Dipole vs Bipole subwoofer

 

Dipole subwoofers

I am very fond of open baffle speakers as you have noticed. I would never put midrange in the box, it sounds way better on open baffle. Woofers are a different story. Most of my systems are based on closed box woofers when it comes to the low frequencies. However, I heard open baffle bass systems, like Legacy Whisper, and I liked the uncoloured fast clean bass. So I decided to experiment with open baffle / dipole subwoofers.

Simple 15" woofer on open baffle of reasonable size has quite a cancellation and lacks low frequencies because of that. Baffle either has to be enormous in size, or bent. H-frame or U-frame is often used to minimize the baffle while to prevent cancellation. My experiments with these baffles did not yield satisfactory bass. Neither slot loaded baffle satisfied. I constructed big closed box with two 15" woofers on opposite sides. Each woofer can be powered by separate amplifier. This box can be configured as bi-pole or di-pole based on the polarity of woofers. I experimented with bi-pole vs di-pole configuration, and not only by listening, but further by measurement I found di-pole subwoofer much better sounding. Here are some pictures from the above mentioned subwoofer and it's frequency responses in bi-pole or di-pole configuration.

 

And these are the frequency responses I obtained by behringer ultracurve. No eq or inductor used, just quick and dirty measurement. First is di-polar configuration, second is bi-polar. You can see much better low frequency extension in di-polar configuration, plus much flatter response. In di-polar configuration the woofers are moving together (box is isobaric), no pressure changes are inside the box, woofers are not slowing each other. In bi-polar configuration, frequency response has nasty peak, I do not know why, plus it lacks real deep bass. Membranes when move, they are significantly acting on each other, slowing the vibration at low frequencies. I did not like the sound at this configuration. There are significant vibrations on the walls of the box too, because there are large changes in the pressure inside.

  

 di-pole

   

bi-pole



Later, I constructed two more di-pole subwoofers, one for each side. Each with two 15" woofers. Below are few pictures from the construction, which was even simpler, not even the closed box. Just two front and back baffles connected in the corners. This slides inside the stand, which is not touching the baffles. This di-pole subwoofer is not even hermetically closed, no need to. Again, I verified that in di-pole configuration the frequency response was flatter, went deeper and had no ugly peak. Sounded way better too. In the final stage I wired those two woofers in series to present easy 8 ohms for the power amps. You have to be careful and realize that back woofer is physically in other direction, so its polarity is reversed (on top of its polarity reversal as di-pole, you get the point).

On top of the di-pole subwoofer sits 2.5 way MTM Betsy WOW with Selenium horn tweeter. All great sounding speakers. Whole system is bi-amplified by two JVC SuperA amps.

 

 

 

   di-pole

 

   bi-pole

 

 

All frequency responses were done without any eq or crossover for the woofers. In the final application these will be powered by separate amplifiers through active crossover set at 160Hz. You can see the bi-pole peak much higher in the second much smaller subwoofer. In di-pole configuration, however, even compact subwoofer performs well and with little eq goes quite low.

Just to be absolutely clear about the terminology, what I mean by bi-polar and di-polar subwoofer, I attached the illustration with arrows. The arrows indicate the polarity by which membrane moves. Not the orientation of woofer itself, although that is important too, to reduce the second harmonic distortion, but that is a different issue I do not want to brink here.
First picture is bi-polar subwoofer, where the membranes move opposite each other. The polarity of sound pressure created to the front and back is the same. However, there are big changes in the pressure inside the box, as the membranes are working against each other. The whole box is shaken, not stirred.

 

 

 

 bi-polar subwoofer

 

In the second case, woofers are wired so, that their membranes are moving in unison with each other, thus creating no changes in the pressure inside the box. Membranes are not fighting with each other, box does not vibrate at all (could be made of flimsy thin material?). However, the sound generated to the front has opposite polarity to the sound generated to the back, yielding narrower beam due to the side cancellation. Efficiency may be lower in the di-pole arrangement, but the sound is not thrown widely against the walls and above mentioned advantages in flatter frequency response and no box vibration are additional benefits. Plus it sounds way better to me too. Enjoy!

 

 

  di-polar subwoofer

 

How much amplifier power do you need

Before the amplifier power recommendation was made, quite a few listening tests were performed in typical living conditions. Most of the listeners preferred the music at these tests at the average listening level of 80 to 84 dB. Tests proved that listening at this spl does not feel tiring even at prolonged listening and does not cause any psychological discomfort. 

The main deciding factor on spl is Nmin (minimal spl in room), which is equal to Nnoise (background noise) plus the dynamics (D). 

Nmax  =   Nmin   +    D

In the city, where the houses or flats are near the roads, railways, construction sites, the background noise is on average 40 to 50 dB. This, at the average Nmax 80 to 84 dB allows the dynamic range of the music for 30 to 40 dB. 

If you want more dynamics, you have to raise the spl, which leads to irritation of your spouse, and neighbors, and even you being tired. 

What should be the wattage of the amplifier recommended for normal listening level? Because high quality amplifier needs to handle peaks without the distortion, and music is full of short peaks, transients, the Nmax was raised to 94 dB. 

Acoustic power can be calculated using equation:

P  =  wcoA / 4

where w is level of acoustic energy (J/m3)

co is speed of sound (340 m/s)

A is absorbing surface (m2)

Absorption can be calculated as:

N  =  10log  w/wo

where wo is threshold of hearing (3.1E-15 J/m3)

w  = wo * 10N/10

Then we can calculate the required electric power  from acoustic power Pa  and efficiency of the speakers (Eff):

P  =  Pa /  Eff

For the size of three typical rooms (42, 54 and 82 m3), typical sound delay of 0.35, 0.40 and 0.50 s, and assuming speaker efficiency Eff ~ 1%, we get the required amplifier electric power as 4.92 watts, 5.68 watts and for third largest room 8.57 watts. Yes, less than 10 watts. 

For highly efficient horn loudspeakers, with Eff ~10%, required amplifier power is 10x lower, yes, less than 1 watt. 

  

  

subwoofers and satellites

This post is my justification for selecting 150Hz crossover point to split the frequency range for subwoofers and satellites.  

Audio chain consists of three parts. First: signal sources, that is your cd player, record player, FM tuner, cassette deck or reel to reel player, mp3 player, streaming device and so on. Second: signal amplification, this includes all the signal manipulation like preamplifier, equalizer, active crossover, all the way to power amplifier. Third: speakers. 

Speakers are the last part of electroacoustic chain. Speakers have a very difficult task of converting the electric signal from the amplifier into the sound image as closely as possible to the original sound in the recording. 

No single speaker is capable of reproducing full range of frequencies in concert hall. Life like reproduction of low frequencies, like organ, base, tympany, big drums, and so on, requires the speaker with large membrane area moving lots of air. Just like the instruments generating the low frequency sounds. This speaker, on the other hand, has large membrane mass, and can not radiate high frequencies effectively and with proper directionality. Lack of pistonic behavior, cone breakups and beaming limits the upper range of large membrane woofer. 

Effective radiation of high frequencies requires small membrane speaker, with light membrane, which on the other hand, can not produce mid or low frequencies. Therefore, it is more practical to split the frequency range into bands. Small 2-way bookshelf speakers are quite common. Midbass driver is accompanied with tweeter. Such speakers can sound good, but they lack low frequency extension. Big floorstanding speakers with woofers, midrange drivers and tweeters are next step up, and can be very good sounding. This is classic approach. However, other approach, where one builds separate woofers enclosure, one or two, cross it low enough so that it can not be localized, and uses smaller bookshelf speakers to reproduce the rest of the frequencies above the subwoofer. 

The point here is to cross the subwoofer low enough, such that our ears can no longer localize its position. Whether one uses just one or two or more subwoofers, or places them under the bookshelf or some place else is the main reason to call these woofer boxes subwoofers. Unlike in classic 3-ways speakers, crossover point between woofer and midrange driver does not allow separating the woofer enclosure far from mid and tweeter. 

Lets discuss optimum crossover frequency for subwoofer. As is shown in the Figure 1 below, authors suggested fd1, the crossover point between the subwoofer and the bookshelves, also called satellites, to be between 70 and 200 Hz. 

Our ears allow us to recognize the directionality of sound source. This is rather complex process based on binaural hearing. Signal source coming from the left side reaches left ear sooner and with higher sound intensity, as sound to the right ear. This is true for most of the frequencies. 

Our ears are horizontally placed on our head with the goal to be able to localize the sound as precisely as possible on the horizon for survival reasons. When our ancestors roamed flats of Africa, they needed to listen carefully where the predators are hiding. The sounds from top or bottom where less important. Therefore horizontal behavior of speakers is of utmost importance. Minor vertical irregularities can be easily tolerated. 

Let's return back to the discussion to the ideal subwoofer crossover point. As shown on the Figure 3 below, there is a difference in phase of arriving audio signal between the left and right ear when signal comes from the side. This phase difference is increasing with increasing frequency. The phase difference at low frequencies is negligible. This is because wavelength is much bigger than the distance between ears. 

Similarly with the phase difference between left and right ear, the sound intensity depends on frequency too. Our head presents much bigger barrier for higher frequencies, as shown in Figure 4 below. Please note that the sound level is identical between the left and right ear below certain frequency. This means we can not localize the sound below 200Hz. Wait a minute, you say, when you set your subwoofer at 200Hz, you clearly heard where it was placed. Well, this is because there was still significant signal above 200Hz, since every crossover has some slope. If you would use brick wall filter, the result would be different. Other reason for being able to localize the subwoofer may be its poor quality. Distortion is basically the generation of upper harmonics. Distorting subwoofer is producing higher frequencies above its crossover point. 

So we have arrived at the point, where if the subwoofer is executed well, that is low distortion without any chuffing, we can state that ideal crossover frequency would be anywhere between 70 and 200Hz. To be on the safe side, I decided on the 150Hz for a long time ago, so all my active crossovers are built this way. Most of the woofers I worked with have no problem covering from Fs to 200Hz, behaving pistonicaly. Some people argue that subwoofers crossover point should be set very low, like 40Hz or lower, just to extent the range of main speakers, hence the name 'sub', but I disagree with that. You should build your classic 3-way tower properly, so it does not need subwoofers. I am not discussing the needs of HT and explosions, I do not care about that crap.  

There are countless ways to make crossovers, passive or active. Two examples, one for active, one for passive, is shown below. 

One can do completely passive crossover for subwoofer, but the inductors are substantial. If one insists on using just one amplifier, there is no other way around. 

I prefer active. Active crossover brings little more complexity, but offers lot more flexibility. While if you are using passive crossover, you need to use amplifier capable of driving woofers, as well as bookshelves. However, if you have active crossover, you can use big classAB on the woofers, but small tube or solid state classA amp on satellites. Much less power is required for the frequencies above 200Hz. Depending on the efficiency of midrange and tweeter, even few watts may be plenty. Other advantage of active crossover is no need to match efficiencies between woofers and bookshelves. There are other advantages too, if you are speaker builder. Building bookshelves is lot easier that building big classic 3-way speaker every time you want to try new mids or tweeters. 

Happy listening!

 

Tekton tweeter array MTM

 I was intrigued by the unusual tweeter array Tekton uses in their speakers. So I build something very similar. I used cheap but well performing 5" midbass drivers, and cheap but equally well performing onkyo tweeters. I used modified crossover. Here are some pictures from the build. They sound and measure very well. Tweeters, to my surprise do not beam, no lobing issues either. There is lobing in midrange in vertical axis, but all good in horizontal. Speakers have well balanced pleasant sound, with exceptionally clear highs. Highly recommended. 

MTM with Vifa 13WH-00-08 and AMT tweeter

I got some old Vifa midwoofers back from a friend, and after all those years they still perform admirably. Its hard to find such a well behaving midrange with almost no breakups. Look at the fr response, it needs no crossover. 

Such great midrange requires good tweeter, so I added small Dayton AMT tweeter. I prefer smaller tweeter for better of axis response. I made small closed boxes with heavily braced foam board. Here are some pics and some measurements. I experimented with value of the capacitor for the tweeter, and after some initial measurements spotted small narrowing of the of axis response with just 3.3 uF cap (pictured), so I increased it to 4.5 uF final. Resulting speaker is the most neutral sounding speaker I made. They are precise, dynamic, yet true to the signal...adding nothing of its own. These speakers are now used in guest room to evaluate new amplifiers I build for the sound quality. So revealing and yet neutral these speakers are. Highly recommended. 

 

Burson Audio V6 Classic review

This is review of dual opamp V6 Classic from Burson Audio

This is my current set up for the review. I use Pioneer DV-300 dvd player as digital source, because it was discovered by lampizator and others that it provides excellent waveform. Even Goldmund discovered that and parked little Pioneer inside bigger box and charged 10x more for it. Off course it has analog outputs, but the sound on this output is boring, bland, like from behind the curtain. External DAC is great improvement. I do have Schiit combo, but that will not be part of the review. First of all, it uses only smd parts with no socket, plus I am not going to open it. I have even better DAC, at least to my ears Cirrus Logic sounds more balanced. Schiit seems to have lifted treble, which may sound pleasing to some, but not good for review.
Besides, Cirrus Logic has one dual opamp on the output, conveniently in gold plated socket. Easy to replace. I have tried half a dozen of good opamps, many years ago, no need to repeat that. It came with 5532, but I tried 2134, 2604 and others. I believe 49720 is there for now. I will be comparing standard 49720 opamp to V6 Classic from Burson Audio.
When it comes to headphone amplifiers and headphones, I have quite a few, but I selected Lehmann audio amp and HE1000 headphones, as this combo provides clean well balanced sound. I believe the DAC should produce cleanest most accurate sound possible, and the headphone amp and headphones should provide pleasing non-fatiguing sound. I like B1 buffer and ACA combo, but ACA is not free of distortion, so I will be using cleaner amp. Other amps I have include Musical Fidelity tube headphone amp, Naim, ACP+, Schiit, and others.
Headphones I like include Sennheiser hd580, hd600 and hd598, HiFiman he400, he500 and he1000. I will be using he1000.

I would like to achieve the cleanest signal possible from the DAC, therefore I require treble to be as clean as possible. I can not stand sharp distorted highs. I do not like woolly mushy soft highs either. I like to have clean sparkly crystal clear highs. Bass lines needs to be deep but not overpowering. Mids easy to follow. 

Second aspect of good sound I like is imaging. What I mean by imaging is that my brain does not have to work hard to follow any desired musical instrument or voice. If it is hard to follow separate instruments, music image collapsed and listening fatigue will follow. 

I may not be able to explain what exactly I like when it comes to sound, but I know when I hear it. 

So next I will be comparing sound of two output stages, one with LME49720 and second with V6 Burson Audio Classic. 

I started listening comparison with Jesse Cook cd's. These are very dynamic recordings, plenty of guitars, and drums, not to mention nice triangles, chimes. I listened few best songs over and over, made few notes, then switched to Burson, listened again, and made notes again. 

One thing I noted on 'free fall' recordings, songs 3 and 6, triangles were more standing out, easier to follow with Burson than with 49720. In vertigo recording, second song, Burson was clearly a winner, 49720 seemed little boring. Harmonica in third song was more real with Burson, three guitars seemed like in the air, much more enjoyable. 

When it comes to stereo separation, both had the same impression. When it comes to dynamics, Burson seemed a little better than 49720. Bass lines were comparable. 

When it comes to Bela Fleck, plenty of deep bass lines, especially 'flight of cosmic hippo' type. Perhaps 49720 may be better in bass department, but it may be just an impression because highs are a little less pronounces in 49720 than with Burson. 

In separate session I listened to plenty of choral and early renaissance music. I like Hildegard Von Bingen for pure polyphony lines. When it comes to the sound quality, I must say both output stages, either with 49720 or V6 Classic sounded about the same. See, this type of music is happening mostly in the midrange. Not much to distinguish in sound quality. Both sounded great. 

I like period instrument recordings, they offer window into that music era. Minstrelsy music group is using flutes, guitars and simple drums to accompany the vocals. Burson V6 Classic was a little more musical sounding then 49720, which was a little sterile. The difference was not big, but noticable and I was always looking forward to put V6 back in to enjoy the music more. Small, but noticeable improvement comes from V6 Classic. 

    Last listening session was done with bunch of Dire Straits and Mark Knopfler recordings. I like the music, always did, so it was fun. Granted, in some recordings they went overboard with loudness war and pushed it to the clipping and severe compression. I skipped those songs, what a pity. Luckily there is plenty of slow well recorded songs to listen and compare 49720 to V6 Classic. 

Songs like 'telegraph road', 'love over gold', and 'private investigation' offer great spectrum of instruments, from acoustic guitar, steel guitar, keybords, synthesizers of all kind, to great drums, with plenty of hi-hats, cymbals and chimes... 

I must say that V6 Classic excelled in these high pitched bells and whistles.  Although 49720 is clean low distortion opamp, easy to listen to, it is on sterile side in comparison to V6 Classic. Burson is more musical, juicy and fun. Just listen cd 'on every street', song 6 'you and your friend' and you know what I mean. That steel guitar just brings shivers down the spine. 

While 49720 can give you 95% of the recording, Burson V6 Classic can give you 98%. 

Betsy wow with Ribbon

This is nice and easy project which yields extremely good sounding bookshelf. Betsy wow (without whizzer) goes quite deep in closed box (Ikea bookshelf), so the sub is optional. But if you want to use sub, it can only help. I started by getting the dust cap out of the betsy wow, then glueing in brass phase plug. This way betsy is not attempting any hights. Then I added bsc to flatten the response, and G2 ribbon tweeter to continue past 20kHz. The result is pretty flat response, flat phase response, and very coherent sound. Only one cap is used on G2 ribbon, because its crossed at ~8kHz. This speaker is high efficiency too, and with 8 ohm resistance it presents nice load to any amplifier. It likes small class A amplifiers, but will handle anything. It gets loud, but its clean. Its no party speaker, but with the sub, it can be easily for music and ht.
Here are some pictures from construction.
Enjoy!