Understanding directivity plots

The directivity plots shown in this blog will be unfamiliar to many, but they are not difficult to understand. They are essentially a frequency response plots  which measure from on axis right out to 90 degrees.

Here is one example:

The x axis shows frequency the same as a response plot, but the y axis shows the angle. Think of it this way. Imagine you want to depict a mountain range graphically. You could take a slice through and show a line at that point, but it would only show a small portion of the shape of the mountains. This is very much like a frequency response plot. They only tell part of the story in a very two dimensional way.

If you really wanted to show their shape in one illustration, you might use a contour map. It will have lines where the elevation at each point on the line is the same. It is as if horizontal slices were taken at different heights, and the line is the outline of each slice seen from above. A directivity plot is just like this, but we use SPL instead of elevation.

Interpreting the plot above

The plot above shows an open baffle speaker and I've chosen this one because it includes many of the features you will see with various designs, not normally all in one plot!

Region A - Here you see ideal constant directivity behaviour from 200 - 1k. Dispersion is around 100 degrees. This is very good performance from a small speaker. I don't know of any other type of speaker regardless of size that can control directivity in this way down to 200 Hz. Notice how all the lines are flat at all angles. This means the sound waves that are seen by the room are not altered in tone by the speaker.

Region B - Here the speaker begins to beam from 1 - 2k. This indicates a problem that needs some attention with this design. Ideally we'd like to see a gradual widening in this range, instead of becoming narrow then wider again.

Region C - Again it is constant with a wide dispersion of 150 degrees. Since this only occurs over a restricted range this is a problem. The room will see a little more energy here.

Region D - The most rapid narrowing occurs here then there is a narrowing of response. Above about 9k the tweeter is starting to beam. This is because this speaker used a 3" full range driver.

The most interesting part is the directivity control that comes from an open baffle arrangement.

What does the ideal directivity plot look like?

Something like this would be ideal. True constant directivity from about 200 Hz up to 20k. The ideal dispersion would be about 60 - 90 degrees where the response would be 6 db down. You can find that point by looking at the colour scale, 6db down equates to orange. On the vertical scale, look for 30 degrees off axis to find a total dispersion of 60 degrees.

See more dispersion plot examples on the waveguide shootout page >

Software used

Arta - download >

Arta was used for these plots. The demo version is free to use but saving files is disabled. You can export plots such as these on the demo version.

Looking inside a Faital pro compression driver

While working on an Econowave, I discovered the compression driver (Faital pro) had failed. Opening it up to investigate, I found the wire connecting the voice coil to the terminals had come loose. It was a simple quick repair job, but a little fiddly. In the process I had a look at the diaphragm and novel phase plug:

Here you can see the phase plug (orange). The centre hole does not let sound through. The radial slots around the outside allow the sound through.

Here you can see the diaphragm (lighter coloured part). The circumferential groove is the moving diaphragm, and you can see how the phase plug shown previously matches up. This unit was used in an Econowave.

Other compression drivers with a look inside:

BMS 4550

B&C DE250

Econowave deluxe - part 2

Both boxes are now complete enough for testing, with a port in the side.

Edges after round over and sanding.

Now they just need a little more filling and they will be ready for finishing.

Initial impressions

These have many of the qualities I like about my Miniwaves, however when it comes to achieving a certain effortless quality in the midrange, these go to another level. They could easily fill a large room. They can play very loud and clean.

Econowave deluxe

This is a deluxe Econowave that I've been involved in building. This one belongs to Roger, one of a handfull of DIY enthusiasts that have been getting together in Melbourne to test out some waveguide speakers and active crossovers. Compared to working in isolation, we get to experience more things without having to buy and it's a lot of fun. Above is the not quite finished first box. It needs some sanding, round-overs and finishing. 

Faital pro compression driver on an 18 sound waveguide. The compression driver has a novel phase plug and I believe the cost is similar to the B&C DE250 that I use. 

Drivers - Acoustic Elegance TD10M

This is one driver that I'm keen to hear. It does appear to be exceptional.

All the MDF panels CNC cut. This made it all very simple. 

We started by gluing the baffles. A larger cut means no routing necessary to flush mount the drivers. 

You can see the internal brace and the corner pieces that will help when the front edges are rounded over. Rather than use very large clamps, we opted to use screws to hold it together while the glue sets. The screws can be removed and filled later. 

The rear terminals will be flush mounted. 

Assembled first box.

Mostly complete first box. With the panels having been previously cut, all of the above took the better part of a day to complete. This speaker will be featured in the next waveguide shoot out.

Previous waveguide shoot out >

Continue to part 2 >

Where can I find reticulated foam in Australia?

You can source reticulated foam for waveguides and horns in Melbourne from Flex Foam in Box Hill North.

In contrast to other suppliers, they were a pleasure to deal with. One said that I would have to pay $300 - 400 to get a chunk of foam as they have to buy in bulk. Or use layered sheets, each 12mm thick, not a very attractive solution. Shown above is enough foam for 6 waveguides, ranging from 6" - 12" in size. The cost is quite reasonable.

Why foam?

Earl Geddes has demonstrated that horns and waveguides sound better when filled with foam. Not just any foam - reticulated foam is needed to have the right balance of absorption without too much attenuation. The foam is intended to absorb to a greater degree the reflections within the horn that cause sound quality problems. I have not yet tried it, but by all account it is a worthwhile improvement.

What to ask for 

The foam you want from Flex Foam is called "Mericell." It comes in different densities, you should ask for 30 ppi (pores per inch). It is also referred to as a filter foam, but it's technical generic name is reticulated foam.

Does it really work?

When I have had a chance to test it out properly, I will be reporting my results, both measured and subjective.  I have another shoot out planned where we will compare different waveguides and drivers. The challenge at this point is to find a good way to cut the foam.

Review: Linkwitz Orion open baffle speakers

For sale: The speaker reviewed here is currently up for sale on the StereoNet Classifieds >

In the world of speaker design, if you haven't heard of Linkwitz, you haven't heard of anyone.

Not too many designers can boast that they have a crossover named after them - arguably the most popular crossover is the Linkwitz-Riley 12 or 24 db/octave. After a long and distinguished career, Linkwitz has created a speaker which combines everything he has learnt. It is named the Orion.

Recently I had the chance to hear the Orion with all current upgrades included.

What is the Orion?

The Orion will be familiar to DIY enthusiasts. It is a 3 way dynamic driver open baffle speaker. It includes a rear firing tweeter so that dipole radiation is maintained full range.

Listening Impressions

Very refined, very high resolution, great clarity. The sound stage is very wide and deep and the sound is very immsersive and there is considerable envelopment. I can see how many have called this speaker their last. And one very strong point for those who will place them in a living room, they excel in a normal untreated room.

They come with a large multi channel power amp recommended by Linkwitz and an analogue active crossover, which is a very elaborate crossover, one of the most sophisticated you will ever see. The box is well built with solid hardwood and the system has been upgraded numerous times to keep pace with successively improvements discovered by Linkwitz.

How does it compare to other high end speakers?

There are two very distinct differences. The first is the sound stage, the second is the bass. Both are very different to conventional box speakers. The sound stage is larger than just about any speaker you are likely to hear. Even in a small room this can be achieved. The sweet spot is also larger. The bass seems to be less room sensitive and so high quality bass can be achieved in a normal room without treatment. One of the main advantages of a speaker such as this is that it can be placed in a room without any treatment.

SBIR - Speaker Boundary Interference Response

Speaker Boundary Interference Response (SBIR) is one aspect of speaker/room interaction that is often ignored in home audio. What is it? In a nutshell, it refers to one way in which the nice looking anechoic response of a speaker gets messed up in a real room.

Here is a simple illustration, showing the direct sound and a reflection off the front wall:

The reflected sound wave causes cancellation, resulting in a dip in the response where the difference is equal to a half wavelength. That dip can be calculated:

f = 172 / x where x is the distance (reflected - direct) in metres

For the example above with a 1m offset from the front wall:

Direct = 2.6m
Reflected = 4.6m
x = 2m

f = 172 / 2
f = 86 Hz

Hence with 1m offset from the wall, a dip will be seen at 86 Hz.

If you know the frequency

If you are looking at an in-room plot, and want to know if a particular dip is related to a boundary issue, use the formula to find the distance.

 Wall offset = f  / 86 (metres)


Avoid similar offsets

If the offset from each adjacent boundary is the same, the dip will be worse. Hence it's best to avoid having similar offsets. The worst case scenario is one in which the offset from the floor, front and side walls is the same.

Confusion

In looking at the frequency response, it's easy to misinterpret. Where you see a dip, is it caused by SBIR, room modes or phase shift between a sub and mains? The phase issue can be eliminated by measuring a woofer only so there is no crossover with a sub included. Then you can calculate the points where you should see SBIR dips. It is possible to combine a room mode peak with a boundary interference dip and get them to cancel, although often this won't be possible.

Dealing with SBIR

In recording studios where SBIR gets the most attention, speakers are often soffit mounted to avoid this issue. The entire speaker is flush mounted into an angled baffle. Woofers can also be placed near the floor to avoid floor bounce. Room modes should also be considered at the same time because the best position for SBIR considerations may not be better regarding modes.

Acoustic Treatment

One way to tame SBIR is with acoustic treatment, where the reflection is damped with some absorption that is adequate to work down to that frequency. Another strategy which might be useful in small rooms is to mount the speaker against the front wall and use some treatment to tame early reflections. It won't work well without treatment.

It's also important to balance imaging considerations. The challenge is that the best location for imaging might not be the best for bass. This is where 3 way systems using active woofers come into their own. The midrange driver can be placed for best imaging, while the woofer can be close to the floor. There is more flexibility to achieve the best of imaging and bass response.

Here a problem with many stand mounts and floorstanders becomes clear. When placed in a real room without paying attention to boundary issues, the lower midrange will often become recessed while room modes introduce peaks in the bass. This means the bass will boom, but the lower midrange will lack fullness. It's something that many audiophiles have adjusted to, but those who seek accuracy in their systems will often not know what they are missing.

Bybee Quantum Purifier - snake oil?

Are they snake oil or are they a genuine upgrade to your system?

Extraordinary claims

These are a passive device that are claimed to improve just about any part of your system. Insert them anywhere in the signal chain and it is claimed that you will see improvements.

We are told that they "operate on the quantum mechanical level to regulate the flow of electrons that make up the signal (picture a metering light regulating freeway traffic flow). " This would suggest that they convert ordinary cables into super conductors, however, this is then contradicted soon after where it is mentioned that they strip away excess energy and dissipate it as heat. There is a simple common name for this device. It is called a resistor, and they are much cheaper. They perform specific function in crossover networks and electronics, but they never come with such grand claims and a premium price. Resistors are dirt cheap. 

The marketing text is seductive and they are quick to mention right at the start that it based on the work of a respected physicist who develops technology for the military. If he works for the military, he must have some serious credibility!

Do they measure up?

A moderater on DIY audio decided to measure one of these devices to see if any validity could be found in their claims. The results are included here:

DIY audio - measurements

What was the verdict?

Sy who peformed the measurements says in summary:

The physical specs and the physics claims are sheer fraud, at least in the few places that they're not meaningless gobbledygook.

The noise reduction claims are interesting. Somehow, the devices are supposed to reduce the noise of other parts in the signal chain. Experiment did not support this! Nor did experiment support the implied increase in electron velocity (I'm rather proud of the hack I used to measure that!). 

Subjective reviews

A 6 moons review has a very different tone.

6 moons review:

My findings were not as dramatic as some reviewers have reported but these devices proved to make clear and significant improvements without any downsides except for the hefty price of admission - $880pr. 

Enjoy the music.com:

Based on my experience with the interconnect purifier, their sonic effect is clearly cumulative with additional devices upstream from the speakers. The magnitude of purity and focus engendered by this device would typically only be the result of a major system upgrade. 

In my opinion

I will admit that I have no experience with this product and I have not measured it. Some will for that reason dismiss my opinion immediately. That is their choice. 


My father often said "a fool and his money are soon parted." In the pursuit of audio perfection, many audiophiles become vulnerable to many clever seductions. The industry makes a great deal of money and audiophiles are a quirky bunch who after spending their money, seem reluctant to admit they have not actually achieved an improvement. Most consumer products that don't deliver on promises result in quick returns. However, in this industry, a manufacturer can get away with a dubious product and it's customers who have gained nothing real are the ones who speak up in their defence. They well know that those who buy the products do not know enough about physics to see through the ridiculous claims.

So why do people think they hear a difference? The main reason is that they want to hear it, especially after having spent money. When you listen for a difference, it is very easy to think you actually do hear an improvement. The brain is very good at finding something new in a familiar piece of music, even with no change. It's a placebo effect. There is another reason for wanting to hear a difference. There is a certain pride many audiophiles feel in being able to hear things others can't. They want to be able to hear it, and don't want to admit they hear no difference. With this kind of belief and value system in place, the brain obliges by providing a perceived difference. The attention becomes focused on details not noticed as much before. There is a real change, but it has been provided by mental processes only. It does not exist, except between the ears.

I suggest if you are considering such a purchase, instead put your money into something real. Invest in some acoustic treatment for your room. The returns are very real and beyond question. No one ever does blind tests for acoustic treatment. There is a reason for that!

Active surround box - update 2

See part 1 >

Almost done. The wiring works out to be more complicated than expected!

 

My solder holder. I previously used a clamp to solder RCA plugs, pots, etc. Makes soldering easier. 

 

Connections to MiniDSP. This is a header which can be snapped off to get the right size. Quite fiddly to solder and easy to apply too much heat. You may well ask why use shielded interconnect cable inside this box? It really isn't necessary as the box acts as a shield, but extra doesn't hurt. It ended up being much more difficult, so from this point I'll simply use hookup wire. Much easier to work with.

EDIT: I decided to ditch this shielded cable and replace it with unshielded twisted pairs. 

 

Pots have a little lug that require a mounting plate. The smaller hole is for the lug that prevents the pot from rotating.  Just a small sheet of acrylic I had handy.

Passive attenuators inserted in between MiniDSP and the power amps. You can see one of the headers that goes into the power amp module. Input connections to MiniDSP have not bee added yet.

Active surround box - update

Read more about this project here:

Active surround box

Drilling can be fiddly. The back plate has been sanded down and re-sprayed due to some scratches, and I didn't like the gloss finish.

Bottom panel - also has been re-sprayed to avoid exposed edges and rust. Heat sinks will go on top of the ventilation holes.

Brackets to attach the heatsinks to the case. Aluminium.

Part 2 >

A sad day for DIY audio - in honour of Zilch

It is a sad day for DIY audio. One of the great contributors to the DIY audio community, Zilch, has passed away.

Zilch was behind the Econowave project, a speaker concept generously offered to the DIY community as an alternative to the ubiquitous floorstander.

He will be sorely missed.

You can read a eulogy offered by a friend he worked with extensively on the Audiokarma forum.

Eulogy - RIP Zilch

Should I bi-amp my speakers?

This question comes up frequently on audio forums, where the original poster asks:

"Should I bi-amp my speakers. I have extra channels in my AV receiver that aren't doing anything. Is it a good idea?"

The short answer is "no, you shouldn't."

Just to clarify, here we will talk about passive bi-amping, where the existing passive crossover remains and one channel drives the tweeter while the other drives the mid. This is different to active bi-amping, in which an active crossover is used and replaces a passive crossover. Each driver has it's own amplifier and the crossover is inserted at line level before the amplifiers. This is a good idea if done correctly, but requires measurement tools to implement.


Reason no.1 - there is no advantage

No extra power is delivered, since the passive crossover remains in the signal chain. While your receiver might in fact deliver 2 x 100w instead of 1 x 100w to each speaker, the extra 100w is still lost in the passive crossover. One channel feeds the tweeter, and about 85% of the power that would normally have gone to the mid is filtered out. The other channel feeds the mid, and about 15% of the power that would have gone to the tweeter is filtered out. The net result is no different.

Some who have misunderstood passive bi-amping, may claim that clipping can be avoided. This is an advantage of active systems, but with passive bi-amping, both amps if identical will clip at the same time.

There are no magical benefits to passive bi-amping and there are no real ones either. 

Reason no.2 - there is a downside

The extra channels in your AV receiver that are not used are not wasted. Affordable receivers are not well designed to deliver a continuous signal to all channels at the same time, the power supplies and heatsinks are simply not up to the task.They are counting on surround channels having an easier task. If some of those channels are not used, that adds headroom the reduces the load. This is a good thing, not an idle waste of resources. If those channels are made to passively bi-amp the mains, they will not only use up that headroom, but they will present a heavier load than was intended for those channels. When pushed, the amp will clip sooner.

Less is more

If you don't need any of the surround channels, give your receiver a break and let them remain unused. Less in this case is definitely more. You will no doubt hear some report benefits and subjective improvements, as you will with many tweaks, snake oil or otherwise. The simple truth is that we are easily tricked, especially when we want to believe. The appeal of this tweak is that it's so easy to do, and if you've asked this question, you are probably looking for a simple easy way to get an improvement. I suggest instead that you have a look around here, and I will show you other ways to get a very real improvement. If you are new to the world of audio, then some of the improvements you can experience are startling. Let me suggest a few things that just might make your jaw drop:

• DIY subs - suddenly the sub you couldn't afford looks quite unimpressive
• Learn the tricks of integrating your sub
• Add some serious bass traps - what you thought was good bass will soon sound like rubbish in comparison and you will be surprised how good your existing sub can sound
• Consider building some good DIY speakers - the result could be a real eye opener
• Pay careful attention to speaker placement - small changes can have dramatic results
• Build some acoustic treatment - the difference is not subtle

Some of the biggest improvements you will ever experience are there and in relative terms they are not exceptionally expensive. 

Can a 6 ohm amp drive 4 ohm speakers?

It's never a good idea to driver speakers with an amp that isn't designed for the load. If your amp or receiver is intended for 6 or 8 ohm speakers, a lower impedance can cause problems.

If you do connect speakers with a lower than recommended impedance, your amp will probably not blow up immediately. Consider this a temporary pairing at best and if you insist on an imcompatible pairing, you will need to take some precautions as outlined below.

The reasons behind limits

When speakers are connected to an amplifier, those with a lower impedance present a greater challenge. The lower the impedance, the higher the current and power output. Dedicated power amps have a relatively simple task and are normally able to handle a 4 ohm load which is adequate for almost any speaker. AV receivers cram a great deal more into one box and also have 5 - 7 channels or more. The most popular budget to mid level receivers are bound by economical restraints and so are forced to cut some corners on the most expensive parts - the heatsink, transformer and the case itself. The result is the power supply is not able to supply the extra power demands that come with a lower impedance. Typically with a 4 ohm load the amp would output 50% more power. Downsizing the power supply means the amp will run out of power and the amp will clip. The rest of the amp is then designed for either a 6 or 8 ohm load which is less demanding. Downsizing the heatsink means that the output devices will heat up quicker if driven hard for an extended period.

In a nutshell, features sell AV receivers. The hidden parts that make an amp stable into 4 ohm loads are much less impressive, hence it is only higher end units that can drive such a load.

What happens if you push the limits

If you push an amp, two things can happen. The first is that it will reach it's limit and then run into clipping. At this point, the sound quality takes a hit and there is the risk of damaging the tweeters. The highest risk occurs with music where the level is continuous. Movies present a lesser risk because of the greater typical dynamic range. The second thing that can happen is that the amplifier overheats and goes into thermal shutdown. If this happens, you haven't damaged your amplifier yet, but you are shortening it's life and can expect it to fail if you don't make some changes.

Other factors

There are two basic ways to avoid having problems:

1. Control the volume sensibly - you will need to reduce it significantly.

2. Avoid the build up of heat

Firstly, the amp should not be placed into a closed rack and if this can't be avoided, fans should be installed that come on automatically (without fail) when the system is in use. Ideally recommended clearances around the amp on all sides should be followed, perhaps to a higher standard than found in the manual. Avoid sources of heat. Never place another component on top or block ventilation openings.


Alternative solutions


1. More efficient speakers

Speakers with higher efficiency require much less power. Commercial options like Seaton Sound and JTR offer value for money solutions. DIY solutions such as the Econowave speaker are also viable alternatives.

2. Dedicated power amplifiers

Dedicated power amps will handle 4 ohm loads with ease as this is the task they are designed for. Affordable options include Behringer A500 which is a studio amp with 230w into 4 ohms. For a little more, the Alesis RA500 can do a similar job. Emotiva also offer very good value for money multi channel power amps which make most receivers look like toys.

Multi channel Power Amps & Crossover

This project contains 4 power amp channels and a digital active crossover module in one box:

Above: power amp modules and MiniDSP sitting in place, not yet connected.

This is a simple low cost project that challenges two criticisms made of active speakers in the past:

• they are more expensive
• they clutter up a rack with many extra boxes

Low cost

The MiniDSP crossover costs $100. This is comparable to the cost of a decent passive crossover for a 2 way speaker. It's easy to spend more, and this is excluding the "development cost" where multiple versions of the crossover are built. 

The amp channels only cost $200. The case which is shown above only cost $52. The total cost is around $400, or less for those spoilt individuals in the US who get everything cheap! 

In the past, a more typical choice would have cost around $500 for the lowest cost crossover (Behringer DCX), then around $600 for a pair of Behringer A500 studio amps. This option would also cost considerably more once a host of XLR cables and connectors were purchased. 

It isn't too difficult to find power amp modules such as these on ebay, which come pre-built and only require inputs and outputs to be wired up.

Single box solution

This option is far neater and more compact with no Vegas-style flashing lights. Cable runs are minimised and kept shorter, and the need for many different connectors is eliminated. This is the ideal situation. Traditionally, audiophiles have been led to believe that separates are better. Often this will be true, but only where better quality parts and design are used at a much greater price. All other things being equal, a single box is better due to shorter cable runs. It is certainly a more cost effective solution.

This project will feature two RCA inputs and two 4 pole speakon speaker outputs. This prevents accidental wiring mistakes where the tweeter and mid outputs are mixed up. 

More details and photos to come when complete. 

Speaker cables >







Update:

Build photos here

How to avoid being misled by speaker sensitivity ratings

Audio enthusiasts are very often misled regarding speaker sensitivity ratings. Manufacturer's vary greatly in the degree of accuracy in the way they use this rating. They typically fall into one of three categories. Firstly, those in the minority who are brutally accurate. Secondly, those with slightly optimistic ratings and thirdly those that are downright misleading. The latter can be a problem if you need efficient speakers to work with a low powered amp. In this article you can arm yourself with sufficient knowledge to read into the specs and determine the truth. You will be able to look at the most misleading and inflated numbers and determine their true sensitivity.

Understanding voltage and watts

Speaker efficiency can be rated in two ways. Voltage sensitivity is defined as the output in decibels at one metre with 2.83V input. As I will show, this rating can be misleading because the power input varies according to impedance. 2.83V into an 8 ohm load equates to 1w of input power, but the power input will be different into any other impedance. I prefer the other method of rating sensitivity, which shows 1w input so that different speakers can be compared.

2.83V into 8 ohms = 1 watt
2.83V into 4 ohms = 2 watts (Voltage rating is 3 dB higher)
2.83V into 2 ohms = 4 watts (Voltage rating is 6 dB higher)

If you are comparing speakers, I suggest you convert the Voltage sensitivity into a 1w equivalent so a fair comparison is made. In some cases this may be difficult where a complex impedance load is presented.

How to estimate sensitivity

If you know the drivers used in a speaker and have access to the data sheet, you can estimate it's sensitivity.

In this example, the Vifa P17 is shown. It is a good example because it is a well behaved driver with it's sensitivity shown correctly. Some data sheets are also quite optimistic and you have to look at both the nominal sensitivity and the response plot and note the conditions under which it was measured. This is a text book example with a smooth response and a nominal 88 dB rating that agrees with the plot.

If this driver were used in a 2 way speaker, it could achieve no greater than 88 dB 1w1m if it were designed to be placed right up against a wall. In this scenario, the wall provides reinforcement of bass and midrange frequencies.

If it were designed as an audiophile stand mount or floorstander, then it would be placed typically into the room. This means boundary gain is given up, and a typical design would allow for this in the crossover. The result is at least 3 dB lower sensitivity. We could now expect 85 dB 1w1m. The effect referred to here is bafflestep compensation, and it is included in every well designed speaker that is not specifically intended for near wall placement.

If another identical woofer is added to cover the same range, in either a 2 way or 2.5 way design, the sensitivity can now return to 88 dB 1w1m. An equivalent voltage sensitivity would be 91 dB since the impedance would now be 4 ohms.

Estimating without data sheets

Where you only have basic specs, here are some suggested ratings.

Speakers with single vented woofers (add 3 dB for a second woofer), sensitivity 1w1m:

5" woofer ~ 65 Hz (-3 dB): 85 dB
6.5" woofer ~ 40 Hz (-3 dB): 85 dB
8" woofer ~ 40 Hz (-3 dB): 88 dB
10" woofer ~ 25 Hz (-3 dB): 85 dB
12" woofer ~ 22 Hz (-3 dB): 86 dB

For a sealed box, the extension is reduced by one octave for a given efficiency. So if the above 12" woofer were placed in a sealed box it would have the same efficiency but around 44 Hz extension.

Optimistic ratings are common

Do you find these number surprising? They are significantly lower than typical ratings published. The problem is that it's very common to show very optimistic numbers. A manufacturer faces a dilemma. If they show an accurate rating, they will be unfairly disadvantaged while others compare their speakers to inflated ratings. Suppose they were to show a rating of 85 dB 1w1m while another show a voltage sensitivity of 92 dB. Many will assume that the higher rated speaker is more sensitive, but the two speakers could in fact have matching sensitivity. I have seen examples of manufacturers who quote a sensitivity rating that is higher than the sensitivity of the bass driver used.

Hoffman's law

The relationship between speaker size, extension and efficiency is referred to in Hoffman's Iron law. You can't have small size, deep bass extension and high efficiency in one box. Where two of these three parameters are chosen, the third becomes a given. Suppose you wanted to achieve 40 Hz extension and 92 dB sensitivity from a speaker. Having chosen two of the three parameters, the size becomes a given. One would need a sensitive driver, most likely a 12" driver with a raw sensitivity of 95 dB before baffle step. A vented alignment would be required, and the box would then need to be as large as required to tune down to 40 Hz. Choosing the right driver will keep the box as small as it can be. If the box volume were restricted to one cubic foot then the choice is restricted to a 10" or 12" sealed low sensitivity driver or an 8" vented alignment with around 85 dB 1w1m.

It's also worth noting that I've discussed anechoic extension figures. Typical specs may show numbers which are based on an in-room guesstimate.

Further investigation

If you would like to investigate this topic further, you might start by downloading driver data sheets, paying attention to the nominal sensitivity, impedance, response plot and any comments on how the measurements were made. Also note that some measurements are taken at different distances. Measurement at 1 ft increases the rating by around 9 dB compared to 1m. Infinite baffle measurements exclude baffle step considerations, and some drivers are measured with no baffle at all which tends to create early roll off.

You might also like to simulate a range of drivers in WinISD, which shows the performance with 1w into half space conditions. This will give a good indication without considering baffle step.

Confused?

This topic can quickly become confusing for many who don't get involved in the technical side of audio. Understand that if you are comparing two speakers with an 8" vented woofer with about the same extension, they will have about the same sensitivity, even if their sensitivity ratings are very different.

Speaker cable: what matters

I suggest you first read my previous article which addresses the question of whether speaker cable matters. While this article implies that it does, you have to ask if it does warrant attention in your case.

General considerations

Aesthetics
Does it match your system and appeal to your preferences?

Pride of ownership
Are you really willing to pay for the name?

Safety
Amplifier damage can occur if the leads are shorted while the amplifier is on. Most cables offer no protection here, pro cables being the exception.

Wiring errors
It should be easy to see + and - so you can wire them up correctly.

Child proof
Do you have children who might pull them out or possibly even short leads out?


Secure connection
This can be an issue with banana plugs - not all of them offer a secure connection.

Cost
Don't be fooled - cost is not the best indication of quality.

Sonic considerations

In terms of sound quality issues, I suggest that there are three parameters that warrant your attention - resistance, capacitance and inductance. The impact of those parameters are relatively subtle and I don't consider other issues to deserve attention.

Resistance

Ideally resistance should be as low as possible. This can be achieved with short cable runs and thick cable. The cable gauge required is related to distance. If you have a long run of thin cable, then losses can result. Resistance also has an impact on the damping factor an amplifier can provide. It could be tempting to simply use very thick fat speaker cables, but in practice this is not necessary and this would likely make compromises in other areas.

Capacitance

Cable capacitance is one parameter that can create problems for some amplifiers. Naim is one notable example. Some of their products are not stable with cables that have relatively high capacitance. They are a relatively rare exception to the rule and most amplifiers should not have a problem.

Inductance

Inductance has an impact on high frequency extension, where a higher value causes roll off. This is likely to count for a large part of sonic differences heard between cables, where high inductance may cause treble roll off. As a result we would like inductance to be as low as possible, especially with long cable runs.

The inverse relationship between L and C

Inductance and capacitance have an inverse relationship - you can only lower one by raising the other at the same time. Spacing + and - apart in a figure of 8 configuration reduces capacitance, but increases inductance. This makes for a general purpose cable that is safe with any amp, but falls short of the ideal. A better choice except in the rare case where the amp can't cope is to lower inductance and accept a higher capacitance. This is what most high end speaker cables aim to do in various ways. This is done with some attention to cable geometry, where both conductors are closely coupled to reduce inductance. One of the best cables for this is actually not a dedicated audio cable.

The ideal cable is?

High power antenna cable! RG213/U which is a coaxial cable. The coaxial geometry achieves a very low inductance and this is essential for it's intended application where very high frequencies must run over some distance with minimal loss. The centre conductor has low resistance and the shield can be used for the - cable.

Other popular choices include Cat 5 network cable configured in various labour intensive ways or ribbon cable with a series of + and - cores running in parallel.

RG213/U simply requires lead out wires on the ends and some form of termination.

My own recipe

My own recipe for audio cables includes:

1. RG213/U coax for low inductance
2. Lead out wires on the ends
3. Pro 4 core speakon connectors to suit my active speakers and prevent wiring errors or shorts
4. Techflex sleeving for an attractive finish
5. Heatshrink to finish it all nicely

I will be displaying them when they are finished. The connectors are different to what is normally seen in home audio.

Does speaker cable really matter?

My answer to this is similar to many other issues in audio - it depends. Speaker cables generally get more attention than they deserve. Compared to the bigger issues like room acoustics, the quality of recordings you listen to and the accuracy of your speakers, the differences between cables are relatively minor. The real question that most should ask is "are the differences worth my attention right now?" If you can answer all these questions with a definite yes, then continue on to focus on cables.

1. Is your room acoustically treated, including as many bass traps as you can fit and a sensibly chosen mix of diffusion and absorption? If not, address those issues first, the entire system will improve dramatically.

Required reading:
Take the free bass trap test
Confessions of a bass trap luddite

2. Do you have the best speakers you can afford?

3. Does your system impress you with the way that it can place musicians in the room, or even transport you to another venue? If not, you have other more important things to sort out first. A system should image well with even the most basic speaker cable.

4. Can your system reproduce bass with depth, power and a tight punch without any hint of boom? Don't accept missing or poor quality bass!

5. Can your system reproduce all the kinds of music you enjoy equally well?

6. Do you have albums that you enjoy both for their musical content and for the quality of the recording? If not, you're better off searching for new music.

Did you manage to get this far and answer yes in all cases? By now you've probably realised I view speaker cables upgrades as the icing on the cake for a system that is already well rounded and well put together. However what many audiophiles like to do is like the teenager who goes and spends all his money on the best tires and mag wheels that he can afford. Unfortunately his bomb car will never really gain much benefit!

Now the next question is what matters about speaker cable?

DIY active speaker cable

Active speakers are best served with different speaker cable. Pro audio has the answer:



Here is a speakon chasis socket (left) and a speakon plug (right).

What makes it ideal for active speakers? This cable allows 4 cores, and once wired correctly there is no way you can mess anything up. You can't accidentally invert the polarity of one driver. You also can't hook up the tweeter to the bass signal. That is a nice feature.

 

You can see the strain relief on the right. The cables you would use with this connector are similar to mains power cable, but with 4 core rather than 3. You can get such a cable at Decibel Hifi for a reasonable cost, but there is nothing to stop you using any cable that appeals.  Multicore is simply more convenient.

 Above you can see the parts. Strain relief on the bottom prevents pulling the cable from putting strain on the ends of the wires. Cables can be screwed in, or soldered.

 

 

I bought 8 connectors as they were going cheap on ebay. It's worth looking there since you can often get good deals. Retail prices are a little steep at $15 for each connector and $10 for the chasis sockets. I got there for $3.80 each and the sockets were $20 for 4 including shipping.

I will be adding techflex sleeving which is reasonably priced at Chrome Audio.