Pi Subwoofer Project Part 4: The Dreadnaught

Check out the 3 Pi sub's performance graph. The measurement shows frequency response in the horizontal and amplitude in the vertical axis. This measurement is what is called a "ground-plane" measurement. That means the measuring mic is placed on the floor at a distance from the sub, with the sub also being on the ground. Additionally, the measurement is done out-doors. This method gives essentially a reflection-free response -- no echoes or bounces from any surface. In terms of frequency response, that's like measuring the speaker in theoretical free space, so you can actually see what the sub is putting out, by itself. We do this because normal sized listening rooms will interfere with the output of the sub at different frequencies as measured at the microphone. You will get all sorts of phase cancellations and "room effects." Since those effects are different in different sized rooms, we need to eliminate the room from the measurement to have a performance baseline.

Pi Speakers performance graph www.pispeakers.com

Here's something interesting. Trace the graph from 40Hz down to 20Hz. There is about a 6dB rolloff in amplitude in that octave. Typically, when you place a speaker in a room it will gain 6 or more dB per octave descending in frequency response. Therefore the 3 Pi sub in most rooms will actually provide flat response down to 20Hz. So the design takes into consideration that "room-gain" gets you flat performance down to 20Hz, which is all you need.

The other end of the sub's frequency range, typically determined by low pass filters incorporated in the plate amplifier built into the sub, is variable. You wouldn't normally use it as high as 200Hz. If the sub is far away from the main monitors, 50-80Hz is good. If it is close or flanks the mains, a higher low pass may be used, up to 150Hz or so, if used in stereo pairs along side and behind the mains.

As an update, here are shots of the 3 Pi subwoofer we made with the final touch added, "grotesque industrial (truck bed liner) black." Yep, I preferred the naked look, 20-20 hindsight and all... We could have simply given it a nice tung oil finish, the wood has excellent grain, but this has a certain dreadnaught appeal. At least we completed the project as planned. ; ) The sound is as magnificent as it was during the Earpocalypse, performance down to 20Hz. Final touch:

Pi Subwoofer Project Part 3 here!

Using Ribbons on MARS

Dr. Johnzo Blevins kills a VO

Here is a brief on using ribbon mics in situ at MARS. A lot of it is procedural, but it’s a combination of technical things and learning proper studio procedures. Signal flow can’t be haphazard either. I’ll break it into bullet points. 

·      Mount it properly. All microphones should be handled gingerly, but especially ribbons. The holder is mounted on the stand before the mic comes out of the box. The box remains on the cart or other stable surface until the clip is properly threaded on the stand (do not cross thread.) Reverse the procedure to put away the mic. Never hold any mic near the ribbon or capsule. Use only a suspension mount (Royer/white box). Ribbons are always stored vertically. (The ribbons will warp!)

·      Place the included sock over the mic when moving it. Wind from motion can break the ribbon. Also be aware of ventilation. After the mic is in position, remove the sock for use and always use a windscreen.

·      Phantom power will break or stretch any ribbon when shorted through a bad cable or during patching, even if all cables are good. Simply turning phantom power off on outboard mic pres is not enough. The way our half-normaled TT bay is implemented, phantom power will be present at mic lines, even if patched elsewhere. Normally, C24 mic pres 1-8 have phantom on, 9-16 off, so there will always be phantom power present. Additionally, some pres are difficult to confirm whether or not phantom is on, especially for students or those with poor eyesight. If one of those is on, 48V will be present in multiple locations, not just where you assume it would be on paper. Even if all phantom power is turned off, it is a rare session that does not use a condenser mic, which requires it; so that’s a moot solution.

DW Fearn talks about phantom damaging ribbons through patching and bad cables in the following video, start it at 16:37 for the good part:

Note that he said the 48V exposed ribbon “mic will make a trip back to the manufacturer.” He said that several times. I have met this man several times. He does not repeat anything unless it is really important.

The solutions, in brief, are as follows and all are equally important:

·      When setting up any session, you must be trained not to plug in mic lines at the panel. The Second Engineer confirms that the mics and cabling are correct in the Studio as per the signal chain. The Engineer will confirm with instructor’s approval, in the Control Room, that the patch bay is correct and all gear in the chain is set correctly for getting a basic sound. This includes mic/line switching and phantom settings. Pro Tools inputs must not be armed. No mic lines are plugged into studio panels before the above have been done and the Engineer walks through the studio to confirm mic placement and cabling. This is procedural but crucial. It is also professional.

Engineer and assistant confirm signal chain

·      At this point it is mandatory that a Cloudlifter is inserted between the ribbon mic and the panel, before any mic cables are plugged in. Since a complex session will have many cables, it is important that the Second confirms the Cloudlifter is connected to the correct mic, before plugging anything into the panels. The Cloudlifter requires phantom, which is counterintuitive, so the ribbon channel must have 48v. As you see it is easy to make an error and ruin an expensive mic.

·      Playing around in an expensive facility designed to record musicians with expensive microphones is not an option. Getting a job as a sound professional requires practice, knowledge, experience, discipline and ears. Break a ribbon in the real world and you go home and don’t come back.

Be prepared to get on your knees to check cables. It’s good practice because you need to get good fast at understanding signal flow. You have to internalize the logic of it. No one in the recording/sound world wears a suit because it is not beneath any professional to kneel in front of a patch bay, panel, kick drum, guitar amp, piano, double bass, etc..  You are aspiring to be sound artists, but you are also serving artists, musicians, talent. Lose the attitude and learn the techniques, and while you are at it, learn to wrap cables correctly, too. (I’ll post a video of that some time soon.)

Check this link for an example of a session plan showing special treatment for ribbons.

All Students

Guys, I am taking a leave and won't be teaching next semester. It wasn't until fairly recently that I got official confirmation on this, or I would have mentioned it earlier. I want to thank you for working hard, doing a great job in my classes, and for supporting the sound aspect of the program by signing up and closing out the advanced courses.

In the near future, I will create some open posts in which you will be able to ask sound questions. Feel free to ask. Have a great remainder of the break.

Mamet Makes Spector/Pacino Flick

Phil Spector envisioned Pacino as the best actor to portray him. So David Mamet took Phil's advice for this upcoming HBO flick. This teases well. But the scope of Spector's life is the real cinematic puzzle, not the sad-sensational end. Who's going to tackle that? Maybe Mamet's piece will be the pilot to a series, starting at the end and ending at the beginning: Spector's father committing suicide by rerouting the family car's exhaust. 

Though no one knew it at the time, Spector's first hit record with his own group, The Teddy Bears, was about his father's sad end.

Spector on the right:

Post Class Bounce Ur Vidz

When you finish your Lola, or polish your other work, and want to output video with your mix, this is what you do:

Your grades are now posted, BTW.

Photoz for your scrapbook:

Sound Prdn Grades and Work

Sound Production grades are now posted. Please pick up your work from the pick-up box in front of my office. Have a good break and stay tuned to the blog.

Post class, sorry, still working on the grades.

Try not to scatter projects around plz.

And The Nail in The Ear Awards Go to...

This year, the Nail in The Ear sound award recipients ("award" used loosely) were selected by Expert TAs Mitch and Ruthie. The presentation was made at the Earpocalypse. Congrats! I have only blurry cell phone pics of the winners accepting the great honor:

Ruthie (holding "awards") and Mitch making the presentation

Kyle and Margie

        

All the honored recipients: Kyle, Merlyn, Margie and Cameron

My modded classic Shure model 55. You can still
buy these. It served hosting duties well

Pi Subwoofer Project part 3

The sub got built with just enough time for the glue to dry and was immediately thrown into service for our Sound Production final.

Some things learned might be of interest to newbie box builders, like us:
Followed proper T-nut installation instructions and those suckers go-in like a dream. Ruthie and I used 2 dabs of Gorilla Glue on each T-Nut for mounting the LAB 12 driver. However, the 4 T-nuts used to mount the speaker grill used Phillips head screws, not Allen head which were used for the woofer, so we could not get enough torque to pull them all the way in. After cranking them half way we applied 2 shots with a hammer. All seems well.

We built the box in the hallway and in the tech closet at school. That meant I had to find a lumber yard which had Baltic birch ply and could do straight cuts (no table saw at school.) Nearly impossible in Manhattan. I finally found a little storefront wood guy who had a RATTY table saw; but he had a 5X5 sheet of wood, so I took the chance. His cuts were fairly consistent except for one panel which was a little small in one dimension. I only noticed it during the dry fit. Of course we went with it anyway. I have to say that is nice strong wood. It looks nice too. The panels are laid out so the edges could be seen from the top, which I like. If all the panels were the perfect size it could have been an acceptable "furniture" or home use build. I realized after seeing it together that the box is a rather nice side table.

We went glue and screw. I bought 4 big clamps because I wanted to use the no screw method, but now I know you need a nice work table, 1 million clamps and skill to do that. So, screw it.

We never had time to apply the truck liner paint I bought, so we ran it raw. We will try to finish it with paint this or next week.

Wood blocks were added to the bottom to attach casters. I did not know if they would be rattlers, but in use, they were quiet and great for a mobile sub.

As a guy with little experience with a router, I am proud to say I made the port cutout the correct size to accept a snug press-fit of the tube, flush with the front of the baffle. We ran a bead of Gorilla Glue around it on the inside to assure an air tight seal and long term stability.

We loaded the woofer in after an overnight dry and immediately put it into service. It kicked ass. It was run with another, smaller sub, both crossed low, in hopes of smoothing some room modes. (Modes make bass seem loud or not loud enough in different parts of a listening room.) Because of time constraints, I was not able to do the usual room set-up with measurements. Placement of the 3Pi near mid point of a side wail, the other sub was offset mid point along the front wall.

One other thing: We used some 1 1/4" stock ripped into a right triangle for corner bracing in the cab. I literally found this wood on the street already cut like that. My bad is that I did not calculate how much volume that took up in the cab and adjust. I am pretty sure it would be negligible, though.

I want to thank the student builders who contributed effort into this successful build, particularly Ruthie, Mitchell and Samir.  

I thought the performance during the Earpocalypse final was excellent, as heard in the overture to our program, Stravinsky's Firebird Suite (excerpted, that was the last piece of music before James Impellizeri.) Big Bass drums and tympani on that.

Here are some pics. If we get it painted, I'll post those for before/after:

After cutting the circles with a router, we
checked the LAB 12 and the port tube for fitment

Apparently I am half-way decent with a router

Back view shows the 300 watt digi plate amp.

Personally, I like the industrial look, but this
could be finished with veneer for home use
if you build one. We will try to further the
industrial look with truck bed liner
paint if there is time.

Here's the CAD drawing with dimensions

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