November 28, 2010

How to measure your crossover with Holm Impulse

Coming soon

How to measure your subwoofer (and find out what the room is doing)

Coming soon

How to measure your subwoofer (without the room)

Coming soon

Measurement techniques - an introduction

There are various methods, depending on the application. Here are some quick suggestions.


For choosing the placement of a subwoofer:
Farfield measurement - place the mic in the listening position and measure with REW. You will see many peaks and dips as the room response is included.


For choosing subwoofer filter settings to integrate with the mains:
Nearfield measurement - place the mic as close as possible to the cone while avoiding contact. The room is excluded and you will see a smooth result.


Crossover design:
Gated response to avoid room reflections - Mic @ 1m with speaker and mic elevated

Types of measurement

1. Farfield Measurement

A farfield measurement is taken at a greater distance than most other measurements and shows the influence of the room. When full range measurements are made, the top end will be generally rolled off because most rooms absorb more of the top end. The bass will usually have large peaks and dips in the response. +/- 20 db swings are not uncommon.

Here is an example:















The dark blue line shows the speaker response, but light blue is a farfield measurement. Notice the roll-off  above 1k and the many peaks and dips. This is the influence of the room.

This type of measurement is mostly used for the bass range. It is useful in consdering the impact of room modes. Farfield measurements are best for choosing the location of speakers and subs. 

2. Gated free field measurement

The above chart also shows a gated measurement at 1m. Before any room reflections arrive, the software gates the data and stops recording. This gives a result similar to an anechoic chamber. The problem is that the bottom end is limited. If early reflections are present, the bandwidth will be limited. If the time window is shortened, the lower measurement limit is increased.

The speaker should be placed so that the first reflection point is as far away as possible.This means elevating the speaker and mic, but not beyond the vertical midpoint of the room. Otherwise, the ceiling will introduce an earlier reflection.

If done outdoors, a fullrange measurement can be done if the speaker is elevated high enough.

3. Nearfield measurement

Nearfield measurements are valid only for bass up to 100 Hz. The room response is swamped due to the mic being very close to the cone - as close as possible without risk of contact.



 

In this example, a sealed Rythmik servo sub is shown with different crossover settings. The response anomalies at around 160 Hz are as a result of a chamber that was installed for a bandpass experiment. Normally this measurement would be even smoother.

4. Ground plane

Ground plane measurements are useful for bass. A speaker is placed on the ground (typically outdoors) and the mic is usually placed on the ground at 2m away. Unlike a nearfield measurement, ground plane can operate up into the midrange and a location should be chosen that is free from reflections. Gating should still be used, but in a suitable location the useful low frequency limit will not pose a problem. Where reflections pose a problem, a ground plane measurement can be combined with a nearfield.

5. Half space

A half space measurement may be chosen to avoid reflection problems, but it can add problems of it's own. The speaker is mounted in a baffle that is flush with the floor/ground. It's important that it be flush, or baffle diffraction will contaminate the data. If it's not possible to flush mount, acoustic treatement can be used and ideally this should be flush. Otherwise the problems may in fact be more than those this method tries to avoid.

Compared to a conventional gated freefield measurement, this method avoids the first floor reflection and the need to elevate speaker and mic. The gating will only need to avoid reflections from walls.

Adding them together

No practical and accessible method can do it all. An anechoic chamber or a gated freefield measurement high up in the air are the ideal choices, but they don't suit the casual enthusiast with 2 hours on a Sunday afternoon available. The solution is to combine various measurements and use the one that fits.

Further reading


How to measure your subwoofer (without the room)
How to measure your subwoofer (and find out what the room is doing)
How to measure your crossover with Holm Impulse

Why measure? Using a microphone vs doing it by ear.

You might need to take measurements for:
  • finding the best place to put a subwoofer and other speakers
  • crossover design
  • find out what is causing boomy bass in your room
  • find out if the new tweeter really is a drop in replacement
  • to see if anything is wired out of phase
  • to choose the best settings to integrate your subwoofer
  • find out what those bass traps are actually doing.
Every system can potentially benefit

November 14, 2010

Flat response - does it really matter?

Coming soon - I'm going to talk about the importance of a flat response.














This chart shows my current speakers measured in two ways. The dark line shows a gated measurement at 1m - this is the kind of chart normally shown for a speaker. The light blue line shows a farfield measurement taken in the listening position. The differences are that the 1m measurement includes only the sound from the speaker, while the listening position also captures the influence of the room.

The farfield has a 25 db range, while the gated measurement only 15 db. Considering that the response overall is sloping down, the speaker is actually fairly flat.

3 way budget active system

I'm currently "in-between speakers." Having sold the TLs, I'm using some cheap speakers while I get around to putting together something more serious.

Drivers:
Vifa D19 poly dome
Vifa C17 paper cone (modified)

Rythmik servo subs
Crossover: Behringer Ultradrive DCX 2496

Here is the crossover in it's current state:














So you can see the crossover is at 3k and the polarity of the tweeter is reversed (otherwise there is a dip).

Many people think that the crossover will be where you set the filters. This is usually far from the truth. The electrical filters and acoustic driver response are added together. The settings used to get this response are:

Mid: 3rd order butterworth low pass @ 2k
Tweeter: 4th order Linkwitz Riley @ 2.4k

This is quite some distance from the 3k crossover point. In order to get this crossover to work, I also used a little parametric EQ around 2.4k.

The sound is quite different to what I have been listening to. I've been previously running the system time aligned using the auto align feature on DCX. I don't trust the delay settings used, so I have taken them out. The imaging is clearly different. It's possible to make big changes here quickly and easily, but it can take some time to find settings that you are happy with.

November 13, 2010

What size is this driver?

Ever seen a driver where you can't tell the size from the specs? Perhaps they forgot to mention it.


Here is a general guide. You can tell from the "effective piston area" (SD) in the specs

3" driver - SD: 30 cm2
4" driver - SD: 57 cm2
5" driver - SD: 85 cm2
6.5" driver - SD: 135 cm2
8" driver - SD: 235 cm2
10" driver - SD: 350 cm2
12" driver - SD: 500 cm2
15" driver - SD: 800 cm2
18" driver - SD: 1180 cm2
21" driver - SD: 1550 cm2
24" driver - SD: 2200 cm2

Note: Pro driver with an accordion surround might have a higher SD for a given nominal size. The nominal size is usually based on the outer dimension. 

November 11, 2010

Bass traps vs EQ

Coming soon

Room EQ example

Update
I strongly recommend that you aim to add bass traps to your room. Not convinced it's worth it? Take the free bass trap test.














Black: Raw response
Red: After EQ

This setup includes sealed stand mounts crossing at 80 Hz to sealed Rythmik servo subwoofers, where the mains sit on top of the subs. Behringer Ultracurve was used for EQ, with one PEQ cut for the midbass, then the auto EQ feature was used to smooth things out a little as well. In this case, attempts to boost the dips were manually defeated.

In this particular room, the listening area is small and the two critical seats have only a slight variation.

It would be difficult to get this result with acoustic treatment, but this does illustrate a point. Room EQ is effective where acoustic treatment is not. It becomes increasingly ineffective where acoustic treatment is best used - above 100 Hz. I will be aiming to improve the region above 100 Hz with acoustic treatment down the track.

November 10, 2010

November 9, 2010

Bass management

Bass management is a tricky and complex subject. Loudspeaker designers spend a considerable amount of time with professional tools and a great deal of experience to design the passive crossover in a 2 way speaker. The challenges involved in the crossover between multiple speakers and a subwoofer are also considerable. One could argue that it is more difficult and more complex than the 2 way passive crossover, yet this task is left to the consumer who tries to do it without the right tools and usually without an understanding of the basic issues.

A textbook solution

Here is a vented subwoofer (red) and a sealed main speaker (blue) along with the same speaker vented (green). They are based on a typical 6.5" driver.


Here we have a 4th order crossover. Electrical filters are 4th order low pass on the sub and 2nd order high pass on the mains. A 4th order Linkwitz Riley crossover sums to a flat response where filters are 6db down. So with the sealed box we get a flat response, but the vented version isn't quite right.

Now what happens if we were to apply 4th order filters to both subwoofer and mains?



This arrangement now suits the vented design better, since it's now close to the 6db down point, while the sealed box will have a dip.

Electrical vs Acoustic
It's important to understand the difference and how they interact. Placing a 2nd order filter on a driver at 80 Hz might not result in what you expect. As shown above, the sealed box may have 2nd order roll off below this point, so the summed response is 4th order. If that filter is applied to a vented version with roll off lower down (green curve above), then the result is more complicated.

Measurements are needed

To ensure that you actually know what is going on, you need to measure the speakers using the nearfield method. In most cases, where you want to aim for a textbook crossover, sealed mains are the simpler solution, ideally with drivers that have an fs one octave below the desired crossover.

Alternative solutions

The textbook crossover isn't necessarily the ideal. Once you start to consider the impact of room modes, things get more complicated. It can be beneficial to actually overlap mains and subs. One example has been shown here:

Subwoofer integration example >

November 8, 2010

10 most significant speakers

It's always a little controversial choosing which speakers make the list. Here are my picks based on influence and their contribution to speaker development. Some might squeeze their way in for their novelty appeal. They are included in no special order.

1. Geddlee Summa

The Summa is now well known in many mainstream circles. You probably won't read traditional reviews or see them in flashy hifi magazine ads. Dr Earl Geddes has made it his life's work to find a way to get high end sound out of a high efficiency high end with compression drivers. The Summa is his ultimate statement speaker which breaks with convention in using pro drivers. It has a 15" midrange driver and a compression driver loaded into a waveguide of the same size. Earl isn't the only one to take this kind of approach, but I believe at the moment he is taking the most refined approach to it that I've seen.



By the way, it comes in a much prettier glossy red apparently.

Gedlee Summa website >

The Summa appears to follow on from the JBL 4430



These were designed with a similar approach, particularly in terms of the polar response. This was one of JBL's most successful studio monitors. More info on the Lansing Heritage website >

2. Danley Synergy horn

While directed at the pro market, Danley's Synergy horn is an inventive and very clever design with home audio applications. Like most things that Tom Danley comes out with, most people will be scratching their heads trying to figure out what is going on inside on of his boxes. Danley's designs are unique and take some time to grasp.




What you see here is a compression driver mounted into a conical waveguide. This is combined with the four midrange drivers mounted onto the same waveguide. The clever part of this design is that all five drivers are time aligned without using DSP or active crossovers. They also match polar response. So you have the ideal point source driver without the compromises associated with coaxial drivers. This is a pro audio speaker with very high efficiency and yet it also has the kind of accuracy that audiophiles seek. The waveguide loads the mid top drivers down to 300 Hz with 100 db efficiency up to 1kHz where the sensitivity will be around 108 db.

3. Magico Mini



Definitely an eye-candy speaker. It should come as no surprise that Alon Wolf is an industrial designer. Many other speakers have since copied the construction and style. You probably don't want to know the price.

4. B&O beolab 5

And the sci fi novelty award goes to ...



This speaker takes a very different approach to some of the others. It is a 3 way design with omni directional dispersion and an automatic room EQ system. Dome tweeter and mid are loaded into acoustic lenses and a 15" woofer covers the bass. It's an active powered speaker that only needs a front end.

While omni might seem like the wrong way to go, it does have clear advantages. Firstly, the dispersion is consistent. The sound stage and sweet spot are very large. A speaker such as this can make the sound work for more than just one small listening chair. It's a more social speaker, perhaps less intended for the audiophile who sits in their "man cave" in their solitary listening chair.

5. Linkwitz Orion

You've probably heard of the Linkwitz Riley crossover, but you might not know that one of it's creators has designed a very unconventional state of the art dipole speaker. He sells speaker kits called the Orion.



His website is a wealth of information, not only about the design process but also many aspects of audio reproduction. The Orion is a 3 way active design with analogue filters. It has a 1" Seas dome tweeter, a Seas magnesium 8" midrange and a pair of Peerless XLS 10" subwoofer drivers for the bass.

While there are many who love the sound of ESL and other dipole panel speakers, dynamic driver dipoles overcome many of the problems. Higher efficiency and higher output can be achieved, and the amplification requirements are not so difficult.

6. Rogers/BBC LS3/5a

This was a BBC design, but it was the Rogers version that stands out in the 70s.



Rober Greene describes it as a speaker that "redefined the possible for small speakers." *1

7. Tannoy Monitor Gold

The Tannoy Monitor Gold is a driver that is still highly sought after. Second hand units fetch a price that few will pay for a new driver.





It is a coaxial driver that is well designed for hifi use. Typical pro coax drivers fall well short. You may notice at the back of a magnet there is a dome. This is in fact a compression driver dome. You will then see five tubes shown in section that allow the dome to load into the dual conical profiles. The cone itself then becomes a waveguide and both act as a single point source driver.

8. Quad ESL 57

No list would be complete without the Quad. Despite obvious limitations in output, extension and a tendency to beam in the top end, it seems to find it's way into in any of these lists due to it's midrange transparency.

9. Klipschorn floorstanding speaker



This speaker has been in continuous production for over 60 years! It is a design to be placed in a corner and has very high efficiency. Klipsch are probably the best known of all horn designs.

10. B&W Nautilus


It was tempting to include the Wilson Audio Watt Puppy, which is a highly influential speakaer with some simlar features. However, this speaker strikes me as more innovative. While the Watt Puppy seems to use stock drivers, B&W not only make their own but the drivers themselves are innovative. Diamond tweeters have entered their range and the midrange driver is a unique design. The Watt Puppy uses a midbass driver for the midrange, where a dedicated midrange driver is less of a compromise. The cabinet features a matrix bracing system and a unique style which blends form and function in a very creative way. It's a very well rounded and impressive design.

*1 Greene >

Introducing the Econowave speaker


Update:
After hearing some Econowaves and various driver and waveguide/horn combinations, I have found that not all drivers and waveguides are created equal. This comes as no surprise. You might like to read about a series of waveguide shootouts which include measurements of speakers like these, and listening impressions.

Waveguide shoot out 1 >
Waveguide shoot out 2 >

Are you looking for a speaker that ...
  • is something different - not another slim floorstander!
  • plays nicely with valve amps
  • is room friendly - suits a room that can't be acoustically treated
  • knows how to rock in a way that other speakers can't
  • won't hurt the hip pocket
  • is ideal for home theatre, not simply another hifi box
The Econowave might be for you. It's a low cost DIY speaker that many are now building and talking about in the US forums.



Demo video



Read about it on the Audio Karma forum:

Econowave thread > (sign up required to view images)

or on Parts Express >

Warning: Information overload!
Don't expect to read it all in one sitting. Or 10 sittings. Read as much or as little as you like, but if you want to simply build it "monkey see, monkey do" style, then start here:

Standard version >

Deluxe version >

What about horn coloration?

If you've heard a compression driver based system before, you've probably experienced the coloration that most dislike. Earl Geddes has shed some light on the problems with horns and new designs have overcome the nasty shortcomings that many associate with horns. This is something that you need to experience.

Read about some listening impressions here >


What are the advantages of the Econowave?

1. Controlled dispersion

The waveguide maintains a flat response over a controlled angle such as 90 degrees and we aim to cross to the woofer at a point where it has a match polar response. A conventional two way with a dome tweeter and cone mid can't achieve this. The polar response will shift at the crossover. The only way to avoid the shift is to operate the tweeter down to where it has an omnidirectional dispersion. This isn't a desirable trait.

Controlling dispersion means a better interaction with the room. The econowave usually has narrower vertical dispersion to avoid ceiling reflections.

2. High efficiency

A typical two way speaker is limited to around 87 db efficiency. This tends to limit dynamics as well as output level. Power is cheap but power compression and poor thermal dynamics are limiting factors. Higher efficiency is necessary for an effortless clean dynamic sound. The econowave has around 95 db efficiency and will sound almost twice as loud at modest input levels. This difference will increase with higher power input, since the Econowave will likely have higher power handling as well due to the use of pro drivers. The raw efficiency of a compression driver is around 108 db.

3. Easy to drive load

Typical driver choices will result in an 8 ohm load, which is easy to drive. Typical floorstanders have a 4 ohm load. Most AV receivers aren't suitable. With conventional hifi speakers, one has to spend more on the power amps. An Econowave speaker could eliminate the need for an amplifier upgrade and more than pay for itself.

My plans

I may in fact build my own Econowave. If I do, it will be an active 10" version and the result is likely to become a surround speaker. The longer term plan is to build a 15" active version with the B&C DE200 compression driver and Acoustic Elegance TD15M. It's a toss up between starting with a budget version first, or simply going for the deluxe version. I'm also planning to build my own waveguides, so this wouldn't really be an Econowave project, but the design approach would be similar.

Do you want to buy one but can't build it?

You have two options. One is to source all the parts and have a cabinet built locally. The other is to purchase a Geddes kit. They are designed by Dr Earl Geddes who appears to be leading the field and is a pioneer when it comes to waveguides. If they fall within your budget, seriously consider the Geddes kits. Even if you don't buy them, at least read some of the papers.

Geddes speaker kits >
Geddes papers >

Which Econowave should I choose?

I have a couple of suggestions:
  • avoid the Selenium D220Ti and other titanium compression drivers - much better drivers are available
  • avoid the Dayton round waveguide/horn - the Pyle unit is better sounding and measures very well
  • adding reticulated foam is highly recommended, but must be allowed for in the crossover
  • even when using a poorly made adaptor, better drivers like B&C DE250 and BMS 4550 sound better than the Selenium which is intended for screw fit waveguides and horns

How to get rid of power up thumps

Do you have a problem with thumps when you power your system up or down? It can be irritating and for those with an active system, possibly risky as well. There are a number of solutions.

Find the source

You need to find out what is causing the problem but a process of elimination. Does the sound come through the subs only, or through the entire system? Most good quality consumer electronics don't suffer from this problem. Power amps generally have muting relays that intercept the problem upstream. This means you might have the problem with your sub, that will most likely have no turn on delay. A common cause of the problem is a pro EQ unit like Behringer Feedback destroyer. Strangely, their active crossover unit can cut out the problem upstream.
Solutions

Many leave the offending component on permanently. This is a poor solution for a number of reasons. Firstly, you might have a power failure. THUMP! Secondly, a family member might turn it off. THUMP! There is also the concern over wasting power and the life of your unit will be cut short. Electronics components have a limited life and leaving them powered up all the time will shorten their life dramatically. It's also wasteful of power. Overall, this is a poor solution.

Another commonly offered solution is a power up routine. Ask about fixing your thump issues on a forum and someone will tell you to power things on manually in the right order. Not only is this a pest, it fails the idiot proof test.

Solution 1

Power sequencer. This is a device which can power things on and off in a sequence with delays. You press one button and your system comes to life with the amplifiers powered up after everything else. Thumps eliminated. When you power down, the sequence is reversed. If all else fails, you can always use this solution, and the cost isn't crazy at AU $330. Here is one Australian unit:

Mclelland Power Sequencer
(GNL3300 POWER CONDITIONER & SEQUENCER) >


Another option

Thoroughbred Sequenced Power Interface >


For those outside Australia (we have 240v), the Furman is what you want:

http://www.furmansound.com/product.php?id=PS-8R >

Solution 2

Muting relays. If you have DIY amps, then ideally you want to have muting relays which you can get in various kits. You might search for "speaker protection" as these often include relays. After about 5 seconds after power up, you hear a click when the relay switches on. On power down, the relay shuts down before thumps get through.

Subwoofers are more difficult. Plate amps don't generally have relays and will stay on after the system has been shut down. A power sequencer may be the only solution where the plate amp is set so that auto on/off is disabled. If you have a pro amp for your subs, you may consider your selection carefully. The popular Behringer Europower amps offer limited thump protection. There are alternatives at a higher cost that can remove this issue.

Open baffle bass

What's so special about open baffle bass? I ask this question every time I see someone planning a diy open baffle bass speaker. But first, what is it?

Here is one example. It's called a W frame dipole. In this case, it's also push pull mounted. One driver has the polarity reversed so that while one moves out of the magnet gap, the other moves into it. It's a method to reduce 2nd order harmonic distortion. 
A dipole does not contain the rear wave from the driver and since it is 180 degrees out of phase, they both cancel. The result is a null to the sides and much less bass. An open baffle dipole has some unique traits:
  • no box spring to limit excursion, so less power is required to reach xmax and there is a high risk of bottoming drivers
  • dipole roll-off due to the acoustic cancellation typically adds 6db/octave loss below 120 Hz (frequency depends on baffle size)
  • velocity source - the room is not pressurised
  • dipoles are a low Q bass source
The ultimate bass?

Many believe that open baffle bass is the best you can get. It's claimed that they are less impacted by room modes and therefore have a smoother in-room result and better sound quality than the alternatives. If this were the case, then it would be worth the extra expense. However, I contend that open baffle bass falls short on both counts.

Smoother in-room bass

The side nulls are said to cause less room interaction. This sounds logical enough, and they do certainly measure differently in a room. However, simulations and measurements have shown that they don't necessarily result in a flatter response in room. It is in fact highly system dependant. You might find a dipole measures better, it might measure worse.

Better sound quality

I have had mixed results here. There are some drivers that sound better in a dipole arrangement, including those not intended for the application. However, in one particular comparison, I found no advantage. My listening comparison used two Rythmik Servo kits, each in a sealed box. Both had EQ to ensure I was comparing the same response. I used a familiar track well suited to subjective comparison of bass. Side by side, trying to pick a winner was like comparing many electronics components. I would have struggled to pick a difference in a blind test. Even testing sighted, expecting the dipole to have an edge, it was neck and neck. At that point, I abandoned open baffle bass. I had one sealed box at the time and one open baffle. When I converted the dipole to sealed, I noticed an immediate increase in low frequency authority. That might sound like a contradiction until you consider that the test was set up to level the playing field. When going back to a range of content, including music with more demanding bass and movies, the improvement was clear.

Still considering open baffle bass?

If you still want open baffle bass, then I suggest doing a test for yourself. You'll need to measure and EQ to match fairly. In many cases you are most likely to find that one modest sealed box will match four drivers in a dipole arrangement.

November 4, 2010

What is bafflestep compensation?

An issue that many don't quite understand is bafflestep compensation. It relates to where a speaker should be placed in a room. A speaker that is mounted on a wall, such as a surround speaker, won't require bafflestep compensation. One that is placed into the room away from walls, such as a stand mount, will require bafflestep.

What is bafflestep?

Bafflestep is a compensation in the crossover for the loss of bass and lower midrange that you get when a speaker is moved away from a wall.

Why is it needed?

Mount a speaker flush with a wall and the wall itself will reinforce the sound. Imagine a speaker suspended up in the air. That is called full space radiation. It radiates in all directions and so gets a bit lost. Now put it on the ground and that is now divided up in half. We call that half space. That's a bit like a surround speaker on the wall. Now move it away from that wall and it no longer has that reinforcement. The bass and lower midrange efficiency response will go down, however, above a certain point in the midrange, the speaker baffle will reinforce the sound. This depends on the width of the baffle, where a wider baffle will reinforce the midrange to a lower point, below which the response will step down. This needs to be compensated for in the crossover, otherwise the bass will be weak and the lower midrange will sound thin. The speaker will sound thin and overly bright.

Two ways to compensate

There are two ways to deal with bafflestep:

1. 2.5 way crossover

Let's say you have a 2 way speaker with a tweeter and mid. You can add a second woofer to compensate for the bafflestep loss. You simply apply a first order low pass filter at say 200 Hz.

2. Bafflestep compensation in the crossover

You can also simply alter the crossover design so that there is some EQ to reduce the midrange above a certain point, then the tweeter would be padded to reduce it's level. The downside is that efficiency is reduced by around 3 db.

How does your speaker handle bafflestep?

If you intend to build a diy speaker design someone else has published, it's important for you to know how it was designed.

Is it designed without bafflestep? In that case, you can use it for a surround speaker or against a wall without any change. If you want to use it as a stand mount placed well into the room, then you need to compensate.

Is it designed with bafflestep? Then you can place it into the room without change. If you want to put it on a wall, say as a surround, you will need to remove bafflestep.

How to add bafflestep

1. Convert a 2 way to 2.5 way

Add a matching woofer as described previously.

2. Bi-amp a 3 way speaker

That allows the woofer to compensate by increasing the gain. You might need to change the crossover point.

3. Modify the crossover

You will give up some efficiency in doing this and hence need to drive it with more power.

How to remove bafflestep

Essentially, this is the reverse of all previous suggestions

1. Convert a 2.5 way to 2 way

Take out the second woofer

2. Change the gain on a 3 way biamped speaker

3. Modify the crossover

Further reading

Rod Elliot describes bafflestep clearly, along with some charts that will make it more clear:

ESP Bafflestep article >