Question re near-field low-frequency measurement

[mefisto]

Hi Ian,

thank you for your reply.

Yes l actually met Marco through Rich in London back in 2018. I enjoyed listening to Marco’s system.

Regards

Ian

Ha, ha, small world, eh?

Hi M,

I think you might be able to use your 15M woofers because they are easily be used up to about 1500 hertz. You could therefore use the wave guide Robert S has recommended with a 1.5 inch compression driver.

. . .


An f3 of 40-45 hertz is readily obtainable in 5-7 cu ft .

Thank you for your generous offer, but I would rather continue with the current project. I do not feel like starting a new enclosure before at least attempting to finish the current project and ascertain how I like it. I already have an enclosure and I am rather satisfied with it, provided that my measurements are correct. In that regard, I am rather prejudiced against ported enclosures, as I always have the impression that the bass is not "natural". I understand it is subjective, and perhaps, as it was suggested to me, I have not heard a "properly designed one", but I have to go with what I like even if it is not (audio) magazine perfect. Furthermore, in a few weeks I might have an ALTEC H808, which, according some measurements can be used below the rated 800 Hz cross-over recommended and used by ALTEC, with the right driver, see below.

Kindest regards,

M

[Ian Mackenzie]

What is the volume of your enclosure?

[mefisto]

Hi Ian,

I do not know how to estimate it, as the apparent volume depends on the fill. Saying that, the raw volume is about 100 l. My best estimate of the driver volume is 6-8 l. Bracing perhaps another 1 l. The fill is positioned about half-way between the front and rear, where the velocity should be maximum, and from there towards the front both sides and bottom are covered. The fill is about 6-8 in thick fiberglass.

Kindest regards,

M

[Ian Mackenzie]

Well it’s too small for a descent coffin.

The 15M can work in a sealed box that side.

Btw notions of bass reflex versus seal become blurred due to the difference in the slopes of each type. This has a profound audible effect on perceived bass quality. A bass reflex loudspeaker goes lower and flatter than a typical sealed system.

Ad a result the effect of room boundaries grossly impact with a significant bump in the bass of a reflex system compared to a sealed system. Use of a graphic EQ or Dirac room EQ can equalise out the worse of these problems to the point you won’t discern which system is which. The bass reflex system generally wins in terms of dynamic output.

Another variation of bass reflex system is a passive radiators which are a diaphragm with a voice coil or magnet. A well designed passive radiator will outperform a port in a compact enclosure. For this reason they are employed in hi end near field monitors. A key. Benefit as elimination of port tube pipe organ resonances. In large enclosures the this is far less an issue because the ports are relatively short.

The earliest American loudspeaker with a passive radiator was the Electro Voice Interface A. This was an excellent loudspeaker back in the day and got the jump on implementation of the T/L design of bass reflex systems. Up till then bass reflex implementation was largely empirical.

[Robh3606]

The earliest American loudspeaker with a passive radiator was the Electro Voice Interface A. This was an excellent loudspeaker back in the day and got the jump on implementation of the T/L design of bass reflex systems. Up till then bass reflex implementation was largely empirical.

Hello Ian

You sure about that?? The JBL's go back into the 60's

Rob

https://www.lansingheritage.org/html...alogs/1966.htm

[mefisto]

Hi Ian,

Well it’s too small for a descent coffin.

The 15M can work in a sealed box that side.

Btw notions of bass reflex versus seal become blurred due to the difference in the slopes of each type. This has a profound audible effect on perceived bass quality. A bass reflex loudspeaker goes lower and flatter than a typical sealed system.

Ad a result the effect of room boundaries grossly impact with a significant bump in the bass of a reflex system compared to a sealed system. Use of a graphic EQ or Dirac room EQ can equalise out the worse of these problems to the point you won’t discern which system is which. The bass reflex system generally wins in terms of dynamic output.

Another variation of bass reflex system is a passive radiators which are a diaphragm with a voice coil or magnet. A well designed passive radiator will outperform a port in a compact enclosure. For this reason they are employed in hi end near field monitors. A key. Benefit as elimination of port tube pipe organ resonances. In large enclosures the this is far less an issue because the ports are relatively short.

The earliest American loudspeaker with a passive radiator was the Electro Voice Interface A. This was an excellent loudspeaker back in the day and got the jump on implementation of the T/L design of bass reflex systems. Up till then bass reflex implementation was largely empirical.

Thank you for the explanation, specifically your reference to the "room boundaries effect" and the "passive radiator". Admittedly I heard only a few bass-reflex system since most of my friends have horns even for the lower octaves, and all of the bass-reflex systems had the pronounced bass, which I was referring to. About two months ago, I met a gentleman who has been professionally involved in audio design and listened to his speaker with a passive radiator, and I did not hear the pronounced bass,. In fact, the bass was amazing.

So, the notion that I "have not heard a competently designed bass-reflex (until then) may be correct.

Now, it is my understanding that a design of such a system is extremely difficult.

Kindest regards,

M

[Ian Mackenzie]

It’s actually not difficult at all if the correct process is followed.

As l have pointed out elsewhere what you decide to do is your choice.

What’s important to realise is that your own disbeliefs are not necessarily valid to anyone else’s situation. This is unfortunately is not uncommon in diyers. Because something didn’t work for them they enshrine the failure as a protected value. In other words they protect the disbelief by being precious about it.

In the case of bass reflex design the very best studio monitors world wide use this design principle.

[Ian Mackenzie]

Instead of throwing the baby out with the bath water the most common issues can be worked out with some problem solving skills.

1. Is the right woofer being used in the correct enclosure type, volume and tuning?

2. Has the enclosure been evaluated in different positions to determine if it’s the location?

3. Has the listening position been evaluated against other positions?

4. Has the size and construction of the room been investigated to determine if it is a source of the issue? For example small square room constructed with block, stone or fabricated concrete can be a source of low frequency issues.

If a systematic approach is applied the source of the issue can readily be identified.

[Ian Mackenzie]

Hello Ian

You sure about that?? The JBL's go back into the 60's

Rob

https://www.lansingheritage.org/html...alogs/1966.htm

As far as the plublished papers by Thiele and Small yes.

Bass reflex goes back to Novak and others theories. People had their own approaches but no one had applied T/L data to predict and design a system using Thiele’s high pass filter theory on alignments. Thats my point.

Electro voice were the first to apply computer simulation to T/L data using equations that when applied to a scientific calculator were able to output data points for a graphical curve of an alignment for a particular driver.

My father who was an engineer used an HP 45 in the early 1970’s for complex calculations of pressure vessels. This was all quite an innovation back then.

https://www.hpmuseum.org/hp45.htm


The reference to Thiele is mentioned in EV literature.

It was D.B Keele Jnr at Electro-Voice who took Thiele’s work a step further with a set of 6th order electrically assisted alignment.

I have paperback book somewhere called Bullock on Boxes that provides a detailed history of all players involved in the bass reflex system.

https://worldradiohistory.com/hd2/ID...-Page-0060.pdf

https://worldradiohistory.com/hd2/ID...-Page-0060.pdf


https://www.audioheritage.org/vbulle...l=1#post412740


http://www.xlrtechs.com/dbkeele.com/...Alignments.pdf

[Robh3606]

As far as the plublished papers by Thiele and Small yes.

Hello Ian

No not that the earliest use of a passive radiator. The PR-8, 10 and 15 are in use well before the EV system were manufactured. I should have been more clear not the particular alignment. That's why I posted the catalog link.

Rob

[Ian Mackenzie]

Oh l see.

Yes and Celestion had the Ditton 66 and the Ditton 25.

Passive radiator loudspeakers becoming more common with near field monitors and some consumer bookshelf systems systems.

Time for a beer in Outback Queensland.

This reminded me of Route 66 going down the Mississippi.

Nindigilly Pub is Queensland's oldest hotel located in its original condition and position on the banks of the Moonie River. The license was issued in 1864 after operating as shearer's accommodation for the Nindigully Station. From the late 1800s the Nindigilly Pub was a Cobb and Co coach change over station.

Despite its population of only nine people, The Gully attracts a regular flow of locals (nearest eight kilometres) and tourists from all over Australia for a cold beer and a chef fresh prepared meal - the kilogram T-bones are legendary or try the new two kilogram plus road train burger (feeds minimum of four people).

[Ian Mackenzie]

Time for another beer

Club Boutique Hotel An Unexpected Outback Queensland Oasis

12 craft beers on tap. Wait a while country.

Sound by JBL

[Ian Mackenzie]

Hi Rob,

In the LMS manual there is an explanation of how to calculate the correct way to adjust the port measurement so it be included in the woofer nearfield measurement.

For most this is a more convenient approach than a ground plane measurement.

[Ian Mackenzie]

M,

In an effort to bring you clarity of the application if your Altec 515 woofer l present to you the following information and guidelines:

1. Try and avoid accepting information from fellow diy friends as gospel for your particular situation. Often well meaning diy users are merely quoting you their experience or observation in their own situation. If you ask five diy people their experience or a question you will seldom get one concise answer in agreement. Diy users are notoriously inaccurate with measurements of TL data.

This leads to doubt of what to do when in fact the answer is best found on the Altec data sheets. Start with a manufacturers data. The dig person then starts to overthink the issue with reasons to believe others opinions. This can lead to dissatisfaction with their project.

Most of the Altec 515 drivers have very similar TL data. If your driver is one of these it’s very straightforward.

Attached is the official Altec TL data.
Using a reliable model l present two different simulations of the Altec 515B.

Without offending diy people when using a with a large VAS differences of 20% or less in enclosure volume aren’t going to make a really noticeable difference. With such low QTS and the high QMS the driver characteristics are largely influenced by its electrical QES properties. Hence its low QTS. This is indicative of an efficiency driver with a low mass cone and small Xmax making it suitable for larger front horn loading with a small rear chamber.

2. But there are some non maximally flat bass reflex and close box tuning that can be used if a compromise is acceptable.

In the first comparison the 515B is compared in two different volume sealed enclosures. The volume is the net volume. As mentioned above differences of 20% in enclosure volume aren’t an important consideration. As can be seen increasing the enclosure a significant order of magnitude does not really extend the bass response falling at 13 db per octave below 100 hertz.

In the second comparison a maximally flat alignment based on the drivers TL parameter set QB3 cut off at 18 Db per octave below 83 hertz. This alignment is compared to a larger custom enclosure with tuning to provide a TL response at the lowest point with a 24db per octave cut off below 47 hertz with a gradual fall the the passband below 200 hertz. Some users find this type of custom enclosure an advantage in small rooms that are dominated by low frequency room modes.

Conclusion

The 515B is relatively miss matched for a sealed enclosure of bass reproduction is required at realistic levels without a subwoofer.

The 515B can be used with a Thiele Small maximally flat alignment if a limited output below 100 hertz is acceptable. A custom enclosure tuning can be utilised with a non flat alignment is acceptable. This enclosure however will only perform effectively with mid bass above 47 hertz. If desired a sub woofer could be used to fill the bottom two octave.
Note this driver has only a modest X max which will limit its maximum bass output. The benefit of this driver is its efficiency making it a suitable match to SET amplifiers.

You could use this driver with a 500 hertz Altec horn which is generally its application.

[mefisto]

Hi Ian,

M,

In an effort to bring you clarity of the application if your Altec 515 woofer l present to you the following information and guidelines:

. . .

Conclusion

The 515B is relatively miss matched for a sealed enclosure of bass reproduction is required at realistic levels without a subwoofer.

The 515B can be used with a Thiele Small maximally flat alignment if a limited output below 100 hertz is acceptable. A custom enclosure tuning can be utilised with a non flat alignment is acceptable. This enclosure however will only perform effectively with mid bass above 47 hertz. If desired a sub woofer could be used to fill the bottom two octave.
Note this driver has only a modest X max which will limit its maximum bass output. The benefit of this driver is its efficiency making it a suitable match to SET amplifiers.

Thank you very much for your time spending on the analysis above. By a strange coincidence, it came shortly after I reviewed Gary Dahl's posts regarding his implementation and some of our private communication about his decision to abandon the idea of ALTEC 515 woofer that he intended to use initially, first for AE TD-15M, and finally for ALTEC 416. He arrived at a similar conclusion as you did and actually ended using a subwoofer for the reasons you have stated.

Interestingly, the originator of the Beyond the Ariel thread has decided against a subwoofer: https://www.diyaudio.com/community/t...2#post-7472176.

As you and I briefly discussed in our private communication, I rebuilt the enclosure for TAD1601A per your advice and finished a wave-guide for a Heil AMT. If I can trust my measurements, which is questionable, the Heil measures flat (+-2.5-3 dB) from about 400 Hz to 22,000 Hz. Thus, I made a crude 2nd order cross-over at 650 Hz. The cross-over is definitely not to your or other people on this forum standards, but one has to start somewhere.

Kindest regards,

M