RETURN II

There are far more than five senses available to you in this awful wonderful human sensorium and one of them is the sense that You Could’ve Done Better. But this was not one of those times.

You ever see something and think “I bet I could make that, but better, and more cost-effective”? You ever think “I could make a portable, hi-fi, PA speaker with shit-your-pants bass, noise-complaint SPL, and art-gallery looks”? You ever sit at home and wonder “what if I went all out? What if it was way too big and way too loud and way too pretty?” Yeah, me too. This time I did it.

SCOPE

This was probably a time that I should’ve done less, but didn’t. In scoping out a project on commission I usually discuss

  • Portability
  • Loudness
  • Bass/quality
  • AssAesthetics

The discussion should and usually does occur late at night over libations which contributes to some amount of scope creep—in a good way—and in this case we started at:

  • Portable enough
  • Loud enough the neighbors want to come to the party too
  • Yass bass
  • Museum worthy

I was thinking of a very reasonable design—1 cu ft, 36V battery, maybe 2 W6-1138 (but with Neo woofers for weight). But then something terrible happened. I saw a targeted Facebook ad for the Soundboks 2; it was was full of shitty marketing claims and absurd dBSPL/battery life statements and poorly mixed dubstep (like, dubstep is fine, just don’t mix it badly or use it to tout sound quality). Here are some reference claims:

I’m not an acoustic engineer, but—wait, no, no I am. These are bullshit metrics. What kind of half-assed sound company specifies a “dB” value but no reference for the units (SPL? Re?), distance, or weighting (A? K?). I could fart at 122 “dB: for a battery life of 40 hours if I’d put the mic by my arse.

For the un-initiated this is the equivalent of saying “Oh yeah my car is really fast, it’ll do 120.” 120…what? MPH? KPH? Like when you drive it? Or when you throw it off a cliff?

Anyway fair to say this bothered me slightly and the new goal was to make a speaker that was better than the Soundboks 2. A portable party in a box. My specific objective goals were:

  • 122dBSPL (Re) @ 1m in the passband
  • Passband 40Hz to 20 kHz
  • f3 @ 38 Hz
  • Directivity controlled ± 4 dB up to 15 kHz

*For the sake of clarity, if not otherwise specified, all dB numbers in this document will be dBSPL @ 1m relative to 20 µPa.

ACOUSTIC DESIGN

Speakers assembled to front face

BASS

On the spectrum of “large/efficient” and “small/inefficient” for a constant bandwidth target, there are three main real-life ways to achieve this in the range of “reasonably portable.”

“Pro” speakers in a large box, i.e. lots of magnet, low moving mass, stiff surround. Think FaitalPro12XL

  • + efficient as hell
  • – generally 8 ohms
  • – $$
  • – Fs is often quite high

“Tang band” style in small box, i.e. lots of magnet, lots of coil, tons of moving mass, allowing for really low free-air resonance and massive linear excursion space.

  • + compact 
  • + always impressive for size
  • – $$
  • – low efficiency

“Dayton Audio” style in a medium box: Medium BL, medium mms, heavy magnet

  • + cheaper
  • + pro-sumer design means well controlled directivity, well designed in-band response
  • + reasonably efficient
  • – heavy
  • – has potential to be “worst of both worlds”

Here’s a quick comparison of the three designs plotted at a very reasonable 100Wrms:

Simulated FR @ 1Wrms

The Iron Law clearly demonstrated here: The Dayton design is in the middle for sensitivity but sacrifices on size to get extra bass. The Tang Band, which will never have the sensitivity of the DA or the FP design, loses a little bass to be small, but has overall good LF extension. The Pro design is huge and efficient but loses on LF extension. But wait! This is battery powered! We’re voltage limited! How do 2 4 ohm drivers shake out against 1 8 ohm driver?

@ Battery Nominal Voltage

The Dayton Audio design clearly wins out (dotted lines are theoretical response vs Pmax/Xmax limited response). The final question: can we kill the Soundboks? If we disregard all concerns for safety, in theory—nearly:

At 1100 Watts of input power (rms), 2 DCS205s are capable of outputting 121.2 dBSPL @ 1m @ 100Hz. The Xmax limitation cuts heavily into the bass output below that. But this design will sound better, look better, and be smaller, so DCS205 it is!

The final choice for bass—sealed, ported or passive? A simple one; sealed sucks for battery-powered. The port would’ve had to have been huge to handle the requisite volume velocity with grace, so after modeling every single DSA, I opted for 2x DS315 12″ PRs + 100g to tune to 38 Hz. These are quite wonderful passives—huge xmax, Rms for days, low enough Fs, and their ID matches the DS205s.

Full send:

MIDRANGE

Knowing that we’re in the neighborhood of 115 dBSPL@1m @ 50V input makes things a bit tricky from 300Hz to 20kHz. Pro sound options are mainly focused on output efficiency, with the sacrifice being directivity and flatness of response (DA PK165-8 below, which had neither the efficiency nor the response I was looking for):

What a terrifying directivity curve

which would essentially demand that you cross it over at 2kHz—untenable for a tweeter capable of 115 dBSPL.

Luckily, there weren’t that many options, and when one can’t sacrifice loudness, quality, or size (i.e. directivity), you must pay a lot of money. I landed on the beautiful and beautifully expensive FaitalPro M5N8 which measures like:

Their claimed efficiency of 99dB @ 1W/1m is a little short of the truth (95dB@1W/1m) but they manage an easy 117 dBSPL with xmax and plim constraints:

Throw it in (a 3D printed back volume to separate it comes in later):

HF

There was really only one option: the Peerless by Tymphany BCS25SC08, a silk dome tweeter + a (small) horn for efficiency. 98 dB, 100W of power handling (they get ragged before that, though), and the horn rather small so the directivity actually rather pleasant (this is in 30 deg intervals compared, the midrange plot was at a 45 deg intervals).

Throw that in the bag too:

And then for a back volume, the passives:

Next up is the electrical design. Stay tuned!