Low Frequency Wave Propagation

[ToneJockey]

I have what I think is a basic question for some of the more knowledgable members here. I was in a discussion with a peer when the subject of low frequency wave (bass tones) came up. From what I thought I knew on the subject is that

1. Bass frequencies (say under 40 hz) will not develop (properly)in small rooms because of the length of the wave.

2. If they do develop (under the proper conditions) they would travel farther than higher frequencies.

My questions are

Will smaller speakers say the JBLs LSR4326P or any near field monitors be able to reproduce these frequencies.? My friend has a set of small speakers and said that he could not hear the bass very well but when he stepped out of the room into a room farther away, the bass made itself known.

He is under the impression that high frequencies travel farther than low. I told him that I believe the lower ones travel farther because of their increased length which causes them to be reflected less. Is this a correct view?

Thanks for helping with our education in the physics. When I entered this phrase into google I got every kind of medium except air!

[JBL 4645]

This is true the wave length travels further. My room is only 14 feet x 10 feet x 8 feet and if I where to open the door from the room leading to the kitchen the lower tone even at 32Hz (sine wave) is enough to buzz the drums! The window in the kitchen rattles and buzzes. Now than there is a second door that is directly inline with the door leading to the main room now I open the door to the bathroom and the tone gets larger and the window sounds like its going to drop out of the frame any moment soon.

I’ve played other sine wave tones even at 18Hz and the slow buzzing on the window LOL, I don’t care what my neighbour thinks above she probably thinks it’s a washing machine.

The sub I use is JBL 4645.

I think the highest like 20 KHz has a very small wave length of no more than 1”?

That’s like when you play the tones going upwards from 1Khz to 20Khz its changes in pitch amplitude, correct me if I’m wrong, and starts to get fainter and fainter as it nears 10Khz.

This why most users damage there HF drivers or tweeters by excessive EQ on the high frequencies BAD! Or they have poor hearing ether way I’ve seen smoke coming from a friends Realistic Hi-Fi loudspeakers around 1986 they had a12” bass driver a mid range and high range. It was damn funny and not as expensive personally I thought he was nuts showing off like that.

A week later when he replaced the burnt out tweeters and he blew them again, LOL, what a moron!

http://govguru.com/random/default.aspx

http://www.acousticalsurfaces.com/ac..._IOI/101_2.htm

[Ian Mackenzie]

I have what I think is a basic question for some of the more knowledgable members here. I was in a discussion with a peer when the subject of low frequency wave (bass tones) came up. From what I thought I knew on the subject is that

1. Bass frequencies (say under 40 hz) will not develop (properly)in small rooms because of the length of the wave.

2. If they do develop (under the proper conditions) they would travel farther than higher frequencies.

My questions are

Will smaller speakers say the JBLs LSR4326P or any near field monitors be able to reproduce these frequencies.? My friend has a set of small speakers and said that he could not hear the bass very well but when he stepped out of the room into a room farther away, the bass made itself known.

He is under the impression that high frequencies travel farther than low. I told him that I believe the lower ones travel farther because of their increased length which causes them to be reflected less. Is this a correct view?

Thanks for helping with our education in the physics. When I entered this phrase into google I got every kind of medium except air!

A small system if it does extend to the desired LF cut off frequency will be so inefficient that its output power would have little useable benefit.

The small system with a higher LF cut off may have a subjectively superior dynamic performance.

It depends on the room boundaries and the construction of the walls etc. HF absorption and sound barrier is more easy to obtain than LF.

If you are seated at a room boundary and the loudspeaker is also located at a boundary you may hear LF signals because of what we call room gain.

Studies have proven than if the listener is directly sitting on or over the sub woofer the effect of detecting very low bass is possible (global).

There are some useful Harman papers on bass in listening rooms (over on the corporate Harman www page) that you might find interesting.

I would not loose sleep over attempting to produce bass below 30 hz in the home. A better appoach is to use a dedicated home theatre sub. A true VLF sub is big not a cheap item.

[JBL 4645]

Ian

It’s puzzling that sometimes I can get a nice low end from the JBL Control 5 and they do have restraints on the frequency dynamic range output but surprised never the least.

Tones have ranged down to around 50Hz to 40Hz on some films, but I know where it’s going start chuffing at the port due to its technical tolerances.

[Steve Schell]

The speed of sound in air will vary a little depending on barometric pressure, temperature and humidity. Since sound waves propagate through molecular collisions, when those molecules are closer together the collisions occur more frequently and the soundwaves travel faster. I have generally used 1,130 feet per second as a useful average in calculating wavelengths.

Dividing 1,130 by the frequency of interest gives us the wavelength. For 100Hz. the wavelength is 11.3 feet; at 20Hz. the wavelength is 56.5 feet. We can see that in the bottom couple of octaves, our room dimensions become comparable or small in relation to the wavelengths involved.

When sound energy reaches a room boundary, it either passes through it (through boundary movement), is absorbed by it (through friction), is reflected by it, or most likely a combination of all three. The extent of each is determined by the boundary's resistance to being moved (i.e. its impedance). Brick or concrete block construction is high impedance and reflective of low frequency wavelengths. Drywall or lath and plaster construction (like my poverty cabin) is lower impedance and lets much of the low frequency energy escape. I think this can be a good thing, as long wavelengths folding back on themselves in small rooms results in much chaos- cancellations and strong standing waves that result in very unequal distribution of sound pressure. This is one of the vexing problems of listening to our hi fis indoors. Curiously, if the room is small enough (like a bedroom) the bass can sometimes be more satisfying than in a larger room, as the volume of air in the room is modulated by the speaker, as in a car stereo with big woofers.

Maintaining flat power response requires a quadrupling of air movement with each descending octave; low frequencies have a long duty cycle. Since they contain much more energy than high frequencies of equal loudness, they travel farther in air before being attenuated. This is one reason the neighbors complain about the boom boom not the tweet tweet.

I tune pianos at a large church equipped with a powerful pipe organ. Many times I have been sitting in my car in the parking lot and felt the bass pipe fundamentals shake me and the car, more than 100 feet from the church.

Very few speaker systems maintain flat power response down to the 20Hz. range. My big horn subwoofer comes close, and the energy seems equally strong in most parts of the house as well as outside in the yard. It seems to me that in the bottom octave the sub is essentially propagating into half space, the ground being the only effective boundary it sees.

[rs237]

Hello Steve
you write from your big horn subwoofer. Details would interest me over it.

regards
juergen

[ToneJockey]

Thanks guys,

As I thought you have pulled the curtain back a bit on these factors.
This is a complex subject with so many variables like room size, boundary
construction and frequency response.

I think the sub is a good thing to have if you really want to help in this area but as you have said, there are certain caveats that will have to be considered.

[Andyoz]

Just following on from Steve's points...

Basically, what you are talking about is the "pressure zone" that occurs below the frequency of the lowest room mode (also called "room cut-off frequency"). Within the pressure zone, travelling waves cannot exist and the pressure in room is "pumped" up and down in sympathy with the loudspeaker diaphragms.

Whether or not you can actually hear sound once you are in the pressure zone is highly dependant on a few factors. The damping of the room modes is very important but also things like the type of bass loading of the speaker (sealed or bass-reflex) and the boundary surface constructions.

If the room modes are well damped, then the audible transition into the pressure zone may not be too abrupt. Conversely, if you have strong room modes, levels could be 5-10dB down once you are inthe pressure zone. This is another area where soffit mounting helps as it inherenty increases LF output. So, in a heavily damped room with soffit mounted speakers, audible bass within the pressure zone can be very linear as there are no room mode effects to cock up the response.

[Steve Schell]

Juergen, I have designed two front loaded horn subwoofers to date, and they are similar in most respects. A single 15" driver is loaded by a 2:1 compression ratio and a W folded exponential horn of about 20' length and 15Hz. flare constant.

The earlier one (pictured here) exhausts out the side and is intended to be aimed at a room corner. When this is done the room expansion out of the corner actually approximates the 15Hz. flare to a total path length of 35' or so.

It is remarkable what horn loading can accomplish. The response curve was measured at the box exit with a one watt input into an Altec 515-16G driver. Note that the lower limit of the chart is 10Hz. It can be seen that the SPL is 113dB at 30Hz., and is still 100dB at 15HZ. Room modes are partly at work here, as other measurements I made showed more uniform response from 20Hz. to 80Hz. with about 108dB average sensitivity.

As can be seen in the pictures, the sub was built in four boxes that bolt together. Each is roughly 4' by 4' by 2'. It was held together only by screws but didn't buzz or leak. It is currently disassembled and stored, but will probably be put back in service in a friend's system in a few months.

[rs237]

Steve,

thanks for the pictures. Great work .
do I see that correct, you use the compression directly at the driver, without compression chamber?

regards
juergen

[Hoerninger]

Steve,

thank you for sharing.
Are there any issues concerning the low frequency power?
Needs someone care for glass damage, wall rattling or even destruction?
___________
Peter

[Steve Schell]

Andyoz, I didn't read your comments carefully enough earlier but they do make perfect sense. Thanks.

Juergen, in my sub the driver is mounted to a flat baffle board with a rectangular cutout of 67 square inches area. This creates a 2:1 compression ratio for the 15" driver, which has a piston area of about 133 square inches. The rectangular cutout mates with the first throat section of the same size and shape, then the horn expands slowly from there.

The "front chamber" as it is called would be the small area between the cone and the baffle board. This volume functions as an acoustical capacitance and will act to create a high frequency rolloff. A front chamber volume of this small size would have a negligible effect in regard to the subwoofer's passband. It matters a lot in high frequency compression drivers though, so the diaphragm is placed as close to the phasing plug as possible.

The rear of the cone looks into a sealed back chamber of 16 cubic feet. I'm not sure if this volume creates effective annulling of the horn's throat reactance, but when I tried experiments to reduce the volume the lowest response peak began moving upward in frequency.

Hoerninger, that is a good question. We are applying repeated pushing and pulling forces to the building structure, no doubt. No broken windows so far, though they do rattle when the sub gets loud. Mostly the sub is being fed less than one watt of power, so it is not like some prosound application with a 1,000 watt amp. I have noticed a few hairline cracks in the plaster of my living room that weren't there the last time I painted. I don't know whether the sub has created these, or perhaps some house settling or the occasional earthquake.

I have read posts from prosound guys talking about high SPL testing in buildings and observing effects to the structure. One post (Tom Danley's I think) discussed major damage to an old house that was scheduled for demolition anyway.

[Hoerninger]

Steve, interesting reading.
This low octave roughly 20 Hz - 40 Hz seems to have a special impact on the house structure. Without it it seems to be less destructive.

I want to add an example. Some weeks ago I played the horn combination I mentioned once including four bass horns with an amp 2 x 800 watts. (These horns do not work in the lowest part 20 Hz - 40 Hz.)

I was alone in the audience and listened to "Ride of the Valkyries" from the film "Apokalypse Now".
It was loud, very loud ... The floor was vibrating, my chest took the bass.

But there was no resonance from the large glass front, no hearable noises from the hanging ceiling or anything else. And there have not been any complains afterwards.

I have never played the horns that loud again. The music teachers are always very satisfied when using them.
____________
Peter

[JBL 4645]

Steve

Very impressive I take it you can still swing a cat a around the room, that is huge!

Also doesn’t a dedicated floating floor give the impression that the loudspeaker is a larger room, where the bass wave will travel the length of the room than go underneath the floating floor thus creating a deep tone? I saw this many years ago at the music & technology course that I was doing.

[Steve Schell]

JBL4645, my house uses a perimeter foundation and has a floor that is suspended over the ground about three feet. When I bought the house the floor was very weak and flexible, and the speakers I played seemed to have very little bass. I later strengthened the sub flooring and added a layer of 3/4" plywood, and the bass has been subjectively stronger since then. The repairs were made before I began building horn subs though, so I don't know what effect the stronger floor has on energy below 50Hz.

After building the first sub, a strong initial impression was that the listening was occurring in a much larger space than my small living room. Recordings made in vast spaces were more believable. I think that our brains associate low frequency extension with large spaces, and flat response to 20Hz. results in a bit of a suspension of disbelief that one is sitting in a small room.