This is an old, old, trick that I learned from a guitar store owner in the 1960s. If you take your solid-body electric guitar and press the end of the headstock against a door, it will amplify the sound of your playing very considerably.
I do this all the time with my Strats on 6-panel pine doors, but it works well with hollow doors, too.
This also confirms that the headstock and neck have a lot to do with the resonance and tone of a guitar.
I know a lot of people already know about this, but this is for those who have maybe never tried it.
I am not sure what the DAF (Door Amplification Factor) is, but I am going to do some measurements and get back to you. In my house, the solid pine doors are loudest, followed by the hollow slab doors. The metal entrance doors are not very good at all.
Here are some data on the DAF (Door Amplification Factor)
First, some notes: Hearing is a logarithmic function with respect to frequency. When measuring SPL (Sound Pressure Level), normal ears can hear a 1 KHz tone at 0 dB SPL. But we can't hear a 82 Hz tone (the low E string on a guitar) until about 40 dB SPL.
Amplitude in dB can be confusing. Perceived loudness is what you hear. If you have a 100W amplifier and you want the sound to be 'twice as loud', you need to increase the power to 1kW (1,000W). Just doubling the power results in a 3dB increase, and although audible it is not dramatic. 1dB is the smallest change that the average listener can hear.
With that out of the way:
I used a digital sound level meter to measure the loudness of the guitar alone and the guitar amplified by a door.
The meter was 14 inches from the guitar and the door for all measurements. (Speakers are usually rated at a distance of 1 meter.)
The same location was used as the contact point between the guitar headstock and the door. With a six-panel door, the upper part of the right hand middle panel joint was contacted at a 45 degree angle. (See photo). Significant pressure was applied to the guitar in the direction of the door during testing. I would guess the force to be about 2 - 4 pounds.
I used three different Stratocasters: an ash body Strat, an alder Strat with a big "swimming pool" route, and a laminated mahogany/black walnut/spalted maple Strat.
...and tested with a 6-panel pine door (6PP)
I strummed an open G chord on the guitar in the same way for all testing. The open G is what most guitars are tuned to, mechanically, and the frequencies center around 196 Hz.
I put the meter on MIN/MAX and captured the maximum SPL for each part of each test. These can be compared.
Guitar Guitar with 6PP Door Net Gain
Ash Stratocaster 76.6 dB 79.8 dB 3.2 dB
Alder "swimming pool" Strat 76.4 dB 78.9 dB 2.5 dB
Mahogany/Walnut/Maple Strat 76.9 dB 79.4 dB 2.5 dB
RESULTS: The ash Stratocaster saw a 3.2 dB increase in loudness with the 6 panel pine door. 3 dB represents a doubling of power. It is significantly louder than the guitar by itself.
From a statistical standpoint, there's a big standard deviation due to the small sample group, so it's fair to say that the three guitars saw the same loudness increase due to the door. (The difference between the readings is less than 1 dB, which is not detectable).