Speaker-Microphone and Room Reverb Effects

[mikebse2a3]

I had become curious about the "Speaker-Microphone-Tube Amplifier and the Room Reverb Effect" claims put forth by Bob Carver and others but any actual test/measurement data has not been presented to the public that I could find. So I had decided to perform some test with my system/room in an attempt to measure this phenomenon and understand any relation to how any perception of the effect would manifest itself.

 

I will post the measurements and results in later posts.

 

miketn

 

 

At this time I would like to present some comments by Bob Carver about this phenomenon for background to anyone interested in this subject.

 

 

http://www.enjoythemusic.com/superioraudio/equipment/0512/bob_carver_cherry_180_tube_monoblock_amplifier.htm

 

It is well known that a loudspeaker can work backwards, converting sound pressure into an electrical signal at its input terminals. Many years ago, it was Roger West who mentioned having used a Sound Lab ESL as a microphone, though I'm not sure with what degree of success. Carver believes that his amps can take advantage of this effect and says that:

 

"The extra cool thing about this amp is that it has the ability to listen to the room via the principal of reciprocity. The loudspeaker the amp is connected to behaves like a microphone and "listens to the room." That signal is then sent back to the input to be mixed with the forward signal to help generate a more delicious sense of acoustic space by including the room in the final sound in a way other amps can't."

 

I expressed my doubts to Carver about the audibility of this effect, noting that the speaker microphone effect couldn't amount to more than a few millivolts. Carver's response was as follows:

 

"You are right - about 3.1 mV at 90 dB SPL at our listening position, then multiplied by the gain of the amp, 32 X = 0.0992 volts rms. With a high quality speaker about 89dB or so sensitive at 14 feet away we have about 8.5 forward volts at the speaker, and 0.0992 "speaker microphone" volts. Finally, 0.00992/8.5 = -38.6 dB, just barely noticeable, but quite significant after we get used to listening to it."

 

"As low as it is. All tubes amps do this a little bit by their nature, transistor amps don't do it at all, and this amp does it a lot by intentional design. It's easy to hear the "oom signal if we hook up a small speaker on the end of about 150 feet of wire and go outside. Hook it across the main speaker in your living room with the amp turned on, but no input. Then have a friend come into your living room and clap his or her hands, stomp around, make noise and sing happy birthday. While you are 150 feet away outdoors with the door closed so you can't hear your friend from the room, hold the small test speaker in your hand and listen to it. You will hear the sound of the room. With a solid state amp, you will not hear a thing, just silence. All tube amps that have a non-zero output source impedance do it, at least a little bit as you surmised. Transistor amps don't do it at all because the almost zero source impedance shorts out any "speaker microphone" signal. Yes, my amps do it a lot by virtue of the current feedback loop. With this amp, lower idle current (as read on the meter), makes the amp do it more. Higher idle current makes it do it less."

-------------------------------------------------------------------------------------------------------------------

 

 

You can begin listening at 50 minutes into the video for his comments about again this phenomenon.

 

https://youtu.be/LQ9USqpclWc

 

"So what happens in a vacuum tube amplifier is the amplifier makes another sound that's related to the sound that it heard, in other words the amplifier is able to listen to the room because it is hearing reverberation, echoes, time delays, all the of the components associated with a venue. So what happens is a loudspeaker speaks and the room speaks back to the loudspeaker, the amplifier hears it by the signal going around the feedback loop and out it comes again..............The real delay is the acoustic delay and that delay makes it sound spacious and big to our ear/brain system.

[mikebse2a3]

This is the Sound Source Drive Level used to drive the Listening Room.

 

Sound Source: Jubilee KPT-KHJ-LF

Source Drive Channel:  First Watt F3

Scope Range: .5v/Div

Source Drive Signal Frequency/Level:  315 Hz @ 3v p-p

Edited May 28, 2015 by mikebse2a3

[mikebse2a3]

This Test is for reference comparisons with later Test Results.

 

 

Speaker-Microphone:  Jubilee KPT-KHJ-LF

Amplifier:  None (This is just the speaker connected to the scope as just a microphone)

Scope Range: 10mv/DIV

Measured Level: 315Hz @ 6mv p-p

 

This reverb level is down -53.97db / 0.2%  versus the 3v p-p sound source drive voltage.

 

 

What this represents is under the reference drive level used this is the maximum voltage the Speaker/Microphone itself could generate due to nothing but the scope loading it.

Edited May 28, 2015 by mikebse2a3

[mikebse2a3]

This Test is for reference comparisons with later Test Results.

 

 

Speaker-Microphone:  Jubilee KPT-KHJ-LF

Amplifier:  None ( A 4-ohm non-inductive resistor is paralleled with the Speaker-Microphone for a reference voltage result for comparisons)

Scope Range: 5mv/DIV

Measured Level: 315Hz @ 3mv p-p

 

 

This reverb level is down -60db / 0.1%  versus the 3v p-p sound source drive voltage

Edited May 28, 2015 by mikebse2a3

[mikebse2a3]

This Test is for reference comparisons with later Test Results.

 

 

Speaker-Microphone:  Jubilee KPT-KHJ-LF

Amplifier:  None ( A 2.66-ohm non-inductive resistor is paralleled with the Speaker-Microphone for a reference voltage result for comparisons)

Scope Range: 5mv/DIV

Measured Level: 315Hz @ 2mv p-p

 

 

This reverb level is down -63.52db / 0.066%  versus the 3v p-p sound source drive voltage

Edited May 28, 2015 by mikebse2a3

[mikebse2a3]

This is the Speaker-Microphone (Jubilee KPT-KHJ-LF) connected to the First Watt F3 with no Input Signal.

You are seeing the Speaker-Microphone Effect..!

 

 

 

Speaker-Microphone:  Jubilee KPT-KHJ-LF

Amplifier:  First Watt F3

Scope Range: 5mv/DIV

Measured Level: 315Hz @ 1mv p-p

 

 

This reverb level is down -69.54db / 0.033%  versus the 3v p-p sound source drive voltage

Edited May 28, 2015 by mikebse2a3

[mikebse2a3]

This is the Speaker-Microphone (Jubilee KPT-KHJ-LF) connected to the Cary CAD2A3 SE (4ohm tap) with no Input Signal.

You are seeing the Speaker-Microphone Effect..!

 

 

 

Speaker-Microphone:  Jubilee KPT-KHJ-LF

Amplifier:  Cary CAD2A3 SE (4ohm tap)

Scope Range: 5mv/DIV

Measured Level: 315Hz @ 4mv p-p

 

 

This reverb level is down -57.5db / 0.133%  versus the 3v p-p sound source drive voltage

Edited May 28, 2015 by mikebse2a3

[mikebse2a3]

This is Testing using the K402/TAD TD4002 as the Speaker-Microphone and Driving Source.

 

This is the Sound Source Drive Level used to drive the Listening Room.

 

Sound Source: KPT-K402 / TAD TD4002

Source Drive Channel:  First Watt F3

Scope Range: .5v/Div

Source Drive Signal Frequency/Level:  2kHz @ .5v p-p

Edited May 28, 2015 by mikebse2a3

[mikebse2a3]

This is the Speaker-Microphone (K402 / TAD TD4002) connected to the First Watt F3 with no Input Signal.

You are seeing the Speaker-Microphone Effect..!

 

 

 

Speaker-Microphone:  K402 / TAD TD4002

Amplifier:  First Watt F3

Scope Range: 5mv/DIV

Measured Level: 2kHz @ 1mv p-p

 

 

This reverb level is down -53.979db / 0.2%  versus the .5v p-p sound source drive voltage

Edited May 28, 2015 by mikebse2a3

[mikebse2a3]

This is the Speaker-Microphone (K402 / TAD TD4002) connected to the Cary CAD2A3 SE with no Input Signal.

You are seeing the Speaker-Microphone Effect..!

 

 

 

Speaker-Microphone:  K402 / TAD TD4002

Amplifier:  Cary CAD2A3 SE

Scope Range: 5mv/DIV

Measured Level: 2kHz @ 1mv p-p

 

 

This reverb level is down -53.979db / 0.2%  versus the .5v p-p sound source drive voltage

Edited May 28, 2015 by mikebse2a3

[mikebse2a3]

Some comments about equipment used and test results.

 

Of course both the (Jubilee KPT-KHJ-LF) and the (K402 / TAD TD4002) are "High Efficiency Systems"

 

The Cary CAD2A3 SE is a 2A3 tube based mono amplifier which has no global negative feedback and would be considered to have a High Output Impedance by design.

 

The First Watt F3 is a Nelson Pass single gain stage amplifier with a small amount of global negative feedback with about a 1ohm Output Impedance which is on the rare side of Solid State Amplifier Designs.

 

 

While using sustained Tones of either 315Hz or 2kHz as Test Signals doesn't display any Time Delay Effects it does show the Speaker-Microphone Effect as far as listening to the room and the resulting effect when connected to these amplifiers or resistors tested.

 

It is also very important to remember that these results are with the Speaker-Microphone (ie: Jubilee KPT-KHJ-LF or K402/TAD TD4002) connected to amplifiers with ZERO Input Drive to them. I believe there is good reason to believe that if the loudspeakers had been driven (as most recordings would be providing) the ability of the loudspeakers to act as a Microphones would be severely limited.!

 

To quote member Ken (AEA Audio) thoughts on this: "The concept of using a speaker as a microphone to capture some “echo” while it is playing music cannot be done, for many reasons. The biggest reason is that the sensitivity of the speaker (ability to pick up sound) is almost zero when the cone is extended under force from the voice coil. Think of the effect of putting your hand firmly on the cone while trying to use the speaker as a mic. The pressure will keep the cone from vibrating, so it can’t pick up sound. The same thing happens when the cone is under force (being  pushed) by the voice coil, causing any ability to pick up ambient sound to be greatly dampened."

 

I believe the test results support the fact that neither amplifier is amplifying (at least of any significance) the Speaker-Microphone signal based on voltage levels read when compared to the (resistor loading or no load) test used as reference points where no amplification was capable of happening.

 

As far as the perceived audibility of this effect and considering the worst case test result had the Speaker-Microphone signal -53.979db below the Drive Signal which equals only 0.2% of the Drive Signal Level I suspect no one would hear the "Speaker-Microphone Reverb Effect" in any typical listening room with typical music recordings with the equipment I tested.

 

It would be very interesting to see Test from Bob Carver and his amplifiers since he reports enhancing the Speaker-Microphone Reverb Effect with his designs.

 

 

miketn

Edited May 28, 2015 by mikebse2a3

[DrWho]

Mike, you might want to try different frequencies while you're at it.....room modes are going to have a large effect on how much coupling you should expect.

I've seen much larger numbers in the past....sometimes as high as a volt when using subwoofer drivers.

The thing that confuses me is that the original acoustic wave completely reflects off the speaker diaphragm when using a low output impedance amplifier. Basically the diaphragm is very rigid. How is this any different than the speaker diaphragm moving a little bit, and then getting amplified right back? If anything, I'd expect the higher output impedance amplifier to have less reflected energy since the higher cone motion acts like an absorber. If the amplifier actually amplified the signal to be louder, then it would be inherently unstable.

Edited May 28, 2015 by DrWho

[mikebse2a3]

Mike, you might want to try different frequencies while you're at it.....room modes are going to have a large effect on how much coupling you should expect.

 

I agree Mike and did try some different frequencies but it should also be noted that this was a real world test where the room mode patterns would exist to some degree although the sustained tone does allow them to fully develop.

[tube fanatic]

Nice overview Mike.  Wonder if there's a way to get Carver to become involved in the discussion!

 

Maynard

[Chris A]

I expressed my doubts to Carver about the audibility of this effect, noting that the speaker microphone effect couldn't amount to more than a few millivolts. Carver's response was as follows:

 

"You are right - about 3.1 mV at 90 dB SPL at our listening position, then multiplied by the gain of the amp, 32 X = 0.0992 volts RMS. With a high quality speaker about 89dB or so sensitive at 14 feet away we have about 8.5 forward volts at the speaker, and 0.0992 "speaker microphone" volts. Finally, 0.00992/8.5 = -38.6 dB, just barely noticeable, but quite significant after we get used to listening to it."

 

It seems to me that what would be audible is a decay after an impulse (like a floor drum head hit), looking for the characteristic room bounce delays (i.e., room length, width, and height) coming back to the speaker, then comparing the "speaker-microphone" response to the original source input, looking for added reverb effect.  This would include the time delays associated with each room acoustic reflection.  Broad band impulses would simultaneously show full spectrum response.

 

Remember that room acoustic reflections are down several dB based on distance from the reflected surface.

 

Also note that the TAD driver has about an 11 ohm minimum impedance at 650 Hz and 6-10 kHz on the K-402 horn.  The bass bin has a minimum impedance of 7.5 ohm from 20-100 Hz.  Those are the regions where I would expect to see that greatest coupling to either amplifier.  The F3 output impedance being stated at 1 ohm.  I don't know what the Cary output impedance is.

 

Do you have a junction transistor amplifier (i.e., a "SS amplifier") that you could also use that has really low output impedance - like 0.1 ohm or even much less?  It would be interesting to see the difference.

 

Mike--thanks for posting the results.  I know that it takes real effort and time to do this, and the community benefits from being able to see the results.  Measurements speak louder than mere words and the human hearing system hears differently than simple measurements can show, clearly.

 

Chris

Edited May 28, 2015 by Chris A

[DrWho]

 

Mike, you might want to try different frequencies while you're at it.....room modes are going to have a large effect on how much coupling you should expect.

 

I agree Mike and did try some different frequencies but it should also be noted that this was a real world test where the room mode patterns would exist to some degree although the sustained tone does allow them to fully develop.

 

 

Right, and a fully developed null at the speaker means less voltage....

 

And would it be a pressure null or a velocity null that would cause the speaker to move more?

[Chris A]

This is probably the easiest way to hear the differences between amplifier types, e.g., the reverb effect:

 

http://www.libinst.com/Audio%20DiffMaker.htm

 

In addition, saving the difference file will permit multiple analysis techniques to be applied on it for not only quasi-steady-state signals, but also transients.

 

Chris

Edited May 28, 2015 by Chris A

[Ski Bum]

Nice expose Mike, much more thorough than my own tests.  I hooked a single ended amp with output impedance ~3 ohms, no source, volume cranked, to one of my fortes.  I recall reading 1.4 mv (power supply hum) when the room was silent, then blasting music through the other forte on a big ss amp to unhealthy spls and seeing 2-3 mv.  At more sane levels, ones more in line with what the small amps could actually deliver, I was getting only marginally more voltage than the power supply hum, never cracking 2 mv.  It's really happening, but it's buried way, way down there.