Why can’t I hear my THTLPs?

[DaveMacKay]

I have two minimum-size (15"x18"x72" with 12” RSS315HF-4 driver) THTLPs in my listening room. They are upright with the mouths firing towards the ceiling.

I have the THTLPs connected to my stereo and use them for listening to music. I typically listen at levels of 75 to 85 dB.

I know that they are working:

• With REW, I can see how they perform in my room (see attached graph).

• If I turn them off, the music seems hollow.

But I can’t hear them making any sound. Even when I put my ear right against the THTLP (near where the driver is located) I can’t discern them making any sound.

Is that how they should be performing? Do they only become more pronounced at higher volumes? Or might I have a problem that needs investigating?

[J_Dunavin]

Try unplugging just one.
Make sure they are not playing out of phase with each other

[Bill Fitzmaurice]

The reasons are equal loudness and localization. It takes a 100dB tone at 60Hz to sound as loud as an 85dB tone at 600Hz. Even with the subs at a relatively high level the sound will seem to be coming from the mains, because of the distance between your ears. When that's relatively long compared to the wavelengths being heard you can directionally located where they're coming from. When that's very short compared to the wavelengths being heard you can't. A 60Hz wavelength is 18.75 feet. Even elephants don't hear very low frequencies, which they use to communicate over very long distances. They sense them with their feet via ground conduction.

[DaveMacKay]

Deleted message. Taking new measurements.

[Bill Fitzmaurice]

Do it again with two different mic positions. That should give two different results. If not something is amiss. The peaks and valleys are caused by boundary reflections. They should shift with the mic placement.

[DaveMacKay]

Try unplugging just one.
Make sure they are not playing out of phase with each other

Thanks for the suggestions. In pursuing them, I've taken some measurements with REW and created charts showing the results.

This shows how my two subwoofers perform together, orchestrated by a MiniDSP 2x4HD:

both on.jpg

This shows how each subwoofer measures individually when the other subwoofer is muted:

individual.jpg

This shows the affect that changing the phase has on each individual subwoofer:

one at a time.jpg

To take these measurements, I set 1 subwoofer at 0 degrees phase and set the other to 180, then switched them vice versa.

This shows the affect that setting the phase to 0 or 180 on both speakers has:

both 0 or 180.jpg

Based on that data, I have changed my subwoofer configuration so that the LF sub is inverted (i.e., phase at 180 degrees) and the RR subwoofer is as 0 degrees.

new config.jpg

So .. I know that the subwoofers are working and --- thanks to your suggestion --- I've improved my configuration. Thank-you.

However, it still seems odd that I don't hear sound emanating from the subwoofers.

[DaveMacKay]

The reasons are equal loudness and localization. It takes a 100dB tone at 60Hz to sound as loud as an 85dB tone at 600Hz. Even with the subs at a relatively high level the sound will seem to be coming from the mains, because of the distance between your ears. When that's relatively long compared to the wavelengths being heard you can directionally located where they're coming from. When that's very short compared to the wavelengths being heard you can't. A 60Hz wavelength is 18.75 feet.

Thanks Bill.

I do hear satisfying sound at the low end, it just doesn't sound like it comes from the subwoofers. Your explanation has taught me why that's the case.

[DaveMacKay]

I think I uncovered the problem: I had the gain structure set wrong. The signal I was sending to the SPA250DSP plate amps was of insufficient voltage.

I describe the process I went through in setting the correct gain between the different components in my system in this post https://audiokarma.org/forums/index.php ... n.1054984/ on Audiokarma.

TLDR ... the minimum sensitivity of the SPA250DSP plate amp I'm using with my THTLPs is 320 mV. Because of the gain structure I'd implemented (-20 dB on the MiniDSP and -10dB on the plate amp), the signal being sent to the THTLP was often below that threshold. Changing the gain structure to - 5dB on the MiniDSP and -25 dB on the plate amp eliminated that problem.

[Bill Fitzmaurice]

I should have mentioned this before, but the only way you can really hear a sub with very low THD is when you turn off the mains, or if the sub is some 20dB louder than the mains. Subs are meant to be felt, not heard.