A Midrange Horn for the Altec 288B (2)

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Written by: Bjørn Kolbrek
Category: Horns
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An Exponential Midrange Horn for the Altec 288B

Measuring

 

Please note: this is not a construction article. It describes the process of building a large plywood horn, but the information is only provided for your interest and inspiration, and as an example on how to build horns.

Background

The background for building this horn is actually quite complicated. Back in 2003-2004 I came in contact with Thomas Dunker, who was one of the proponents of horns (and tube amps) in the "horn and triode revival" of the 90s. One of our main topics of discussion was how to build a good midrange horn, covering the major part of the human voice fundamental range. One horn that did this, was the Western Electric 15A, covering 80Hz-5kHz (approximately) with a single large horn and a small compression driver. How was this possible? Common horn wisdom says that a horn will cover approximately three octaves (or one decade).

So we set out to gather as much information on horns and horn theory that we could, especially from the time when horns were full range devices, for use with mechanical phonographs or theatre speakers. 

The project evolved and expanded to become a far larger undertaking than either of us had imagined. Hundreds of articles, books, theses and patents piled up. Software was written, measuring equipment built. And as a more or less direct result of our investigations and research, I decided to start on a MSc in Acoustics, and now I work on my PhD. 

The Driver

Back to the midrange horn project. First of all we needed to select a suitable driver. A pair of JBL 2470s for each horn (using a Y-split) was considered, but later rejected. We ended up with a modified version of Altec 288B. This driver is quite close to the field coil 287, which was designed for a cross-over frequency of 300Hz. 288B is specified for 500-16000Hz at 40W input. It has an underhung voicecoil, and somewhat larger clearing to the phase plug than 288C and later versions. Which means less HF-response, but better LF performance. As I have mentioned before, 288C is a better choice than 288B in terms of HF response. 

The 288B also has a larger diaphragm than WE 555W, which was used down to 60Hz, without crossover. It should  be suitable for a lower midrange horn at home listening levels, provided of course, that the horn has good resistive loading of the driver in the working range. 

Preliminary Design

Since proper acoustic loading of the driver, especially near cutoff, was important, this was taken into account in the design. One measure of the horn's loading properties is the power factor, or the cosine of the angle between the resistance and the reactance of the throat impedance. The power radiated by the horn is proportional to the power factor, so the power factor should be as close to one as possible in the working range. Near the cutoff frequency, all horns have a large mass reactance. But the shape of the resistance and reactance curves can be manipulated by changing the way the horn flares at the throat end. The profile will be similar to a hypex horn, which flares slower at the throat end.

The principal parameters for the horn were:

  • 200Hz cutoff frequency, 
  • Unity power factor in the pass band,
  • Modular design to facilitate experimentation

And since the internal flare of the compression driver is very much part of the horn, the design has to include that portion too. The 288B internal flare has a cutoff of about 208Hz, which is too high for a horn that should start  out with a low flare. So we rebuilt the drivers with a new internal throat section of constant diameter. This means that the horn, as it is, will not work with any stock driver on the market!

The profiles of the horn are shown below. The first part is Hyperbolic-Exponential with T=0, to match the driver plane wave throat well. The next parts are exponential. Total horn length is 95.5cm, and the mouth is 65 by 65cm. 

HornProfile

Below are simulated throat impedance and power factor for the horn, mounted in free field. 

Simulated throat impecance

Simulated power factor

The power factor is remarkably flat from 400Hz and up. Of course, this is only a simulation. Throat impedance measurements have not been done on this horn yet.

When the basic outline of the horn was decided, and the interface dimensions set, design of the specific parts began.

The Middle Section

 The middle section was designed first. This was expected to be a constant factor in the experimentation to follow. The profile was corrected for the curving of the wave fronts, the wave front areas were calculated by numerical integration. The horizontal profile was taken to be conical, since this would simplify the calculation of wave front areas, and also make the actual wave fronts behave in a simpler way. The main part of the expansion was to be in the horizontal plane for the same reason: to keep the wave front as little curved as possible.

The Throat Section

The throat section will do the transforming of the cross section from round to rectangular. Based on an old Bell Labs patent (which was the basis for the WE 31A horn), this transition is made by going through an eye-shape, as shown below.

Throat

The Mouth Section

The mouth section is slightly exponential in the horizontal plane, and purely exponential in the vertical plane. Due to the strong curving of the wavefronts, it was not attempted to correct the shape here; it proved too difficult and time consuming at the time, and the project had already taken too long. We decided on a pure exponential horn, with a square mouth of 65 by 65cm. 

Construction

 Construction began with the middle segment. Since the walls of a rectangular horn do not meet each other at a constant angle, we had to cut that angle correctly. For the middle segment this is not too complicated, since two of the side walls are conical. Here is how we did it:

1) Make a jig with a slope equal to the angle between the horn axis and the wall.

THorn 01

2) With the side wall on the jig, cut the profile on a band saw.

THorn 02

3) Then mount the side walls on a jig, and start fitting the top and bottom walls.

THorn 03

THorn 04

4) When the first panels are in place, laminate several layers of plywood on both sides. We used 4mm 3-ply wood for the curved sides, and 8mm 5-ply for the straight sides. Build up to 16mm thickness. Use stiffeners inside the horn to prevent sagging of the walls due to the tension in the plywood.

THorn 05

The middle segment is then fitted with flanges that fit the throat and mouth segments. These flanges are made from 16mm plywood covered by 2mm aluminium at both sides to get a smooth surface. Pegs are used to align the segments.

The throat segment is quite complicated. It consists of a welded steel shell with flanges that fit the 288B at one end, and the middle segment at the other. Inside this steel shell, the transition from round to rectangular cross section is cast in tin. Here is how it was done:

1) The shape of the transition is calculated at every 5mm. The shape is exported to AutoCAD and printed in 1:1 on paper. The paper is glued to "architect cardboard", 5mm thick foam-filled cardboard, and cut out. A hole is drilled at the center of each piece. The pieces are then stacked on top of each other on a rod.

THorn 06

2) The shape is smoothed, filled with putty etc, to make it nice and smooth. 

3) A paraffin positive mold is made from this cardboard plug. 

4) A plug of RTV silicon is cast in the paraffin mold.

THorn 07

THorn 08

5) This plug is then centered in the steel shell, and hot liquid tin is poured around it. 

THorn 09

6) When the tin has cooled, the plug is removed, and we have the transition.

THorn 10

7) The extra tin is sawn off, and the inner surface primed with a thick filler/primer (we used Capalac Unigrund). 

Next up is the mouth segment, which is curved in both directions. This is much more complicated than the middle segment, and requires a good jig. Below is a picture of Thomas Dunker and me holding the middle segment and throat segment, with the mouth segment jig on the ground.

THorn 11

The jig is required to hold the force of several layers of plywood pressed towards it. We started with the least curved sides. A massive amount of bracing and clamps is required, see below. Also note that we had already made the mouth frame, to hold everything in place. We laminated 4 layers of 3-ply wood. After this was done, a similar procedure as with the middle segment was used to cut the wall corners so that the two last walls would fit properly.

To prevent the walls from sticking to the frame or jig, we used some thin plastic foil between the parts.

THorn 13

When the side walls were laminated and cut, the top and bottom walls were laminated in a similar way. As can be seen from the photo, this was a quite complicated procedure. Threaded rods, wing nuts and plywood braces, in addition to clamps were used. We had to use all we could gather of clamps, big and small. Note the plastic foil to prevent the parts from being glued to the jig or braces.

THorn 14

The finished mouth segment.

THorn 15

The next step is to bolt the segments together and make a smooth transition. Then the horn is painted with a primer/filler, and finally two layers of strong paint. For the final mountings, a thin foam gasket is used between each segment. 12 6mm bolts are used to connect the throat and middle sections, and 14 6mm bolts are used for the middle/mouth segment connection. Pegs are used to align the parts.

THorn 17

The modified 288B driver ready (with gasket) for mounting on the horn.

THorn 16

Performance

The horn was measured outdoors, on a jig made from an office chair for rotation. 

THorn 18

THorn 19

Frequency Response

Below is the frequency response for various driver configurations. The blue and red lines are for one set of 288B, with and without rear cover respectively. The black curve is for another pair, I think with rear cover. It is evident that there is a difference between the drivers. Some drivers have a silver shorting ring, evident in the better HF response in the black curve. 

You may wonder why the LF response is falling off so early, even if the throat impedance is high in the region, and that I have shown here that the 288B with plane wave exit has a flat frequency response down to 100Hz. Well, it is because of the directivity of the horn.

THorn OnAxis

The near field measurements also show this, here the response is flatter.

THorn Nearfield

Directivity

Horizontal directivity:

THorn-DirH

Vertical directivity:

THorn-DirV

Distortion

Distortion at 100dB SPL at 2m distance. The typical rising 2nd harmonic distortion is visible. Higher order distortion is comfortably low.

THorn Dist1

Closing Remarks

It was of course very exciting to hook up these horns and play music on them for the first time. And we were not disappointed. They sounded very nice. It turned out that having the conical expansion in the horizontal plane was the best, and this gave a very wide sweet spot. The horn fills the room better than the AH425s, and having a large part of the voice fundamental range in one horn certainly has its advantage. 

The project started somewhere in 2006, and finished in 2010, a month before I moved to Trondheim and had to pack them down. They are perhaps the part of my old system that I miss the most.

Thomas has finished his pair, though, and they are playing beautifully. Below is a picture from his setup. The low frequency system is JBL 4560 with JBL 2220A woofer, and the HF system is University 4408 horns with P.Audio WN-D34 modified for 0.5" exit. Crossover frequencies are around 300 and 3000Hz, tri-amplified with active crossover (Behringer DCX2496). 

rch 01

Update, Fall 2013: The horns are up again in my system.

©2006-2012 Bjørn Kolbrek and Thomas Dunker

 

Latest blog entry

Horn Loudspeaker Simulation

Horn Loudspeaker Simulation

Many readers may be familiar with the free software Hornresp, which, given driver parameters and some geometrical parameters for the horn and cabinet, can simulate a wide variety of horn, direct radiator and bandpass systems. This may be enough for most people, while others (like me) are curious about the math and algorithms behind it

Simulating horns and horn loudspeakers has interested me for a long time. I found Hornresp back at the time when David McBean first made it available (around 2000 I think). At first you could only get the full functionality by paying a small fee, otherwise the throat area of the first segment was fixed. He later told me that he actually ended up giving the full version for free to those who asked, and if I had known that I'd probably started using it in earnest earlier. But my first midbass horns were designed using Hornresp (with the original limitation), as was my later horn subwoofer and other horns. But around 2005-2006 I got interested in horn theory and other methods of evaluating horn performance, like power factor (the cosine of the phase angle between resistance and reactance of the throat impedance). First I made a program that would read the data exported from Hornresp and plot the power factor. Later I started looking into wave fronts in horns, and combination of horn types (like multiple segment horns with Hypex segments), and for that I needed to write my own horn simulation software. 

I first made contact with David McBean in 2004, and we started corresponding regularly, discussing many aspects of horn simulation. This was extremely helpful when I started writing my own horn simulator, and I can't thank David enough for making Hornresp available, and for guiding me through the rough terrain of horn simulation. Although he never showed me more than a couple of snippets of code, through discussion and references I was able to make things work and learn things from the ground up. 

My horn simulator, HornCAD™, evolved into a complex program that could use the Mode Matching Method (MMM), simulate all types of segments, rectangular horn, and curved horns. It could use measured compression driver data, and export dxf files for CAD use. Thanks to the MMM, directivity simulations were quite accurate, and I used it extensively during my time at Celestion. 

But since HornCAD™ was born as a tool to learn horn simulation and to explore various ways to simulate horns, it is full of unfinished functionality and bugs that are hard to track down due to the size of the code base. Therefore it has never been released to the general public. That may change in the future, though... But enough about my background as a horn simulator writer!

Simulation Basics

The point of this article is to give an introduction to horn loudspeaker simulation. While it may seem like black art for most people, it is more similar to electrical circuit simulation. So in this little series, I plan to explain how to implement a simple "Hornresp-ish" code in Octave/Matlab. It will of course not have all the bells and whistles of Hornresp, but will be able to simulate the power response, electrical impedance and diaphragm displacement of a basic horn loudspeaker. 

Horn loudspeaker simulation typically consists of simulating:

  1. The radiation from the horn, especially radiation impedance, but also directivity.
  2. The horn itself: throat impedance as a function of mouth radiation impedance, and the output volume velocity as a function of the input volume velocity.
  3. The acoustical circuit apart from the horn: front and rear chambers, vents etc.
  4. How the loudspeaker driver behaves when connected to the acoustical load made up by the horn, chambers, vents etc and thereby the power delivered to the horn, the electrical input impedance of the system, diaphragm displacement etc. 

We will cover all of this, but also things like simulating multiple segment horns, horns with special profiles like the tractrix, and maybe even look at tapped horns. 

Glossary

The list above contains many words that may be unfamiliar, but if you want to learn horn simulation, you need to know what they mean. So here is a short glossary:

  • Radiation impedance: When something vibrates with a certain velocity, it pushes the air, and the air pushes back with a certain force. The ratio of this force from the air to the velocity of vibration is the mechanical radiation impedance. In this analogy, force is analoguous to voltage and velocity to current, and just like voltage divided by current gives electrical impedance, force divided by velocity gives mechanical impedance. 
  • Volume velocity: velocity times area, cubic meters per second.
  • Acoustical impedance: A similar analogy can be made in acoustics: with pressure analoguous to voltage and volume velocity analoguous to current, the ratio between them is acoustical impedance. It relates to mechanical impedance through the area of the vibrating surface (which may not be solid, but can be an imaginary surface, for instance the horn mouth). Mechanical impedance is acoustical impedance times area squared.
  • Acoustical circuit: an analoguous circuit of acoustical components like ducts, horns and volumes. It can be drawn like an electrical schematic, and we can calculate its performance based on electrical circuit theory. This is a fundamental method for simulating electroacoustic systems.

Simulation: The Nitty Gritty Details

Here are links to the parts in the series (the links will go live as the parts are added):

  1. Part 1: Radiation and T-Matrix
  2. Part 2: Adding a driver
  3. Part 3: Multiple segments and more T-matrices
  4. Part 4: Other horn profiles and curved wave fronts
  5. Part 5: A simple tapped horn model

The Bass Transmission Index

The Bass Transmission Index

Evaluating the accuracy of low frequency reproduction isn't as straightforward as one may think. There is talk about "fast bass", and PRAT (Pace, Rythm And Timing) is sometimes mentioned as a description. In such a discussion someone will invariably point out that a 20Hz sine wave isn't "fast", and that the issue of fast or slow bass is meaningless. It may seem that way using these simple arguments, until you experience accurate, detailed bass reproduction. But how do we quantify it? Obviously not by measuring the rise time of a 20Hz sine wave! 

This is actually a very real problem. For the mix engineer, perhaps the biggest problem is how a loudspeaker with poor temporal response alters the perceived balance between rhythm section instruments like kick drum and bass guitar. Though not exclusively the case, loudspeakers with ported cabinets (bass reflex loading) are more likely to exhibit this type of problematic behaviour, due to the use of resonant elements to increase bass extension. An incorrect mix made on loudspeakers like this may not transfer well to other sound reproduction systems, and can't be corrected later during mastering, since the two instruments occupy the same part of the audio spectrum.

Lara Harris, Keith Holland and Philip Newell have done extensive work on quantifying bass reproduction accuracy, resulting in Lara's PhD work and the Bass Transmission Index (BTI). The BTI is an objective measure of a loudspeaker's ability to accurately reproduce low-frequency musical content. The metric aims to describe how well a loudspeaker reproduces the temporal envelope of a dynamic and harmonically-complex signal, something that cannot easily be evaluated from the usual frequency response (magnitude plots). 

Like the STI (Speech Transmission Index), as used in speech intelligibility measurements, the BTI is based on a metric called the Modulation Transfer Function (MTF). MTF-based methods such as these pass an amplitude-modulated signal through the system, varying the rate of the temporal fluctuations across a range of values that are likely to feature in the real-world signals that the system will encounter. The preservation of modulation depth between input and output is used as an indicator of how well the system can reproduce these temporal variations. The figure below illustrates the concept, showing modulation depth for input (mi) and output (mo) signal envelopes.

MTF

The BTI evaluates the MTF in 10 frequency bands from 16-160Hz, with 7 modulation frequencies from 0.8-11.7Hz. The algorithm computes a matrix of modulation index scores between 0 and 1 for each combination of frequency band and modulation frequency. This grid of numbers can be used to calculate an overall average score, and is visualised as a grayscale intensity image where white = 1.00 (perfect reproduction). As a rule of thumb for interpreting these BTI intensity images:

  • Inconsistent shading in the horizontal direction (left-right) indicates variation in the frequency response magnitude (this is variation across frequency bands).
  • Inconsistent shading in the vertical direction (up-down) indicates variation in the faithfulness of envelope transmission (this is variation across modulation frequencies).

The figure below shows an example BTI intensity image and mean score for a bass-reflex studio monitor. Frequency bands (centre frequencies) are on the x-axis, and modulation frequencies on the y-axis.

BTIplot

Recently I have worked with Lara to make her BTI research code production ready, and it has now been officially made available on GitHub. It runs in Matlab and Octave (although not all the advanced options are available in Octave, and some requires certain Matlab tooboxes), and demonstrates a reference implementation. Here are the links:

BTI Toolbox on GitHub

Lara's LinkedIn post about BTI

My LinkedIn post about BTI

 

Big Bend Bass Horn: Driver Update

Big BenD Bass Horn: Driver Update

[Previous: Installation; Main] 

This update is actually long overdue, and related to a project at Celestion I wasn't able to finish: a cone driver specifically designed for bass horns. It follows the ideas I outlined under My Approach to Bass Horn Design, and is a 12" driver designed for a compression ratio of about 1:2. As I wanted it to be easy for Celestion to put it into production, I used as many standard parts as possible, either directly or something that could be easily machined from standard parts. But I also added a feature that aren't easy to find in modern drivers: an underhung edge-wound voice coil. 

Here are a some of the features of this driver:

  • Underhung 3" edge-wound voice coil
  • Copper sleve on pole piece
  • Focused magnetic gap
  • Low moving mass
  • Vented pole piece and back plate
  • 250W power handling
  • Large ferrite magnet
  • Inverted dustcap to allow for phase plugs if desired

And for those who worry about the high power rating being detrimental to other qualities important for horn speakers, rest assured: this power rating was a result of the voice coil size and venting, not of "beefing up" the driver (which typically makes the moving assembly lighter) to take higher temperatures and forces. 

Here's a side view of the driver, without the front segments. It's built on a Celestion FTR chassis. 

New driver

The magnet system:

bigBendDrivers3

The gap flux is slightly above 1Tesla, which is quite good for a gap this size. It takes a substantial amount of magnet to produce that, especially when you lose gap width to a copper cap. 

Comparisons with old DIY driver

Below are a couple of photos comparing the new driver to a DIY project I used to begin with, referred to as 12" DIY driver in the performance measurements. The DIY driver used the motor system from a pair of Celestion NTR08-2009D 8" woofers I found in the bin. They had the cones cut out, but the motor was salvageable (even the voice coil), and I used them to build a pair of 12" drivers using available parts. I used the lightest 12" by 2" cones I could find, and a fairly soft spider. They turned out to be quite good, with the BL^2/Re being a good match for my bass horns. But the new drivers are more robust, and also give a very good performance and produces very clean bass in the Big BenD horns. 

Comparision with old DIY driver

Comparision with old DIY driver (back)

 Parameter  Old DIY driver  New driver
 Re [Ohms]  4.46  5.8
 Le [mH]  0.065  0.086
 BL [N/A]  12.3  17.0
 Mms [g]  39.2  64.5
 Rms  [Ns/m]  3.4  0.63
 Cms [m/N]  4.36e-4

 1.88e-4
     
 

Now I just hope Celestion will finialize this project and put the drivers into production, as I think this would be a good driver for bass and midbass horn use, especially for domestic use.

[Previous: Installation; Main] 

Big BenD Bass Horn: Installation

Big BenD Bass Horn: Installation

[Previous: Performance Measurements; Main; Driver Update] 

Finally, the last article about the Big BendD bass horn! This part will cover the installation and setup of the horn, with some comments on the subjective performance at the end.

Installation

Although the DIY 12" driver showed the most promise during outdoor testing, I still wanted to try the Altec 515-8G when setting up the horn. It was mounted in the rear chamber, the chamber was filled with wool, and the rear wall covered with pieces of an acoustic celing tile. 

RearChamberAltecWoolBackWallDamping

Then came the process of carrying all the parts into the living room, setting it up, adding gaskets, bolting it all together, installing the drivers, wiring it up and put the rest of the system back together. In the process I had help from my good friend Harry. This is really a two-person job, because carrying the big parts into the house isn't easy to do by oneself. The parts were all designed to fit through a standard door, but they are still a bit difficult to move around. 

Setup01Setup02Setup03

Setup01Setup05Setup03

Setup01Setup08Setup03

Setup01Setup11Setup03

With two people working, the setup was done in a couple of hours. There are a lot of bolts, about 70 per horn, so or ratchet spanners got a real workout. 

The complete setup is shown below. The white middle/throat sections blend in with the walls, making the bass horn less dominating in the room. It does work, some people have not recognised it as part of the bass horns.

Axi2050Proto1

The First Test: Where's the Bass?

Just setting up the horn and putting some music on, without any crossover or EQ, left us wondering: Where's the bass? Isn't this supposed to be a bass horn? There's a lot of lower midrange, but the sound was a bit thin. 

Altec95dBInRoomThe measurements made it obvious what the problem was: the output fell below 70Hz, and apart from a peak at 50Hz, the response was more like a midbass horn than a 30Hz bass horn. What was going on? 

We swapped the Altec for the DIY 12", and that helped significantly; the better impedance match between the horn and driver was definitely an issue in the presence of room modes. Which turned out to be the real problem.

Room Modes

 My living room is about 7m long and 3.5-4m wide. This means that the second mode in the length direction and the first mode in the width direction are both at about 50Hz, which is also clear from the measurement above. Between these two modes, energy transfer in the room is quite limited. A wider room would definitely be an improvement, but you have to work with what you've got. 

In the modal region in a room, the response can vary quite a lot from position to position. Some listening positions have a smoother response, while others place the listener at peaks or nulls of modes. To find the best starting point for EQing the response, I measured the response in the room at a 2.5 by 3m grid, every 0.5m. The results for all positions are shown below. 

RoomRespGrid

It turned out that the response was quite smooth about 3m from the horns mouths, somewhat behind the middle of the room. The responses at this line are shown below. Annoyingly, the response drops quite sharply below 45Hz, due to the lack of modes between 25Hz and 50Hz, but apart from rebuilding the room (which I'm sure the landlord wouldn't like), there's not much to be done. 

RoomResp3m

I have applied some EQ to lift the response a bit, but one should be very careful with EQing dips caused by room modes. And I'm not going to spend the rest of my life in this house, so hopefully my nest home will have a more beneficial distribution of modes int eh listening room!

Listening Tests

So, after all this work, how does it sound? The lack of output below 45Hz is only noticable on music where you know there should be something down there. Apart from that, the response in the listening position is quite smooth, and it has the traits of bass horns that I have been missing for so long: proper impact - even at low volumes -, and responsive, detailed and tight bass. Low frequency details in the recordings are quite easy to hear, and there's no overhang or resonance. It sounds effortless even at very high volumes too. The horn integrates well with the fast and detailed midrange of the Axi2050, making it a good combination. I also tried a delay-derived subtractive crossover, as described in the Horn Book, and it made a worthwile improvement to the coherence and naturalness in the lower midrange.

All in all, I'm very satisfied with the performance. 

[Previous: Performance Measurements; Main; Driver Update] 

Big BenD Bass Horn: Performance Measurements

Big BenD Bass Horn: Performance Measurements

[Previous: Belts and braces pt. 4; Main; Next: Installation] 

 With one horn finished, I took it outside to do some frequency response measurements. First thing to do is to measure the horn under conditions similar conditions to the simulations. This is a very important part of designing speakers, if you want to use simulation tools in the process. You need to verify that your simulation is correct, and if not, in what way. It is especially important if you are writing your own simulation software. I think people are getting better at it, but there have been many cases on DIYaudio of people complaining about their simulations being wrong, when the actual problem is that they have not simulated what they have actually built. 

Corner

The actual condition of a horn built into an "infinite corner" (3 infinite baffles perpendicular to each other) isn't easy to achieve in practice. The best I could do was to use the walls of my house and garage:

Testing1

And then put some extra sheets of plywood between the buildings to try to close the gap.

Testing2

So, finally the first measurements, with and without baffles. The difference isn't huge, This may be because the garage is still close enough in terms of wavelengths to contribute to the baffle effect. (I'm not sure if the levels are actually correct, trying to do the calibration in ARTA gave some confusing results.)

MeasuredCornerTxt

So how well does this fit with the simulations? Actually pretty good, see below. The Response below 200Hz is almost spot on, the  response at higher frequencies deviates, probably because of the simple model used for simulating the curving, and because I used a different driver than in the measurements. 

What I'm quite happy with is that the curving approach worked as intended: there are no sharp dips and peaks or suckouts in the response, and it doesn't roll off until about 500Hz. This creates a nice overlap with the midrange horn. At the lower end, the response starts to fall off rapidly below 30Hz, which was the intended lower limit. The response is a bit uneven, but we'll see that this changes with a different driver.

OutdoorResp

Wall

The second test condition is in front of a wall. Neither of the two are fully representative of the operation conditions in actual use, but it will show the effect of placing the horn in front of, instead of flush with, a wall.

Testing3

The effect of one missing side wall and the increased distance to the back wall is evident: a loss of level at low frequencies that was predicted by both the simulations and scale model measurements. 

MeasuredWallTxt

Driver Tests

The next measurements were done with REW as I found it easier to do a level calibration there than when using ARTA. Four different drivers were tested:

Altec 515-8G, a 15" driver built in a Celestion FTR-3070 chassis, A 12" guitar speaker, and a 12" high efficiency woofer built using various parts available. The results are shown below. 

There is a clear difference between the drivers; the guitar speaker clearly fails and has a very peaky response (not all musical instrument drivers are suitable for bass horn use, even if Dr. Bruce Edgar had good results with EVM-12). The 15" drivers perform about the same. The best results comes from the 12" driver I built. This isn't actually very surprising, since it has the best impedance match with the horn of all the drivers. 

The dips in the response at 483Hz and 870Hz are from standing waves in the empty rear chamber.

FR105dBWall

Distortion

Following is some distortion meausrements of the drivers tested. I measured at 95, 105 and 115dB SPL at a 2m distance. Only the results for 115dB (114dB for the 12" prototype) are shown.
Altec115dB

15inDIY115dB

Guitar115dB

12inDIY115dB

The 15" drivers are quite similar in response, but the Altec clearly has lower distortion. The guitar driver has very high distortion (not surprisingly, since it has a stiff paper surround and only 1mm overhang on the voice coil), and is best left to what it was designed to do: create distortion for electric guitars. 

The 12" DIY driver has the smoothest response, and while the distortion is slightly higher than the Altec, they are both quite at "sane" listening levels.  

Altec95dB

12inDIY95dB

[Previous: Belts and braces pt. 4; Main; Next: Installation] 

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