz) frequencies because the sound wave is too long or hasn't had a chance to complete a full cycle. Also, sitting too close to a subwoofer in a larger room will produce the same effect. What happens to SPL also in this situation with distance.

2) Smaller drivers give you tighter bass. As you move to larger drivers it is too difficult to move the driver back and forth quickly, creating "less tight" or muddy bass.

3) Ported subs are better for HT, while non-ported, sealed designs are better for music.

If anyone wants to expound on these and use as many facts as possible, that would be great. Feel free to add more info, but please, no one-worded answers. Let's have some factual backup with the answer if possible.
Roly

[ThomasW]

1) FALSE, headphones reproduce 20Hz
2) FALSE, urban myth exploited by marketing people.
3) is this really important now?

Sorry but the internet isn't a place to get an education. Instead it's a place to exchange bits of info. If you really want to understand this stuff, start with some books on acoustics and loudspeaker design.

I recommend, F. Alton Everest, "The Master Handbook Of Acoustics"

[Rolyasm]

Sorry but the internet isn't a place to get an education.

http://www.ooen.net/

http://www.elearners.com/?tsource=it&tid=68143

Sorry, but I had to post these. You know that statement isn't true. Now Thomas, I really do appreciate all of your help, but you don't have to reply to my stupid posts if you don't want to. I posted this specifically under Starting Block. Look at other posts: Do amplifiers sound different? Rear Speaker Distance? Receivers Vs. Seperates? All of these people are looking for an education and opinion. I realize this may be a bad day for you, and you may be particularily annoyed by me, but I don't think I need to be chastised for asking questions.

I am reading Sound System Engineering by Don and Carolyn Davis, but it isn't a very easy read. And, I know these seem like really stupid questions to you, but when I talk to a professional, national speaker builder who has a difference of opinion than you, I get curious about how these so called myths get started, or how much is your opinion and how much is their opinion. Really, on a conversation with a speaker engineer, I was told this. No I can take your word, or his, but I chose to ask some questions. I am really a pleasant fellow although not well educated about electronics and such. Not sure what questions are appropriate and what are not. Ya, sometimes I get on here and ask some dumb questions, but some are not so dumb. I enjoy the threads and enjoy reading the responses. Not sure how to make you happy, but I will try to read more and perhaps ask very specific, higher-level questions. Sound good?
Roly

[ThomasW]

It's called physics, not opinion....

1) One doesn't need to reproduce the entire wavelength to hear the note. Thus one can 'hear' low frequencies in a room smaller than the length of the wave.
(Note, this is why you can measure subsonic frequencies from your sub in your room)

2) primary importance isn't the diameter of the speaker, it's the ability of the speaker to 'accelerate/decelerate'. Given proper motor design and enough power, one can accelerate a manhole cover at the same rate as a 8" woofer.

Ciao'
Thomas

[chrispy35]

Rooms can have nulls where certain frequencies are attenuated due to cancellation of sound waves.

Given two rectangular rooms of the same length:width ratio, the smaller room will have a higher frequency cancelled than the larger room due to the difference in room size. Perhaps your friend may have been trying to indicate is that the smaller room is more likely to exhibit an audible null at certain locations in the room when compared to a larger room. Some speakers may be unable to produce any usable output at the frequency of cancellation for a larger room.

Chris P.

[Sim reality]

I was about to answer the questions:

1) I agree with chrispy, in small rooms the reflected waves are almost the same amplitude as the incident wave so it does produce a lot of "null" nodes in the room. The problem is not that the room need to support a full wavelength (the wavelengths we are talking about are only 6-12 inches anyways, I assume you have room bigger then that).

2) "tight" bass is defined by the ability of the magnet to control teh driver. Smaller drivers are easier (cheaper) to control at low SPL but as the volume goes up the smaller driver needs greater excusion which makes the driver a lot more difficult to control. If you are looking at the exact same price the smaller sub will be "tighter" but this has more to do with cost and quality then the actual size.

3) Once again we are back to control of the driver... Seal boxed make it easier to control at lower SPL and ported allows for more movement at any particular pwoer. However Velodyne uses a servo-assisted feedback mechanism that some would argue produces just a controlled (if not more so) driver movement as a sealed sub.

Without indicating specific subs it's really difficult to make general statements. Best to listen critically to them and then choose the sub you like the most.

[dyazdani]

1) I agree with chrispy, in small rooms the reflected waves are almost the same amplitude as the incident wave so it does produce a lot of "null" nodes in the room. The problem is not that the room need to support a full wavelength (the wavelengths we are talking about are only 6-12 inches anyways, I assume you have room bigger then that).

A 20Hz wave is about 55ft...I'm not sure what frequencies you are referring to.

6-12 inches corresponds to wavelengths of 1.1-2.2kHz

So basically the common freq range of 20Hz-20kHz is 55ft to a small fraction of an inch.

[Rolyasm]

How about an example: Everyone has heard the big car subs. Who is the bass louder to, the person in the car or the person 30 feet away, or 55 feet away? Any difference?
Roly

[Kal Rubinson]

How about an example: Everyone has heard the big car subs. Who is the bass louder to, the person in the car or the person 30 feet away, or 55 feet away? Any difference?
Roly

The person who doesn't want to hear it. :M

Kal

[Sim reality]

A 20Hz wave is about 55ft...I'm not sure what frequencies you are referring to.

6-12 inches corresponds to wavelengths of 1.1-2.2kHz

So basically the common freq range of 20Hz-20kHz is 55ft to a small fraction of an inch.

Sorry... I didn't bother to verify the wavelength... Not that it matters anyways...

I must have been thinking about problems with duel subs... ops:

[dyazdani]

The person who doesn't want to hear it. :M

Kal

That's a good one :rofl:

The intensity decreases with the square of the distance, so it's going to be the person in the car, unless it is a stretch limo, then there could be an exception :

[Boombox]

It's called physics, not opinion....

Really? Not to be mean or anything, but I don't understand what physics you are talking about....

1) One doesn't need to reproduce the entire wavelength to hear the note. Thus one can 'hear' low frequencies in a room smaller than the length of the wave.
(Note, this is why you can measure subsonic frequencies from your sub in your room)

Well according to the physics I know, you can only produce notes (discrete frequencies) if the wave does produce an integral number of nodes; from the first fundemental up to x number of nodes. These are notes. The rest is junk or call it noise which in a musical context will give the character of the instrument producing the notes or harmonics. This is what I understand Rolyasm questioned in point 1 of his post. I say, most certainly. You will not hear the 20Hz, but you'll hear a low frequency noise. If the wave is allowed to complete its 18m cycle, the full information of the 20Hz wave is represented, which includes the amplitude.

http://en.wikipedia.org/wiki/Harmonic_series_%28music%29

With point 2, its a trade off between the efficiency of the driver to move sufficient air and the power available. Smaller drivers are less efficient to move large amounts of air requiring more power, therefore most manufacturers use larger drivers. In general, yes, smaller drivers will give you "tighter bass". It is in fact not too difficult to move larger drivers back and forth, it just cost more...

[Kal Rubinson]

You will not hear the 20Hz, but you'll hear a low frequency noise. If the wave is allowed to complete its 18m cycle, the full information of the 20Hz wave is represented, which includes the amplitude.

Is that why headphones and car stereos don't work? The ear response to varying pressure changes at its external opening and cannot sense what is happening elsewhere in the room except by inference mechanisms. So, if the pressure varies at a rate of 20Hz (and at sufficient level), the ear will transduce it.

Kal

[Jack Gilvey]

http://www.adireaudio.com/Files/TechPapers/WooferSpeed.pdf

[dyazdani]

Well according to the physics I know, you can only produce notes (discrete frequencies) if the wave does produce an integral number of nodes; from the first fundemental up to x number of nodes. These are notes. The rest is junk or call it noise which in a musical context will give the character of the instrument producing the notes or harmonics. This is what I understand Rolyasm questioned in point 1 of his post. I say, most certainly. You will not hear the 20Hz, but you'll hear a low frequency noise. If the wave is allowed to complete its 18m cycle, the full information of the 20Hz wave is represented, which includes the amplitude.

I think you are confusing a couple of wave concepts, maybe? The nodes you are mentioning as "notes" (fundamental, harmonics, etc) are usually described in transverse waves. They can be created by such things as vibrating strings, pipes, etc. e.g. musical instruments. The wave propogates along the string, but the medium (air) moves perpendicular to the string. Nodes in rooms are a little different I think, though they are still created by combining incident and reflected waves.

Sound waves are longitudinal, meaning the wave moves in the same direction as the air molecules. Kal hit the nail on the head when he mentioned the pressure variation.

If everything other than the pure note is "junk," then why wouldn't you listen to music in an anechoic chamber? Am I missing something? Maybe I misunderstood your point...

[Chris D]

Hmmm... instinctively, I don't think that's accurate. I think ThomasW's statement is correct, that you do not have to produce an entire wavelength of a frequency in order for the sound to exist. Otherwise the frequencies audible in any room would be constrained by the room size. In my 12 x 22' room, I can create frequencies down below 12 hz. But the best demonstration of what we're talking about are my Etymotic earphones, inserted in my ear. The "room" becomes only maybe 1" long, but I get a full range produced in my ear.

[Kal Rubinson]

Hmmm... instinctively, I don't think that's accurate. I think ThomasW's statement is correct, that you do not have to produce an entire wavelength of a frequency in order for the sound to exist. Otherwise the frequencies audible in any room would be constrained by the room size. In my 12 x 22' room, I can create frequencies down below 12 hz. But the best demonstration of what we're talking about are my Etymotic earphones, inserted in my ear. The "room" becomes only maybe 1" long, but I get a full range produced in my ear.

Yeah. What similar to what I said above but I left off the smiley.

Kal

[ThomasW]

Really? Not to be mean or anything, but I don't understand what physics you are talking about...

Something relatively simple and straight forward where a frequency generator is used to create a 20Hz sine wave, and the readout on a frequency counter fed by the microphone placed 1" from a loudspeaker dustcap shows 20 Hz.

[chrispy35]

Hmmm... instinctively, I don't think that's accurate. I think ThomasW's statement is correct, that you do not have to produce an entire wavelength of a frequency in order for the sound to exist. Otherwise the frequencies audible in any room would be constrained by the room size. In my 12 x 22' room, I can create frequencies down below 12 hz. But the best demonstration of what we're talking about are my Etymotic earphones, inserted in my ear. The "room" becomes only maybe 1" long, but I get a full range produced in my ear.

Frequencies won't be constrained by roomsize but the frequencies at which cancellation occurs will be. For any room, frequency response for the bass frequencies is not going to be flat everywhere in the room due to standing waves within the room. With headphones, you're essentially at the very back wall of the 'room' which I don't think suffers from cancellation (correct me if I'm wrong). You need to be at some point in between the front and back walls to get a standing wave at a certain frequency determined by room dimensions.

We're hearing changes in pressure, it just so happens that those changes in pressure travel at a finite speed from the source (hence the wave) and thus are suseptible to amplitude enhancement/cancellation if the conditions are right. With headphones, the conditions just aren't right.

Chris P.

[AptosJeff]

Folks, there are many misleading statement being made in this thread, along with the useful information. It would take a while to sort through it all, even though there are members here who have the knowledge to do it. I suggest if you really want to know this stuff, pickup a book on the fundamentals of acoustics, and scan through the sections on resonators and architectural acoustics. Or find it on-line, but try to get solid engineering sources. Some suggestions:

1. Music, Physics & Engineering by H. Olson - easy reading, but dated on audio subjects.
2. Acoustical Designing in Architecture by Knudsen & Harris - Harris has designed some of the best concert halls in the country, imo.
3. Fundamentals of Acoustics by Kinsler & Frey - the classic introductory engineering text.

On the other hand, if you just want quick answers, ask a specific question and the experts here can respond without writing whole chapters on acoustics.

[dyazdani]

There are also some good resources online. Simply Google for what you want. Many physics sites have material pertaining to wave propagation, sound, etc.

[soho54]

1)In small rooms a person will not hear the lowest (20-@35Hz) frequencies because the sound wave is too long or hasn't had a chance to complete a full cycle.

False. The wave doesn't magically terminate midway though it's cycle when it reaches a wall, it keeps on coming. In a small room lower freq are boosted if anything.

Also, sitting too close to a subwoofer in a larger room will produce the same effect.

False. The pressure wave keeps on coming. Ths SPL will also be increased due to the closer proximity to the driver.

What happens to SPL also in this situation with distance.

SPL decreases the further away you are from the source. Same as always, nothing changes.

Also room modes have nothing to do with sounds ability to propagate at <20hz in a small room. 18hz down 10db in your seating position move over a foot or two. Now 12hz has 3db gain but you've got 18hz back to normal. The levels may change due to in-room position, but the <20hz waves are always present.

2) Smaller drivers give you tighter bass. As you move to larger drivers it is too difficult to move the driver back and forth quickly, creating "less tight" or muddy bass.

False. See Sim reality's post#6 answer 2, and ThomasW's post #4, part 2. They hit the nail on the head. No need to re-type it.

3) Ported subs are better for HT, while non-ported, sealed designs are better for music.

This is a broad generalization at best. The best application for each situation will always be different, and based on equipment, environment, and personal preference.

How about an example: Everyone has heard the big car subs. Who is the bass louder to, the person in the car or the person 30 feet away, or 55 feet away? Any difference?

From the loudest down - in car, 30ft, then 55ft. That is why they measure as close as possible to the drivers in SPL competitions and not 30ft off. EDIT: They are actually measured from the front passengers seat area. I haven't gone to one of those in 13 years.

Visualize sound as waves in a pool produced by throwing a rock in the middle. The waves slowly dissipate with distance traveled. With sound this is predicted with the inverse square law of sound.

[Rolyasm]

It makes sense. And I won't ever tell you what company's engineer told me this. Kind of scary though.
Roly

[soho54]

Kind of scary though.

That's the way it is with everything. That is what's truly scary. 8O

A doctor who skims by with a D+ is still a doctor. Just because a person works in a particular field or took a few tests years ago doesn't mean they know what they're doing, or that they are any good at it. :B

[dyazdani]

A doctor who skims by with a D+ is still a doctor. Just because a person works in a particular field or took a few tests years ago doesn't mean they know what they're doing, or that they are any good at it. :B

That's absolutely true.

[Rolyasm]

You know what they call a person who can't make it through MD school?

[Chris D]

"janitor"?

[dyazdani]

I was thinking "cadaver"

[Rolyasm]

A dentist. :rofl:

[David Meek]

Lawyer? :

[Clive]

Okay how about a COP and that would be me!!! On that note I believe the subject has gone off track and exhausted. Thanks for all your inputs :T