s ▁have ▁some ▁f ret ▁mark ings ▁as ▁a ▁visual ▁aid , ▁particularly ▁for ▁higher ▁pitch es . ▁ ▁Am pl ification ▁▁ ▁Sol id ▁bod ied ▁E UB s ▁produce ▁very ▁little ▁sound ▁without ▁electronic ▁ampl ification . ▁H ollow - b od ied ▁E UB s ▁produce ▁a ▁quiet ▁tone ▁that ▁is ▁loud ▁enough ▁for ▁individual ▁practice . ▁However , ▁since ▁h ollow - b od ied ▁E UB s ▁do ▁not ▁have ▁a ▁large ▁reson ant ▁cav ity ▁like ▁a ▁double ▁bass , ▁they ▁cannot ▁reproduce ▁the ▁lowest ▁notes ▁of ▁the ▁instrument ▁without ▁an ▁ampl ifier . ▁To ▁ampl ify ▁the ▁E UB , ▁the ▁string ▁v ibr ations ▁are ▁sens ed ▁with ▁a ▁pick up . ▁▁ ▁Early ▁E UB s ▁used ▁magnetic ▁pick ups ▁similar ▁to ▁those ▁in ▁electric ▁gu it ars , ▁or ▁per cuss ive ▁magnetic ▁dia ph rag m ▁pick ups ▁( e . g ., ▁the ▁A mpeg ▁Baby ▁Bass ). ▁ ▁Many ▁modern ▁E UB s ▁use ▁pie zo elect ric ▁pick ups ▁located ▁in ▁the ▁bridge ▁or ▁a ▁combination ▁of ▁pick up ▁types . ▁The ▁signal ▁from ▁the ▁pick up ▁is ▁usually ▁pre am pl ified ▁and ▁equal ized ▁with ▁a ▁pre am pl ifier ▁and ▁then ▁sent ▁to ▁a ▁bass ▁ampl ifier ▁or ▁a ▁PA ▁system . ▁For ▁practice ▁in ▁a ▁hotel ▁room ▁or ▁a partment , ▁an ▁E UB ▁can ▁also ▁be ▁connected ▁to ▁head ph ones . ▁ ▁Some ▁E UB s ▁have ▁a ▁h ollow ▁reson ant ▁chamber