September 14th, 2009

Infinite Baffle Loudspeaker

Infinite baffle speaker

Since I started being interested in audio, I’ve seen all kinds of speaker designs, weird arrangements and weird geometries. From all those speakers, in terms of extremely low frequency reproduction the best ones where the infinite baffle subwoofers. Those setups go easily down to frequencies like 10Hz. You may say these figures are useless with a musical program and normally i would agree but if we are in a quest for perfection i would say it’s mandatory. Even if our music doesn’t have much information below 27Hz (lowest note on a grand piano, though pipe organs go even lower) it is important to have the roll off region in frequency response at least an octave lower.

The reason for this is that the speaker and the cabinet box form a high pass filter and like with all filters it will add lots of delay in the slope region which may affect the quality of the reproduction. For Home Theater applications response down to teen frequencies becomes a necessity. If you look at the spectrogram of a .50 Barret rifle shooting sound you will see the most information is in the 10 to 20 Hz area. You may not hear the 10Hz but you will certainly feel it, much like if you would be standing next to the rifle.

The principle of IB (Infinite Baffle) is to install the woofers on a panel that is wide enough so that the waves coming from the back of the woofer never meet the waves coming from the front of the woofer. Looking at the wavelength of a 10Hz sine you will see you need a very big panel. However IB can also be done if you put the woofer in a box big enough so that it has zero damping. Custom HT rooms have the woofers mounted on a sealing or floor and have the attic or basement act as that big cabinet box.

In this project I’m presenting the author build a speaker on a panel to fit a door way and have an entire room to isolate the back waves from the woofers. He is referring to the Hoffman’s Iron Law that says: having efficiency, low frequency response and box volume one must sacrifice one of the three to have the other two. IB of course sacrifices box volume. There are some who argue this Law, because it actually holds true if you place the subwoofer in an open environment. Closed spaces have gain that will  your low frequencies even if your speaker loses efficiency, and here by speaker i mean the woofer plus cabinet. Woofer’s efficiency is constant.

Another thing is nowadays Hoffman’s Iron Law can easily be defeated with powerful woofers and equalizers. There are woofers that can take tons of power and have huge excursion and can go really low in small enough boxes. Returning to the IB setups you will also need high excursion woofers and high pass filter on your amplifier with a corner frequency between 5 and 10Hz, sometimes higher depending on the woofer your using. You will need high excursion because the woofer won’t be damped at all so it will be free to move. At high powers and low frequencies it will move allot.

In this project the author didn’t built a subwoofer but a full-range infinite baffle speaker. I don’t see the point of this expect for a center channel. He used 8 x 12″ woofers for the low frequency part. If the woofers aren’t such quality it’s better to use many of them, this way the stress will be divided between them. When i first saw the baffle i thought it is too thin to hold the eight woofers. But then i saw the bracing on the back, and that will make it more rigid for sure.

The tweeters are piezo which in my opinion are not that great. Also the crossover i think could use more work. I always liked higher orders crossovers because they give you allot more dynamic range. The whole system reaches 96dB/W/m, which is very nice. Is a nice and fun build, and even if it won’t live up to your expectations at least you will have a sonic weapon.

Infinite Baffle Loudspeaker: [Link][Via]

router-roundover

In the first part i presented some of the woodwork on the satellite speakers and a first schematic of the crossover. These days i had time to finish the front baffles, made the edges round, cover the speaker with dark colored carpet and install the drivers and crossover for testing and voicing.

The rounding of the baffle’s edges is done using a 8mm round-over bit on the router. Make sure you get the board fixed so that u can push the router in the edge.

roundover bit 8 mm router bit

Once this is done i applied the first layers of black paint and glued the baffle on the rest of the cabinet. While i was waiting for it to dry i carpeted the back plate. When you chose to use carpet it’s a good idea to install the back plate a little inside the cabinet and this way to leave an edge. This will allow you to glue the carpet and not leave any visible marks at the joints. After the back plate the sides, top and bottom will be carpeted in one piece.

backplate-spkr spkr back plate 2 spkr side spkr side 2

The front baffle was cut with 3mm more on each side so that the carpet would go right at the same level

The satellite speakers are almost done at this point, it is time to install everything inside for the first tests. I chose to fill the cabinet about 70 to 80 % with wool and this way lower the total Q of the speaker to somewhere around 0.8. This can be a high value but since i will use active filter on the subwoofer matching will be easier.  The first version of the crossover network was done in air just for testing. It seems the waveguide gain was slightly higher than expected so the initial L-pad had to be changed. Also the cut frequency was little too high for the tweeter. Since i wanted the option of bi-amping i decided to add a tweeter protection circuit i had in my schematics notebook which I’ve seen used in some professional speakers. After these changes in crossover came another session of listening tests. There was still a part in the upper midrange (voice sibilance) i found to be too emphasized. Adding a resistor of about 1.5ohms in series with the inductor in the high pass filter lowered the Q and smooth the sound.

filtru1 filtru 2 zobel

In the last picture you can see the Zobel network which is an impedance equalization

These are 2nd order filters so you need to reverse polarity of the tweeter, connect the plus of the tweeter to the minus of the filter and vice versa. Otherwise you will have a big dip in the frequency response caused by the phase shift of the filters. Another thing to be careful about is not to connect ground of the high pass and low pass filters if you’re using bi-amp connector. The strap at the connector takes care of that. The radiator i used on the TIP transistors may be an overkill but better safe than sorry.

I did an experiment with a baffle step circuit. From a certain frequency the directivity of the speaker becomes focused and this may appear as an increase in sound pressure level. A baffle step attenuates the response a little from that frequency up. Usually i don’t use it for i didn’t find it necessary. In this case however i got better response on bass at the expense of overall efficiency. However once i will add the subwoofer to the system this circuit might not be necessary. In this case the circuit is made of a 2.7mH inductor in parallel with a 8 ohm resistor connected before the crossover. Baffle step circuit is impractical when using bi-amping though.

Here is simulated baffle step responses using Edge software:

baffle step uneq baffle step eq

To be continued:

– Measurement of on-axis and off-axis frequency response

– Building the subwoofer

Bookshelf Speakers

I will present you here a 2.1 speaker project made from some leftover materials. Nothing fancy but i will try to get the most out of everything. In this first part i will give information about the overall design and the first work on the satellite speakers. Drivers i used were those i had laying around, a pair of 8″ woofers with nothing written on them, probably Chinese stuff (I will have to measure some of its parameters), a pair of 6.5″ mid-bass drivers i saved from some commercial speakers and a pair of horn-loaded soft dome tweeters from PAudio model PCT-300.

I plan on using the two 8 inchers for a subwoofer, i will have to measure some parameters to determine what box to build. The satellite speakers will have one mid-bass driver and one tweeter in a closed box. The box should not be bigger than 10l in volume.

Some of the Thielle – Small parameters of the 6.5 incher mid-bass are as follows:

Fs: 65.41Hz, Qts: 0.62, Qes: 0.76, Qms: 3.43, Vas: 18.9l, Sd: 150 cm^2, Xmax: 3mm, Re: 7.4 ohms, SPL: 90dB/w/m, RMS Power Handling: 50W

6.5 inch mid-bass

A closed 10l speaker models with a -3db band starting 85Hz with a Qtc little over 1. Below 85hz there is a 12db per octave attenuation so it will be pretty easy to match it with the subwoofer. I can lower the Qtc with heavy fill hopefully to a value below 1. Because of small dimensions of the cabinet the driver will never exceed maximum excursion.

predicted lf response

Predicted low frequency response in WinISD

Now off to the wood work. Tools needed: Router, Jig Saw, Circular Saw, Electric Drill, Clamps, Screws, Glue, etc.

Lucky me i found a pair of old speakers measuring about 9l of internal volume. Almost perfect for my application. I only kept the top, bottom and the side panels. I make a new back panel and cut hole for the speaker terminals. I want the option of using separate amplifiers on highs and mids so i use bi-amp speaker connectors. I plan on painting the front baffle black and finish the rest in some dark colored carpet.

9l old speaker router back panel back panel connector

You can see here what i kept from the old speakers, the router getting ready, the back panel in position and the back panel with the connectors installed.

Now follows the routing of the front baffle. I use MDF for it’s easy to work with and has pretty good acoustical properties. First i prepare the router. I make the compass from a thinner board of MDF and i attach it to the bottom plate of the router. After this i measure the desired radius and make a hole in the board. This hole will be the center of the circle being routed.

router w/ compas cutting baffle baffles front baffle

Better measure 5 times and cut once…

The drivers are flush mounted and the front baffle’s edges will be rounded to minimize diffraction. The distance between the acoustical centers of the two drivers is 17cm. This distance corresponds with the wavelength of approximately 2kHz, which will be pretty much in the middle of our crossing point. This alignment avoids phase problems in this region. Also loading the tweeter with horn provides time alignment with our woofer.

Before talking about crossover lets look at the high frequency driver a bit:

pct-30

As you can see it is pretty sensitive at 93dB, mine actually reaches flat region at 92 dB. Because of the waveguide there is a big boost on the higher mid-range. Also note the fs of 1.4Khz. This means we should cross this tweeter not lower than 2.8khz. I decided to cut it at about 4Khz with 12db/ octave slope and this way make it linear down to approximately 2kHz. Considering the gain provided by waveguide i will cross the woofer at about 1.4kHz 2nd order Butterworth again.

This is how crossover looks like… on paper for now:

xover schematic xover graph 1 xover graph 2

In schemtic you will see the Zobel network in parallel with the woofer for impedance equalization and the L-pad on the tweeter to adjust for the increased sensitivity.

To be continued:

Finishing cabinets, round-over the edges of baffles

– Building crossover, install and voicing

July 25th, 2008

Annoying Beep Prank Device

Annoying Beep Prank Device

The idea of an annoying beep device it’s pretty original and I’m sure it works on everyone. So here is how it works, the thing is damn small because it only contains the ATTiny13, a piezzo speaker a resistor and a capacitor, thus beeing easy to hide; you carefully hide the device into a friends house. The device will start emitting a beep sound at random intervals of time. The time between beeps is anywhere within half an hour to nine hours.

Your friend will start looking for the source of the beep among his gadgets like mobile phones, tv, etc, not knowing that the beep is coming from the prank device. Just imagine yourself into this situation, it will drive you nuts :). And there’s another feature of the device, the small battery will last forever because the device only draws 200uA.

Annoying Beep Prank Device: [Link]



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