I have a pair of older freestanding speakers and they take up quite a lot of space. They are 29 inches tall and 18 inches wide. So I thought "Hey, kill two birds with one stone and make a 'bricks and boards' bookshelf out of them to save some space." I know if I crank the volume they will vibrate the shelves pretty badly but for normal listening they should be fine right? I have some junk on top of them now and it doesn't seem to hurt anything but will having them so close to books and stuff significantly effect the sound? I know the weight wont be an issue as I have sat on them before (and like many on 4chan and I am quite fat) and they don't budge.
Picture of random old speakers from google.
If they were a decent pair of well build speaker before, the size of the cabinet would of been tuned for the specific driver.
Volume and port sizes (size and depth) matter (look up diy speaker building/golden speaker ratio/port calculations etc)
It's quite possible they will sound like absolute shit.
That's not really true, for example Sonus Faber only makes legit speakers, no HTIB or multimedia shit like many previously respectable brands (B&W, Focal etc) have been pushing lately.
Whether they're the best bang for your buck is another question altogether.
Old stereo speakers would probably be designed to stand in free space. When you put a speaker up against a wall, on the floor, in a corner, in a cabinet, etc, you're making the bass and midrange louder than what was intended. This is because treble is directional like a ray, and bass is less directional or omnidirectional and behaves as pressure. Sound pressure that would normally fill the air behind the speaker is instead sent to the listener. Basically the more you cover the speakers, the less accurate they will sound.
Resonances also decrease sound quality. You'd want something to put in between the speakers and the shelves like >>52355152
said so you aren't transferring energy to them, especially to the point of vibration.
Also I'm confused, is this going to be a 29 inch tall shelf? Don't place these on the floor. The tweeters are ideally at ear height because of the directional qualities of treble. This is why normal people but speakers on shelves instead of shelves on speakers.
Old speakers are great and it would probably cost a lot of money to buy a modern equivalent.
But if the speakers are nearing 30 years old, the capacitors need replacing to sound their best.