What gives some pianos a Honky Tonk sound?
There is a complex answer to this seemingly simple question. First, is this really a desirable sound? When you learn what it can do to a piano you may rethink your answer! I am an experienced piano technician who started tuning in the mid-1970s, and have run into the need to both prevent and create the sound. Here are variances on how it is achieved and the problems that go along with it.
The first problem, to me (it's a matter of taste), is WHY? Why would anybody want the instrument to sound like anything other than, say, a PIANO? I keep my instruments in good tune, and all of my recordings are done on conventionally tuned pianos with no anomalies. Again, that's my preference. This sound started sometime in the mid to late 1940's when certain artists were "rediscovering" ragtime music. They also rediscovered the pianos the music had been played on, as uprights were not regularly manufactured past the 1920s, and many had not been regularly maintained since the ragtime era. During that time there was a definitively larger ratio of pianos to technicians than exists now, so good piano care on a consistent basis was hard to come by. Anyhow, somebody thought this might be a cool sound, and by the mid-1950's, many record companies were producing albums played on pianos made to sound "honky-tonk." The origin of that very term has been obfuscated over the years, but educated speculation by myself and my peers would suggest that the application of this term to a drinking establishment where such music was played more likely came from the musical term than vice-versa, and that the musical term likely emanated from the sound of the piano (which really does not resemble geese in flight!). Therefore, the application to a drinking establishment would, in a literal translation, mean drinking establishment where one can hear an out-of-tune piano.
How to achieve the sound needs to start with how not to. There are three common techniques. One is to make the piano hammers harder. I have seen countless tragedies where somebody has inserted thumbtacks into the tips of the poor defenseless hammers. Since a hammer is essentially a piece of felt bent over wood at a tension of over 60 psi, the tack will likely split the felt in a short time, leaving only the wood exposed. Then, either the wood or the still-inserted tack will break the strings due to the hardness. The older the piano, the more true this is. So how do you harden the hammer? Realize that this is permanent before proceeding. Make a mixture of one part lacquer to five parts lacquer thinner. Remove the piano action and place it on newspaper, leaning forward a bit so there will be no dripping to the action's moving parts. Paint on a coat along the fronts of the hammers only, and allow 12 hours to dry. Then a second, then a third. Your piano will be out of commission for two to three days, but the resulting sound will be both louder and brighter. The coats in the lower register where the single and double strings are should be a little lighter to help avoid breakage from the percussive hits. This should be redone about every five years, depending on how often it is played.
There is a less permanent, but tinnier device known as the mandolin rail. It is available from nearly any piano roll manufacturer or many piano supply companies, and can be retrofitted to most upright pianos. By either pulling a knob or pressing the middle pedal of an upright/console, you will get the extremely thin and brittle honky-tonk effect. However, the brads hanging on the end of this device can chew up the hammers a bit, and when the felt splits and the brads fall off (attached to their thin canvas fingers), they can fall into the action and cause the necessity for a technician to remove them from harms way and lecture you on the reasons for not using this device. Also, it depends on gravity and is therefore useless on a grand piano (thank goodness). To find one, look up [piano supply "mandolin rail"] on Google.
The last method is detuning. This was most commonly used by artists such as Billy Rowland and Dick Hyman (both as "Knuckles" O'Toole), Winifred Atwell and Del Wood. The center strings are tuned as they should be, then the alternate string (in the bass and tenor) or one of the side strings (in the treble) are detuned either up or down just a little to create the gaudy effect. Since not all of the strings are playing on the same frequency, it tends to soften the piano a bit, as some of the wave sets between the strings tend to cancel a large number of the harmonic overtones the note would normally produce, rendering the piano softer and unplayable for virtually anything except "honky-tonk" ragtime. One of the prerequisites for trying this method is to have your own tuning hammer and mutes, and good knowledge of how to set pins. Again, why would anyone want to voluntarily do this to a piano? I still don't have an answer as to why it is appealing to some people, but I accept it for what it is.