Speaker IV – In-Cabinet Speaker Padding

I finally got around to a final modification to the “built-in” speakers of the entertainment center (AKA Speaker IV).  The way the speakers sit in the cabinet leaves a gap of about an inch between the baffle and the door.  That gap is an acoustic mass that has to interact with the speaker output, creating resonances.

Solution: add some acoustic foam around the front of the speaker to seal the gap.  So the foam cut-outs for the drivers are something of a hatchet job.  It works.  Well.  The mid-range, such as voices, are so much clearer, more intelligible.  Little details that were masked before are now discernible.

2016-01-18 17.25.57Drivers are a 2″ G2Si ribbon tweeter and Peerless 6.5″ HDS series mid-bass.  Cross-over is acoustic 4th order LR at 2.5 kHz.  The mid-bass is in a sealed cabinet tuned to 80 Hz.  Why so high?  The intention is for these to be crossed over to a subwoofer at 80 Hz as “small” speakers.

Response tested sitting about 32″ off the floor – pre foam.  (I’ll do an update when I do a post-foam test.)  Test level is 1 watt, measurement distance 18″.  Measurement window is about 3.5 msec, so the response is smoothed.

S4 right zoomedThe distortion levels are respectable.  The distortion above 1 kHz is mostly 3rd harmonic (green) which suggests the ribbon tweeters would sound best crossed over above 4 kHz, as well as the mid-bass could be crossed over lower as part of a 3-way design.  (Will have to confirm with additional measurements of the individual drivers.)  Alas, not a big enough opening in the entertainment center for 3 drivers.

S4 Right Distortion

Seas 22TAF/G (H1283)

I used these in my brother’s speakers, Speaker III.  I’ve always had a soft spot for 3/4″ dome drivers.  I used them in my first design, Speaker I, listened to them for years.  When I switched out to a pair of Polk RTi4’s with 1″ tweeters, one of my first impressions was the very top end lost some air, some imaging.  I could localize symbols to the speakers, which I didn’t recall with my own design.  I have always – and possibly erroneously – attributed that to smaller diameter tweeter.

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This Seas 3/4″ uses an aluminum/magnesium alloy for the dome, fabric (Sonomex) surround, and ferro fluid for additional power handling.  Resonance is 1100 Hz, and sensitivity rated at 92 dB.  Testing was therefore done at -2 dBW.

Frequency response is very smooth, and quite extended.

Seas 22TAFG -2dBW freqz

Distortion isn’t as a good news story.  Performance here is generally average, with the good news that its primarily second order distortion.  I believe the distortion suggests that a 3 kHz or higher cross-over is the best choice, and as low as 2 kHz would likely work (my Speaker III crossover is 2.5 kHz).

Seas 22TAFG -2dBW distortion

Vifa (Peerless) D27TG-05-06

The D27TG-05-06 is a conventional 1″ fabric dome tweeter with ferro fluid (pictured below on the left).  Resonance is a respectable 1 kHz with a Qts of 0.77, sensitivity is advertised at 92 dB.  Testing is done at -5 dBW (should have done at -2 or -3 dBW).

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The frequency response is quite smooth, with a few dB of gradual roll-off on the top end.

Vifa D27TG05-06 -5dBW freqz

What is even more impressive is the distortion performance – this unit is 6 years old, been sitting around in a box!  Distortion is at -50 dB from the fundamental through most of the band above 2 kHz.  The third harmonic only rears its head at 4-5 kHz.  This tweeter could easily cross over at 2 kHz at 2nd order, and maybe 1.8 kHz at 4th order if you wanted to push it.  That explains why this tweeter has had such a long life and is still available.

Vifa D27TG05-06 -5dBW distortion

Dayton Audio Horn

Dayton Audio produces a 1″ polyamide compression driver and several threaded horns.

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The combination compression driver with horn is very efficient.  I estimate about 105 dB at 2.83 vRMS / 1 meter based on the measurements, which matches the manufacture’s specs.  Thus all testing is performed at -15 dBW.  The frequency response has a gentle roll-off of about 2 dB / octave until 12 kHz.  Then it drops off more quickly, and shows resonant behaviors.

Dayton horn freqz -15dBw

The distortion curve is quite surprising.  It is completely dominated by second order harmonic distortion!  Reportedly second order harmonic distortion is the least offensive.  I’ll need to build something from this and find out how it sounds.  Recommended cross-over is 1.6 kHz, and that looks about right provided the slope is 2nd order or better.

Dayton horn distortion -15dBW

Tang-Bang TW25-1744S

This is a ceramic dome tweeter with a neodymium magnet.  I picked these up based on the manufacturer’s claims of low distortion, extended response (to 30 kHz), and the compact mounting dimensions.  A plastic mesh grid covers and protects the dome.  Its the center tweeter shown below.

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Here’s the first unit frequency response

Tang-bang freqz 10 msec

Very smooth, and extended to 700 Hz, but a combination null / peak at the top end.

The distortion measurement

Tang-bang distortion

A little higher than expected.  Other tweeters and systems that I will be showing have the harmonic distortion as low as 50 dB below the fundamental.  The distortion is rising below 2 kHz which limits the crossover frequency and slope to be 2.5 kHz at 3rd order.

Tweeter Testing Methodology

Finally getting comfortable enough with the Room Equalization Wizard software to start some serious testing.  Another audiophile has set the bar high – Zaph Audio.  He has done large tests of tweeters, mid-ranges, and woofers.  One of his claims is harmonic distortion is a key discriminator in the sound of speakers and their drivers, and is an important factor in setting the cross-over frequency.  So my goal is to mimic his methodology.  Part of the Zaph Audio approach is consistency – testing all of the drivers using the same process.  So even if it turns out the process has some hidden defects, the ranking between drivers are likely to remain valid.

I started by building a large baffle with a replaceable 12″x12″ panel to which drivers can be mounted.  Flanking on the left/right are two of the GIK Acoustics panels borrowed from the living room.

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The problem with this setup is the reflection from the ground.  The driver center is about 30″ above the ground, and the microphone is 18″ away.  Doing some math, you can show that the round trip travel time will be about 3.85 msec for the ground reflection compared to the main impulse response.  Here’s an example.  Notice the “blip” at 3.85 msec?

Example Impulse Response with Reflection

Why is the extra blip a problem (as well some other smaller, extra blips)?  It distorts the frequency response calculation.  Show below is the result with the full 10 msec of data:

Dayton horn impulse 10 msec

Next, with the data limited to a 3 msec data record:

Dayton horn impulse 3 msec

So how to get a longer, clean data record?  How about laying the board down so that the driver faces up?  The ceiling is 10′ high in the room.  There is a lot of equipment in the room which will generate reflections.

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Here’s the resulting impulse response – clean for a full 10 msec!  (Alright, not perfectly clean, but significantly better.)

Example Impulse Response NO reflection

The resulting frequency response, compared with the upright measurement with a 10 msec data record.  The imperfections in the curve – likely the result of the finite test baffle – are sharper and more exaggerated with the reflection present.

Dayton horn freqz compare 10 vs 10

For completeness, repeat the comparison, 10 msec data record on the floor vs. 3 msec data record upright with reflection.

Dayton horn freqz compare 10 vs 3

What else needs to be controlled?  For the distortion measurements to be comparable, the input power should be set so the driver output is at a consistent value – I’ll use the same value of Zaph Audio of 90 dB at 1 meter.