PowerPoint Presentation

Horn - Introduction

Horns have been used since the beginning of audio reproduction technology.PROHorn loudspeakers offer outstanding sensitivity (or efficiency) RESULTING IN EXTRORDINARY DYNAMICS. For example:

AvantGarde Duo 104dB/W/1mJBL CMCD-81108dB/W/1mCerwin Vega EL36C106dB/W/1mAverage106dB/W/1m

A typical bookshelf speaker79dB/W/1mDifference27dB/W/1m

Doubling the input power increases the sound pressure level (SPL) by 3dB. Therefore, a typical bookshelf speaker will deliver 106dB using 512WATTS OF ELECTRICAL POWER!Note: 27/3=9And 29= 512

Horn - Introduction

CON

They are bulkyThey are expensive but require less power for the same SPLOften exhibit an irregular radiation pattern unless properly designedHave an uneven frequency response unless PROPERLY designed

Horns Examples: commercial systems

Alura 100dB/W/1m $15,000/pair

AvantGarde Duo 106dB/W/1m $60,000/pair?EXPENSIVE indeed!

Exponential Horn Project Design (1)The project consisted of designing and building three enclosures.

One vented enclosure for 30Hz to 200Hz (200Hz is the max low-pass frequency for most subwoofer amplifiers) using a 15 cone driver.One exponential horn enclosure for 200Hz to 1800/2000Hz using a 6 cone driver.One enclosure for frequencies above 2,000Hz using one or two 1 compression drivers/horns.

Exponential Horn Project Design (2)DRIVERS

This was a project on a tight budget. The drivers were selected for their specifications (Thiele/Small parameters) and affordability.

Woofer: Dayton Audio PA380-8 $75.8 Excellent woofer for the price with a low Qts of 0.24, a BL-product of 23.44Tm, a 3 voice coil and a sensitivity of 98.5 SPL/W/1m

Woofer: Eminence Alpha-6A $44.99 Inexpensive midrange driver with a large BL-product (8Tm), a low Mms (diaphragm mass=7gr) and a sensitivity of 93.6 SPL/W/1m

Tweeter: Eminence 1 compression driver with horn (discontinued)

Exponential Horn Project Design (3)Cross-OverThe speaker is tri-amplified using a Behringer Super-X PRO CX3400 stereo 3-way active crossover featuring Linkwitz-Riley 24dB/Octave filters.

Design ToolsThe 15 driver vented enclosure was designed using BASSBOX PRO, an excellent commercial software package for designing sealed, vented and bandpass enclosures.The exponential midrange was initially design in Excel, based on a collection of white papers published by a horn expert.Later in the project, I discovered HornsRep, a free 3-dimensional finite-difference wave propagation modeling software package. Free it is. Easy to use, it is NOT, but proved to make excellent predictions!

Area Expansion for Midrange Horn

Exponential Horn Project Design (4)MeasurementsWithout access to an anechoic chamber, measuring accurately the performance of a loudspeaker is difficult as multipath reflections degrade the measured response. The state of the art technique uses the MLS (Maximum Length Sequence) methodology. It is implemented in the DRA LAB MLSSA analyzer and Audio Precision One analyzer. However, this equipment costs between $5000 and $30000.Nevertheless, similar results can be obtained using:A calibrated measuring condenser microphone $60A microphone preamp with phantom power $50A quality PC audio card (96kHz/24bit)$150SoundEasy software package$300SoundEasy is many things but it is NOT EASY for sure. This being said, this is an incredible piece of work with immense capabilities in terms of measurement and design. It uses a full MLS implementation and calculate the impulse response of the measurement set-up eliminating sources of errors associated with PC sound cards, amplifier, cabling and microphone preamp. A true MARVEL!

Exponential Horn Project Design (5)

Measurement Techniques

I used two techniques for measuring the frequency response of the speaker.The Ground Plane MethodThe speaker being evaluated is laid on its side on a flat surface away from any wall or other sources of reflection, with the measuring microphone 2 to 3 meters away. This simulates a 2 radiation environment*.The Near-Field MethodA measurement is made with the measuring microphone 10mm away from the dust cap and similar measurements are made for each port and/or additional driver. The data is then summed digitally.

*I live in a cul-de-sac. It involves placing the speaker in my driveway 4 to 5m away from the garage door with the microphone on a stand in the street. Professional indeed, but with stacking enough measurements to compensate for the birds, the dog barking, etc It is possible to get good measurements.

Loudspeakers measurements (1)I mentioned in earlier slides the difficulty in measuring accurately the response of loudspeakers. The biggest issue is reflections from the floor, ceiling and walls. The best way to obtain valid measurements is to locate the speaker in an anechoic chamber. From the picture on the right, it is clear that such chambers are EXPENSIVE.

Lets look at how reflections affect the measured (and HEARD) response of loudspeakers in the next few slides and discuss methods of measurements without using an anechoic chamber.

Loudspeaker setup in an anechoic chamber

Loudspeakers measurements (2)Geometry of measurement set-up

Loudspeakers measurements (3)Geometry of measurement set-up

Loudspeakers measurements (4)Geometry of measurement set-up

Loudspeakers measurements (5)

The measured signal at the microphone is the sum of the direct and reflected wave. Therefore:

Wm = W + Wr or,

Wm = A (cos(2ft) + K R cos(2f(t+Td))

And with T = 0 Wm becomes:

Wm = A (1+KRcos(2fTd)) and

Wm/W= 1+KRcos(2fTd))

Geometry of measurement set-up

Loudspeakers measurements (6)Lets assume that the reflection coefficient R is 0.8, d=2m and h=1m. The plot below represents the ratio Wm/W. The combination of the reflected waves and direct waves results in variation of +/-50% of the measured signal at the microphone!

Wm/W+/-50%d=2m and h=1m

Loudspeakers measurements (7)Same plot as earlier slide but presented in dB

Wm/W-2/+1.5 dBd=2m and h=1m

Loudspeakers measurements (8)R is 0.8, d=5m and h=1m Wm/Wd/H=5d=5m and h=1m

Loudspeakers measurements (9)Wm/Wd/H=10d=10m and h=1m

R is 0.8, d=10m and h=1m

Loudspeakers measurements (10)As d/H increases, the measured response becomes less and less affected by reflections. This is the GROUND PLANE METHOD.Wm/Wd/H=50d=10m and h=0.2m

R is 0.8, d=10m and h=0.2m

MITIGATIONS THE GROUND PLANE METHOD

By placing the loudspeaker on its side on a large hard surface such as concrete (R~1) and placing the measuring microphone at a distance of 5m or more, it is possible to obtain a good measurement of the frequency response up to 3KHz or more. This is the simulation of a 2 environment. In this case, the actual amplitude is half of the measured amplitude (or -3dB). This technique is known as the ground plane method.

Wm/Wd/H=50d=10m and h=0.2mLoudspeakers measurements (11)This technique could be used up to 20kHz by correcting the measured response, using the data displayed in the graph in the upper left of the slide.

Loudspeakers measurements (12)If the measuring microphone is placed very close to the driver (10mm from the dust cap) and the driver at least one meter away from the floor . The amplitude of the reflected wave becomes negligible when compared to the direct wave. The difficulty with this technique is in recombining the measurements from each driver and port into a single measurement. This is the Near-Field Method. With care, this technique works very well.Wm/WNear-Field d=1cmNote the scale!

MITIGATIONS Near-Field Measurement

Loudspeakers measurements (13)NOTE TO THE HI-FI NUTS

HI-FI NUT, Definition: A HIFI nut is an idiot with more money than neurons happily buying the latest speaker cable at the modest cost of $10,000 or spending with immense joy $3,000 on a POWER CORD. Yes, you read well. A $3,000 POWER CORD! Do you really think that changing the 3 feet of copper wires from the socket to the amplifier will make a difference? Then, what about the 100 feet of wires connecting the socket to the breaker box? I can guarantee you that these wires are made of the cheapest material the builder could get away with. And what about the 100 miles of wires from the breaker box to the power plant? You dont believe me? Open, any HI-FI magazine at Barnes and Nobles. Half the ads are for speaker wires and other snake oil products. LOOK AT THE GRAPH AND SEE WHAT THE FLOOR REFLECTION DO TO YOUR MEGABUCK TOWER SPEAKERS seating 5m away from the speakers. The problem will certainly be resolved with a new power cord.

PortsTweeterMidrangeWooferSuperTweeter*Exponential Horn Project Design (6)Note: You may notice a strong resemblance with the $15,000 Alura speakers. However, I didnt know the existence of these speaker at the time. GREAT MINDS THINK ALIKE

Measurements - Bass EnclosureFrom a scanned hardcopy as all data (modeling/measurements, etc.) are lostPorts 1,2,3Near FieldWooferSummation

Port ReflectionsReflections from Back of EnclosureFlat Response (+/-3dB) from 35 to 300Hz

Measurements - Bass EnclosureFrom a scanned hardcopy as all data (modeling/measurements, etc.) are lostPlease note that the curves are plotted in relative amplitude as the measuring microphone is not calibrated for ABSOLUTE SPL.The bass enclosure is build with MDF. Each panel is routed, glued and screwed for maximum rigidity. Bottom, top panel as well as left and right panels are braced together to the back panel using MDF. The extensive bracing prevent panel vibration and reduces internal reflections (not enough).The bass enclosure provides a smooth response over the design frequency range (0 to 200Hz). The sensitivity (computed from model) is 98dB/W.Note the notch in the near field woofer measurement at the tuning frequency of the ports. This is typical of vented boxes (or Bass Reflex)The internal reflections can be attenuated by filling the box or padding the inside walls of the enclosure.In any case, the port reflections are beyond the design frequency range and 20dB below the woofer output.

Minimum woofer motion at port tuning frequency

Exponential horn measurements**Scanned from hardcopy

This is the only surviving hardcopy showing the horn frequency response. Unfortunately, it includes modeling of the horn enclosure with other drivers and a modified horn enclosure rendering the plot very confusing3D Finite Modeling using HornResp (dashed black)Measured Frequency Response (solid black)

Exponential horn measurements*This is the model and actual frequency responses of the horn. The data was recorded using the ground plane method.*Scanned from hardcopy3D Finite difference Modeling using HornResp (yellow)Measured Frequency Response (cyan)

Reflection from the back of the enclosure

Exponential horn measurements*Fabricating a horn is a time consuming, difficult and painstaking woodworking project. Such poor result after so much work is very disappointing. However, ALL IS NOT LOST *Scanned from hardcopyThe frequency response of the horn is ABSOLUTELY, COMPLETELY, HORRIBLE!

Surprisingly, it sounded quite good playing music with the bass and tweeter enclosures. Not only to me, but to anyone who listened to it...

First observation: The finite difference modeling from HornResp makes an excellent prediction of the terrible horn response. Frankly this is quite astonishing. I am indeed IMPRESSED!

Second observation: This is a very poor design

The deep notches above 1kHz are the result of reflection off the back of the midrange enclosure. (They look bad but are not heard normally.)Notch

Exponential horn measurements*However, when I compared the model prediction to the actual data, it became apparent that the horn behaved EXACTLY as it should. The problem is that it is a VERY POOR DESIGN.*Scanned from hardcopyThis horn was based on a series of white papers published by a supposed horn expert. (I will not mention the name)

I had not discovered HornResp yet. I began experimenting with the modeling software as the horn enclosure was almost complete and I was puzzled by the model predicted response.

When I began taking measurements of the horn, I was quite disappointed by the results but was unsure as to the cause of such poor data. Could it be a measurement problem?

Exponential horn measurements*BUT All IS NOT LOST YET*Scanned from hardcopyLESSON#1: Horns behave in a very complex manner. I dont believe now that there is a Cookbook approach that can deliver an acceptable outcome.LESSON#2: Never (ever!) underestimate the power of computer modeling. The measured frequency response of my poorly design horn is very complex. It is quite astonishing that HornResp provides an almost identical predicted response. This is no accident!LESSON#3: It appears from these results that my measurement techniques are reliable. (This is good news as measuring speakers is tricky business)LESSON#4: HornResp is a reliable modeling software. (User friendly, it is not!)

Exponential horn measurements*Horn Modification not done yet. Project is on standby*Scanned from hardcopyALL IS NOT LOST: The curved surfaces make building a horn a difficult and time consuming woodworking project.IT IS VERY DISAPOINTING to realize that all this hard work results in such a poor outcome. HOWEVER, it appears, from HornResp modelling, that the horn enclosure can be easily* modified to produce the RED response using the same driver. NOTE: HornResp indicates that it may be possible to design a horn with a 200Hz to 1.6kHz frequency response using an affordable driver (Eminence Alpha-6A $50). The +/-3dB ripples are perfectly acceptable.Modified Horn*i.e. a new horn section can be added to the throat of the existing horn, resulting in a longer horn with a smaller throat and higher compression ratio. A new driver enclosure needs to be fabricated.+/-3dB103dB/W

Exponential horn measurements**Scanned from hardcopyThe model indicates that the modified horn will cover the 200 to 1.6kHz range. As a rule of thumb, a mid-range horn works well within a 3-octave range (200x23=1200Hz). Therefore, a 1.6kHz frequency extension may be optimistic and we may have to limit the horn to 1kHz or 1.2kHz. (pending measurements)

A good quality 2 compression driver can easily cover the 1kHz-20kHz frequency range. The RADIAN 760PB (~$250) is a perfect example. (111dB/W@1m) (Quite expensive though)

RADIAN 760PB OUSTANDING FREQUENCY RESPONSE

Spl and powerHorn Modification not done yet. Project is on standby

SPL and POWER The midrange horn efficiency is 103dB/W@1m. When compared to a typical bookshelf speaker of 79dB/W, this is 24dB more.

To increase the Sound Pressure Level (SPL) by 3dB, the electrical power to the speaker must be doubled.

24dB/3=8 and 28=256

Therefore, 1W of power into the midrange horn produces the same SPL as 256W into a bookshelf speaker! (or 32W into a good floor-standing speaker-SPL=88dB/W)

Nothing equals the sheer dynamic range of a horn speaker!

Hornresp modelling - example

FAITAL-PRO W6N8-120 6 Midrange Driver

Bl>10 - Strong Motor for its classLow cone massLow Le Good for high frequencyThe combination of a strong Bl, low mmd and moderate Le makes the W6N8 6 driver from FAITAL-PRO a good candidate for horn loading ($129.00 at Part Express)

Hornresp modelling - exampleHORNRESP - INPUT PANELHORNRESP - HORN DIMENSIONS76CM

Hornresp modelling - example

Cone displacement almost reaches Xmax=5mm at 100Hz and 100W input power. A second order high-pass filter is recommended below 200hz125dB!HORNRESP SPL (100W input power)HORNRESP Cone displacement (100W)

Hornresp modelling - exampleFrequency Response (1W@1m) and Electrical Impedance in a hybrid (conical/exponential) horn using the Faital-Pro W6N8-120 6 midrange driver1W@1m

Wavefront simulation

Note the changing animation speed

Wavefront simulationWith baffles

Note the changing animation speed

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